Forges 101

[Mikey98118]

Yes.

[Mikey98118]

I have noticed a recent uptick in activity on this thread, but fail to see a corresponding increase in questions. Your worries aren't state secrets, or even surprising to us. So you might as well fess up, and get some answers. We all promise to be kind; really we do

[Frosty]

It's funny, as tired as I got of answering the same questions over and over the recent slow spell is kind of sad. Reading the recent posts I see we have a fellow in Ymber who has indeed read past posts and only needs a little clarification. 

Kind of cool eh?

Frosty The Lucky.

[Trevor84]

Oh don't you worry, I promise that I will come running with some confusion soon enough... 

I started school last September, college classes after 20 years out of school. I hadn't even completed grade 10! Through disability I was blessed to have financial support, (even if I did have to debase myself and fight tooth and nail). 

I am taking the human service worker certificate, its a one year course but that's BS! Every instructor says that it should be a two year due to the course requirements. 

 

Blah blah blah! 

 

The first semester was rough but got me two B's and a C. 

 

I just finished two heavy classes (counselling strategies and case managment) A- and a B+! 

 

I struggled, from chronic pain from degeneration and constant muscle spasms and then the massive emotional journeys through my life. LoL, 

 

 

Yuup, human service worker, blacksmith, rocket tryentist. 

 

I hope you are doing well aaaaand I am working my way through redesigning my little shop. I am almost finished my 2x72 grinder and then I will be servicing the old ir ovens! 

 

You happened to catch me at the perfect time... I just walked into the shop full of excitement and aspirations for some productivity. 

Now I feel obligated to to hurry up and get caught up now that I blabbed about new tools and shop designs. 

 

Ear plugs are going in, smock is going on and my goals are specific! 

Sincerely, Trevor (Lone Tree Forge) 

 

Ps, if I don't reply for the day it is because I have become hyper focused and on task. 

 

[Mikey98118]

16 minutes ago, Trevor84 said:

rocket tryentist.

Good one, Trevor; I like that

46 minutes ago, Frosty said:

It's funny, as tired as I got of answering the same questions over and over the recent slow spell is kind of sad.

Yes, we just never seem to be satisfied...I have had some of the same problems. We must do a better job of coaxing?

[Frosty]

I see can folks devoting a life's pursuit of the tryintific method. You are the Father of Tryence Trevor! I'll be honored to be able to say I knew you back when. 

I'm thinking we need to come up with something new that nobody understands so we can explain and describe it to folk. How about a coal gun burner? Guys would need to make crushers to powder it and feed mechanisms before they could start on the burners themselves. Hmmm?

Frosty The Lucky.

[Mikey98118]

 

Whether you build or buy your gas forge, it is likely that you will need to provide your own sealing and heat reflecting coating on its internal surfaces.

Hot-face materials:

Plistix 900 is a fire clay powder consisting of 94% aluminum oxide, and 1.7% silicon oxide, with 1 to 5% aluminum phosphate as a binder; it is in many ways the premier example of a thin hot-face seal coating and is recommended for use on cast refractories and ceramic fiber blanket (but not over ceramic fiber board), it is a general service sealant that forms a protective thermal barrier for ceramic fiber blanket insulation; it air sets to a hard surface.

    Plistix is sold as powder, and is mixed with tap water to a consistency of sour cream; then dabbed on from a disposable paint brush in 1/16” thick layers (each layer will coat 1.2 square feet per 1lb. of powder).

    After application, seal equipment interior with plastic, and allow to dry slowly at temperatures above 60 degrees to a hard set. Post drying, fire the first layer and all subsequent layers can be applied with brush strokes, prior to drying and firing. Bring the coated equipment up slowly to temperature, to avoid cracking and other damage from thermal shock. Air dry and fire every subsequent layer before adding additional coats.

Kast-O-lite 30 insulating castable refractory can be troweled or cast in a 1/4” to
 1/2” thick flame face layer, giving a large measure of thermal protection, along with mechanical armoring from thicker layers, in case you are moving crucible tongs in and out of your equipment, and for general shop safety; it is use rated to 3000 F, is alumina based for flux tolerance, contains mini silica spheres to provide insulating voids, and is very resistant to thermal shock; it weighs 90 lbs. per cubic foot (compared to 146 for standard alumina refractory). Kast-O-lite 30 has been the favorite refractory for construction of home-built forges and casting furnaces for more than twenty years.

    This refractory hardens gradually enough that the edges of equipment surfaces can be scraped with a straight edge, so that those surfaces meet perfectly, allowing two part forges and furnaces to run with little flame leakage.

    Kast-O-lite 30 has a moderate insulating value of great importance for protecting insulation from heat damage; when coupled with a re-emissive (heat reflective) finish layer, it will greatly lengthen the working life of secondary ceramic fiber insulation. 

    Kast-O-lite 30 will stick to most materials. Use cooking spray, Crisco, or car wax on plastic or wood molds as a release agent. I have also used glass jars as forms, and then shattered them by heating to red incandescence, followed by a water quench, after the refractory cured. Cardboard and wax candle molds both burn away conveniently.

Heat reflective coatings: There are inconsistencies found in advertisements for "heat reflective" products; this is a legitimate label, if inexact. when, advertisements go further, and label various products as IR reflecting, they depart from reality. Yes, there actually are substances that reflect infrared energy; the most notable being gold, followed by silver and aluminum. But the difference between cause and effect is important. Actual IR reflectors are only useful as ultra-thin coatings on optical devices, such as light filters in welding helmets, or camera lenses.

    High-emissivity coatings can be used to more effectively transfer heat through a crucible wall (as a thin coating), or to redirect energy, forming a heat barrier in thicker coatings; to illustrate the importance of the point, we will define a typical thin zirconia coating as one millimeter or less (.039") and thick coatings as three to five millimeters and up. The critical difference between a heat barrier and standard insulation is that the higher the heat level the more effective high-emissive coatings become, while insulation typically lose efficiency as heat levels rise. Also, the thicker the coating the more effective a heat barrier becomes. Induction "furnaces" for instance, use crucibles made of nearly pure zirconia refractory, which is transparent to high frequency waves, but is so efficient as a heat barrier that with secondary insulating refractory between the electric coils and an inch or less of it outside the coils, a crucible becomes the whole furnace.

    The way a re-emissive coating works, is that it absorbs heat so redily that it quickly becomes incandescent. Think of a thin layer of tiny zirconium oxide particles exposed to a high heat source, and radiating that energy in all directions; now picture another layer of particles next to the first, with still other layers behind them. Each layer radiates heat in all directions, but the heat source only comes from one direction, so at every additional layer some heat gets subtracted as it is radiated back toward the heat source.

    So, a thin re-emissive coat will transfer lots of energy through a crucible wall, while the portion of heat it radiates back into the equipment is then re-radiated back at it, while the thicker coating on equipment surfaces reduces heat transfer that would otherwise happen through conduction. Re-emissive coatings are a simple but elegant form of recuperative energy generation. By converting combustion heat to radiant energy emission, heat gain from combustion is mostly retained on interior surfaces before the heated gas is lost out of exhaust vents, while heat loss through conduction is greatly reduced, with the added benefit of reducing heat stress on ceramic fiber insulation.

    Efficient heating equipment is designed for radiant energy to do most of the heating work, with part of the combustion heat saved up on the radiant surfaces, so that direct heating from a flame becomes  a secondary heat source on the work. By the time your equipment interior reaches white heat, less than a third of combustion energy is directly heating metal parts or a crucible, while radiant heat is doing most of the work. Once you understand these principles, why movable exterior baffles coated with a high-emissive layer trumps looking for the best exhaust vent size should become obvious.

    Zirconium silicate (zircon) is about one-third silica, so it takes a thicker layer than zirconium oxide (zirconia) to do the same re-emission job, but then it is also far less expensive and much easier to employ. It is a fact that the smaller the particles of zirconia the greater the percentage of heat re-emission they create (as low as 68% to as much as 95%). The zirconia particles trapped in the silicon matrix of commercial zirconium silicate are minuscule.

    Zirconia crucibles employ very crude particulates, and yet they are so effective as insulation that they become the entire furnace, when wrapped in a high frequency coil, and insulated by a further layer of loose zirconium oxide. So, the thicker the re-emissive layer on equipment interiors the better—always providing you use it in a manner that will stay attached.

    Hot-face heat reflectors can be as minimal as re-emissive coatings over ceramic board and rigidized blanket, or painted on a 1/2” thick layer of Kast-O-lite 30 cast refractory. But an armored tile of 5/32” to 3/16” thick made of homemade zirconium silicate “clay” has become a superior option, thanks to Tony Hansen’s famous Zircopax formulas on digitalfire.com: https://digitalfire.com/4sight/material/zircopax_1724.html

    If you are willing to take responsibility for understanding and using raw materials, there are a number of alternative choices, which beat the heck out of commercial heat barriers; not only costing far less money, but sometimes giving better performance at the same time.  So called IR reflectors (actually high-emissivity coatings) will be of especial help in raising efficiency while protecting interiors of heating equipment; let's lay them out.

    The most effective commercial heat reflection coating claims "up to" 90% IR “reflection.” But, "up to" is actually a cover for the nasty truth that their formula can also mean as low as 68% heat reflection; it’s all a matter of zirconium oxide particle size.

