Well, here it is. I won't have to keep on typing the same post over and over again! Thanks for the new forum Lionel! Oh, yes it's my first post here!
Here's something I wrote for an online book project, explaining the basics of ceramics, refractories, and other high temperature goodies.
Before discussing various aspects of high temperature furnaces and other equipment, it is helpful to understand how ceramics actually work. Simply put, ceramics are mostly metal oxides, such as aluminum oxide (Al2O3), silicon dioxide (SiO2), calcium oxide (CaO), etc. To specify an oxide of a metal, commonly the –um, –ium, or -on at the end of the element’s name is dropped, and replaced with –a. eg; alumina, zirconia, silica, calcia, etc. Ceramics can also be composed of more complex mixtures, such as kaolin, a type of large grained clay, Al2O3 . 2SiO2 . 2H2O. A list of commonly used ceramic materials and their properties will follow soon enough.
2) Acid, Neutral, and Basic compounds
Ceramic compounds fall into these three broad categories. Where R represents a metal and O represents oxygen, chemicals with the formula R2O and RO are bases or fluxes (eg calcia), chemicals with the formula R2O3 are neutral compounds (eg alumina), and chemicals with the formula RO2 are acids or glass formers (eg silica). At high temperatures, which depend on the compound in question, fluxes attack acids or glass formers, lower their melting point, and together form a glass. Soda lime glass, for instance, is easily melted in a furnace because of the large amounts of soda and lime, while pure silica is much harder to melt. Neutral compounds do not flux other compounds and are not easily dissolved by fluxes, with some exceptions such as boria (B2O3). While glass is great for making household glass items, pyrex, and pottery glazes, in the furnace itself glass is undesirable.
Clay particles will adhere to one another when wet and dried, but most particles will not. If clay is used, the ceramic will probably not need another binder, but if pure alumina, for example is used, some sort of binder will also be needed to keep the powder together until it has heated enough to form a ceramic bond.
Ceramic objects and ceramic bonds are created by high temperature firing of powders. The powders are usually slip cast, pressed, or extruded into the shape needed, and then dried and heated. At high temperatures the mobility of molecules and ions within ceramic objects increases, and eventually gain enough mobility to diffuse across various grains of the ceramic powder, fusing the separate grains into one monolithic object. This is a ceramic bond created by what I will call solid state sintering. As the grains fuse together, they shrink towards one another, decreasing the porosity of the ceramic object, causing it to shrink. Another type of bond that occurs is called liquid state sintering (by me at least). Here, some of the components of the mixture melt into a glass, which envelops the non-melted particles, and begins to dissolve them. Eventually the solution saturates, and sometimes higher melting point crystals form within the solution, also knitting the ceramic together.
1) Binders and aggregates
Refractory compositions have two essential parts; binders, which are the glue used to hold the composition together, and aggregates, which are the main bulk of the composition. One could make a composition composed solely of a binder, of course, but as binders are usually more expensive than aggregates, and aggregates add desirable properties such as thermal insulation they are almost always used. Certain aggregates work best with certain other aggregates and binders; large amounts of both acidic and basic components should not both be used in the same refractory composition, as the composition may melt. A small percentage of components which are fluxed and melt are sometimes actually beneficial because they form a glass gluing the rest of the particles together (perlite for example).
2) Commercial refractory compositions
Commercial refractories mainly use binders based on three categories of anions; aluminates, phosphates, and silicates. Calcium aluminate, or ciment fondue, is created by the heating of calcia and alumina by charcoal in a blast furnace; the two components melt, are poured into ingots, crushed, mixed dry with the aggregate, and later cured by the addition of water. Calcium or aluminum phosphate binders are created by adding reactive alumina, or calcia to the refractory aggregate, and then adding orthophosphoric acid solution, forming a metal phosphate and water. Some compositions using alumina and phosphoric acid remain puttylike and unhardened until being fired. Furnace cements sometimes use alkaline silicates (eg sodium silicate) as a binder. Other refractory compositions, such as are used in arc furnaces for steelmaking, use phenolic resins which decompose to glassy carbon as a binder. There are many different commercial refractory aggregates; most are neutral compounds because of their greater compatibility, such as alumina, silicon carbide, etc. Where the binding agent is not ceramic (such as phenolic resins) the acidity or basicity of the aggregate is not important, and thus basic oxides with very high melting points such as CaO or MgO can be used.
3) Homemade refractory composition
Obviously one simply duplicate a commercial refractory composition, but it is usually more feasible to use inexpensive compounds, such as clay. Various recipes have been made which use Portland cement, which contains a high percentage of calcium silicates. The calcium oxide in Portland cement is obviously a flux at high temperatures, and thus lowers the melting point of the composition, but cement does allow the composition to cure at room temperature, whereas pure clay only reaches its full strength after being fired to its maturing point. The outer parts of a clay based refractory furnace will never reach full strength, but this is usually not a problem. Essentially the recipe consists of approximately 15-30% clay; ball clay, kaolin, or fireclay work, but not bentonite, which looses its strength at high temperatures. The rest of the composition is composed of aggregates such as sand, whose main advantage is its low cost, and perlite, which decreases the strength of the composition because of its porous nature, but greatly increases the insulation value. Paper, wood chips, sawdust, or other combustibles can also be added to increase the insulation value as they burn out. Obviously the composition used will be based on what is available; if one can obtain alumina or other higher quality refractory materials, they should of course be utilized.
Now that you've waded your way through that, or breezed over it, here's the reason for this post. I'd like to have everyone post all of their refractory formulas that they know of, and describe in a scientific manner how they have worked. This will give us a tried and true collection of good formulas, techniques, suggestions, and ways of obtaining the needed compounds, which will hopefully be a good reference for those who are building furnaces.
Here's my "first" suggestion for someone to try, which I probably will try myself, but multiple perspectives always help.
The Three Component Refractory: by volume-
1/3 clean fire clay
1/3 fine white clean silica sand
1/3 chunks of clean white diatomaceous earth
The clay is the binding agent, the silica sand should decrease the shrinkage and the cost, because it's less expensive than clay, and the diatomaceous earth provides some insulation because of its light and porous nature. The mix only would need to be dampened until it loosely held together, then rammed into place.
*You must use a clay that will mature at a very high temperature, and the higher the temperature, the better. Otherwise, the clay may melt, when really you were trying to melt metals instead. A good clay is kaolin. To check if the clay will work, look at its oxide contents, and make sure it contains no more than a few percent of fluxes. Do not use beach sand, because it probably isn't silica. If the sand isn't white, it's not pure silica, and has impurities, which may flux the sand and cause it to melt. The diatomaceous earth can be found as kitty litter in dollar stores, or as an absorbent floor drying product of some sort. Powdered d.e. will not provide insulation nearly as well as d.e. that comes in small pebbles.