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...Here it goes: What is the process for tempering glass? I tried a google search on it, but found next to nada

Skin_Mechanic wrote:...Here it goes: What is the process for tempering glass? I tried a google search on it, but found next to nada

Briefly, you heat up glass in a chamber, then rapidly cool it by blasting it with air.

This makes a type of "skin" on the glass.

The outside is in compression, and the interior is in tension.

( I just changed the above, I had it reversed)

If you break the skin, by impact, or sandblasting too deeply, bang!

Ok, now your bein' a (information)tease I need the specifics: How high do I need to fire the piece? How much does it need to be cooled? Is it cooled down to room temp, or only to below the strain point? And what's the difference between heat strengthening and fully tempered?

[quote="Skin_Mechanic"]Ok, now your bein' a (information)tease I need the specifics: How high do I need to fire the piece? How much does it need to be cooled? Is it cooled down to room temp, or only to below the strain point? And what's the difference between heat strengthening and fully tempered?[/quote

Nah, I wasn't teasing, you can't temper in your kiln. You can't even heat treat, (think coke bottles, or marbles). All we can do is anneal, take all the stress out. Leaving controlled stress in glass requires expensive industrial equipment. Brock

Skin_Mechanic wrote:Ok, now your bein' a (information)tease I need the specifics: How high do I need to fire the piece? How much does it need to be cooled? Is it cooled down to room temp, or only to below the strain point? And what's the difference between heat strengthening and fully tempered?

You can't do it without a major investment in equipment. The trick is to heat the glass up above anneal and cool it back to room temperature in less than 30 minutes without shattering the glass. Done correctly ( I think Bert mentioned 600 horsepower blowers), the glass heats up uniformly from all sides and cools quickly but uniformly leaving a skin that has very high stresses on the surface from shrinking faster than the core.

Tony

How bout a coupla electric leaf blowers cooling the glass from both sides... Hmmmmmm, there's gotta be a way to DIY sumthin' I may try experimenting on some small pieces.

A tempering furnace costs a zillion dollars. The heating chamber has expensive alumina rollers that move the glass back and forth while it is being heated from both sides. 3/8" glass takes 5 minutes to heat up from room temp to 1100ºF. Then it is rolled in to the quench part of the furnace which is powered by a 750 HP blower. The blower directs cold air through nozzles placed every few inches on top and bottom. If the glass is going to break it does so about 2 minutes in to the quench. There is a sizeable pile beneath the quench in a removable bin. The cooling part of the process takes 7 minutes, so 12 minutes to go from room temp to 1100 and back down to room temp. Of course there are no guarantees that your kiln fired glass will survive the process.

Once I learned what not to do, I have not lost any pieces. I believe that my glass is likely much better annealed than sheet glass from the factory, so my percentages should be better.

An interesting factor is that the thicker the glass, the easier a challenge it is to temper it. The big HP is necessary for thinner glasses. I still never heard of anybody who managed to do it on the cheap.

Brock explained the stress thing. Glass is very strong in a state of compression, stronger than steel. We know all too well how weak it is in tension, though