It's calcining but in a different more stable manner than Portland Cement. I don't know what or if the silica has much effect beyond being fine aggregate. It may not be an aggregate at all, the other aggregate is mostly recycled high fire type ceramics.

None of the literature I found when I went searching discussed very much about the high temp chemistry.

What I did find interesting was as a gunnited refractory it can be shot onto existing refractories at up to medium red heat and the furnace fired back up immediately. What it didn't mention was how much moisture was used for gunnite, hot patching furnaces.

The site was really closed mouth about anything buy performance and applications. Made me think it might be pretty easy to make at home.

Frosty The Lucky.

 

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, 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, 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 is not. 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 rapidly 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 common 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 insulation layers in your equipment enough so that it should stand up well to the heat that will leak past a high emissive coating (AKA heat reflector) and a thin hot-face layer like Plistix 900F® (rated to 3400°F; 1871 °C). Rigidizer also helps fiber insulation to mechanically support a thin flame face 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 sometimes get free delivery, because the product is so light.

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. Other K26 rated bricks 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 aluminum 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 cutoff discs. 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, or hole saws; 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 online 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 electric pottery kilns. To call them friable is to completely understate their fragility; calling them future rubble is more to the point, when 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 insulation, outside of the bricks.

Perlite and sodium silicate. Perlite granules are easily 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 rectangular solids and curved 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).

 

13 hours ago, Mikey98118 said:

An efficient forge is a radiant oven.  

The term is, "reverberatory furnace," even if it's electric. From context I believe the term means it re-radiates the heat energy input into the furnace chamber. I never found a definition and gave up looking.

My comment has no effect on your lesson regarding forge insulation, my intent is to help dispell confusion when folks run into the term "Reverberatory" instead of your descriptive phrase accurate though it is.

Frosty The Lucky.

I totally agree with your term, and your reasoning.

So, I will start including it behind my term (In parenthesis)

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 crack resistance); it weighs 90 lbs. per cubic foot (compared to 146 lbs. for standard cast refractory). Kast-O-lite 30 has been the favorite refractory for construction of home-built forges and casting furnaces for the last 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 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 candles (used as molds) both burn away conveniently. Some vibration during casting will considerably smooth the casting’s surfaces.

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 re-emission coatings become, while insulation typically loses efficiency—over time—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 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 happen through conduction. Re-emission coatings are a simple but elegant form of recuperative energy generation. By converting combustion heat to radiant energy emission, more combustion heat is 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 so that radiant energy to does much of the heating, with part of the combustion heat saved up on the radiant surfaces, so that direct heating from a flame becomes only part of the heat on part surfaces. By the time your equipment interior reaches white heat, about half of combustion energy is directly heating metal parts, or a crucible, while radiant heat is doing the rest of the work.

Hot-face coating materials: Plistix 900F is a 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 sealant that also forms a 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 little 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 one lb. of powder).

    After application on a dampened surface, 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; or applied by, and smoothed while it is setting up. 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.

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 re-emission they create (as low as 68% to as much as 95%). The zirconia particles trapped in the silicon matrix of zirconium silicate powder are minuscule.

    Zirconia crucibles employ very crude particulates, and yet they are so effective as insulation that they become the entire furnace, when surrounded by a high frequency electric 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 alternative choices, which beat the heck out of commercial heat barriers; not only costing less money, but often giving better performance at the same time.  So called IR reflectors (re-emission coatings) will be of especial help in raising efficiency while protecting interiors of heating equipment.

    The most effective commercial heat reflection coating I have used, claims "up to" 90% IR “reflection.” But, "up to" can also mean as low as 68% heat reflection; it’s all a matter of zirconium oxide particle size. Being a naturally mistrustful type, I tried separating the colloidal content from cruder particulates in the top commercial product by spooning some of their thick mud into a partially filled 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. So, if the colloidal particulates are so much more effective why have crude particulates in the content? MONEY!

 Stabilized zirconia: 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 kind; this is an important factor to keep in mind. What is commonly called zirconia "flour" is nearly 100% colloidal, and will give you the full emissivity benefit; but it is 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 with minimum fuss, 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 (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 also 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 crystalline 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 layers 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 an average heat reflection percentage, zircon does not appear to be the best choice for a re-emission coating, but its 75% heat reflection increases with every additional layer painted on a surface. 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 expensively purchased from a laboratory supplier, like Reade Materials.

