Forges 101

[Frosty]

I believe those are the domed ends of the tank rather than an oval forge. 

How do you plan on forming the refractory in the domed ends? Running a ribbon burner pretty well eliminates the benefit of a circular flame path on the ends.

Angling the burner so the flame isn't impinging the floor perpendicularly will provide a more uniform temperature and reduce back pressure against the burner.

You need much better refractory, welding flux will dissolve most hard refractories at welding temps though that one looks okay on that score.

ITC-100 is a poor kiln wash for a blacksmith's forge. It used to be the ONLY thing available and it's better than nothing. There are much better available now, Plistex and Matrikote fire to a hard ceramic flame face that is impervious to welding fluxes and is resistant to abrasion from cold steel being scraped across it.

Frosty The Lucky.

[Espen E]

Thank you Frosty,

 

I was planning to form the ends by hand, but if the round shape don't add anything I will make them flat.

Regarding your answer to the refractory I'm a little confused, are you saying that you thing it will hold up? 

I know that the kiln wash is not the best, but I could not find the ones you mentioned. Guess it will have to do until I need to redo it.

[Frosty]

That refractory should be fairly resistant to welding fluxes but the temp rating is pretty low. You're going to be welding large billets so the temp in the forge needs to be pretty high or billets will have to soak for a long time while the crystal structure grows. 

The blower driven ribbon plans on Wayne's site have been around a long time and are generally over powered. One would probably melt your forge if you aren't careful.

Check with ceramics suppliers for kiln washes. You want high alumina or a phosphate bonded kiln wash that fires into a hard ceramic. 

The problem with ITC-100 being it's intended use. It was formulated as a "release agent" that prevents materials in a kiln or furnace from sticking to the furnace. It's used in ceramic kilns to prevent glazes from firing to the kiln furniture. Without it your pottery would become one with the shelves and whatever it was touching. 

It's a good product it contains approximately 70% zirconia which is nearly chemically inert and has a very high vitrification temperature. There is another characteristic of zirconia that everybody focusses on as a forge wash. it's a poor thermal conductor and has a very high melting temperature so the propane flame just keeps pumping energy into it and because it doesn't do a very good job of conducting it to the hard refractory it's painted on it has to shed energy by radiation. It makes your forge glow at close o the flame's temperature upwards of 3,200f. IF your burner is tuned to the optimum fuel air ratio.

The real problem with ITC-100 is it does NOT fire hard in a forge, it remains chalky and so rubs off. 

Remember the people at the ceramic supply specialize in preventing glaze from sticking to your kiln so they might not understand what you're looking for. Be clear you want a wash that fires hard and is either high alumina or phosphate bonded. The latter is harder to find but it's out there. High alumina is your best bet.

Let's hope that's more helpful than confusing.

Frosty The Lucky.

[Mikey98118]

I find standard exhuast outlets problematic, because their size can't be varied; therefore, they are either too large or too small for efficient fuel use from any burner that can be varied. I prefer oversize exhaust ports, backed up with external  baffle walls; these can be as simple as movabke brick, or doors with refractory, or with movable kiln shelves.

Either type can be moved closer or further from exhaust openings, so that stock can be moved in and out of them, with minimal opening sizes. Thus exhuast gas can move up and out between forge and wall faces, while the bulk of radiated heat can be reflected back into the forge interior.

You may be hampered in what materials are available to you, but that is all the more reason to take your time designing your forge for maximum benefit 

[Mikey98118]

The simplest form of hinge consists of two short peices of flat bar, drilled with maching holes. One hole has a short length of round bar brazed, welded, or screwed into it. the other flat bar swings around the round bar, one flat bar can be attached to a forge shell, and the other to a door. With two such hinges with pins, facing upward, the door can even be taken on and off of the forge. When the swinging flat bar has several holes in a line, how close the door is to the forge shell can be varied. make sure that the peice with a pin is long enough to accomodate all of the holes, without binding the hinge.

[Mikey98118]

Pushing the envelope

I believe that, at present, Chile Forge is the top-of-the-line commercial forge for commercial work on the market. Do I think there is a better forge for doing commercial work than the largest Chile forge? Yup; a properly built ribbon burner forge will get almost as hot for a smaller fuel bill, and should cost about one-six the money, plus sweat equity, to build. Do I believe this is true across the board, size wise? Nope; ribbon burners will always be most efficient in larger equipment.

    A gas forge made from a five-gallon propane cylinder is probably the smallest size I would heat with a ribbon burner, so it makes a good example to use, if we want to dispute my views of limited ribbon burner superiority. Using two ½” high-speed tube burners, instead of a single ¾” burner, and a movable internal baffle wall, will allow this forge to equal the ribbon burner’s efficiency on small work, and that is as good as you can do; this isn’t a win against ribbon burners, but cuts your losses enough to extend how far single flame burners can compete with multi-flame burners. With smaller forge sizes, single flame burners, if well built, will hold their own.

 

Chapter 1: Equipment parts

[Mikey98118]

Why so? every size and design in heating equipment will do best with one burner design or another. No one shoe fits all.

