This is a formula for making excellent refractory material to use to line the walls of a melting furnace. I am routinely melting aluminum and brass and I have been looking for a formula that is strong and lasts a long time. All of the formulas that I have seen that use Portland cement need replacing pretty often because Portland does not hold up under heat. Anyhow, I have finally found a formula that works pretty well. It's hard as rock and has good insulating qualities. You can make bricks or whole furnace linings with it. And.... You do not have to worry about a steam explosion if you don't cure it correctly. I have made shapes from this material and fired them while wet with no problems. If that were not enough: It's cheap! I am very excited about this and I want to share it.


  • Kaowool, also called rock wool.
  • Refractory clay. I use green stripe, but any refractory clay would probably work. Note that this is not just any clay. Refractory clay has a different chemistry than pottery clay.
  • Perlite. This is a mineral material that is in gardening. If have looked into DIY refractories, you have probably head of it.
  • Optionally, Sodium Silicate. Also called water glass.
  • Water


  • Breathing protection. Seriously. The materials that are used are nasty. There is no way you can try this without some kind of breathing protection. Silicosis is a nasty, nasty disease that cannot be cured. Use breathing protection when handling these materials.
  • Face/eye protection. You do not want to get this stuff in your eyes. This is nasty stuff.
  • Some kind of power stirrer. I used one connected to my drill. You will need it because stirring this junk by hand is not an option.
  • Buckets and something that you would use to mix concrete in.
  • Measuring cups. I got mine at Home Depot in the paint department. Buy extras. They really are handy. One measures up to 24oz and the other measures up to 64oz.
  • Some kind of scale that can measure grams. I bought mine on Amazon for less than $10. It only needs to go up to a kilogram or so.

Get it done:

This formula makes about 5 gallons of refractory. If you need more or less, just keep the proportions the same. It seems like the proportions do not need to be exact. Just get it close and you should be OK. If you experiment with it, I would be very interested in hearing about your results. That's the fun part AFAICT.

You need the following:

  • 300 grams of kaowool. That is not very much. A 12"x12" sheet will probably do it, plus some.
  • 64oz (by volume) of refractory clay.
  • 96oz (by volume) of water. Note that if you want a very hard surface, you can use 1/2 the volume of Sodium Silicate 40%. I find that it's not really required, but if you are casting the bottom of a furnace that will be used pretty hard, maybe it's worth the extra expense.
  • 400oz (by volume) of perlite. That's a little less than 5 gallons. (did I mention breathing protection? Use it.)
  • 48oz (by volume) 40% Sodium Silicate. S.S. acts as a binder and hardens the mixture.

How to do it:

  • Weigh the kaowool and break it up into a bucket. It strips off in thin sheets. If you tear those up a little, it makes mixing easier. Not really required, but recommended. (breathing protection)
  • Measure and add the refractory clay to the bucket. (breathing protection)
  • Add the water to the bucket. You can use less. Less is better. I find that 96oz is about right. You can use a little more or a little less, depending on what you want. If you want to use Sodium Silicate, then mix it with the water before hand and add that way.
    • The amount of Sodium Silicate is about half the volume of the water. The total amount of liquid will be about 96oz for this batch.
  • Mix it. No kidding, beat the heck out of it. Mix it completely and then mix it some more. You have to break up the kaowool. This is absolutely the most important thing there is to a successful batch. The kaowool must be completely broken up with NO clumps. I run my fingers through it to check. Also the clay needs to be completely mixed, but if you...
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