There are three kinds of stabilized zirconia available at present. Rather than its previous price of three times that of plain zirconia, they are only about one-third more expensive 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)

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 for mounting 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 as equipment shells. Of course, a few extra thousands of an inch thickness in the shell wall can save you a lot of work, when mounting hardware with sheet metal screws or pop rivets. A 1/8" thick steel shell, is way too heavy; think 1/16" thickness as a maximum; stove pipe is a lot thinner, and some people use it quite happily, with sheet metal screws or pop rivets. But the added thousandths of an inch in helium, Freon, or propane cylinders makes a convenient difference; the same holds true for vehical 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 combination.

    You can hinge one end of a tunnel, “D” or oval forge; or front 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 give 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 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 will 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 bottom 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.

Burner Portals: 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 does not 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, when positioning burner ports at an angle, that requires grinding the entrance hole into an egg shape.

(B) Larry Zoeller (Larry Zoeller Forge) 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 of his five-gallon paint can forge; the limitation is that this method can only be employed to position a burner port at right angles to the 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; he and his friends use a plate suspended between upper and lower threaded rods to mount burners horizontally in a vertical side wall of brick-pile forges, in a manner similar to method “A” above.

Control of secondary air: Now let us 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 their 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, using a sliding choke mounted on the burner’s mixing tube.

    Simply employ a washer brazed to a short thick tube, drilled, which is threaded for a thumb screw, pressing against the burner’s mixing tube. Once the burner is installed, this choke can be slid up against the portal tube's end, to seal the opening against heating from chimney effects, after burner 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. Of course, the builder does not need to make a tubular shape; this kind of shell would also lend itself nicely to oval and “D” shaped forges. Light sheet metal can easily be cut to any desired shape for front and rear faces. By making the outline and then cutting a larger outline 1/2" outside of, and parallel to it. Room is thereby left between inner and outer lines, 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 stronger.

    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 portal 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 portal tube penetrating the two separate sheet metal walls, so that burners can be held rigidly in position.

                                                                Commercial Forge choices

The subject of what forge to buy comes up on a regular basis. While the yes and no choices don't change very much between established brand names, the opposite is true about most forges being offered below  the $400 (and shipping) range (Diamondback single burner model).

The rest of the pack are mostly imported Chinese junk forges, or wanna be amateur manufacturers that do not matter, and are often worse than the China junk. They all come and go, with the single exception of Mister Volcano forges; these are actually worth considerably more than their asking prices. Best of all, is that their burner, or burners, are not only properly designed, but made of stainless steel. So, Mister Volcano burners are well worth transferring into that first home built forge, once you get up the nerve.

beyond that comes Chile Forge; their forges, and their burners are worth every penny of their considerably high prices; that last part is the stumbling block for most amateurs; businesses  can afford them.

What about all those other commercial choices; I can't recommend any of them, that I know of. Is this a harsh judgement? Yes it is; so what?

Here is an example of what I mean about armature commercial forge manufacturers:

"Our entry level model built with customary care and attention to quality and detail. It is meant to last years of use, not months. Please pay close attention to the steel thickness of other forges you may be considering - sheet metal forges don’t hold up well over time. The repetitive heating and cooling of the forge body during operation weakens the thinner bodies and causes them to fail. That is why we use full 1/4” wall material for all of our forge bodies."

But the fact of the matter is that even coffee-can forges last for years, unless they are left out in the rain. Furthermore, the best material for a high quality forge body (shell) is stainless steel sheet metal; not 1/4" thick mild steel pipe! They employ a half brick as the forge floor, instead of high alumina kiln shelf, or high alumina cast refractory, and a single layer of ceramic insulation, instead of two layers.

Do I think this manufacturer is a greedy evil dirt bag? No; what I think is that they are self-deceived about the wisdom of their design choices, and much to fond of their product. For they could have produced a far better forge for the same amount of time and money, if they actually had a clue.

Was that unkind? No; if I wanted to be mean I could have mentioned their joke of a burner

On 9/14/2024 at 3:43 AM, Mikey98118 said:

Some people prefer vertical or horizontal sliding doors, instead of hinges.

I've been thinking about making a horizontal sliding door as per the image below.

The idea being that the round opening of my forge fits entirely behind the square side of the door but as you slide it to the left, the angled opening becomes larger to fit stock as required but still maintains/contains some of the exhaust gases and heat that would usually escape above the stock in a vertical edged, horizontally sliding door. I've also considered using the same diameter curve as the left-hand half of my round forge opening down to floor level, instead of the 45 degree edge, to make an ever-increasing arched opening as you slide it to the left, if that makes sense.

