Lost another piece to stress cracks, now I'm stuck between slow oven drops and quick kiln shuts

I was working on a thick borosilicate paperweight, and it kept splitting right down the middle after it cooled. Some folks at the studio told me to let it anneal super slow, like dropping the kiln temp over ten hours to avoid internal stress. But others said that's overkill for boro, and you can shut the kiln off once it hits 900 degrees for a faster cool without cracks. I tried the long way first, which worked but ate up a whole day. When I rushed it, the piece made it sometimes but not always. I started pre-heating my kiln to exactly 1050 before putting the piece in, then did a slow drop to 900 before cutting power. That mix seems to do the trick for me now. So, which side are you on for keeping heavy pieces intact, the slow anneal crew or the quick cool camp?

3 comments

3 Comments

oliver_jones3219d ago

Remember my buddy who kept blowing through boro ashtrays? He was all about the quick kiln shuts until one split so bad it sounded like a gunshot. That chunk was nearly an inch thick in the center. He swore it was a fluke until it happened again on the next heavy piece. That's exactly what @amyg29 is getting at, thickness totally changes the game. Watching him trash those pieces turned me into a slow anneal believer for anything bulky.

amyg2919d ago

So is the thickness the main thing that decides which method works?

margaretc5910d ago

Well, thickness sure plays a huge part, but don't forget about how the glass was worked before it even hits the kiln. Like, if you've been torching it unevenly or adding layers in a rush, that sets up stress from the start. I saw a guy once who made a thick vase but kept reheating one spot, and it cracked during anneal even with a slow cool. So it's not just about how thick it is, but how even the heat history is. Thickness tells you how long to anneal, but the stress already in the glass tells you if it'll hold together. Makes you think about the whole process, not just the final step.