    Being a naturally suspicious type, I tried separating the colloidal content from cruder particulates in the top commercial product by spooning some of their thick mud into a water glass, and presto; the crude stuff fell out of suspension in the mixture, and immediately sank to the bottom of the glass. So, I mixed in as much more mud as would separate, and painted the thinned-out coating over a previously coated, and heat cured surface. My forge went from bright orange to lemon yellow incandescence with the same burner and regulator setting.

    When it first came on the market, stabilized zirconia flour cost twice as much as the regular kind. Today, there are three different ways to stabilize zirconia, and the price has fallen to about one-third more than the regular stuff; this is an important factor to keep in mind. So, if the colloidal particulates are so much more effective why have crude particulates in the content? MONEY; what is commonly called zirconia "flour" is nearly 100% colloidal, and will give you the full emissivity benefit; but it's not cheap.

    Zirconium oxide flour is the most effective heat reflector available, but it changes its crystalline structure at yellow heat, from cube to hexagon and then back again during cooling, so twice a heating cycle, it also changes particulate size, which is very hard indeed on every other ingredient in a hard cast refractory; not so slowly turning them to dust. And so manufacturers of high heat crucibles (and others whose products justify the added expense) employ stabilized zirconia, mixed with a binder to make tough refractories and coatings.

Zirconia re-emissive coating: Published government sponsored experiments with zirconia coatings back in the nineteen-sixties tried a number of binders; the most successful was orthophosphoric acid (commonly called phosphoric acid); a readily available and inexpensive product that stays suspended in water; it has some interesting physical attributes. When painted unto a surface it is adhesive, and will hold zirconia particles suspended on walls and ceilings; when heated, it polymerizes, as the acid forms esters. Thereafter it remains on the surface in a vitreous (glasslike) form at room temperatures, and becomes soft and very adhesive above 365 °F (185 °C) from then on. Mixed with zirconia flour this is a highly effective heat shield, but isn’t physically tough. On the other hand, it is simple to repair.

Zirconium silicate (powdered zircon crystal), is a substance that came into popular use, while manufacturers waited decades for reasonable stabilized zirconia prices.

Zirconium silicate, consist of silicate and zirconium molecules mixed in a stable tetragonal crystalline structure; it makes an end run around the size-change problem. Both zirconium and silicate are very resistant to flame erosion; they combine to form a tough hot-face coating. Zirconium silicate starts melting and separating out into its two constituents at 4650 °F (2550 °C); finally, it is only about 75% heat reflective as thin coats (.040”). Zirconium silicate is reasonably priced; if mixed with a binder, you can build up thicker walls of it.

Zirconium silicate re-emissive coating: Zircopax 95% by weight to Veegum or bentonite 5%.

Hot-face formula: This recipe came from a potter’s supply store. It has ingredients that physically toughen, resist strong alkalis, and reflect heat. The (ingredients (by volume) are: one-part alumina hydrate; one-part kyanite (35mesh); one-part Zircopax; half part Veegum T or bentonite clay.

    With such a low heat reflection percentage, zircon doesn’t appear to be the best choice for a re-emissive coating, but its 75% reflection increases with every additional layer painted on. If you want maximum protection for a hot-face layer, or the best high-emissive coating for a crucible, stabilized zirconia flour mixed with a good refractory binding agent (ex. calcium aluminate) makes the optimal choice; it is usually purchased from a supplier like Reade Materials.

There are three kinds of stabilized zirconia. Rather than its previous price of three times the price of plsin zirconia, they are about one-third more these days:

Calcium stabilized zirconia (melting point 4892 °F (2700 °C)

Hafnia stabilized zirconia (melting point 4892 °F (2700 °C):

Yttria stabilized zirconia (melting point 4892 °F (2700 °C):

 

These are my favorite choices for finish coatings, but are far from a complete list of what is available. Anyone who use something else should feel free to list their own favorites.

[Mikey98118]

Spring time is forge building time; so this seems to be the right moment to bring up one solution to The Very First Choice to Make (there are other answers, but this one is kinda easy). The most common container used for forge shells is a used five-gallon propane cylinder; it works pretty well, and is "the well worn path." Great; so why not take it? 'Cause we all want something special to come from all that work and worry, right? Trouble is that what this give you is a tube forge, which is a design that is already out of date.

    So, how can you have your cake, and eat it too? By choosing the used propane cylinder as your forge shell, but changing its internal arrangements. Instead of creating the standard tunnel forge, you want to raise the level of the forge's floor more than usual, to create a "D" internally.

     "D" shaped forges give the maximum usable space, for the minimum cubic area to heat, with excellent internal swirl of the heated gases. Choosing a used gas cylinder, makes an end run around building a "D" shaped shell; it has all the advantages of the oval forge I prefer, sans the hassle of building an oval shell, or repurposing an oval steel object ($$$) for the purpose.

    You will want to aim the burner down, and across the floor toward its far corner, from about two o clock height.

[Mikey98118]

Springtime is forge building time, and therefore it is question time for lots of you. The second most important question (which usually isn't asked until way too late) is how to build a fuel efficient forge. Fortunately, its answers go hand in hand with the first question on everyone's mind: how to build a very hot forge.

Here is a typical question posted in the past: "...not all forges are created equal, accuracy of burner construction, alignment of orifices, alignment of mixing tubes in refractory shell, elevation about sea level and many other variables can have an effect on this rate of consumption; BUT I'm just after a kind-of ball park "Yes, that's about right." or "No, that's way off. you should look for issues with your construction that might be causing poor efficiency."

Tat's a pretty good summation. The point about elevation is overstated, unless we are thinking about Colorado; then it might be true. Nevertheless, it isn't vary relevant these days, because burners that can induce way more than sufficient air at sea level, will still induce enough air at any elevation where people can breath. So, if elevation is a problem these days, your burner is a dog...There are lots of "good enough" burners sold now. You still need to learn how to see the good enough burners from the dogs, but that isn't hard.

Next comes the question of how large a forge should your first one be? May I suggest that you consider a three-gallon forge as LARGE,  and a five-gallon forge as HUGE! If you are wise, your first gas forge will be coffee-can size. You will save quite a bit on the cost of materials, and a whole lot more on fuel bills. However, fuel savings isn't the only advantage to a small gas forge. When you stand before it, the heat and glare in your face, and the heating up of your shop will be quite minor too.

Even after you build larger forges, you will continue to employ your smallest gas forge, whenever you can; its is simply a more comfortable tool; and so kind to your wallet

 

[Mikey98118]

So, why do we see five-gallon size forges at hammer ins, and in videos? Because what is going on is usually a group event; not a single artisan hammering away on a single item. Larger forges also look more impressive in videos.

 

[Another FrankenBurner]

Small forge, who wants that?

I did the math, a 55 gallon drum forge with two inches of blanket would only need a dozen one inch burners if we are going with 700 in³ per burner.

As tempting as 8400 cubic inches sounds, I’m going to stick with the smallest forge that will do the job.

[Mikey98118]

It is not strictly true that larger forges aren't as fuel efficient as small forges; no, not strictly true. But what is strictly true is that the larger the forge, the more difficult maintaining efficiency becomes.

This has little to do with the need to heat a larger interior. It has everything to do with the increased area of internal surfaces, which are leaching energy away through conduction. So, to maintain high temperatures on those surfaces, thicker and better insulation, and thicker layers of more expensive re-emission coatings must be added, to further slow conduction losses.

But, we all know that conduction losses are minor, as compared to the heat being lost out the exhaust port, right? That's true, but irrelevant. What is relevant is that conduction losses, help to cool down internal surfaces; this is precisely what you do not want!

The hotter the internal surfaces get the higher up the incandescent scale those surfaces go. A modern gas forge works primarily as a radiant oven, and only secondarily from the flame. At red heat, most of the energy transferred into work pieces are by conduction from the forge atmosphere. At yellow heat, about one-third of its energy is transferred by light waves. At white heat over half the energy transferred is radiant. How is this possible? Most the the light is infrared; not visible.

So, a fuel efficient large gas forge needs more than larger burners. It needs far more attention paid to conservation of its heat; this should not be surprising. After all, the sole purpose of a forge is heat conservation. Otherwise, we would jut use hand torches

[Mikey98118]

 

                                                                Burner sizes

Generally speaking, burner sizes are based on schedule #40 pipe sizes; or rather on their nominal inside diameters (or its rough equivalent in tubing). Classic venturi burners (AKA wasp-waist, such as Ransom burners) go by the throat diameter (the narrowest point of their venturi constriction).

    One naturally aspirated 3/4” burner (that is capable of making a neutral flame) will heat 350 cubic inches of interior volume to welding temperature in a properly insulated forge (two 1” thick layers of ceramic fiber or the equivalent insulation in some other form). Add additional cubic inches can be added for a burner capable of making a neutral flame in a single flame envelope (which can be tuned from no more than a trace of secondary flame into an oxidizing flame). Below is a list showing how this will translate in other burner sizes. If you substitute schedule ten stainless steel pipe for schedule forty mild steel, you can add a little more heat in forges and furnaces.    Controlling secondary air being induced into the forge through the burner opening(s), by the burner flame can raise heat levels up to twenty percent more.

