Forsythe shared information about firebricks that makesme think I could make my own from kaolin and a high temperature grog material,like the slag from aluminum smelting.
The ability to make them in custom shapes is very appealing as is the potential to save money.
I’ve got many dozens of scientific studies and expired patents on the use of aluminum metal and aluminum slag (called “dross”) and Zirconia Toughened Alumina, (“ZTA,”) etc. saved to a hard drive if you’re curious and want to pursue this. Knowing some of the ceramic science behind refractory processing will help to avoid a lot of wasted time, effort, and fuel — and will be a lot more likely to result in usable brick. The starting composition of the ceramic / refractory materials is critical to making good firebrick…. It’s a lot like baking a cake from a recipe, actually, where forgetting to add something (like milk, egg, butter, using the wrong type of flour, or failing to sift the right kind of flour) can ruin it by making it fail to “set up” properly when baked.
Making DIY firebricks from kaolin is certainly possible and not terribly hard so long as you use a fuel source like propane, natural gas, coal, diesel or waste vegetable oil.
I’ve learned the hard way that trying to make/fire good-quality high-alumina firebrick with wood fuel just isn’t worth the effort. You’ve got to fire to at least cone 14 (or minimum of 1400°C ~2600°F) —with a “soaking” hold at temp for an hour or more— for the kaolinic clay to mature and begin forming mullite crystals.
Using wood to try to reach that temp takes many, MANY exhausting hours (like… a full day and night) of literally constant stoking… and is easier to reach temp if you use a forced-air blower… but then you end up impregnating all your firebrick with wood ash, fluxing the surface with sodium, potassium, and calcium, which produces an aluminosilicate-slag glaze (a formation of thermally- and chemically- unstable, low-melt glass.) That drastically lowers the brick’s future corrosion resistance when they finally finish vitrifying, and it can make them crack when used in the temperature cycling of a firebox.
A 55-gallon drum, lined with 4 inches of ceramic fiber —rigidized with ITC-100 or Zircon coating— and fired with two of those 500,000 BTU propane weed burners (available on Amazon for under $50 USD each) will get you to the firing temps you need and mature the bricks in about 6 hours, (
depending on the number you're firing at a time; more bricks = more thermal mass to heat up = more time and more required fuel expenditure) and without ash / flux / slag contamination. (I’d recommend a couple of the 100lb propane tanks, too, rather than trying to run both burners off of a splitter from a single tank. That will allow both burners to run at max pressure and max heat output.)
A way to measure the temp is needed, but it doesn’t have to be a high-precision type S thermocouple made of platinum and rhodium, making them capable of use up to 1450°C, (which are in the hundreds-of-dollars range because of their rare earth metal components.) So… instead, this IR digital thermometer is good up to 1500°C, with reasonable-enough accuracy for firing firebricks, and it’s what I use. (It’s currently $62, and is occasionally on sale or used-like-new for around $45-$50)
a.co/d/gIiOTVV
If you’re using pure kaolin, it’s best to calcine part of the clay (about 60% of it) to avoid firing shrinkage of your bricks. Most kaolins have a relatively high “LOI” or “Loss On Ignition” of around 18-24% volume. That can make it extremely difficult to produce uniformly straight/flat/flush-fitting bricks of any practical building use. “Calcining” is firing the loose, powdered clay in a ceramic container to about 850° which converts it to metakaolin. That calcined metakaolin can then be added to 40% raw kaolin to make bricks that don’t have any noticeable dimensional loss upon firing.
Adding aluminum dross, aluminum oxide powder, or metallic aluminum powder to the kaolin is the way to go, IMHO, if you’re going to go to the effort of making your own bricks to begin with. The best mix to shoot for is the stoichiometric molar ratio for mullite, which is 2.55 parts Al2O3 to 1 part Silica, which works out to 71.89% alumina content (by weight.) Most kaolin is usually around 38-40% Al203, but can be a little lower than that. So, you’ll want to add enough aluminum oxide to bring it up to approx 72 percent. Also be aware that aluminum metal becomes 1.89 times heavier when it oxidizes (because of the 3 oxygen atoms binding to every 2 aluminum atoms [thus “Al2O3”]) …so, for example, adding 10 parts aluminum metal would add 18.9 parts aluminum oxide once wet-milled or fired in the kiln. Aluminum metal powder also expands considerably when it oxidizes, too, which can be useful if you don’t want to bother with calcining any kaolin beforehand. A mix of 40-50% metallic aluminum powder and 50% kaolin is pretty dimensionally stable without shrinkage or expansion after firing.
Another
HUGE tip I would advise is adding 3-5% magnesium oxide powder, which acts in numerous helpful ways: as a sintering aid by lowering the required firing temp; as a densification aid to increase slag resistance; it helps wet and mobilize aluminum metal and alumina (oxide) particles to achieve homogenous material mixture even if/when you don’t get all your aluminum/alumina perfectly milled to small enough particle sizes; *and* it encourages mullite crystal formation in the mix.
The reacted, fired product of equal parts kaolin and metallic aluminum powder with 3-5% MgO comes out to about 76% Al2O3 and 20% silica, which is a good mix with a safe margin of extra Alumina to ensure mullite stoichiometry with the consumption of some of Al2O3 by the MgO in the formation of diffuse spinel crystals.
Aluminum cans are a great source material, because it already contains 1~3% magnesium in the can sheet. (The body of the can is 1% magnesium and the top of the can is 3%.) The max amount of MgO you’d want to add is 10%. 3-5% is considered ideal.
There are also a TON of studies online showing that aluminum dross left over from melted, recycled aluminum scrap is a highly useful starting material for refractory production. …But you’ll need to process the aluminum dross into powder to use it. (More on that in a sec.)
