Sorry for the long wait on a reply here.
Some notes here:
The crack that has healed indicates that the glass broke during heating, but before full fuse. This is likely a case of thermal shock, and could well be because the vents were open, preventing some areas of the glass from heating at the same rate as other areas, ultimately leading to a large enough difference in temperature to cause the hotter molecules to break free of the slower moving cooler particles. After reaching fusing temperatures, the cracks fused together, but the scars remain.
Another possibility is that the base piece of glass is expanding during heating all the way to the side walls of the kiln, meeting resistance, arching upwards, and breaking. It may sound far fetched, but the glass will expand .7% from room temperature until it reaches its softening point. In the case of these pieces, that means that the bottom piece needs about 7mm of space to expand or it will bow. The break does not look typical of that type of flexural buckling, but it could be a factor. Such a break would again heal at fusing temperatures.
Generally speaking, the panels are a little larger than recommended for firing in a kiln of this size. We recommend keeping the panels at least 2.54cm from the edges of the shelf all around. Further to that, we recommend keeping the edge of the shelf at least 2.54cm from the wall of the kiln all around, and the shelf elevated at least 5cm from the floor with posts in order to promote good circulation of heat and allow cooling around and under the shelf. Thermal shock and poor annealing are much more likely without this spacing. You may be able to successfully fire in the kiln shelf condition that currently exists, but to do so would require to slow down the firing both on heating and cooling.
I strongly suspect that your troubles are at least partly related to having the vents open in the kiln. It is remarkable what a difference this can make to the uniformity of temperature therein, and especially during the annealing cycle, the objective is to first remove the strain that has developed during cooling by getting the entire body of glass to be one temperature at the anneal soak temperature, and then cool in such a manner that a temperature difference of no more than 5°C develops throughout the body of glass until the strain point has been reached. There is just no way that such uniformity exists with the vents open.
So, why does it seem to work when the panel is exclusively clear, but not when there are opalescent colors in play? You can get away with an awful lot in terms of heating and cooling when you are dealing with a homogeneous body of glass. As soon as you begin to mix colors and types of glasses, or get considerably thicker, things need to be more carefully controlled. That means improving circulation in the kiln, closing the vents, ideally scaling the pieces down to better fit the kiln, and probably firing a little differently. Your idea of sandwiching the color between two layers of clear would probably help reduce the warping because it would create a more or less homogeneous outer skin and a more symmetrically designed panel (great for putting the piece in an architectural condition).
This is how I would fire a similar piece or pieces in your kiln given the current shelf condition, size of the piece, and assuming that the kiln is actually firing uniformly*:
Rate Temp Time
50 593 :30
222 660 1:00
Full 810 :20
Full 482 2:00
28 371 :00
Incidentally, this is how I would fire this piece in our kilns, assuming that everything were sized as I have suggested in my notes above:
Rate Temp Time
222 660 1:00
333 810 :10
Full 482 1:00
56 371 :00
As for the issue of the marks on the back from the ThinFire, those are typical of works of this scale. The ThinFire cracks open over spans this large. You can get a good, lucid surface by firing directly on fiber paper, onto which you have sifted dry shelf primer and then then brushed it into the weave and brushing excess off to one side.
*Here's some suggested reading that you might find of interest if you have not had a chance to see it previously:http://www.bullseyeglass.com/images/sto ... tes_01.pdfhttp://www.bullseyeglass.com/images/sto ... _paper.pdf
I hope that this is helpful.