(1) One 1/4” burner should sufficiently heat a 44 cubic inch interior from a bean can or two-brick forge to welding heat. If you just want to shape parts or heat treat them, the burner will do for a 60 cubic inch interior.  

(2) One 3/8” or two 1/4” burners can adequately heat 88 cubic inches; enough to weld steel, or melt bronze, in a two-gallon cylinder forge or casting furnace made from a non-refillable Freon or helium cylinder, or a mini oval forge made from half a car muffler.

(3) One 1/2” or two 3/8” burners should adequately heat 175 cubic inches; enough to run a small brick-pile or box forge, or a two-gallon cylinder tunnel forge.

(4) One 3/4” or two 1/2” burners should heat 350 cubic inches; enough to run a refillable five-gallon propane cylinder forge (or the equivalent size casting furnace).

(5) A 1” or two 3/4” burners should heat a 700 cubic inch interior for a small pottery kiln, etc.

(6) One 1-1/4” or two 1” burners should heat a 1,400 cubic inch kiln.

Use of a ¾” burner with a perfect flame (total combustion in a single flame envelope), which is mounted in an entrance port that is set up to control how much secondary air is induced into the forge by that flame, and you can add another 14% to the volumes listed above. Addition of the proper heat reflection coating will raise forge or furnace temperature still further or reduce fuel used to gain yellow heat; putting it another way an optimal burner running in an optimal forge or furnace will do the same work as average equipment with about 30% less fuel.

    So why not use cubic volumes to describe every burner type? Actual numbers will vary according to burner and equipment designs. On the other hand, naturally aspirated burners all have very long turn-down ranges. If you are anxious about using a hot enough burner for your forge or furnace, use the next larger burner size, and turn it down. Later on, if you long to get the burner size just right, it’s easy to change your burner out for a smaller one, but the reverse is not true.

[Mikey98118]

                                                                   Flame positioning

Burner designs have improved in recent years, and are continuing to develop. Refractory product improvements are advancing apace with burners. Both changes are shifting the ground rules in forge design. Top-down facing burners were the practical choice in times past, for good reason; mainly, it combined well with the limits of reasonably priced refractory wall materials, by permitting flames to impinge on forge floors, which needed to be made of tougher materials anyway.

    But, while new materials (including Morgan’s K26 bricks), can be used to improve performance of standard forge designs, they are even better, when combined with greater distance between flame tips and heating parts, to ensure complete combustion before impingement. Therefore, a reduction in scale formation will be gained by pointing single flame burners up and away from the parts in tunnel, "D," and oval forges; or high up on a side wall of box forges, and aimed toward the opposite wall. Clam-shell forges use an opening in the movable brick walls, between their top shell (where the exhaust hole is located), and their bottom shell (where the burner is usually mounted).

 

    Ribbon burners (and other multiple flame nozzle designs) should have no problem completing combustion before their flame paths impinge on work pieces, or forge walls. But single flame burners should be aimed to provide maximum distance, before impingement occurs. Two or three smaller burners provide far more distance for combustion to complete, than a single larger burner. Smaller burners are also easier to find room for, when facing upward from low in the forge shell.

    A neutral flame is not only hotter than a reducing flame, but it is much better for your health; employing reducing flames in a gas forge has long been standard practice, to decrease scaling on heating parts. Greatly increasing the distance between flame tips and parts is a cleaner choice. This does not prevent you from tuning your burners for reducing flames, to be thorough. But barely reducing will do the same job, with correct positioning, as fairly reducing did previously.

 

[Mikey98118]

                                                              Mounting burner ports

Typically, a burner port (entrance) consists of a short steel tube or pipe with about 1/4” larger inside diameter than the burner’s flame retention nozzle’s outside diameter. This allows enough space to aim the burner somewhat within the portal.

    The burner is held in position and aimed with two rows of thumbscrews; each row has three equidistant screws. One of the advantages of these screws is that they can hold a pipe or tube in place within the portal, and resting exactly where the flame is intended to impinge, while the portal opening is being ground into an oblong shape (to allow the tube to be aimed at a desired angle). This method ends up with a very close fit between tube and shell opening, to promote easy silver brazing of the port’s tube to the equipment shell. You are building a burner, so why not employ it to help construct its forge? Why use six screws to hold the burner? After all, commercial forges all use only three screws to hold their burners. You are not trying to maximize profit, but to build the best forge you can. Six screws allow flame impingement to be moved a little way, if your work is less than perfect.

    Alternatively, you can drill and mount a burner port in the shell with three bent flat bars and some pop rivets, or self-drilling screws. Bracketing parts together can end up looking tacky if you do not manage to keep the shell opening tolerances close. Employing screwed brackets can be a be a minor pain, if the burner’s port tube is positioned at an angle.

    Welding equipment parts, such as burner ports unto a steel shell, takes a wire feed machine and a learning curve. Some people are reworded with distortion in the shell, because of welding contraction; it only takes a little time to learn to run a wire feed welder, and somewhat more to bridge gaps with one; but it takes a lot more time to learn where and how much to weld without creating distortion. Neither hard brazing (braze welding), nor silver brazing creates that problem.

    Hard brazing requires an oxy/fuel torch and some skill, or an air/fuel torch with propylene fuel, and considerable skill. Silver brazing can be done with an air/fuel torch, propane, and close attention to setup. But most silver brazing alloys will not bridge gaps wider than 0.005”. However, some aluminum/zinc-based flux core soldering alloys, like BLUEFIRE Low Temperature Aluminum Zinc Alloy Brazing Rods, do bridge small gaps, melt at 728 °F (387°C), and can be used to bond aluminum alloys, stainless and mild steels, iron, bronze, nickel, titanium, zinc, copper, and brass. It works best when their Silver Copper Brazing Flux Powder is employed along with the filler rods (good on mild and stainless steels, silver and copper alloys, and other metals).

    Silver brazing by hand torch benefits from a lower temperature filler with broad melting ranges, such as Ufhauser silver braze filler A-54N (54% silver/ ), which has a broad elastic range (250 °F), and bridges minor gaps (up to 0.012”); it can be considered something of a capping alloy (capable of forming a weld bead), but if heated too slowly it may suffer from liquation (where the alloy separates into solid and liquid zones); it can only be  remelted well above its normal brazing temperatures, afterward. For this reason, alloy A-54N should be heated rapidly through its melting range; it has a melting range between 1325 °F (dark red) and 1575 °F (bright red). If you are joining a thin shell from a tin can to a thicker tube, keep the flame mostly on the tube.

    This filler alloy has a good color match to steel. Reasonable care with a sanding drum or grinding stone in a die grinder or electric rotary tool, will easily produce a sufficiently close-fit in the joint between a burner portal tube and the forge shell opening. If you are silver brazing on stainless steel, I recommend polypropylene fuel gas (if you employ an air/fuel torch), and black brazing flux.

     Old car mufflers are zinc coated, and new mufflers are coated with an aluminum-zinc alloy. Silver brazing parts to this kind of forge shell will ensure lots of damage to the plating. Stay Brite silver solder may be employed afterward, if you don’t want to paint the forge shell. Some zinc-based soldering alloys are zinc-tin-lead (avoid these), zinc-tin-copper (excellent), or zinc-cadmium (use fume rated respirator with these and follow all safety guidelines to the letter).

Note: The main ingredient in zinc flux is zinc chloride (follow safety guidelines listed on its container); it is the only ingredient in many of them; it tends to “tin” the surface of steel, rather than just cleaning it. If steel is freshly cleaned and power buffed with stainless steel wire wheels, it can be zinc soldered without flux, but why do things the hard way? Zinc’s melting point is 787 °F; comfortably below its boiling point of1665 °F. Zinc fumes are easily seen and smelled; avoid them. Unlike lead fumes, it takes a much heavier dose of zinc vapors to cause fume fever. Unlike lead, the body can tolerate a little zinc, but keep your dose tiny; none is best. No metal fumes are good for your lungs.

Caution: Metals give off toxic fumes upon reaching their boiling points. Using zinc coated sheet metal or parts (such as old car mufflers) is okay if you're careful about doing it. The boiling temperature of zinc (the point at which it makes fumes) is

1665 °F (bright red heat). Your forge shell should not get higher than one-fourth that temperature, during heating cycles. But you do need to be careful to keep the shell well away from the edge of the exhaust openings, by not making the openings in ceramic fiber, kiln shelf, or cast refractory next to the shell. But zinc coated flame retention nozzles or mixing tubes need to be stripped of their coatings. There is no need to avoid zinc coated reducer fittings on a burner’s air openings. In other words, keep zinc away from part surfaces that may become incandescent (above 1200 °F or 649 °C).

Note: Preheat temperatures should be kept down to 600 °F (315 °C on zinc coated surfaces, such as old car mufflers, to avoid damage to the existing coating on their surfaces, and to keep scale formation down on the steel; “tinning” the bare steel with a zinc chloride-based flux will help with this. Remove all residual flux with hot water and a clean rag after soldering.

    Larry Zoeller (of Larry Zoeller Forge) is credited for first mounting schedule #40 pipe to a forge shell with conduit locking rings; he calls it a “burner holder assembly.” If you are looking for fast and easy, he sells them for $25 and shipping from his website. Their main limitation is that they can only position burners at right angles.

    Ideally, the burner port’s tube should be completely external to the forge shell; in any case, it should not extend inside the forge further than is needed to secure a locking ring.

