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Topic: Is glass wool tempered or anealed?  (Read 288 times)

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Offline pcm81

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Is glass wool tempered or anealed?
« on: May 15, 2019, 07:33:40 PM »
I have 2 sets of glass wool. Corning and "chinese" Individual strands are of close diameter. 7 thousandths for corning and 9 thousandths for chinese. The Corning glass bends like silk, chinese breaks. I am guessing it is annealing vs tempering issue. Tempted to try to anneal the chinese glass in tabletop electric furnace. Looking for advice and input regarding temperature and cooling rate. Also if i am simply totally wrong... According to wikipedia I am looking at 900F temperature to anneal... how slow do i need to cool it?

Offline Enthalpy

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Re: Is glass wool tempered or anealed?
« Reply #1 on: May 17, 2019, 04:41:05 AM »
Only from Wiki, I have no first-hand observations...

To improve its resilience, glass is quenched at least in macroscopic thickness, from 550-700°C depending on the composition . This compresses glass' surface, so no tensile stress concentrates at microscopic notches and propagates.
https://fr.wikipedia.org/wiki/Verre#R%C3%A9sistance_m%C3%A9canique_:_fragilit%C3%A9
https://fr.wikipedia.org/wiki/Verre_tremp%C3%A9#Trempe_thermique
No word there about a minimum speed to make glass non-crystalline. I believe this limit is trivial.

Bottles are not quenched, the cooling time is 30-100min, reportedly limited by heat diffusion
https://de.wikipedia.org/wiki/Glas#K%C3%BChlung
so the duration varies as the thickness squared. 5mm and 4000s extrapolate to 8µm and 10ms. Or did you mean mils? Sounds huge to me. 6s then.

Offline pcm81

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Re: Is glass wool tempered or anealed?
« Reply #2 on: May 17, 2019, 02:39:04 PM »
Only from Wiki, I have no first-hand observations...

To improve its resilience, glass is quenched at least in macroscopic thickness, from 550-700°C depending on the composition . This compresses glass' surface, so no tensile stress concentrates at microscopic notches and propagates.
https://fr.wikipedia.org/wiki/Verre#R%C3%A9sistance_m%C3%A9canique_:_fragilit%C3%A9
https://fr.wikipedia.org/wiki/Verre_tremp%C3%A9#Trempe_thermique
No word there about a minimum speed to make glass non-crystalline. I believe this limit is trivial.

Bottles are not quenched, the cooling time is 30-100min, reportedly limited by heat diffusion
https://de.wikipedia.org/wiki/Glas#K%C3%BChlung
so the duration varies as the thickness squared. 5mm and 4000s extrapolate to 8µm and 10ms. Or did you mean mils? Sounds huge to me. 6s then.

From what i understood based on further reading:
The initial cooling of glass must be fast to crate glass itself. Slow cooling would result in formation of crystals, not glass. Fast initial cooling "locks" liquid like structures within crystal like boundaries producing glass. This cooling is from 1800 deg F down.
Annealing is then done to release stress. This happens at temperatures 850*F up to 1050*F and cooling must be slow.
Annealed glass can be tempered, which is the process of heating it up to below annealing temperature and quickly cooling it, producing an outside skin with tensile preload on it.
My annealing experiment with wool did not yield soft, flexible glass. The test at 1050*F fused the wool into an anthill type of structure. Test at 925 *F did not change the brittle nature of wool i started with. Looked for "glass work" forums, but no joy. Wondering if corning glass wool is chemically different from Chinese wool.

Offline Enthalpy

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Re: Is glass wool tempered or anealed?
« Reply #3 on: May 20, 2019, 05:09:33 AM »
I believe (but didn't check in length) that any reasonable cooling speed quenches ordinary glass. Thick glass has been made for millenniums, so quenching fibres is trivial.

Glass is formed at a high temperature, but internal stresses build up during cooling through a much lower temperature range, where the material has already some strength. This is typically the annealing temperature. I don't believe (not checked) that annealing brings something before quenching. It makes a difference for alloys because of segregation; they are first "solution-treated".

Quenching creates a compressive stress in the skin.

The chemical composition of glass varies a lot.

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