Shoutout to glassblowers who ace the cooling stage

I just tried making a drinking tumbler from boro glass over the weekend. After shaping it, I popped it into the kiln for annealing. I programmed a drop of 200 degrees per hour, thinking that was safe. But when I pulled it out this morning, the handle had a fine crack running down one side. It looks like thermal stress got to it. Maybe my drop rate is too quick for boro? How slow do you cool your boro pieces? I have a standard kiln with a timer and temp gauge. Would love to hear how you avoid this kind of thing.

3 comments

3 Comments

susanm561mo ago

Actually, my Paragon Caldera kiln runs a 250 degree per hour drop for boro as its standard schedule and I've never had a stress crack. That slowdown to 200 might be letting the glass sit in a tricky temperature zone too long. Sometimes going too slow can cause problems as the different thicknesses in a piece like a handle cool unevenly anyway. I'd look at your initial annealing soak temperature or how evenly you heated the handle attachment, not just the cool down.

valfisher1mo ago

Used to swear by super slow cooling like 150 per hour for my boro mugs. Had the same hairline cracks on handles all the time. Switched to a faster 300 degree per hour drop in my Jen-Ken and the problem just stopped. Guess the piece doesn't get stuck at a bad temp that way.

sethhernandez1mo ago

Man, @valfisher's point about a faster drop fixing cracks is so true. I see this everywhere, like when you overthink a problem and make it worse by trying too hard to be perfect. Slowing your cool to 200 degrees an hour probably let the thick handle and thin wall pull apart at different rates.