    A washer should be provided to slide back and forth on the burner’s mixing tube near the portal, so that it can limit how much secondary air the burner flame induces through the gap between the burner’s mixing tube and the portal wall. A nut can be silver brazed onto the washer, so that a thumbscrew can keep it positioned at the right distance away from the portal edge; limiting secondary air into the forge to only what is needed for complete combustion, without lowering internal temperatures needlessly. Considering air introduced from the burner opening as no different than air from other openings is a sad mistake, since those other openings do not have flames to induce air into the equipment.

[Mikey98118]

                                                                   Exhaust size and shape

One thing backyard casters and blacksmiths both worry over is how large to make exhaust openings on their equipment. Too small and you have high back pressure killing burner performance; too large and you cannot retain enough heat to do your work. Of course, the closer to the "right" opening size your equipment is the stronger the forge or furnace can be built. Just don't confuse the right size for a “perfect” size. If burner output can by varied (turn-down range), there cannot be any such thing as a perfect exhaust opening size. The right size is what is needed to accommodate the burner's highest output (the highest you are willing to take it to).

    Variable is the optimal opening size; all other dimensions can be outright wrong, but are seldom just right, with a burner flame that can be varied. This is one of the many reasons for controlling exhaust flow with an external baffle wall, positioned beyond a large exhaust opening; thus, permitting the least heat loss through radiation, while maintaining optimal atmospheric pressure in the forge.

Note: It is smart to include a ring of hard cast refractory around the exhaust opening, to protrude a little bit; diverting hot exhaust gasses away from the shell, where it would super-heat the metal.

    If you decide on a movable brick baffle wall in front of the forge, keep the bricks at a small distance from the exhaust opening, to allow hot gases to move up and out, between the opening and brick, while regenerating radiated heat from a re-emission (heat reflective) coating on the near side of the bricks, and back into your forge. Keep the stock entrance in the brick only as large as is needed to move parts through.

    This arrangement helps to slow the flow of expended gas in the forge interior to what is needed, and no more; as it gets close to the exhaust exit, the gas speeds up and through the opening; another desirable trade off. So, you are gaining hang time for the heated gas in the forge, and recuperative savings from bounce-back of radiant energy; a win-win situation. A baffle wall also minimizes infrared and white light from impacting your eyes and skin, improving your health and comfort.

Doors: Maximum part clearance can be provided with a hinged and latched forge door (stainless-steel toggle latches are your best choice); it should contain built-in interchangeable baffle plates (cut from high alumina kiln shelves). A door makes building the refractory structures inside of equipment much easier, and permits larger parts to be heated than would pass through a smaller exhaust opening. Best of all, it allows closely contoured movable internal baffles to be employed, which cannot pass through the exhaust opening; this promotes the use of single burners for small pieces, saving money in tunnel, oval, and “D” forges, which are run by two or more burners; on these forge shapes, the door is a big step up from an exterior brick baffle wall; it should include a parts entrance that can be varied in size; for instance, with sever plates cut from kiln shelves, with different openings drilled and cut into them (for passing stock through); these can be exchanged, and held within a pocket structure on the door. These improvements do not all need to be seen to at once, so long as a hinged and latched door is included in the forge shell. On forge/furnaces, the door can be left as is, or can be attached to a single pin hinge, and revolved out of the way.

Sliding doors: Some people prefer vertical or horizontal sliding doors, instead of hinges. People usually employ the new kind of insulating bricks as horizontal sliding doors. High alumina kiln shelves are seven times more insulating than clay fire brick, but not as insulating as the new semi-insulating fire brick now being used for pizza ovens and home fireplaces; but high-alumina kiln shelves are tougher at incandescent temperatures than the new bricks; this is a consideration for something you will end up shoving parts back and forth through. Exchangeable kiln shelve plates, with different part openings drilled and cut into them are fine, but building an elaborate system of moving kiln shelf parts to ape the ability of bricks to infinitely vary their openings comes under the heading of "gilding the Lilly." The additional energy savings it provides, probably is not worth the effort. Make up additional openings in kiln shelve baffle plates sparingly.

    Diamond or carbide coated rotary burrs (and diamond or carbide coated hole saws) are the preferred way to drill holes in kiln shelves. Friction cutoff blades (safest) and diamond coated blades (only of small diameter) are the best ways to cut out straight lines between those holes.

    A hinged and latched door, can also work on a box forge. Yet, movable bricks, trapped in an angle iron, or structural channel frame, will work out better than a hinged door. Furthermore, the channel frame works best, for sliding solid parts up and down on woven wire, and running through  counter balanced pulleys.

    You want to coat the hot-face side of either kind of door with one of the re-emission coatings. You can use a formula of 95% zirconia silicate powder (crushed zircon) and 5% Veegum (or 5% bentonite clay as an alternate); this mixture makes a tough heat reflection coating for wear surfaces. The ingredients should be available in ceramic supply stores. Zirconium silicate can also be mixed with fumed silica to make a tuff and heat reflective coating on hard refractories, or on ceramic fiber products. There are other choices, Like Plistix 900F, but none of them are easily purchased in other countries. Zirconium silicate and bentonite clay should be readily available in pottery supply stores, in many places.

Note: fumed silica in water is also known as colloidal silica. Silica (silicon oxide) is the main ingredient in common glass. However, glass has a much lower melting temperature than silica, because lime and potash are mixed into it, for the express purpose of lowering melting temperatures (the lime), and promoting the process of melting (the potash). Fumed silica easily melts initially, because the powder’s particles are so small that it has a tremendous amount of surface area, to promote the melting process. After the initial firing, this silica becomes glass like, and remelting it would take far higher temperatures. This is why fumed silica easily melts (once) on the surface of ceramic fibers, to rigidize fiber insulation. And why it also works as one of the binders in some high alumina refractories.

Casting furnace lids: All these advantages can also be applied in casting furnace mode, if a round kiln shelf is placed in an angle iron hoop; it can be swung into position above the furnace and swung out of the way during crucible removal. A mall center hole in the shelf allows observation of, and metal to be added to, the melt; it also provides a rest for preheating metal to make sure it is thoroughly dry before placement in the crucible. But the hot exhaust gasses will heat re-emission coatings on the plate’s underside into incandescence, causing energy to be radiated back into the furnace.

[Mikey98118]

Forge Floors: If you don’t plan to do much forge welding, laying down a Kast-O-lite 30 floor over ceramic fiber insulation (or Morgan’s K26 insulating bricks) will be tough enough, and more insulating than kiln shelves. For occasional welding, some people use a steel baking can, filled with the kind of kitty litter that is made from pure bentonite clay bits to shield the forge floor from welding flux, but frequent welding is certain to slop flux onto a forge floor. A kiln shelf, that is trapped in slightly oversized slots in the forge shell, will pay for itself with the ability to be slid out and resurfaced, once its top is fouled with flux. Once the flux cools into a hardened glassine layer, a flap disc can remove it with modest effort, before the widening puddle slops over the shelf’s edge. Do not forget to where safety glasses, and a dust mask, while grinding. If you are also going to use a tunnel forge as a casting furnace, the kiln shelf can simply lay on the round forge wall, when your forge/furnace is in the horizontal position.

Forge Shells: Variable shaped brick forges can be mounted on a suitable (fireproof) surface, with nothing more than metal angles, and threaded round bar, to hold them together. “D” shaped, tunnel, and oval forges are best contained in Sheet metal shells. If the forge has sufficient insulation to keep its outer surface below 400 degrees Fahrenheit, aluminum can be used for its shell; otherwise, steel or stainless-steel is a better choice. Hex shaped forges (i.e., Modified oval shapes) can employ noncombustible fireplace backer board as the shell. If you live in an area where sheet metal has become ridiculously overpriced, old appliances are another source of sheet metal. Coffee cans, car mufflers, propane cylinders, non-refillable Freon, or helium cylinders, all make satisfactory forge shells. Smaller “D” shaped forges were once made from mailboxes, but I do not think they are a worthwhile effort.

 

[Mikey98118]

                                                   Working with castable refractory

There are several hard castable refractories used as flame faces in forges, and casting furnaces. So far, I think Kast-O-lite 30 serves best in both equipment.  You can carefully drill, grind, and scrape it, as the refractory is still setting up; this goes well enough, during the first hour, but far less easily after the refractory completely sets. During the week of curing, the refractory continues to harden, very like concrete.” In fact, castable refractory is a form of concrete; what sets it apart is that the chemically locked water that remains after curing can be—CAREFULLY—steamed out of the finished form by firing. Concrete cannot be fired; it simply explodes when heated; this difference is due to what is used as the binder in concrete; Portland cement. Aside from firing, the more familiar you are at working with concrete the more you already know about working with castable refractory.

    The other differences are that refractory has no rocks used as filler material; instead, refractory has ground up chunks of alumina rubble (aggregates), which help to stabilize the refractory against thermal shock. Insulating refractories, such as Kast-O-lite 30, also contains silica (or alumina) spheres, to create insulating voids (that are also crack interrupters), along with calcium aluminate cement for a binder.

    It was Kast-O-lite 30’s resistance to thermal cracking that first made it popular among home casting enthusiasts twenty-five years back. We were already using Perlite to make our own semi-insulating hard refractory, but its resistance to cracking during thermal cycling couldn’t be matched by other refractories.