All of this has been in my head for months. I really need to find myself a round tuit!

Cheers,

Jono.

2 hours ago, Hefty said:

I really need to find myself a round tuit!

Good Morning,

They are on the back shelf at 'the Paddle Store'. They come in many different sizes.  LOL  Paddles very rarely come as a Pair, mostly singles. The Paddle Store is at the intersection of 2 Creeks, One is the Right Way, the other is the only choice Left. When you are up the Creek that far, you don't need to keep digging, just paddle gently. LOL

Neil

 

there are as many designs for openings as there are people to dream them up. I say, the more the merrier.

    However, what is important, is that a gap between the the sliding door(s), bricks, or whatever, is maintained between it and the forge opening. It is through this gap that the exhausted gases flow up and out, so that no back pressure builds up in the forge, no matter how small the opening is around heating parts.

    Theoretically, the perfect baffle wall, would be a solid barrier, with an opening of the same shape as heating stock, and only a little larger than its size, to slide the stock back and forth through. As usual the perfect is the enemy of the practical. I am only bringing up this example, to illustrate a point.

    Obviously, in this extreme example, there is no room provided to git rid of hot exhaust gasses, so they must pass out between the baffle wall and the forge opening. The point is not to try to emulate the example, but to keep the goal in mind.

    Everyone ends up doing what works best for them. The point is to keep in mind why what goes where

                                                                          The why of burner sizes

Why do we discuss such a wide range of burner sizes, here? Size is relevant to use. What is large as a hand torch, or in a mini-forge will be too small in ceramic kilns, or commercial forges (which use one or more 1” burners). 1” burners provide four times the heat output of a ½” burner.

    Flame control is about more than choosing the right burner design, and proper tuning; it is equally about selecting the proper burner size. These are primarily used as equipment burners. So, understand that heat management only begins with increased heat output. The reason burners are aimed on a tangent, is to cause their combustion gasses to swirl around equipment interiors, creating a longer distance from flame tip and exhaust opening.  

    Obviously, a lengthened exhaust path increases the amount of hang time, for hot gases to deposit combustion energy on internal surfaces. What is not so clear is that the heat gained is not added by these gases blowing an extra foot or two at high speed; it is due to their continuing drop in velocity over every added inch of distance.

    Hot gases begin to slow, as soon as they leave the flame envelope. The flames of two 1/2" burners will use the same amount of fuel to produce an equal amount of heat as a single 3/4” burner; but they will drop velocity much faster in a five-gallon forge or casting furnace, increasing efficiency, because their flames can burn faster/hotter without creating a wasteful tongue of fire out of the equipment’s exhaust opening. Ditto for two 3/8” burners versus a single ½” burner in a two-gallon combination forge/furnace, or two ¼” versus a single 3/8” burner in a one/gallon forge/furnace.

    When forging small parts, further efficiency can be gained by placing a temporary partition in equipment interiors; separating them into twin spaces, and shutting down the rear burner. This is something that cannot be done with a single larger burner, which is centrally located. Combination forge/furnaces can have the forward burner shut down during casting operations, so that its flame is not wasted, from being positioned too high up the crucible.

    If a little is good, than more most be better, right? Usually, that is wrong. The exception is multi-flame burner heads, such as ribbon burners; these burner heads allow multiple tiny flames to take the most advantage of flame speed slowing. However, this technology is not perfected. I believe it is far enough along, that home-built ribbon burners are a practical choice for some beginners.

    But, they are not for me. Not that multi-flame burner heads aren't a cool idea; its just that I'm a perfectionist. Until someone comes up with a perfect flame from ribbon burners, I just ain't interested. That shouldn't stop all you practical people out there

 

Howdy!

I've grown tired of the inefficiences of my current propane forges, and while i greatly enjoy using my coal forge, i would like to have a nice gas forge so i'm in the works of designing a forge and i have some questions.

Preliminary info

The current goal for interior dimensions is 6"Wx4"Hx12"L. I plan to use high alumina Kiln Shelf for the floor and 2 1" layers of kaowool covered by 1/4"-1/2" layers of Kastolite-30 coating with plistix 900f for the walls. i plan to use mikey burners, and due to the relatively low volume compared to the length, i will likely use 2 1/2" burners for an even heat through the forge. I do some knife making, mainly ornaamental and need to be able to reach welding temps. I also would like to make the forge modular, in the sense that i could take the surface with the individual burners off, and replace it with a ribbon or different style of burner if i wanted to or if i am using it to test burners.

