The myth of a long slow cool?

[Peter Angel]

Hello.

I was speaking to the owner of a commercial glass art studio (who works with full sheets of thick glass...his kilns are huge!).

He said that if your kiln is well calibrated and you know the exact annealing temperature of a glass eg bullseye, system 96, gaffer, etc) then you only need to do a long hold at the annealing temperature. There is no need to gradually reduce the temperature past the annealing temperature. In other words a long hold at the anneal temperature and then switch the kiln off and let it cool normally.

The only time you need to do a slow cooling past the annealing temperature is if your kiln is not well calibrated or you don't know the exact anneal temperature of the glass eg a random piece of float glass or a glass bottle.

He said that he and other warm glass studios have been doing this for many years without any problems and they test for stress in the glass. The concept of a long slow cool is a myth.

I'm not saying I agree with him. I'm only passing on what he told me.

Could this be true?

[Tom Fuhrman]

some of that is true. if you can get a copy of the discourses that Frank Wooley wrote on the annealing process. viscosity of different glasses and their chemical makeup cAN have a big effect on what can be combined and shape has a lot to due with it. i.e the large telescopes blanks that Corning produced required a very slow cooling process because of their shape they took over 200 days to cool one and it still cracked. Libensky was also very well educated in doing large casting annealing and he had his own schedules. check out some of his schedules and rationale for doing what he did.
even the shape of molds and their design and vent holes makes a huge difference. The sameis true for injection plastic molding and production and die casting. Thwe oldglass moldsinvoved a lot more than just casting around a given shape. lots of engineering was involved that most are not aware of. that's why a lot of the old iron and steel molds willo not render decent production. I've had many antique molds that were not originally made for decent production and are essentially just large pieces of iron scrap now with a lot of beautiful machining in them.

[Brock]

Once the glass is properly annealed, you still have to worry about THERMAL SHOCK. They are two separate things. And, what you can do with a monolithic clear glass under exact, repeatable factory conditions is a far cry from someone fusing various colours of compatible glass in their home kiln, under somewhat less precise controls.Go for the long slow cooldown, there's a reason we've been doing it for decades

[Bert Weiss]

Last week, I was firing 2 pieces 26" x 30" x 30mm float glass. The power went out after 4.8 hours of anneal soak at 1000ºF, followed by about 4.2 hours out of 4.8 hours falling to 900º. So the glass was 9 hours in to a 9.6 hour anneal process. Power was out for 2 hours, on for 15 minutes, and off another 5 hours. I was able to make an assessment when the power came back on. I decided to just let it drop naturally. It had dropped faster than programmed, but not dramatically faster. There was quite a bit of hot glass in there, keeping the whole kiln hot. I had no idea the power would go back off. The glass is intact a week later. I will be doing a grind and polish on the edges soon. I expect it to play nice. The risk I took was falling too fast from the strain point down to 700ºF. I want to believe, if the glass survived this drop, it is properly annealed.

So this is exactly what you describe, Peter. If there is a place in the schedule to save time, It would be from 900ºF down to 700ºF. However, I always feel comfortable with the schedules I use. The kiln cools slower than my program somewhere below 600.

[Morganica]

Hello.

I was speaking to the owner of a commercial glass art studio (who works with full sheets of thick glass...his kilns are huge!).

He said that if your kiln is well calibrated and you know the exact annealing temperature of a glass eg bullseye, system 96, gaffer, etc) then you only need to do a long hold at the annealing temperature. There is no need to gradually reduce the temperature past the annealing temperature. In other words a long hold at the anneal temperature and then switch the kiln off and let it cool normally.

The only time you need to do a slow cooling past the annealing temperature is if your kiln is not well calibrated or you don't know the exact anneal temperature of the glass eg a random piece of float glass or a glass bottle.

He said that he and other warm glass studios have been doing this for many years without any problems and they test for stress in the glass. The concept of a long slow cool is a myth.

I'm not saying I agree with him. I'm only passing on what he told me.

Could this be true?

Anything can be true, and in some circumstances I'd agree with him, but overall I think those statements are 'way too generalized. If they were always the case, I could pour hot water into a cold glass and it would never break.

Kilns have varying insulation values, glass projects employ varying thicknesses and shapes and colors, kiln furniture and mold refractories can be included or excluded, etc. All those things are going to affect thermal equilibrium of the glass on the downramp, which is what we're really talking about. And people who run glass production for a living are more likely to have precisely calibrated kilns and highly standardized processes than the majority of us kilnformers.

I think his statement is more likely to be true if his ginormous kilns are brick, especially if they're more than one brick thick, and he's working a lot of common designs with flat sheet of a few layers of perfectly symmetrical float glass. If he's doing a 2-foot tall soda lime casting with twists, turns and tentacles that varies from 0.125 to 3 inches thick and a tack fuse with 200 separate components tomorrow in a fiber kiln...not so much.