    Mixing and drying instructions for refractory mixtures will be found on the 55-pound bag it originally comes in; but mixing and curing instructions are less likely to be included with smaller amounts purchased from resellers online. The most used refractory in this equipment is Kast-O-lite 30, because it is use-rated to 3000 degrees Fahrenheit, not inclined to thermal cracking, lighter weight, and semi insulating. Kast-O-lite 30 is known as a gunning refractory, which means that it can be flung on walls of large industrial equipment through special nozzles, rather than only being cast in place; the qualities that makes it good for gunning, also make it useful for spreading in layers as thin as ¼” by hand troweling, onto the inside of curved surfaces.

    Layers of ceramic fiber insulation as thick as 2” (in two 1” layers) can be pushed into shape inside of steel containers, and stiffened with colloidal silica rigidizer. After firing, the stiffened insulation will adequately support up to a ½” layer of hard refractory, in the (bottom) third of a cylindrical wall while it dries. The next day the cylinder can be rotated, and a further third can be covered, and the final third can be covered on the third day. Remember to thoroughly wet down the older refractory area, where the freshly spread refractory will blend into it, to ensure complete adhesion between the new and old layers.

Kast-O-lite has up to one year expected shelf life, if stored in a dry container at moderate room temperatures (50-70F); it should be mixed with no more than 20 percent water, and mixed for three minutes, then poured within ten minutes, into stout water tight forms for best results. A small amount of vibration will improve the casting’s finish surfaces. Keep the casting covered with a damp towel during air curing, which takes between sixteen and twenty-four hours, and keep the casting above sixty degrees Fahrenheit while it is air drying, for a week, with a small incandescent light bulb, or votive candles.

    The first thirty minutes of set up time is the most important, as the mix is changing from thick mud mixture into a solid. If you have cast horizontal mating surfaces for the upper and lower halves of a clam-shell forge, or vertical mating surfaces for the forge shell and door in a horizontal forge, you want to use the edge of a steel square, etc., to flatten the mating surfaces by scraping. If you did a good job of cutting and grinding forge shell edges for close tolerance, this is where it pays off; if not, well…aren’t you glad you read this section before you started pouring refractory?

    What if you did not? It isn’t too late to fix your mistake. Low places, which don’t meet up with mating surfaces can be filled in with Plistix 900F. Thoroughly clean, and then wet the surfaces where you will lay the finish coat. Place wax paper over the top of the cement, and then close the mating surface against it during drying and curing. The wax paper should come away from the dried surface easily; if not, just let it burn away during firing. High spots must be ground away. If you grind too far, just use the refractory coating to correct that mistake. Don’t try to do the whole job at one time. Correct your mistakes one at a time.

    How much firing? Once the chemically locked water is driven out, is firing finished? No; most people consider this to be good enough, but without frequent firing during wet weather, the refractory can still slowly regain some water content from ambient air; necessitating the same careful fire drying routine you used during the initial firing, to keep the accumulated water content from cracking the refractory from internal accumulation of steam pressure; unless you take firing to the next step, which is called calcining. Basically, you heat the fired refractory up to yellow incandescence all the way through the form. It will begin on the inside surfaces (flame faces) and slowly soak through the refractory until it reaches its exterior surfaces (“cold” faces).

    Technically, calcining is the process of removing, by very high temperature (but below melting point), any volatile particulates, and finishing the oxidization of anything that can be oxidized, in a substance. Many of the constituents of a refractory mixture are separately calcined long before being included in the blend. But the cast refractory article may also be “calcined” at the melting point of glass (because some lime is included in the mixture to lower the melting point of its silicon); this to improves strength and durability, while making the refractory far less porous. One example of calcining would be fine porcelain, which is fired at higher temperatures for extended periods versus a ceramic coffee mug, which is minimally fired at much lower temperatures. One of the binding agents in most refractories is silicon. Some of the other constituents in many refractories are materials that contain silicon (like clay, which is likely to contain up to 40% silicon). When a fired refractory product is kept at high temperatures for an extended period, the silicon content begins to liquefy, gluing the other ingredients together more thoroughly, and filling in any micro gaps between refractory particles; effectively toughening and waterproofing the refractory.

    So, “calcining” is a word with a double meaning; its proper use is one thing, and the second use is closer to industrial slang. Despite all the good and honorable intentions of English teachers everywhere, industrial slang follows an extension of the ‘golden rule (“them what has the gold makes the rules”). In this case, “them what has the power makes the rules”). In other words, OEM sales departments choose what are proper industrial terms concerning their products. And, as with so many other lessons from the school of hard knocks, we can like it or lump it, but we aren’t going to change it.        

   

Insulation: It only takes a moment's comparison between heat lost through an exhaust opening with heat lost through forge walls to make it clear that insulating the forge, just to slow heat loss is only half the battle. You are insulating the forge, to help super-heat its internal surfaces into high levels of incandescence; at least into yellow, and hopefully into white-hot ranges. An efficient forge is a radiant oven.   

    The burner flame is best used primarily to create radiant heat transfer; rather than for heating stock directly; get that straight in your mind, or give up all hope of knowing what you are doing with forge design. Why? Because the choices you make about hard refractories, insulation, and heat reflective coatings, need to reflect the need to super heat the forge interior without destroying those materials.

    For many years, ceramic fiber insulation in walls and under the floor has consisted of two 1” thick layers inside curved forge walls, and single layers of ceramic board, possibly with a further 1” layer of ceramic blanket between the board and shell, in box forges.

    Two one-inch layers of ceramic fiber insulation within of a hot-face layer is the minimum insulation that is normally considered adequate for heating equipment; one-inch of ceramic fiber insulation normally isn’t. How much insulation is adequate also depends on other factors, such as how small the equipment is, and how long the heating cycles are. In other words, circumstances can alter cases—but only somewhat.

    Even the cheapest grade of ceramic fiber blanket doesn't melt below 3000 °F. Product temperature ratings come from the level of heat the fiber will withstand without massive shrinkage; this should illustrate the importance of locking the individual fibers in more secure positions by rigidizing; it also demystifies the seemingly magic protection given by a relatively thin sealing coat of high-temperature refractory, capped by a heat reflective coating. When you exceed ceramic fibers use-rating it begins to wither. 

    Ceramic fiber products need both rigidizer and finish coatings to do well in today's gas heated equipment; this is because better burner and forge designs create much higher internal temperatures than were likely twenty years ago. Rigidizer is especially important if you want your insulation to last. On the other hand, between using 2600 °F (1427 °C) rated fiber insulation and rigidizer, you can toughen the secondary insulation layer in your equipment enough so that it should stand up well to the heat that will leak past a high emission coating (AKA heat reflector) and thin hot-face layer like Plistix 900F® (rated to 3400°F; 1871 °C). Rigidizer also helps fiber insulation to mechanically support a thin coating, or cradle a cast refractory layer.

(A) You don't want to use thick ceramic fiber layers; instead of a single 2" thick layer; use two 1" thick layers. Ceramic fiber blanket can be easily parted into thinner layers via delamination, if you mistakenly purchase 2” thick blanket.

(B) Rigidize each layer after installation, and heat cure it with your burner, before installing the next layer.

(C) Form the burner openings before rigidizing each layer. Remember to leave burner openings just a little oversize so that they can be finish coated with a hot-face layer of something like Plistix 900F.

(D) Dispense the colloidal silica rigidizer from a used cleaner bottle with a spritzer top unto horizontal surfaces, and heat-set the ceramic fibers in position, before rotating curved surfaces to position further areas for the same treatment. After firing, those surfaces cannot sag out of position.

(E) Silica rigidizer is colloidal silica (just fumed silica, which remains suspended in water) and common everyday food coloring (to allow you to visually judge how far it is penetrating the ceramic wool); this water born product is easiest to dispense by spritzing. You can always pay through the nose for premixed rigidizer from a pottery supply if you prefer; I buy fumed silica through eBay and Amazon.com and get free delivery.

 

Morgan’s K26 insulating firebricks (distributed through Thermal Ceramics in the U.S.A.) have become a tougher alternative to ceramic board in box forges and a better alternative to ceramic blanket under floors in “D,” oval, and box forges; they are use-rated to 2600 °F (1427 °C), and are available from eBay and other online sources; legitimate shipping charges are small because these bricks are very light weight. Old style K26 rated firebricks are not anywhere near as insulating, nor lightweight; with the addition of a high temperature coating, like Plistix 900, or Kast-O-lite 30, these bricks can even be used as a hot-face layer in forges and casting furnaces.

    There are other improved insulating firebricks on the market now, but I am not familiar with them.

Morgan’s K26 insulating firebricks, and competing brands, can all be cut by hand with a worn out hacksaw, but are more quickly cut with resin bonded discs for ceramic materials. Do not use steel cutting resin bonded discs on brick, or resin bonded ceramic cutting discs on steel. Morgan’s K26 bricks can be holed with ordinary drill bits, but drill smoother with carbide tipped bits; unlike ceramic fiber products, they are semi resistant to hot flux.

    These bricks have become popular over the last few years, so their prices on eBay have effectively doubled, but now they can be purchased from more and more sources, like High Temp Inc., at reasonable prices and shipping charges.

