Devit

[Elaine Smith]

I recently had the opportunity to purchase some glass windows from a church. The glass is a streaky green glass. I have tried fusing several pieces and the glass devitryfied (sp?) rather badly. Some have been reading up in ways to prevent this. I understand I need to decrease the amount of time the glass spends at > 1400 and decrease temp quickly. Any other suggestions ?

[Laurie Spray]

If you have a glass that is not necessarily used for fusing and is prone to devit I would use a super spray or sprinkle clear powder on the surface before firing.

[Stephen Richard]

It is also unlikely to be very successful ever, as older window glass was produced without any concern for compatibility between batches. It will be best appreciated used for projects as cold glass not kiln formed ones.

[Bert Weiss]

To elaborate on Steve's post, glass made in the same furnace in the afternoon, can be incompatible with glass formed in the morning. As the glass cooks, the chemical content changes, enough to effect compatibility. The materials used in the batch can also determine propensity to devitrify. When they formulated for flat glass, devitrification, when reheated was a non-issue. It is true that Laurie's fix of an overglaze can be effective, on compatible glasses, but you do have to get to 1400ºF to gloss the overglaze.

[Morganica]

I'd lean toward the don't fuse camp, myself. If it's been in a church building, the glass has been subjected to everything from acid rain to lots of sunlight to cleaning chemicals for years. That can cause subtle changes in the glass as well as a slight surface etching, which can accelerate devit. (Devit, btw, isn't only caused by heat. It can also be caused by chemical or mechanical agents, or any combination of the three)

If you want to give it a shot, though, use the shiniest, freshest pieces and (obviously) don't fuse different pieces together unless there are swirls of color or texture in adjacent pieces that fit together and show it came from the same sheet. Clean the glass really, really well, if possible giving it a light sandblasting or etching to remove any surface problems. Then fire a test piece and see how it does. Keep out of the "devit zone," typically 1300-1400F, as much as possible, and process for as short a time as possible.

If you get any signs of devit in your test piece, consider using an overglaze. You can make one yourself with laundry borax, just search on the web for the recipe.

I personally don't like the clear powder idea, especially with an unknown glass. The powder probably won't be compatible, which could result in a hazy or crackled appearance. Even if it works, powder still tends to trap bubbles, which can be seen against many colors and gives the glass a slight clouded appearance.