Chris Lowry wrote:So I've been doing my first fusing and I'm getting some devitrification on only some colors. What am I doing wrong? I'm doing some 5-6 layer stacking and some double sheet fusing at the same time.
When you reach full fuse temp and it is cooling down do you open the door to force cool or just let the oven cool naturally?
Am I doing the schedule wrong?
200 an hour to 1000 hold 15 minutes
300 an hour to 1225 hold for 1 hour
500 an hour to 1490 hold 15-30 minutes
naturally to 950 hold 1.5 hours
100 and hour to 700 hold 1 minute
Thanks for the help
So...it's hard to say why you're getting devit without more information. First, make sure it IS devit (most likely it is, but there can be other reasons that the glass doesn't come out smooth and glossy). But let's take it from the top: You're fusing 5-6 "layers" of glass. If by layers you mean 3mm sheet glass, then you've got up to 18mm of thickness, or about 3/4 inch, of glass. I don't know what kind of glass you're using, but a reasonable schedule for soda lime fusing glass might be (from the BE tables and my own schedules):
300dph to 1100F hold 30
400dph to 1240F hold 30
AFAP to 900F hold 3 hours
45dph to 800 no hold
80dph to 700 no hold
270dph to room temp (essentially you can turn the kiln off at this point)
So, basically, your schedule is too fast and you're liable to induce some stress at the rate you're going unless you're working with a really tough glass like float.
On the devit question, though, there are a lot of reasons why glass devits or crystallizes. It can be the chemistry of the glass itself (glasses that aren't made to be held in kilns for long periods of time are prone to form crystals when you fuse or cast them), or the fact that it's subjected to treatment that creates "nucleation sites," i.e., seed crystals that allow crystallization to take place. Those treatments can be chemical (application of acids, for example), abrasive (rough grinding), holding the glass at a temperature that promotes crystal growth.
Most commonly, though, crud on the surface of the glass gives crystals something to form around. Sometimes that can be dirt, oil from finger prints, swarf (residue from grinding that gets embedded in the glass), or microscopic fibers from ceramic fiber paper or kilnwash that embed in the surface of the glass from a previous firing. Once started, crystals will continue to form every time the glass is heated, so the devit will grow.
Different glasses devit at different rates. Transparent glasses tend to devit less than opaque glasses, and lighter-colored glasses devit more than darker glasses. Usually. If you go to the Bullseye website and search on devitrification you'll find some good information on avoiding devit. Primarily, you want to make sure you clean your glass really well, avoiding cleaners that leave a residue, touch the surface as little as possible between firings, vent the kiln when you're using refractories like fiber paper (which can off-gas and leave crud on the glass surface), etc.
What the SprayA (or any anti-devit spray--you can easily make your own much less expensively with a box of borax laundry soap) does is act as a "flux," or low-temperature glass that melts over the surface of the devit to cover it up. It's OK as a final step, but it can be a problem for food-bearing surfaces and it actually is a different glass with different characteristics than the base glass. The sprays are also "sticky" if they drip onto your kilnshelf--they can drip into the kilnwash and wind up taking divots out of the shelf, so be careful.
Bullseye also recommends sifting a very fine layer of clear powder over the entire surface of a piece with devit. That is a better solution (I think), although it can be tricky to get exactly the right thickness; too little and you get an orange peel effect. Too much and you trap tiny bubbles that cloud up the surface. Neither solution is great for curved inner surfaces that have already slumped into a mold; I think the better solution is to try and avoid devit in the first place.