     There are several kinds of refractories used for hard firebricks, but only one formula was historically used for insulating bricks, until recently: that was the pinkish to yellowish bricks made by including a foaming agent in clay to make lightweight bricks that are use-rated to 2300 °F (1260 °C); you see them employed all too often in old gas forges, and still employed in electric pottery kilns. To call them friable is to completely understate their fragility; calling them future rubble is more to the point, if they are used in equipment with rapid heating cycles, like forges and casting furnaces.

    While the strength and durability of various insulating refractories vary widely, all of them have a good insulation value in environments that are at or above 2000 °F; but Morgan’s K26 brick equals that of ceramic fiber blanket products at these temperatures. On the other hand, their K26 bricks can provide some structural integrity, while the blanket can easily be shaped into curved forms and then rigidized into firm featherweight secondary (back up) insulation, outside of the bricks.

 

Perlite and sodium silicate. Perlite granules are usually bonded together into monolithic insulating structures, with sodium silicate; both will quickly melt, If you exceed 1900 °F (1038 °C); these materials do best as tertiary insulation, but can be used as secondary insulation outside of insulating firebricks, if you aren’t going higher than 2300 °F internal working temperatures. Perlite can also be filled into contained areas, such as beds below insulating firebricks, or between them and curved forge shells; it will carry considerable loads, and is perfect for filling up space between solids and container shapes, to keep you from needing to measure and cut other materials to fit; this material is extremely light.

    Sodium silicate is attainable online, and inexpensive bags of Perlite are available in the garden departments of large hardware stores (as it is used for a soil additive).

 

[Mikey98118]

Hot-face coating materials: Plistix 900F is a fire clay powder consisting of 94% aluminum oxide, and 1.7% silicon oxide, with 1 to 5% aluminum phosphate as a binder; it is in many ways the premier example of a thin hot-face seal coating that is recommended for use on cast refractories and ceramic fiber blanket (but not over ceramic fiber board), it is a general service sealant that forms a protective thermal barrier for ceramic fiber blanket insulation; it air sets to form a hard surface.

    Basically, this product forms a smooth high alumina coating; its sole claim to form a re-emission surface is its finish smoothness, due to its small particle sizes. The smaller surface particles are the higher their emission percentages go, when coated on a heated surface. Kast-O-lite 30, when hand troweled in place, forms a very rough finish surface, with quite low emission percentages. A careful job of finishing it with a smoothed over surface of Plistix 900F, will increase re-emission considerably; on the other hand, coating a high alumina kiln shelf with it will produce zero benefit.  

    Plistix is sold as powder, and is mixed with tap water to a consistency of sour cream; then dabbed on from a disposable paint brush in 1/16” thick layers (each layer will coat 1.2 square feet per 1lb. of powder).

    After application, allow to dry slowly at temperatures above 60 degrees to a hard set. Post drying, fire the first layer and all subsequent layers can be applied with brush strokes, and smoothed by hand. Bring the coated equipment up slowly to temperature, to avoid cracking and/or pealing from thermal shock. Air dry and fire every layer before adding additional coats.

Kast-O-lite 30 insulating castable refractory can be troweled or cast in a 1/4” to
 1/2” thick flame face layer, giving a large measure of thermal protection, along with mechanical armoring, in case you are moving crucible tongs in and out of your equipment, and against contact with heating parts ; it is use rated to 3000 °F; is alumina based for flux tolerance; contains mini silica spheres to provide insulating voids, and resistance to thermal shock; it weighs 90 lbs. per cubic foot (compared to 146 for standard alumina refractory). Kast-O-lite 30 has been the favorite refractory for construction of home-built forges and casting furnaces for twenty-five years.

    This refractory hardens gradually enough that the edges of equipment surfaces can be scraped with a straight edge during the first hour of drying, so that those surfaces meet perfectly, allowing two-part forges and furnaces to be run with little or no flame leakage.

    Kast-O-lite 30 has a moderate insulating value of great importance for protecting secondary insulation from heat damage; when coupled with a re-emission (heat reflective) finish layer, it will greatly lengthen the working life of ceramic fiber insulation. 

    Kast-O-lite 30 will stick to most materials. Use cooking spray, Crisco, or car wax on plastic or wood molds as a release agent. I have also used glass jars as forms, and then shattered them by heating to red incandescence, followed by a water quench, after the refractory cured. Cardboard and wax candle molds both burn away conveniently.

 Morgan k26 insulating firebricks (use rated to 2600 °F) are distributed through Thermal Ceramics in the U.S.A.; these bricks have many insulating air voids, but are much sturdier than the old 2300 °F insulating bricks; these were standard up until recent years. You will want to seal their (flame facing) inner surfaces from flame erosion damage, with Plistix 900, etc. These Morgan bricks will supply insulation that is equal to 2" thick ceramic wool blanket. You can use a cheap imported, or worn out, hole saw to drill a hole for your burner to set in. The brick is equally easy to saw to length with old worn-out steel tools.   

Heat reflective coatings: There are inconsistencies found in advertisements for "heat reflective" products; this is a legitimate term, if inexact. when, advertisements go further, and label various refractory products as IR reflecting, they depart from reality. Yes, there actually are substances that reflect infrared energy; the most notable being gold, followed by silver and aluminum. But the difference between cause and effect is important. Actual IR reflectors are only useful as ultra-thin coatings on optical devices, such as light filters in welding helmets, or camera lenses.

    Re-emission coatings can be used to transfer heat more effectively through a crucible wall (as a thin coating), or to redirect energy, forming a heat barrier in thicker coatings; to illustrate the importance of the point, we will define a typical thin zirconia coating as one millimeter or less (.039") and thick coatings as three to five millimeters and up. The critical difference between a heat barrier and standard insulation is that the higher the heat level the more effective high-emission coatings become, while insulation typically lose efficiency as heat levels rise. Also, the thicker the coating the more effective a heat barrier becomes. Induction "furnaces" for instance, use crucibles made of nearly pure zirconia refractory, which is transparent to high frequency waves, but is so efficient as a heat barrier that with secondary insulating refractory between the electric coils and an inch or less of it outside the coils, a crucible becomes the whole furnace.

    The way a re-emission coating works, is that it absorbs heat so readily that it quickly becomes incandescent. Think of a thin layer of tiny zirconium oxide particles exposed to a high heat source, and radiating that energy in all directions; now picture another layer of particles next to the first, with still other layers behind them. Each layer radiates heat in all directions, but the heat source only comes from one direction, so at every additional layer some heat gets subtracted as it is radiated back toward the heat source.

    So, a thin re-emission coating will transfer lots of energy through the surface of a crucible wall, while the portion of heat it radiates back into the equipment is then re-radiated back at it, while a thicker coating on equipment surfaces reduces heat transfer that would otherwise proceed through conduction. Re-emission coatings are a simple but elegant form of recuperative energy generation. By converting combustion heat to radiant energy emission, heat gain from combustion is mostly retained on interior surfaces before the heated gas is lost out of exhaust vents, while heat loss through conduction is greatly reduced, with the added benefit of reducing heat stress on ceramic fiber insulation.

    Efficient heating equipment is designed for radiant energy to do the greater part of heating, with part of the combustion heat saved up on the radiant surfaces, so that direct heating from a flame becomes a secondary heat source on the work. By the time your equipment interior reaches white heat, about one-third of combustion energy is directly heating metal parts, or a crucible, while radiant heat is doing most of the work. Once you understand these principles, why movable exterior baffles coated with a high-emission coating trumps looking for the best exhaust vent size should become obvious.

    Zirconium silicate (zircon) is about one-third silica, so it takes a thicker layer than zirconium oxide (zirconia) to do the same re-emission job, but then it is also far less expensive and much easier to employ. It is a fact that the smaller the particles of zirconia the greater the percentage of heat re-emission they create (as low as 68% to as much as 95%). The zirconia particles trapped in the silicon matrix of commercial zirconium silicate are minuscule.

    Zirconia crucibles employ very crude particulates, and yet they are so effective as insulation that they become the entire furnace, when wrapped in a high frequency coil, and insulated by a further layer of loose zirconium oxide. So, the thicker the re-emission layer on equipment interiors the better—always providing you use it in a manner that will stay attached.

    Hot-face heat reflectors can be as minimal as re-emission coatings over ceramic board and rigidized blanket, or painted on a 1/2” thick layer of Kast-O-lite 30 cast refractory. But an armored tile of 5/32” to 3/16” thick, made of homemade zirconium silicate “clay” has become a superior option, thanks to Tony Hansen’s famous Zircopax formulas on digitalfire.com: https://digitalfire.com/4sight/material/zircopax_1724.html

    If you are willing to take responsibility for understanding and using raw materials, there are a number of alternative choices, which beat the heck out of commercial heat barriers; not only costing far less money, but sometimes giving better performance at the same time.  So called IR reflectors (actually high-emission coatings) will be of especial help in raising efficiency while protecting interiors of heating equipment; let's lay them out.

    The most effective commercial heat reflection coating I’ve used (ITC-100), claims "up to" 90% IR “reflection.” But, "up to" is actually a cover for the nasty truth that their formula can also mean as low as 68% heat reflection; it’s all a matter of zirconium oxide particle size.