Assembling methods

       i would like to do a no weld design for easier assembly and the ability to remove all pieces for maintenance and modularity for future upgrades , maintenance, or testing. Currently the extended portion of the bottom portion of the forge will be mirrored on top and a bolt going through threm to secure the top and bottom but i am not sure how to approach attaching everything mechanically in a clean and efficient manner.

       In addition, i was considering having threaded bolts sticking out through the kaowool, that way i can pour the castavle refractory onto each piece individually an hopufully it will be secured by forming around the threads on the bolt. I'm sure this can cause an issue though of heat insulation and heat passing through the bolt into the shell

Burner mounting and Forge Shape

      My current design employs a box shape. I would like to do an oval but don't have the means to create an oval shaped shell, and am currently unaware of any good existing shells. Maybe a muffler? Plus the oval would be hard to make modular for the previously mentioned reasons. But anyways, to my understanding from previous readings, it is hard to meet ideal flame impedement conditions in a box forge due to the flat sides and no location where there is a greater distance from the flame to the impeding surface. I was considering mounting it coming through the top on the side closest to you in the drawing, and aiming it to far corner of wall and floor, hitting mainly on the wall. I was wondering if i could create a radius in that corner with plistix 900 so the flame does not meet a hard corner. Is this necessary?Not sufficient enough? Or should i go for a D style forge and mount the burners low on the wall and angled towards the roof the arc. And if i were to do this, would i want to aim it at the peak, the far side of the peak from the burner, or before the peak on the side the burners are on. I would assume the latter. Or do you have another recommendation?

Thank you for taking the time to read this, and i look forward to what you have to say.

8 hours ago, Jake18 said:

       i would like to do a no weld design for easier assembly and the ability to remove all pieces for maintenance and modularity for future upgrades , maintenance, or testing. Currently the extended portion of the bottom portion of the forge will be mirrored on top and a bolt going through threm to secure the top and bottom but i am not sure how to approach attaching everything mechanically in a clean and efficient manner.

What you are trying to describe is a brick pile forge; these have become quite popular, since the new series of insulating firebricks came out, making them practical, affordable, and easy to build. Frosty has posted photos of his group's little brick pile forge here on IFI. This particular photo is worth a thousand words

One of the many advantages of this design is that the problem of cracking in internal hard ceramic corners do not apply.

    Uneven heating of corners in forge design also no longer matters, with better building materials and much hotter burners.

    Finally, be sure to mount your two little burners high up on a side wall of the forge; not on the top, so that their flames are aimed at the far wall,  and well below ceiling height. There are also photos available showing how to do that, sans any welding.

Do you want to take it from here, Frosty?

20 hours ago, Mikey98118 said:

I believe it is far enough along, that home-built ribbon burners are a practical choice for some beginners

Sure I'll start with suggestions for Jake's forge AFTER I address your above mistaken statement. Multiple orifice (ribbon) burners aren't for "some Beginners" they are for some forges and purposes. They're also available commercially though require a blower far as I've seen.

Jake: I hate to say it but you're reinventing the wheel here. I don't know how many posts discussing your very issues are still available but skimming and reading through the "propane forge" section of Iforge will provide answers to your above questions. 

I just deleted a long rambling post trying to cover your initial post. I'm a too rambly guy, you can thank me later.

Why so long a forge? You can only realistically forge about 4-6" at a time, any more you bring to forging temp will be damaged through crystal growth. It CAN be fixed but is more the territory of professional heat treaters. If you want to heat treat in it make one FOR heat treating and make another for forging. Okay, that's suggestion 1.

A common mistake almost everybody I know of made with their first forge is making it TOO LARGE. My main shop forge is silly crazy too large but I can partition it to useful size chambers.

What shape forge do you want? Rectangular, cylindrical, D, etc.? Each one has plusses and minuses along with building issues.

What you describe above would be a "D" shaped forge. That only because my preferred term "Vault forge" wasn't popular for anybody but me to use. <sigh>

What you describe above as a "modular" forge is commonly called a "Clam shell" sort of. I don't know of anybody who's built one with a top that lifts off, most are hinged like a clam. It provides easy access to the interior for whatever you need, from placing and removing work to doing maintenance. 

