“Question-when cutting up a Screen Melt, using a tile saw. How do you NOT get devitrification when laying the slices cut sides up?”
Devitrification occurs where there are differences in the surface. This means that the surfaces exposed to the heat must be both clean and smooth. It is not enough for only one of these to be the case, both are required.
First, the sawn edges need to be clean. A good scrub with a stiff bristle brush is essential.
Second, devitrification sprays of whatever kind do not seem good enough to prevent the devitrification. This is probably due to the thin covering of the differences (scratches, pits, etc.) on the surface.
Beyond that, I know of two ways to prevent or reduce devitrification. That is, providing a smooth surface to resist devitrification.
1 – Grind
This can be done with hand pads, grit slurry or machines such as a Dremel with damp sanding pads or belts, wet belt sanders, or a flat lap. The grinding should go down to at least 400 grit before cleaning and arranging to fire.
2 – Clear glass
This method relies on putting a layer of clear glass that is less likely to devitrify than the cut edges over the whole surface. You could use a sheet of glass, although that would promote a multitude of bubbles due to the spaces between the strips and the naturally uneven heights of the strips.
Placing a layer of fine frit on top of the arranged pieces before firing is a way of allowing air out and forming a smooth upper layer by filling the gaps. It is best to avoid powder, as this promotes a multitude of fine bubbles, giving a grey appearance. The layer you apply needs to be an even layer and at least 1mm thick. If you are concerned at getting lots of bubbles, you could use medium frit instead. In this case, the layer will need to be thicker than 1m to get an even coverage. The whole of the surface of the piece needs to disappear under the layer of frit, and that may be a good guide to the thickness of frit to apply.