    Being a naturally suspicious type, I tried separating the colloidal content from cruder particulates in the top commercial product by spooning some of their thick mud into a water glass, and presto; the crude stuff fell out of suspension in the mixture, and immediately sank to the bottom of the glass. So, I mixed in as much more mud as would separate, and painted the thinned-out coating over a previously coated, and heat cured surface. My forge went from bright orange to lemon yellow incandescence with the same burner and regulator setting.

    When it first came on the market, stabilized zirconia flour cost twice as much as the regular kind. Today, there are three different ways to stabilize zirconia, and the price has fallen to about one-third more than the regular stuff; this is an important factor to keep in mind. So, if the colloidal particulates are so much more effective why have crude particulates in the content? MONEY; what is commonly called zirconia "flour" is nearly 100% colloidal, and will give you the full emissivity benefit; but it's not cheap.

    Zirconium oxide flour is the most effective heat reflector available, but it changes its crystalline structure at yellow heat, from cube to hexagon and then back again during cooling, so twice a heating cycle, it also changes particulate size, which is very hard indeed on every other ingredient in a hard cast refractory; not so slowly turning them to dust. And so, manufacturers of high heat crucibles (and others whose products justify the added expense) employ stabilized zirconia, mixed with a binder to make tough refractories and coatings. For maximum re-emission, chose stabilized zirconia flour.

Zirconia re-emission coating: Published government sponsored experiments with zirconia coatings back in the nineteen-sixties tried several binders; the most successful was orthophosphoric acid (commonly called phosphoric acid); a readily available and inexpensive product that stays suspended in water; it has some interesting physical attributes. When painted unto a surface it is adhesive, and will hold zirconia particles suspended on heating equipment walls and ceilings; when heated, it polymerizes, as this acid forms esters. Thereafter it remains on the surface in a vitreous (glass like) form at room temperatures, and becomes soft and very adhesive above 365 °F (185 °C) from then on. Mixed with zirconia flour this is a highly effective heat shield, but isn’t physically tough. On the other hand, it is simple to repair.

Zirconium silicate (powdered zircon crystal), is a substance that came into popular use, while manufacturers waited decades for reasonable stabilized zirconia prices.

Zirconium silicate, consist of silicate and zirconium molecules mixed in a stable tetragonal crystalline structure; it makes an end run around the size-change problem. Both zirconium and silicate are very resistant to flame erosion; they combine to form a tough hot-face coating. Zirconium silicate starts melting and separating out into its two constituents at 4650 °F (2550 °C); finally, it is only about 75% heat reflective as thin coats (.040”). Zirconium silicate is reasonably priced; if mixed with a binder, you can build up thicker walls of it.

Zirconium silicate re-emission coating: Zircopax 95% by weight to Veegum or bentonite clay 5%.

Hot-face formula: This recipe came from a potter’s supply store. It has ingredients that physically toughen, resist strong alkalis, and reflect heat. The (ingredients (by volume) are: one-part alumina hydrate; one-part kyanite (35mesh); one-part Zircopax; half part Veegum T or bentonite clay.

    With such a low heat reflection percentage, zircon doesn’t appear to be the best choice for a re-emission coating, but its 75% reflection increases with every additional layer painted on. If you want maximum protection for a hot-face layer, or the best high-emission coating for a crucible, stabilized zirconia flour mixed with a good refractory binding agent (ex. calcium aluminate) makes the optimal choice; it is usually purchased from a supplier like Reade Materials.

There are three kinds of stabilized zirconia. Rather than its previous price of three times that of plain zirconia, they are about one-third more these days:

Calcium stabilized zirconia (melting point 4892 °F (2700 °C)

Hafnia stabilized zirconia (melting point 4892 °F (2700 °C)

Yttria stabilized zirconia (melting point 4892 °F (2700 °C)

[Mikey98118]

The equipment shell: This is where most people start their construction plans. A sheet metal casing does more than hold refractory materials in place; it is also important as handy place to mount burner portals, exhaust doors, and legs or carriages on. Nothing stops you from using hard refractory materials to help brace a very thin shell from the inside, but the shell should be thick enough to provide a rigid surface to mount parts too, at a minimum. With pop rivets and/or silver brazing, even tin cans can be made to work just fine as equipment shells. Of course, a few extra thousands of an inch thickness in the shell wall can save you a lot of work, during mounting, with self-drilling sheet metal screws. A 1/8" thick wall is way too heavy; think `/16" wall thickness as a maximum; stove pipe is a lot thinner, and some people use it quite happily, using sheet metal screws or pop rivets. But the added thousandths of an inch in helium, Freon, or propane cylinders makes all the difference; the same holds true for exhaust mufflers, or pots and pans.

Caution: Aluminum beer kegs are not a good shell choice because they lose their temper at 400 °F (752 °C), becoming much weaker. Most forge and furnace shells reach 400 °F (752 °C) during normal use. This does not prevent you from using thicker aluminum structures as shells, but thin and soft makes a very bad choice.

    You can hinge one end of a tunnel, “D” or oval forge; or one wall of a box forge, to get increased control of parts handling (ex. Crucible tongs) or to make construction and repairs easier. You can hinge the top and bottom halves of a forge together, so that it can be used as a clamshell design.

    Forge shape is a matter of opinion and we all have one. The most popular shape around is the tube forge; these have become the proverbial "well-worn path"; unless employed in a forge/furnace combination, they are also out of date for anyone but jewelers and knife makers. Oval shapes have been around for more than twenty-five years, and are finally catching on because they are a great improvement on tube forges.

    Oval forges provide more use out of their heated space than a tube shape can. The floor area in an oval forge will end up at least one-third wider than in a tube forge. As burner flames become hotter, the added room before your flame impinges on a ceramic fiber insulated wall has also become increasingly important. Most people face their burners down at an angle so that their flames impinge on a high alumina kiln shelf, or cast refractory floor in tube forges; kiln shelves are cheap, easily replaced, and very tough. Kast-O-lite 30 is semi-insulating, tough, and can be shaped to improve atmospheric circulation (flame swirl).

    The slickest homemade forge design that I have yet seen is an oval mini forge built from half a car muffler. Larger oval forges require sheet metal work, or more expensive containers.

Note: advanced materials, such as homemade tile (made from zirconium silicate and Veegum T), or insulating half bricks, covered in Kast-O-lite 30, can allow forge burners to be aimed upward toward the far side of an oval or “D” forge. The burners should be positioned high on a side wall, and aimed at the far wall of a box forge.

    So, why not build a box forge? If you employ rigid materials, such as ceramic fiber board and/or insulating bricks, a box forge makes good sense. But curved internal and external surfaces make more efficient forges; internally, to encourage even heating from burner flames, and externally to promote better air cooling of the shell.

Caution: When flames are turned up for forge welding, small forge interiors may become super-heated, all the way to their sheet metal shells. At this point, the ability of those surfaces to air cool becomes quite important! Then the air circulation around the exterior of tunnel and oval forges has an advantage over “D” and box forges, with their flat bottoms.

Brick pile forges have the advantage over box forges of changing shape and/or size, as needed; they are held together with steel angle and threaded round bar.

[Mikey98118]

Brick pile forges: When might a steel shell amount to silliness on a forge design? Let us discuss the why of box forges. Assuming that box forges started out as a way to accommodate straight refractory products like firebricks, ceramic fiber board, and high alumina kiln shelves, deciding on a box shape is no great stretch, right?

    And most of us don't want such expensive materials wrecked, so steel cladding comes quickly to mind. If you are running a busy steel shop, that makes sense. If that new forge is going to live in your studio, I would recommend just keeping gorilla types outside of locked doors; this will save wear and tear on a lot of other equipment too. Just post a sign saying "no brats allowed". Keep a whip and a chair handy; brats will not leave quietly.

Which brings us to what should sensibly follow in typical brick pile forge construction:

(1) For most forge sizes: Morgan K26 bricks from Technical Ceramics for floor, walls, and ceiling.

(2) Refractory cement to glue the ceiling brick into one piece, the floor into one piece, and each wall into one piece (six sides in all), Cementing the bricks together, makes your forge more stable, but hampers its ability to change size; so run your forge long enough to decide that its size and shape is where you want it, before cementing anything more than the ceiling bricks together.

(3) The thinnest high alumina kiln shelf you can find to cover the floor. Or a small sack of Kast-O-lite 30.

(4) A metal plate, or hard cement board, over the ceiling bricks is a smart bet, only if the burner or burners are positioned there, and down facing. Or, use a metal plate on one side wall if burners, are mounted high up on it (and cross facing). It takes a metal cover to provide a surface to mount burner ports on, if your forge is ever to include a hinged door.

(5) four sections of angle, and four sections of all-thread, with matching nuts and flat washers to allow the angle to keep the floor, ceiling, and four walls trapped together.

(6) A floor flange, pipe nipple, and six thumb screws to make each burner port used to mount each burner on the forge top, or side. Most people find a floor flange makes a good anchor for a burner portal on brick, or refractory surfaces.

(7) A good burner or burners, with valves, regulator, hose, and fuel cylinder.

(8) Four extra bricks (minus whatever bricks you left out of one wall to leave an exhaust opening in the front of the forge) to make a baffle wall in front of the exhaust opening.

(9) Plistix or some other finish to coat the ceiling, walls with.

(10) A large square pan, nearly full of Perlite from your nearest garden center to place the forge on.

