Avoiding Crystalization (Devitrification, Devit)
I had some problems when Uroboros went out of the 90 COE business and I changed over to Bullseye sheet glass. Opaques often had "the mange" in a few small places, after firing.
I went down the rabbit hole on "Devit" and have made adjustments. Now I have the best surfaces I've ever had. Perfect, clean, beautiful.
Here are some thoughts on how to avoid crystalization issues using your firing schedule.
First a definition: Devitrification is the process of crystalization on a previously crystal free glass. Generally considered undesirable and manifesting as a surface shine disruption of the unpleasant variety.
Using what we know about the crystalization process we can optimize kiln firing conditions to make them as unfavorable to crystal formation as possible. First Crystalization is a low entropy state. Therefore it is a cooling process phenomenon. Adding heat (adding chaos or entropy) is antithetical to crystal formation.
1) Always cool AFAP or 9999 down to the anneal range. I generally put the breaks on at 1000 F.
2) Consider a slow ramp to a shorter hold at a top temperature vs a fast ramp and a long hold. A lot of schedules call for AFAP or relatively fast ramps above where you can temperature shock the glass. Slowing down on the way up helps to avoid the need for a long top hold and has many other benefits, like a superior bubble squeeze.
2) Add a little heat at the end of a long hold at your targeted top temperature. Because of the complexities of heat transfer you can get some surface cooling during a long hold. If you are holding glass at a top temperature for a tack or a full fuse, give it a fast little pop of heat at the end with a hold of 0 minutes.
3) Slump below 8 poise whenever practical. Did you know that the anneal temperature use to be determined by the bend of a glass rod after 4 hours? You can successfully slump with less risk and more beauty and retention of detail below many of the posted slump schedules out there. Bullseye has a ton of schedules by mold. Much can be accomplished below 1215 F.
4) Surface cleaning - I use dawn dish liquid in the sink as a degreaser. Then I follow up with spartan cleaner. Whether that removes crystalization nucleation sites or prevents surface tension disruptions from a surfactant action I don't know but it is foundational.
Figure 1 below is a representation of the heating and cooling process in the kiln. The Y axis is change in volume. Sometimes entropy is also shown on this graph on the y axis. Change in entropy increases and decreases in the same direction as volume on this graph (Entropy also increases up the Y axis and decreases down the y axis). Crystalization is a low entropy state (it is the most orderly state). This concept is important for how to control crystal formation. Crystalization requires 1) cooling and 2) in the right temperature range.
The black shaded range is the range in which crystalization is possible. I marked up the figure in red to show Tx (the viscosity below which crystallization does not occur). Tx for glass is at 8 poise or around 1215 deg F for bullseye glass. Tm is given by various sources as 2 poise for glass.
Crystalization for us is a surface phenomena. A boundary layer problem. 8 poise also happens to be in the range where glass powders start to sinter which implies the start of boundary layer melt.
What this means for the fuser is that basically any temperature above 1215 F that a fuser may use in a firing schedule opens the door for crystalization during cooling.