[Mikey98118]

So what forge shape do I think will serve best with a ribbon burner? A "D" shape forge, with the flame facing upward from the side of a bottom shelf. I would use a separate drilled piece of high alumina kiln shelf, angled slightly inward for the flame face.

[Mikey98118]

Forge Shells

Variable shape 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. Rectangular and hex shaped forges (i.e., Modified oval shapes) can employ noncombustible fireplace backer board, if you live in an area where sheet metal has become ridiculously overpriced. Old appliances are another source of sheet metal.   

[Mikey98118]

. Tin cans, car mufflers, propane cylinders, non-refillable Freon, or Helium cylinders, all make satisfactory forge shells. Smaller “D” shaped forges are made from mailboxes.

[Mikey98118]

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 unproductive. You are insulating the forge, to help superheat 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're doing in forge design. Why? Because every choice you make about refractories, kiln shelves, and ceramic fiber products needs 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 outside 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.  If you exceed 1900 °F (1038 °C), Perlite and the sodium silicate it is usually bonded together with, will both quickly melt; these materials do best as tertiary insulation.

    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 emissive 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.

(D) Dispense the rigidizer from a used cleaner bottle with a spritzer top unto fairly horizontal surfaces, and heat-set the ceramic fibers in position, before rotating curved surfaces to position further areas for the same treatment.

(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 into the fiber); 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.

[Mikey98118]

Perlite and sodium silicate (which it is usually bonded together with), will both 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 firebricks, or between them and forge shells; it will carry considerable loads, and is perfect for filling up space between solids and containers, to keep you from needing to measure and cut other materials to fit.

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

[Mikey98118]

Morgan’s Thermal Ceramic K26 insulating firebricks have become a tougher alternative to ceramic board in box forges and a better alternative to ceramic blanket under floors in tunnel, “D,” and oval 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 as 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.

    Morgan’s K26 IFB bricks can be cut by hand with a 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 cut with ordinary drill bits, but drill smoother with carbide tipped bits; unlike ceramic fiber products, they are 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 price and shipping charges: https://hightempinc.net/product/refractory-bricks-and-insulating-firebrick/ 

     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 unsound nature; calling them future rubble is more to the point, when they are used in equipment with rapid heating cycles.

    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 featherweight insulation, or secondary insulation, outside of the bricks.

[Mikey98118]

Castable refractories: There are several good castable refractories used as flame faces in forges, and casting furnaces. So far, I think Kast-O-lite 30 serves best in both equipment.

[Mikey98118]

Hot-face materials

Plistix 900F® is a fire clay consisting of 94.1% aluminum oxide, and 1.7% silicon oxide, with 1 to 5 % aluminum phosphate; it is in many ways the premier example of a thin hot-face seal coating and is recommended for use over ceramic fiber blanket (not recommended over ceramic fiber board), or solid refractory products: it is a general service sealant that forms a protective thermal barrier for ceramic fiber blanket insulation; air setting to a hard set.

    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 the coating 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. 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 poured in a 1/2” thick layer gives a large measure of thermal protection, along with mechanical armoring, in case you are moving 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 spheres for insulation, and is very resistant to thermal shock; it weighs 90 lbs. per cubic foot (compared to 146 for regular refractory). Kast-O-lite 30 has been the favorite refractory for construction of home-built forges and casting furnaces for over 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 properly, allowing two-part forges and furnaces to run with only minor flame leakage. Of course you can further refine the seam faces with cement facing to stop flames completely.

    Kast-O-lite 30 has a moderate insulating value of great importance for protecting insulation layers from heat damage; when coupled with a re-emissive (heat reflective) finish layer, it will greatly lengthen the working life of secondary 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 with heat and water after the refractory was cured. Cardboard and wax candle molds both burn away conveniently.  

[pnut]

Is there a reason it's not recommended for fiberboard?Personally I think the results were better on the fiberboard than on the blanket in my forge door. Possibly due to me putting it on too thick on the ceramic blanket. I've had no problems with it on the fiberboard. 

Pnut

[Mikey98118]

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) mirror coatings. 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 re-emissive 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 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-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 combustion heat saved up on the radiant surfaces, so that direct heating from a flame becomes only a secondary heat source for 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) flour 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% versus 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're 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 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 colloidal particulates are so much more effective why have crude particulates in the commercial formula? 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 commonly available, but it changes its crystalline structure at about 900 °F, 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.

Simple zirconia based 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 normal 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 easy to repair.

Zirconium silicate: Zircon crystal is a substance that came into popular use, while manufacturers waited decades for reasonable stabilized zirconia prices; it is commercially known as zirconium silicate. Natural zircon crystal is silicon oxide and zirconium oxide, mixed in a stable tetragonal crystalline structure; it makes an end run around the size-change problem. Both zirconium and silicon are very resistant to flame erosion; they are both tough enough ingredients to make a superior hot-face coating. Zirconium silicate doesn’t start melting and separating out into its two constituents below 4650 °F (2550 °C); it is only about 75% heat reflective as thin coats (.040”). Zirconium silicate is reasonably priced, and easily mixed with a binder; you can build up thick coats 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.