Beyond these items there can be a long list of added items if you like, but it will all be add-ons This list is all you need to construct a basically hot, efficient, and safe brick-pile forge.

 

[Mikey98118]

Burner Ports: Once you build a burner, you need to install it in your forge or furnace, which brings us to burner ports. Some people just drill a hole in the steel shell and a matching hole through the refractory layers, but this doesn't provide support for the burner or any way to fine-tune its aim. Most of us attach a short section of larger diameter pipe or heavy wall tube to the outside of the shell, and use six thumb screws, in two rows of three screws each, to trap and aim the burner.

Note that:

(A) Pipe or tubing can be arc or braze welded over gaps in sloppy fits, but silver brazing requires close fitting parts.

(B) Larry Zoeller uses a 1-1/2" by 4" long schedule #40 pipe nipple with two 1-1/2" conduit locking rings, and two 2" by 1-1/2" rigid conduit reducing washers to mechanically affix a burner portal (for his 3/4” burners) into a hole in the forge shell.

(C) Jerry Frost came up with an excellent method of mounting his burners directly onto brick or refractory surfaces; he threads a pipe nipple into a floor plate; a simple and effective method that can be employed on any flat horizontal surface.

Control of secondary air: Now let's discuss the induction of secondary air and unwanted cooling of the equipment. Even single combustion envelope burners can benefit from external cooling air if the burners penetrate extra thick insulating layers (more than 2"), or the burners are very small 3/8" or less, because internal cooling from the cold incoming fuel gas could be overcome during long heating cycles, under these conditions.

    Most burners have at least secondary flame envelopes, so some builders deliberately leave their burner ports unsealed, because secondary air induction (powered by the flame) is needed for complete combustion of such flames. Unfortunately, this usually leads to an overabundance of a good thing, because the flame becomes an even more powerful induction "motor" than a burner's gas stream makes. It takes energy to heat air, so extra secondary air becomes a drag on performance within the equipment; leading to as much as 20% heat reduction. Fortunately, you do not have an if/or choice to make. It is just as easy to control incoming air through the burner port as incoming air through the burner, by the use of a sliding choke mounted on the burner’s mixing tube.

    Simply employ a washer brazed to a short thick tube, drilled and threaded for a thumb screw, on the burner. Once the burner is installed, it can be slid up against the portal tube's end to seal the port against heating from chimney effects after shutdown, and slid closer or farther from the portal tube for secondary air control during operation.  Is this more work? Obviously, but you should expend the additional effort; especially because it is an add-on project, which need not delay getting your forge up and running.

     Existing tin cans and paint cans make cheap and easy equipment shells, with built in bottoms, which makes them irresistible for most first-time builders of portable forges and furnaces. When someone mentions making a shell from light sheet metal and pop riveting it together for more convenient diameters, most of us just shrug off the suggestion. But recently I stumbled across double wall chimney inserts that are filled with...you guessed it; ceramic fiber. Naturally they are too expensive to be tempting, but they got me to thinking...

    Two different diameters of sheet metal forms, pop riveted together, could be filled with Perlite that is glued into a monolithic shape, with plain old water glass (sodium silicate), making a highly insulating and rigid furnace shell for a minor monetary outlay. And since such cylinders can be made into larger diameters than the usual shell sources, they could also contain an extra layer of insulation and still have plenty of room left inside the shell for hot-face and insulation layers.

Oval forges: Of course, the builder does not need to make a tubular shape; this kind of shell would also lend itself nicely to oval shaped forges...light sheet metal can easily be cut to any desired shape for front and rear faces. By making the outline and then a larger outline 1/2" outside and parallel to it. Room is left between inner and outer shell, to cut out tab shapes with a drill and rotary tool; these can then be bent 90 degrees. Holes can be placed through the tabs and the outer shell wall beneath, and the tabs can be pop riveted in place, strengthening the shell into quite a rigid form to add the insulation into; afterward, this form is little heavier than a simple tin can, but far tougher.

    Burner ports can be attached to the finished form by employing a hole saw, to drill through both walls at the desired angle, forming four drilled tabs on one end of the tube, shoving the tube through the holes from inside the shell, drilling matching holes through the inner shell, and employing pop rivets to hold the tube in place. Or, the tube can be silver brazed to the outer shell. With the tubes penetrating two separated sheet metal walls, the burners will also be held rigidly in position.

Forge size: It is a natural desire to go for the largest forge you envision yourself ever needing; and that is nearly always the wrong move. There is no such thing as a forge that is too small; if you outgrow a smaller forge, you will still find yourself using it whenever possible, to save time, money, and heat build-up in your shop. On the other hand, an oversize forge usually ends up collecting dust in a corner--to save time, money, and heat build-up.

    Building costs of forge construction are directly proportional to size. What’s worse is that you may end up choosing second rate refractory materials, and cheap burners, to save on construction costs in larger forges; this can end up tripling the expense of running your forge. The larger the forge interior the greater the heat loss through the walls, ceiling, and floor. The greater the heat loss the more fuel that must be consumed to keep the interior super-heated, which leads to increased heat lost through the exhaust opening.

 

[Mikey98118]

                            A five-gallon used propane cylinder forge/furnace

Most home casters use a five-gallon used propane cylinder for their casting furnace's shell (container body). I normally suggest a two-gallon cylinder as a shell for your first gas forge, but if you also want to build a casting furnace, the five-gallon propane cylinder can usually be picked up for free at most places that sell propane; this is because cylinders must be tested every ten years, to see if they can be legally refilled. There are usually old cylinders lying around, which can no longer be legally refilled. Propane dealers are happy to give away old cylinders, left behind by customers who replaced them with new cylinders.

    The biggest difference between tunnel forges and home casting furnaces, are whether they are positioned horizontally, or vertically. An extra four legs, which extend several inches beyond the forge shell's far (bottom) end, allows that placement; their extended legs also enable the forge to sit several inches above a sand filled pan, so that liquid metal can discharge through the bottom emergency spill hole, and into the sand, in the event of crucible failure. You also need to have the forge shell’s near (top) end; carefully cutting it off, in a flat plane (hopefully using a welded seam on the cylinder as your guide), and hinged back on, with a latch added on its other side, so that crucibles can be lowered into the furnace and picked up out of the furnace with crucible tongs.

    Some people include A hinged door on the exhaust opening end of their forge, for ease of refractory installation and repair; also, for the ability to heat larger parts in the forge than would otherwise be possible). You simply include about one-fourth of the wall to the door for convenient crucible use. You will also want the inside surface of the far end of that forge to be perfectly flat; I recommend using a small round kiln shelf for the purpose, instead of cast refractory; it is simply easier to get right.     

    Instead of a smaller exhaust opening, you drill an emergency spill hole clear through this end (at least 3/4" diameter). Instead of a flat forge floor, make the wall entirely round, and slide a rectangular kiln shelf unto it for forge use; leave it out for furnace use. Use two 1/2" burners (placed a one-third distance between both ends), instead of a single 3/4” burner. You shut down the top burner for casting, and only use it for forge work. You can also place a temporary internal baffle wall on the kiln shelf floor, and shut down the far burner, to save fuel when forging small parts.

    Should you take the second burner out of the forge when it's not running? So long as a down-facing burner's air openings are kept closed, leaving it in place should be okay. But double check the fuel hose to that burner, to insure it isn't heating up, due to the lack of cold incoming propane vapor to cool it down; if its hose is heating up too much to touch, it is safer to remove that burner and hose, to keep it from overheating. As to the burner that is kept shut down; so long as its air openings are kept closed by the sliding choke, there should not be any overheating from chimney effect. The exception to this is a burner with a fan attached. Any fan driven burner, must not be left in place, when shut down. However, it is a simple choice to make the burner in these forges up-facing, since the wall is entirely round, and the floor is slid in and out; this avoids many problems.

    First class refractory and insulation choices are often considered too expensive to be used in larger forges; this is a mistake. Propylene fuel costs about one-third more than propane, if used from refillable cylinders; it also gets about one-third hotter, which means that the burners can be cut back that much, for the same heat level in your forge; or smaller burners can be used, which is even better. Either way, the exchange rate of internal atmospheres will be slowed; this reduces fuel consumption for work heated, since most of the heat loss in forges and casting furnaces is straight out the exhaust port. Employing propylene calls for more expensive materials in the forge and in any flame retention nozzle, to withstand increased flame temperatures. Thus, cheaper building materials costs far more than they save!

[Mikey98118]

    But, doesn’t a propylene fuel cylinder cost a lot more than a propane cylinder? Yes, but if you construct your forge with the intent to switch from propane to propylene fuel, eventually, this expense is not part of your original costs. But, aren’t you forced to switch to propylene, to recover your additional construction costs? Again, the answer is no. Refractory materials within your forge where out from accumulated heat damage, over time. The better the materials used the longer they last.

[Mikey98118]

                                                                            Rigidizer

Common  rigidizer for ceramic blanket consists of colloidal silica (AKA silicon dioxide flour; synthetic amorphous silica; hydrophilic fumed silica; silicic anhydride; pyrogenic (fumed) amorphous silica. It is colloidal (does not settle out of water), both because of its slight weight (2.3 lbs. per cubic foot), and the small size of it’s particles, in flour form (0.2 to 0.3 μm average particle size).