White as a Difficult Glass

Description of the Project

A white 3mm base with 3mm and 6mm decorations made up of mosaic pieces from previously fused glass (all the same CoE). At the end of the firing three corners had broken and their edges rounded. The fourth corner had sharp edges. The tentative conclusion was that there was incompatibility between the white and the previously fired pieces. There were no other cracks visible on the white or between the mosaic pieces. The author did not indicate what the schedule was for either firing, nor what the profile of the last firing was, but asserts white is a particularly difficult glass which does not work well with a wide variety of colours.

My observations are: 

• Compatibility is not an issue on the heat up. It is only a problem at annealing and cooling.
• Breaks on the ramp up (showing rounded edges at the conclusion of the firing) are normally the results of too fast rates.
• Breaks during cooling (showing sharp edges) are due to annealing, compatibility, cooling rates, or some combination of these.
• Previously fired glass can show some shift in compatibility and so needs slower up ramp rates than normal for the profile and thickness.
• Incompatibility between the base and the mosaic pieces would show up as breaks in the white glass under each top mosaic piece.
• Not all glass of the same CoE from different manufacturers is compatible.

Could this have been from incompatibility?

On the way to top temperature the pieces have not yet combined. The incompatibility will only show up during the cooling, as it is the differences in viscosity between the fused pieces that cause the breaks.

Only one of the broken corners has those sharp edges, making incompatibility an improbable cause of the breaks. Further, incompatibility between the base and upper layers present either a crazed appearance at the connections, or simple breaks around the base of each decorative piece. Incompatibility would have multiple breaks all over the base, if not the top too. Finally, if the fired mosaic pieces were incompatible with the white glass, there would have been breaks throughout the whole piece, not just at the corners.

A further possibility is that the corners were very close to the sides of the kiln, because only the corners broke away from the piece,. If it was side fired, much slower rates are required. And all kilns tend to be cooler near the sides on the heat up than toward the centre, even if top fired.

My guess, based on the description, is that the up ramps were too fast, and the anneal was too short and the cool too fast. Unless the previously fused pieces were tested for stress it is not possible to know whether those were stressed before the final firing, which could have caused the break off of the three of the corners. The fourth corner break was on the cool down and is most likely to be too short an anneal and/or too quick a cool.

Is white glass especially difficult?

There is nothing in this piece to identify white glass as an extraordinarily difficult glass, or that a multiplicity of colours added to white would provoke breaks. The problems exhibited are most likely related to fast heat up ramp rates, and inadequate annealing and cooling.