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) mirror coatings. 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 re-emissive 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 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-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 combustion heat saved up on the radiant surfaces, so that direct heating from a flame becomes only a secondary heat source for 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) flour 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% versus 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're 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 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 colloidal particulates are so much more effective why have crude particulates in the commercial formula? 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 commonly available, but it changes its crystalline structure at about 900 °F, 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.

Simple zirconia based 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 normal 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 easy to repair.

Zirconium silicate: Zircon crystal is a substance that came into popular use, while manufacturers waited decades for reasonable stabilized zirconia prices; it is commercially known as zirconium silicate. Natural zircon crystal is silicon oxide and zirconium oxide, mixed in a stable tetragonal crystalline structure; it makes an end run around the size-change problem. Both zirconium and silicon are very resistant to flame erosion; they are both tough enough ingredients to make a superior hot-face coating. Zirconium silicate doesn’t start melting and separating out into its two constituents below 4650 °F (2550 °C); it is only about 75% heat reflective as thin coats (.040”). Zirconium silicate is reasonably priced, and easily mixed with a binder; you can build up thick coats 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.

Please understand that these are by no means the only heat reflecting coatings. You can find others discussed on this forum. Furthermore, some seal coatings, such as Plistex 900 will re=emmit about 75% of IR back from their surfaces.

[Mikey98118]

Forge sizes

The so called one-brick Forge doesn’t exist. Originally, the term was applied to forges made from two hollowed out bricks, mounted facing each other; this size is good for fuel efficiency, when building very small objects, such as mouth harps, or chasing tools. Recently the term has come to be applied to very small box forges made from brick, and heated by canister-mount air/propane burners. These minuscule brick pile forges are about the smallest size that can breathe properly (back pressure issues) with air/propane burners. The original one-brick forges were heated with oxy/propane torches.

You can see an example  of this forge on the Net. Just input "One Brick Forge with Torch:

 

The equivalent size in a tunnel forge would probably be a tomato-can forge. Next size up is the coffee-can forge, one-gallon paint-can forge, or half-muffler forge. Then comes none-refillable Freon and helium cylinder forges. Finally, we come to five-gallon propane cylinder forges, which nearly always turn out to be way to large for a beginner’s forge. As the half-muffler forge demonstrates, all these sizes have their equivalent in other forge shapes,".

[ThomasPowers]

Well the "one brick forge" I built and ran back in the 1990's was built from one soft firebrick. I used it to forge all the nails for my Mastermyr chest and a bunch of hack silver based on viking era finds.  20 below 0 degF windchill  that winter and the one brick forge I could use in my admittedly drafty basement.

[Another FrankenBurner]

I have made a few, on the fly, for odd jobs that didn't need to waste propane.  Even with the Morgan K26 bricks, they crumbled fairly quickly.  I have thought about a sheet metal skin and a coating of plistex to see if they would hold up longer.

Here is bad picture of a half brick forge for a ring I was working on:

 

[Mikey98118]

Corundum sand is simply crushed garnet; fine for grinding on wood, but a poor choice on steel. One of the tricks you need to watch out for is drop shipper advertisements that start out describing abrasive accessories as silicon carbide, and then go on to describe them as corundum sand. You may rest assured that they are just corundum sand; in other words, they are the same cheap garnet found in sandpaper, with blue are green die added to their bonding resin, to help confuse potential victims.  

[ThomasPowers]

Corundum is NOT Garnet!

Corundum is Al2O3  aluminum oxide, mohs hardness of 9

Garnet used for sandpaper is usually Almandine, Fe3Al2(SiO4)3, mohs hardness of 7-7.5

As mentioned Garnet is not a great abrasive for steels.

[Mikey98118]

As we both know aluminum, oxide is the merely chemical designation of a number of synthetic and natural forms of aluminum crystal. What you want to bare in mind is that the term corundum sand is not being used as a legitimate description, but as a dodge, I have also herd it advertised as fused chromium oxide," and then re-discribed as corundum sand in the next sentence, which is just as FALSE as describing an abrasive as silicon carbide, and then following up by calling it corundum sand in the next sentence. The intent to deceive is the obvious point

[Frosty]

Don't ascribe to malice what can be explained by ignorance. People writing ads rarely know anything about what they're selling. It falls under the theory of management neatly. In short, you don't need to know how to do a job to run the operation.

Frosty The Lucky.

[Mikey98118]

One ad can be put down to mismanagement. A whole series of them-by the same company-is better described as deliberate. Furthermore, they described the shank diameter of these abrasive stones both as 3mm and as 1/8" (which is 3,2mm); Not that it matters, since the shanks are actually 2.9mm, which will definitely fly out of a rotary tool with a 1/8" collet.

I did not write this out of spite. I've ordered thirty of these stones, with eyes wide open, because they happen to come in stone diameters down to 4mm; despite their flaws, that is quite handy for my needs. That doesn't mean that other purchasers won't be disappointed, if their eyes aren't open prier to purchase.