The idea of building a refractory table with a U shaped top that can be lifted off was a fun topic here for a while. Lots of guys posted drawings for versions though I don't know of anybody who built one.

Below is a pic of the club's "no weld" forge about 3-4 minutes after lighting from cold. The piece of stock in it was put there by a guy who just couldn't wait. A 1/2" T burner is way more than necessary for this volume forge but they have good turn down range so these don't get run at very high psi.

Frosty The Lucky.

 

On 9/14/2024 at 3:40 PM, Frosty said:

I don't know what or if the silica has much effect beyond being fine aggregate. It may not be an aggregate at all, the other aggregate is mostly recycled high fire type ceramics.

Silica content in refractory and ceramic products serves as one form of binder, among others, and also, depending on amount, as waterproofing.

Clay, which are the most common refractory and ceramic ingrediant, runs around sixty percent silica content. High alumina refactories run between forty and five percent silica.

However,  fumed silica is also used as a binder in some refractory products; probably becuase of their low melting temperatures, in this form.

                                                               Diamondback forge update

For years, I have suggested single burner Diamondback forges to people who don't want to spend the time to know what they are doing, before building their own gas equipment; that hasn't changed, but the forges they offer certainly have; mostly for the better...mostly.

To begin with, they seem to have lowered their prices, while everyone else is raising theirs! There have also been structural changes to the forge bodies, which I consider as clever, and not harmful; they seem to have increased the amount of bolts and eliminated all welding--good!

  They also look to have changed away from ceramic board insulation to ceramic wool--BAD! However, you can update your insulation back to ceramic board, once the wool wears out.

   They offer a three burner forge, with an open side panel; the original use for this feature, was heating horseshoes. If you're not a farrier, the usefulness of a side panel, is likely to be more apparent than actual; however, other forges that have this kind of third opening have problems with warping of the forge body, which is very actual. Its up to you. I'm just saying...

At  these low low prices, what about a two burner forge? I would say, go for it, because you can always divide the interior space, by placing a brick between the burners, leaving one shut off.

Well, if two burners are better than one, than why not three? Because of back pressure, ruining burner performance; this will force you to open that long side panel...

The RimCereal 6-quart Stainless Steel Canister is 9.2” long by 7.4” diameter; its extra length and diameter makes it a very handy improvement over the standard 4-quart containers normally used in coffee-can forges, as its extra length is perfect for mounting two 1/4” burners. It is available for $29 through Amazon.com.

Coffee-can forge/furnaces

There seems to be some confused ideas about coffee-can forges being a cheap and easy way to get into blacksmithing. What they are is an economical way to forge small parts, once the forge is built. C-C forge construction also provides some economy of scale. You can find ceramic wool blanket offered in squares that are large enough to work in a C-C forge, so you spend less money for it. Castable refractory can be purchased in five-pound bags, economically providing good flame faces to protect the ceramic fiber insulation; these make smart choices; but the most significant savings come from minor fuel consumption; their use in casting furnaces preceded their use in miniature gas forges.

 

    This equipment is compact, making it highly portable, and handy for those with limited space; jewelers, can use it to forge chasing tools, or small hammers, and then employ it as a small casting furnace. This combination of forge/furnace is also handy for makers of upmarket folding knives, and other small metal art objects.

    Primary insulation layers made of mixtures of Perlite and water glass (sodium silicate) are going to melt in short order, if you heat your forge up to yellow incandescence; only employ perlite and sodium silicate in tertiary layers of insulation, with ceramic wool or insulating bricks between it and, a primary layer of castable refractory.

    Perlite and furnace cement will break down more slowly, but they still cannot hold up to direct flame impingement. You could mix Perlite and castable refractory as a secondary insulating layer, but then you would have spent enough money to buy that square of ceramic wool blanket; there isn’t much economy in that move.

    The infamous plaster and sand 'refractory formula' is such a major heat sink that you will want to throw your forge in the garbage, before this so-called refractory even has a chance to crack apart!

