After testing a variety of grogs, we determined that a 50/50
combination of Christy Grog, 12M and 20M, pretty closely replicates the
glass gradation. And when we make a brick using the same Redart fireclay and the grog mix, we can form a brick using the same 16.5 percent water we used with the glass, so the gradation looks good. Our standard brick not using glass is therefore:
800 grams Redart Fireclay
400 grams Christy Grog 12M
400 grams Christy Grog 20M
264 grams water
Now we form, dry, and fire a brick using grog and the same firing profile. The brick looks good, with no visible cracks. It's interesting that in a small-scale kiln using standard clay raw materials, one can fire a brick this quickly. The absorption of the grog brick at the same firing profile is 10.4%, and the brick feels pretty whimpy.
Also, the color of the grog brick is kind of a salmon. The glass brick fired at the same temperature has developed a nice brick red:
The brick on the left contains 50% glass, the one on the right 50% grog. Same firing profile.
I don't know why the red would develop so nicely with the glass content. I've always thought that oxides like iron develop specific colors depending on the temperature and potentially other oxides present. In this case the glass seems to have in some sense "wetted" the iron to bring out a rich red at low temperatures.
Now we keep firing clay/grog bricks at higher temperatures until we reach the 6% absorption figure required by the ASTM specification. This took more firing than I expected. Here's the data:
So we had to fire to 2100 degrees F to get the same absorption with the grog that we achieved at 1850F with glass!
Now we can do the strength testing then begin the energy analysis.