I did as you suggested and made up three samples with 50 colemanite and 50 of each sample. The colemanite had more chips from the can than I realized in it--so there will be some brown spots but that won't change the major chemical reaction significantly. I also made up three samples of that glaze PeterH posted. The reason I have to do another bisque is because I did not have all the clay samples for testing glazes ready. I had gotten some more cone 6 bodies since I did the original bisque. So, should be very interesting. I had also made a lot of little bowls and cups to test things like those above that I thought might make a mess. I bought some kiln wash from a local vendor....it disturbs me....it's not the silica/alumina wash I am used to. So I didn't want to risk having to grind shelves.
Thanks for all your input. As soon as I have those fired samples, I will post them!