    The second “cheap and easy” idea about C-C forges is that you can simply run them with canister-mount torches. There are high priced dual-fuel (meant for propane and propylene) torch-heads that have stainless steel “flame tubes”, which are so thin that they quickly oxidize away in the super-heated environment inside of a forge. Most propane torch-heads have brass flame retention nozzles, which will melt inside of a forge. So, the torch cannot be placed in a sealed burner port. Instead, it can only be placed in an oversized hole, if its flame is weak enough, or aimed toward the hole from outside of it, if it is one of the hotter burning models. Either way the torch’s flame nozzle is either destroyed, or must be used inefficiently; the usual answer for this problem is to replace propane with propylene fuel canisters, at twice the price, to provide sufficient heat!

    A better choice is to push the thin-walled stainless steel flame tubes, of dual-fuel torch-heads into a thicker walled stainless steel tube, to slow down high heat oxidation losses, and to leave a flame retention nozzle in pace, after the inner thin layer is mostly oxidized away. This configuration can be placed into a forge’s burner orifice. To prevent oxidation losses on the original nozzle’s outer surface, this spacer tube must be interference-fit into the stainless steel flame tube; no air gap between these two parts can be permitted; this also has the advantage of maintaining the outer tube in place, after high heat oxidative erosion has done its worst on the thin layer’s inner surface.

    All of these thin flame tubes have narrowed openings, which are needed to maintain a flame; this area will oxidize away quite rapidly, when placed within a forge. So, an additional short outer tube is added, creating a full-fledged flame retention nozzle, and the bull-nose end of the flame tube is cut away, as it is no longer needed, or desirable. The better commercial torch-heads have screw on torch tips, eliminating the need to cut away the bull-nose section of their flame tubes, and permitting them to continue being used for delicate heating tasks.

    If you are going to all the trouble to build a burner, or repurpose a commercial torch-head (and you certainly should), you will want to place it in a forge that is worthy of it, right? Now you have another consideration, because a 3/8" burner is the largest size you can use in a coffee-can forge; by the time you have constructed it, you might not want to place such a nifty little burner in a tin can forge. So, you may decide to spend a little extra, to use a stainless steel container. 3 lb. coffee-cans (used for years as coffee-can casting furnaces, and later as forges) are about equal in size to 1 gallon paint cans (these are 6.6” diameter by 7.5” long), or #10 food cans (which are longer, but narrower), or some of the four-quart, or even five-quart, stainless steel containers; this last is the choice that is being recommended for forge/furnaces. Of course, a stainless steel stock pot, or kitchen canister is far stronger than any tin can; able to support exterior parts from, and will last forever. You can search for likely containers by imputing “stock pot” or “stainless steel canister” into your search engine.

A nu steel 4qt stainless steel canister is a perfect size and shape for a Coffee-can forge; it is as long as any coffee can at 7-1/4” and a little larger diameter at 5.9” outside diameter; available through Amazon.com.

Oggi Stainless Steel Kitchen Canister is 5-quart size; it is 8” in diameter and is 8” high, including an air tight locking lid, which saves a lot of work, since its clear plastic section can be replaced with hard refractory, and turned into a proper lid for a casting furnace, and forge door. The added twenty percent of interior space, combined with the existing lid mechanism more than repays its price of $25.49 through Amazon.com.

6-quart Stainless Steel Canister is 9.2” long by 7.4” diameter; its extra length and diameter makes it a very handy improvement over the standard 4-quart containers normally used in coffee-can forges and casting furnaces, as its extra length is perfect for mounting two 1/4” burners; it is available for $29 through Amazon.com.

The main difference between a tube forge and a casting furnace is that the forge lays horizontally, and the furnace stands vertically. With a little added work on its legs (to keep it up above sand box level in casting mode), along with the addition of an emergency drain hole in the bottom face to let liquid metal escape into the sand in case of crucible failure (or a second crucible for gold or silver), and a door in front/on top, which moves out of the way; a forge/furnace can be made to do both tasks quite well.

    One of the hard facts of equipment design is that there is no free lunch. Everything is a tradeoff. Being able to cast and forge in one piece of equipment must be paid for with some limitations on what can be done with the door/lid and the forge floor; the larger the forge, the more serious these limitations become, but in a coffee-can forge/furnace the limitations are minor, because its forging capacity is limited to begin with. Thus, the lack of a flat floor section presents little inconvenience. Of course, a separate flat section can be cast, and placed within larger forges, for heating stock, and removed during casting.  

