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My homemade furnace refractory recipe.

After years of experiments I developed inexpensive versatile and convenient refractory recipe. It is highly refractory. Insulates heat well and is easy to make and repair.

The core of this technique is porous ceramic.

For the additional heat insulation I use 1" thick Kawool graded to 2600F. The clay mixture can be prepared few days before you make the furnace but I do not recommend to store it wet for a long time. Not because I now it's bad but because I did not try this and have no idea if it works. You may store dry mix for a long time and it's handy for repair.

For the furnace body I use any suitable metal can, barrel, tin or garbage bin.

First determine how high and how wide the hot chamber will be. Add 2-3" wall thickness for the refractory layer(+4-6" to total diameter). Than add thickness needed for the Kawool layer if you plan to put it there. The same calculations applies to the bottom. I do not give you specific dimensions because everything is dictated by the crucible size and your burner. I would not recommend to make too big chamber as it will drop the efficiency. The 2" gap between the crucible wall and the furnace wall is about right in my opinion.

Trim your metal body to desired size. Cut a hole in the side wall for the burner at the desirable height. I like when the burner opening is at the bottom of the chamber. Put Kawool lining on the bottom first. Than put several layers of newspapers. It will burn after first run and provide a layer of carbon that allows you to disassembly the lining and refractory in case you need it. Otherwise the lining may bake into one piece with refractory. Than add side layer of Kawool and put few more layers of newspapers on it.

Next step is to make refractory mix. Take a bag of animal bedding (wooden shavings). You can get it at any pet shop or Wal-Mart for few dollars. Measure the approximate volume of the refractory you need to make and take about the same volume of COMPRESSED bedding. Add about half of this volume of Kaolin (I use EPK Kaolin) in the bedding. Be careful the clay is very light and easily go airborne. Use dust mask. Carefully mix the bedding and the clay for 10-15 minutes. The shavings must be covered with the dry clay evenly and some clay should remain loose. 

Now comes the important moment. You add water. There are two ways to add water. Quick and risky and slow and reliable. For the first few attempts I recommend to add water slowly drizzling it on the mixture and constantly mixing it.  The clay should create smooth and sticky layer on every particle. If the mixture looks like a pile of wet shavings add some clay. Spread it and mix in. At the end you should get more or less sticky mixture. It is crucial NOT to add too much water. This will create too weak refractory and will dry too long.

Now you can ram the refractory in the furnace. Put some mixture on the bottom and compact it with the wooden stick. I use a piece of 2X4. Make desirable layer (at least 2"). Than start to mold the walls. I tried to ram the mixture between the insert and the walls or mold the walls layer by layer by hand. It works either way. Just make sure you compact the mixture well. There should not be any visible air pockets.

When you completed the furnace plaster the inner wall with some Mullite-Kaolin mixture. Use 1/1 ratio and wet it to the slurry consistency. Do not apply it thicker than 1/8" or it will peel off.

Leave it for few hours to settle. Charge the furnace with the barbecue charcoal brickets to the top. Lit the bottom pieces before you fill the chamber if your burner opening is too small to lit the charcoal through it. You did not forget to leave the hole for the burner, did you? Ok! Your furnace is full of charcoal and the bottom pieces are lit. Now connect your blower and blow at the rate when the charcoal burns slowly, quietly. If the burner opening is big enough you may need no blower. But I prefer to blow. It spreads the heat evenly and the furnace dries faster. Try NOT to heat it to the point where the charcoal ashes start to melt and sip into the walls. That's very bad and you will have to redo the refractory. You need just to heat it up till the water will evaporate. It takes about 2-3 hours for the 2" layer to dry completely. Than make dry run (no crucible inside) with your regular burner and ramp it up to the maximum temperature. Let it cool down slowly, cover the exhaust hole with some Kawool.

Now inspect the walls, lightly scrape off loose particles. Be VERY careful the refractory is very delicate at this point.  Apply the second layer of clay wash on the walls and the bottom after the furnace is completely cooled down. Now it is ready for use.

Eventually it will crack and sag. If the hole is big plaster it with bedding + Kaolin mixture. Have some dry mixture handy. If it is hair crack or small defects - plaster it with the clay wash. The whole idea is to keep the walls porous. The wooden particles had burned after first hot run leaving the holes in the clay. We want to keep those holes. If you add to much dense plaster on inner wall it will crack and may sag down. So try to keep things light. If you have Alumina Oxide add it to the wash. It is highly refractory and will serve better than just Mullite-Kaolin.

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