We ran the tests, assigning our code number of L4567 to the project. Of course, putting some in water was the first obvious test. As expected, it swelled in a manner similar to bentonite. Then we sieved 100g through a 50 sieve mesh removing almost exactly 50% aggregate, which looks similar to grog. We fired the aggregate to 2200F (upper left) and it is refractory, with few particles melting. Is it brick grog? No, we poured some water on the fired aggregate and it hydrated the same way limestone does (upper right). The 50% finer material was slurried and allowed to settle. It did not do so, even after a week (bottom), showing it is clearly colloidal, likely sodium bentonite. The bentonite portion fired dark brown and melting, the same behaviour as typical raw bentonite. So what is this cat litter? Dry raw bentonite grains sized to ~30 mesh mixed 50:50 with limestone aggregate of the same size.
On the left is G2826A3, a cone 6 transparent glaze (an improvement on the 50:30:20 classic Gerstley Borate base transparent recipe substituting Gillespie Borate, reducing its percentage and increasing SiO2). Despite the improvements it exhibits this strange cracking and crawling. The G2826A1 on the right uses a frit to source the boron instead, clearly a better idea. These tiles were fired to 1700F. The problem is likely the ulexite mineral in the Gillespie Borate - it is known for this behavior of suddenly shrinking and then suddenly melting (the latter of which is just starting). Since Gillespie Borate is plastic and suspends slurries well we thought calcined kaolin would be better than raw kaolin in the G2826A3 recipe (to minimize drying shrinkage). However, it did not improve the situation. All of this being said, this recipe is still working reasonably well at cone 6 (likely stopping and holding it at 1700F exaggerates the problem).