Forge: Final Refractory Research

Ok, so the experiment with the clay and perlite is finally finished. Perlite already contains a lot of alumina, so I tried just mixing clay with powdered perlite (I used the bottom of a metal cup to crush it; a better option would have been a ball mill). The first brick was even parts (by volume). It cracked into two pieces and had a lot of spalling. The second was 2 parts perlite to 1 of clay. It broke in two, with only a little spalling. The last was 3 parts perlite powder to 1 part clay. The surface melted slightly, but there was no cracking or spalling. (I believe the last brick was tested after I started using fuel pellets, so the temperature was probably considerably higher than with the first two.)

So, my plan was to line the next forge with the 3 to 1 mixture of perlite powder and clay. I was also going to add some non-powdered perlite, but only pieces between 1mm and 3mm. I ran into a problem with this. Powdering perlite takes a long time. Also, perlite is mostly air, which means that an 8 quart bag of perlite yields only about 2-3 cups of powder. So, I started looking around for alternative refractory mixtures again.

This time I found a mixture that looks really good. The recipe is 2 parts clay, 2 parts silica sand (beach sand is primarily silica; play sand at hardware stores is also mostly silica), 1.5 parts perlite, and 1.5 parts Portland cement. I used this mixture, but I added the 3-4 cups of powdered perlite to the mix, and I also increased the portion of regular perlite, to improve its insulating properties.

I have not tested this mixture with heat yet, but I have finished the forge and will be testing it once it is totally cured and dried.