One of your risks when slumping is a slumping tear — something that happens most often on large-diameter slumps. Here's an example:
We know this is a tear rather than thermal shock because the glass did not break completely apart. The ends of the break stop before reaching the edge, and slumping temperatures aren’t high enough to fuse broken pieces back together.
So, what's a slumping tear?
As the glass heats and enters the transition range — the temperature range in which glass changes from a solid to a non-brittle "liquid" state — the top surface of the glass (closest to the elements) softens before the glass on the bottom surface. This is because the heat must travel through the glass to reach the glass closest to the shelf.
When slumping, this can mean the top layer becomes soft enough that the glass can no longer support its own weight, so it begins to sag. The bottom layer, however, is still brittle and hasn't had sufficient heatwork to stretch. As the top surface stretches, the bottom surface tears.
The solution? A reverse anneal.
The goal of annealing is to allow the glass to transition from a liquid to solid uniformly. The goal of a reverse anneal is the opposite — to transition from a solid to a liquid uniformly.
If the annealing for your project looks like this:
FULL ➡️ 900° F 🛑 2:00
0100 ➡️ 725° F 🛑 0:00
Then the reverse anneal, added to the initial heating at beginning of the schedule, looks like this:
XXXX ➡️ 725° F 🛑 2:00
0100 ➡️ 900° F 🛑 0:00
Where XXXX is whatever initial heating rate you would have used before.
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