    Another limitation in forge/furnace design is burner positioning. While the flame can be pointed in several ways within a forge, the flame in a casting furnace is aimed to impinge on the furnace wall as far away as possible, without impinging on the crucible (since flame impingement on a crucible promotes cracking). If the flames in a forge were aimed this way, they would not burn for a long enough distance before impinging on work pieces, if the burners should be pointed downward, toward a floor area. In these days of greatly improved castable refractories, it is better to aim them up and slightly inward, to ensure the longest possible exhaust path before impingement on work pieces, while also keeping the full force of a flame from impinging on crucible walls.

    When the forge doubles as a casting furnace, the use of two burners changes from a smart choice, into a practical necessity; this allows the burner toward its rear to be run alone, while the forward burner (which now becomes the top burner), is shut down, rather than wasting some of its heat (it being positioned too high on the crucible for efficiency). But that single burner needs to provide enough heat to do the job; thus, for bronze casting, you may want the rear burner to be 3/8” size; not ¼”. The larger burner can always have its gas pressure lowered to reduce its flame down to the equivalent of a ¼” burner for forging.

    Surprisingly, 3/8” burners are much easier to build correctly than ¼” burners; this is mainly due to the gas orifice. The smaller the gas orifice the greater the difference between a desired diameter and what may be available at any given time. All the other differences between what is optimal and what is available in part dimensions become exaggerated in miniature burners, too. But, the lack of exact gas orifice diameters is the chief restraint to building ever smaller burners; can it be done? Yes. Is it worthwhile? Probably not. If you use a 6-quart size stainless steel canister, that 3/8” rear burner can be used to full effect, when forging.

 

                                                           Fine tuning heating equipment

Fine-tuning burner performance completely is usually done while running it in its intended equipment, and only after adding finish coatings, and a front baffle plate, door, or brick wall for an adjustable exhaust opening to the forge and/or furnace, along with adjustable secondary air chokes installed on burner mixing tubes. These things are needed to raise internal temperatures high enough to ensure complete combustion. Sounds backward, doesn't it? But the thing is that perfect performance only comes in equipment that has been turned into a radiant oven. Even as the burner’s flame is best judged in a cold forge, final evaluation of the burner’s effects in the forge, is judged by looking at the exhaust, and the level of incandescence on internal surfaces.

    The burner is merely part of the forge; if performance only revolved around the burner, most of what we have learned about constructing heating equipment would be "gilding the lily”—It is not.

    Back when I was still writing Gas Burners for Forges, Furnaces, and Kilns, I raised the temperatures in my first forge enough that it changed from orange to lemon yellow incandescence, merely by refining the high-emission coating it was painted with (by separating crude particles from its colloidal grade particles, using water). A few weeks later, lemon yellow jumped up to yellow white by stopping all secondary air from entering the burner port; this can be further refined with the addition of a sliding secondary air choke on the burner's mixing tube, and a baffle wall in front of the exhaust opening. It has been stated that good burner performance is a delicate dance of different forces; ditto for the equipment it heats.

Mikey, is it worth a little extra expense to still use ceramic wool behind castable in a coffee can forge?

I'm at a point where I'd love to knock together a quick and dirty coffee can forge but my reasons for wanting it are competing variables:

-I'd love a cheaper forge using materials I already have (all castable refractory), BUT

-I want something that will heat up quick and be fuel efficient for small projects (ceramic wool would be less of a heat sink at the start of a session)

Is there any economy of (small) scale with heating up all castable in a coffee can forge compared to a larger forge or is it fairly proportional due to the smaller burners?

This is one of my biggest issues, I'm cheap but also impatient!  And I struggle to find sensibly sized pieces/rolls or ceramic wool at reasonable prices in Aus. I know there are times when we need to compromise these design decisions, but I wonder if I need to just spend the money and wait to be shipped too much ceramic wool!

Cheers,

Jono.

Those are all excellent questions, and fortunately they have practical answers. To begin with, the difference between good enough and best performance in your forge's insulation diminishes, along with the size of the equipment.

  So, using straight refractory in such a small forge, rather than complicated insulation layers is only a very small no-no

  Better yet, that refractory can be made pretty insulating with the addition of silica or high alumina spheres in its mixture; these should be available through a supplier of concrete, as lightening concrete is what they were invented for.

  Best of all, the addition of Perlite, a common soil additive will do a similar job. I recommend a smoothing layer of straight refractory over the layer with Perlite; smooth is very good for heat reflection from flame faces

This being so, then why do I go on and on about how to do things the difficult way? Because I'm a picky-butt

BTW, the formula would be one-third Perlite by two-thirds refractory.

That is by volume; not by weight.