I'm hoping that someone heard the tidbit that I need.
I'm told that right angles and other sharp angles need extended annealing time to be successful. The trick that I need, is how much longer?
The piece is approx 12" tall, 8in wide max, with a 2" square channel through the middle. The square core IS solid, and too small to be made holllow with what I currently know. I've been running on the 8 inch Libensky sched from Glass notes, and doubling the hold time, taking the ramp to strain at 1.5x.
Could there be a simple starting point (all castings + glass + ovens are different), from which to extend the sched?
Something like, for 2 internal angles, extend the annealing soak by 75%.
Being on the cutting edge is one thing, dancing on a razor blade, quite another.
Thanks
Greg
tricky casting annealling Q
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charlie holden
- Posts: 260
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- Location: Atlanta
It is hard to imagine what your shape is like and where the right angles are.
I made some rectangular boxes with two inch high, 1/4 inch thick walls, where the top of the walls were buried in the bottom of the mold while the bottom of the box was exposed to the air. So to cast them the glass started on the plug that formed the interior of the box and flowed down the sides to what ended up being the top of the walls.
This sets up a situation where there is not much glass for strength, right angle inside corners and lots of variation in insulation thickness around the glass. I broke seven of them before they started surviving. Even though the glass was only 1/4 inch thick I had to anneal for 4 inches thick. I arrived at that by figuring that the glass walls needed to anneal for a thickness of at least 2 inches since their tops were buried but bottoms exposed. Then I doubled that schedule to account for the right angles.
The thicker the glass around the weak point the more strength it has to counteract stress. You just have to try things and then look at the results with a stressometer to see how it worked. Cold working will often be when you find out how much stress is too much.
ch
I made some rectangular boxes with two inch high, 1/4 inch thick walls, where the top of the walls were buried in the bottom of the mold while the bottom of the box was exposed to the air. So to cast them the glass started on the plug that formed the interior of the box and flowed down the sides to what ended up being the top of the walls.
This sets up a situation where there is not much glass for strength, right angle inside corners and lots of variation in insulation thickness around the glass. I broke seven of them before they started surviving. Even though the glass was only 1/4 inch thick I had to anneal for 4 inches thick. I arrived at that by figuring that the glass walls needed to anneal for a thickness of at least 2 inches since their tops were buried but bottoms exposed. Then I doubled that schedule to account for the right angles.
The thicker the glass around the weak point the more strength it has to counteract stress. You just have to try things and then look at the results with a stressometer to see how it worked. Cold working will often be when you find out how much stress is too much.
ch