So we’ve established that we can make a brick with absorption of less than 5 percent by soaking for 60 minutes at 1850 F.
The profile has been one minute to 1100 F, then holding at 1100 F for 30 minutes to assure organic burn-out, then four minutes to 1850 and hold for 60 minutes.
The absorption has been good, and the brick surfaces look good, but there are cracks on the long edges of the bricks. The assumption is that going from 1100 to 1850 in the absurd time of four minutes is simply too much for the clay/glass mixture, so it cracks.
The first thing we try is slowing the profile down to two hours from 1100 to 1850. The cracks are much smaller, but about half of the bricks we make still have cracks. Here’s a picture of a crack:
The vertical dimension is 1 inch overall
Since we are trying to prove that we can make viable bricks quickly with 50 percent glass, we can’t have cracks. And once a satisfactory kiln profile has been established, the next step will be to test the bricks in flexure, so we want no cracks, which, even if they are small, will weaken the bricks both because they are voids in the brick profile and because they act as stress concentrators.
Even slowing the kiln down to a three-hour ramp from 100 to 1850 does not completely eliminate the cracks. The ceramic literature talks a lot about the two stages of quartz inversion, so we further modify our profile to account for the quartz inversions, slowing to 180 degrees F per hour for 45 degrees on either side of 573 F and 867 F, the two quartz inversion points. That doesn’t solve the problem.
We don’t want to slow the kiln profile any more, so we think about the actual possible causes for the cracking. Maybe it’s not simply from going too fast.
Two other possible causes come to mind. First. no kiln is perfectly uniform in temperature. And this kiln has very hot elements mounted in only the top and the bottom. The thermocouple is mounted in the middle of the kiln. So maybe, through both temperature stratification and radiation heat transfer, the long sides are heating and shrinking faster than the short sides and middle of the brick, and hence cracking.
The bricks are being fired standing on the long sides for maximum strength during firing.
Second, during firing, we believe that the bricks containing glass get somewhat softer than bricks containing grog. As described in a previous page, we are depending on lowering the viscosity in the glass to encapsulate the particles of clay and give the final strength rather than the chemical evolution of the clay. An expected result of that process is a softening of the whole mass during firing. So it may be that during firing the soft brick, standing on its side, gets "torqued" enough for a little split to open up.
This test kiln was built with quartz support tubes on 2-inch centers, because that is the dimension of the support tubes in a roller hearth kiln I saw at a tile manufacturer. We wanted to simulate a roller hearth furnace with this project. The owner of the roller hearth has since pointed out to me that the rollers in the roller hearth are always turning, which helps with the stress on the piece by never giving it a chance to concentrate in one place, so this kiln, if anything, should place greater stress than expected in a manufacturing environment.
So we try firing with the brick laying flat and it appears to have worked! The first two bricks made laid flat have no visible cracks and an average absorption of 4.6 percent. We will now try to squeeze down on the profile to find the fastest possible profile.