Adding a small amount of kyanite as seed crystals also works very well to propagate mullite evolution throughout the blended mixture. Kyanite is available from most pottery supply stores online. (Kyanite and mullite are the same 2.55:1 alumina:silica chemical content — it’s just that kyanite forms under geologic pressure in the earth, and it converts to mullite when fired at relatively low temperature under atmospheric pressure in a kiln.)
You’ll also want to ball-mill your clay and alumina materials together beforehand. Having particle sizes under 40 μm will ensure strong sintering bonds and aid in mullite formation. Several studies have shown that ball-milling the aluminum and the kaolin together down to as low as 3.5 μm will cause mechanically-activated mullite crystals to form — even before firing — and can lower the required sintering temperature by at least 100 degrees Celsius. Sub-micron particles can even be sintered at 1200°C and still achieve maximum mullite evolution… but it’s almost impossible for a ball mill to achieve that <1 micron particle size (the falling, impact-action of the milling balls mechanically welds very small particles back together, keeping them in the ~3.5 μm minimum size range.)
Also be aware that metallic aluminum powder is explosively flammable (it’s the chief active ingredient in thermite and high-explosive fireworks) and it’s not something you’ll want to take lightly or handle carelessly. Adding the kaolin to the aluminum metal before milling it down to fine powder will greatly reduce any flash combustion risks. (You’ll still want to be careful about the offgassing hydrogen if you decide to wet-mill the aluminum powder into aluminum oxide. The aluminum strips the oxygen atoms off of the water, leaving behind pure hydrogen that will seep out of the milling jar. Good ventilation in the milling area is paramount to avoid accumulating that extremely flammable hydrogen gas. Also: be sure to wear a respirator mask when handling any materials of these superfine particle sizes produced by ball milling.)
However, the reason you wouldn’t want to use aluminum slag itself as grog without processing it into powder is that aluminum slag (AKA “dross”) usually contains pockets of metal within the clumps of oxide… and that metallic aluminum will have a thermal expansion mismatch with the oxides in which it is embedded. Any leftover aluminum which doesn’t continue to oxidize and expand with the addition of oxygen atoms to their molecular mass — will expand and contract with thermal cycling. Both of those things will cause cracks and spalling either when you first fire the brick and/or when put them into use after firing. Any aluminum metal you do use in making the brick will need to finish oxidizing during that first brickmaking kiln-firing, before the ceramic sintering is completed and the clay is vitrified. The way to ensure that is by A) milling the dross to a fine powder [and knowing/controlling the amount of metallic aluminum you’re adding,] and B) ensuring that the MgO powder addition is in direct-surface contact with any metallic aluminum used.
Pure aluminum forms an oxide “skin” in nanoseconds when exposed to ambient air, and that oxide layer prevents further oxidation of the metal beneath it. The magnesium oxide breaks the aluminum oxide skin’s surface tension, allowing the molten aluminum to burst out of its oxide layer and continue flowing into the clay body, diffusing evenly into the brick, and finish oxidizing during the kiln firing.
wet-molded bricks dry quicker when you use metallic aluminum powder, because some of the aluminum will react with the water and evolve exothermic heat, drying the brick from the inside-out, which is an added bonus of using the metallic aluminum. ensuring the bricks are bone dry before firing will lessen the chance of spalling during firing.
You could use something like a Cinva ram to compress the green bricks using minimal water for forming, but that's another thing you'll need to build DIY. My opinion: not worth the effort for the very small amount of dry clay compaction you'll be able to achieve. Brick presses for dry pressing clay are necessarily mammoth hydraulic beasts. It's next to impossible to achieve the necessary compressive forces with simple cantilevered mechanical leverage when using dry clay.
Anthracite coal particles will most definitely burn out at these firing temps, unless you somehow achieved total reduction (0% oxygen) in your kiln during firing… in which case, they would form silicon carbide and aluminum carbide around the pores left behind in the brick… but that would be quite difficult to do, even if you were trying to do it on purpose. (The carbon will offgas and react with the molecularly-bound water’s oxygen being released from the uncalcined portion of your kaolin clay, forming carbon dioxide bubbles that then exit the brick, leaving behind a network of pores.)
Sawdust is a lot more commonly used than anthracite nowadays, and some people even use plastic pellets or polystyrene beads for the burnout material. (Polystyrene beads are seriously messy to deal with, though, because they static cling to
everything)
Another method is dish soap (like the saponin used for bubble-forming in the refractory industry) and paper fiber, (like from toilet paper or paper towels) whipped into a frothy mix and then poured into molds. I’ve heard that one works well for extremely lightweight bricks and cast shapes, but I haven’t seen it done in person, and I suspect that it would be difficult to achieve uniform porosity from one brick to the next, because the foamy bubbles will migrate upward through the slop and slowly collapse as you work. It's said that you want to only mix small batches at a time and immediately pour all of what you make into molds.
That “Self-Reliant Potter” free e-book I linked in the refractory/firebrick thread is really helpful for seeing how to form and stack the bricks for firing. There’s also a handful of firebrick recipes, though none of them are the high-alumina type.
Another good tip is to make a hinged metal brick mold that’s held together on one corner by a spring latch. You can then use grease on the mold interior to quickly form each brick and pop it out of the mold to dry. The standard, old-fashioned, wooden slop molds can be frustratingly slow to extract each newly-formed green brick. Accidentally mashing your thumbprints into your carefully compacted green bricks (because they’re sticking inside the wooden form and having to be forcefully pushed out) gets really old really fast when you’re trying to churn out several dozen or a couple hundred brick units.
