Re-heating Thick Glass

[Stephen Richard]

I posted an off the cuff kind of response on how to begin to think about the rate of advance that might be used on any subsequent firings to the first full fuse.

I have now looked at the Bullseye schedules for thick slabs and at stone's rates of advance for single pieces of glass and have come up with a theory.
The theory runs to the effect that the rate of heat up of evenly thick, rectangular pieces should be no more than twice the rate of the initial annealing cool.
The data cited comes from Bullseye (various places) and Graham Stone's book.
Theoretical support so far:
6mm glass is annealed at 80c/hr; subsequent rate of advance from cold - 150C/hr = a multiple of 1.9 [stone heats 6mm at 320C/hr, a multiple of 4]
12 mm glass initial rate of cooling - 55C/hr; stone heats at 140C/hr = a multiple of 2.5
19mm glass initial rate of cooling - 25C/hr; Stone heats at 50C/hr = a multiple of 2.0
25mm glass initial rate of cooling - 15; Stone heats at 25C/hr = a multiple of 2.0

This of course would apply only to glass slabs uniform in thickness and without tapers or other significant variations in dimensions.

What empirical evidence is there to support or deny this kind of theory? Is it a discussion worth having?

Stephen

[charlie]

[quote="Stephen Richard"]I posted an off the cuff kind of response on how to begin to think about the rate of advance that might be used on any subsequent firings to the first full fuse.

I have now looked at the Bullseye schedules for thick slabs and at stone's rates of advance for single pieces of glass and have come up with a theory.
The theory runs to the effect that the rate of heat up of evenly thick, rectangular pieces should be no more than twice the rate of the initial annealing cool.
The data cited comes from Bullseye (various places) and Graham Stone's book.
Theoretical support so far:
6mm glass is annealed at 80c/hr; subsequent rate of advance from cold - 150C/hr = a multiple of 1.9 [stone heats 6mm at 320C/hr, a multiple of 4]
12 mm glass initial rate of cooling - 55C/hr; stone heats at 140C/hr = a multiple of 2.5
19mm glass initial rate of cooling - 25C/hr; Stone heats at 50C/hr = a multiple of 2.0
25mm glass initial rate of cooling - 15; Stone heats at 25C/hr = a multiple of 2.0

This of course would apply only to glass slabs uniform in thickness and without tapers or other significant variations in dimensions.

What empirical evidence is there to support or deny this kind of theory? Is it a discussion worth having?

Stephen[/quote]

that's interesting.

also, looking at annealing rates, doubling the thickness halves the rate. not only that, but the rate change is 2x the previous rate for the next ramp (usually around 100-150F span).

there must be a constant 2x or 1/2x in the equation somewhere.

regards,
charlie

[Lauri Levanto]

Let us first look at annealing. The purpose of annealing is to prevent the formation of permanent "frozed in" stress. Byllseye studien tell that this is achieved when the thermal difference between the coldest and hottest part is less than 5 C.
Annealing soak does that. Annealing proper maintains this 5C gradient ontil the glass is solid enough. After that BE permits 10C delta during cool down. It is enough to prevent thermal shock.

When reheating, our interest is only to avoid thermal shock. To keep the ingot in one piece. The same 10 C difference applies.
When has the core of a thick piece achieved the temperature surface-10 C ?
It depends of the thickness.Over a shorter distance the heat moves fast.
Over a double distance to the core, the heat has to travel double length.
That indicates double time.
However, heat like water, runs faster on a steep slope. When we doubled the distance, the steepness wasalso halved. The heat travels slower.
Therefore double distance with half the rate needs 4 z time. I agree with Graham.

On the way up viscosity diminishes, that is elasticity increases. It tolerates more without shock. I would start with the delta 10 C . At annealing range one can go at least double so fast. After annealing range the is no risk for thermal shock.

-lauri

[Stephen Richard]

So Lauri, you suggest a rule of thumb of twice as fast a heat up as the initial rate of cooling until the Annealing temp is reached. Then double the initial re-heat rate.
Do I understand you correctly?
Stephen

[Bert Weiss]

I haven't studied Steve's theory, but it can't possibly make sense to me. I know from reading this board, that I can heat glass faster than most people here. There are a few reasons for this. One being that my kiln has elements strung an inch apart over my entire roof, that are controlled by relays that are on or off 120 times a second, yielding proportional control. Another being that I am often heating float glass in either a single or double layer. Float has a little lower COE than the fusing glasses, and has a perfectly consistent thickness. On the other hand, I program my anneal schedule timing much the same as everybody else. (my temperatures are for float) So my ratio of heatup to anneal cool would be quite different. I heat a single layer of glass up to 12mm thick at 600dph, which is conservative because it never fails. I have not bothered to find the edge where I am going too fast for any glass I fire. I heat 2 layers of 10mm at 300dph to fuse them and for a reheat I go at 112.5 dph. Again, I have no clue how much faster I could go, but I have no real need to find out.

[Brock]

. . . Bullseye schedules for thick slabs and at stone's rates of advance . . . not float.

[Bert Weiss]

. . . Bullseye schedules for thick slabs and at stone's rates of advance . . . not float.

Stone uses float glass as his standard. You have to adjust temperatures for Bullseye or other glasses.

There is a significant difference between a thick slab of one Bullseye glass and a slab made of multiple glasses. The stresses during heatup are quite different, yet they anneal the same, which is exactly my point.

[Brock]

Stone uses float glass as his standard. You have to adjust temperatures for Bullseye or other glasses.

Maybe you haven't actually seen his book. He gives specific schedules for many types of glass.
You should read it.

[Alexis Dinno]

Stone uses float glass as his standard. You have to adjust temperatures for Bullseye or other glasses.

Maybe you haven't actually seen his book. He gives specific schedules for many types of glass.
You should read it.

No need to be rude: Stone does use float glass as his standard (see page 6-30 of Firing Schedules for Glass). Stone provides adjustment constants for several other kinds of glass, but the schedules are all based on float.

[Brock]

That wasn't rude. Rude is when you call people epithets.
Maybe this is an exception to the, "change every thread to float" we normally get.
It gets very tiring after, oh, 9 years or so . . .

[The Hobbyist]

I haven't studied Steve's theory, but it can't possibly make sense to me.

Then why are you posting a reply? Is it just to tell us again how great your kiln and process are?

I have no interest in nor do I ever use float glass. So you will never see me posting an opinion on threads that are about working with float.

Jim

[Bert Weiss]

I haven't studied Steve's theory, but it can't possibly make sense to me.

Then why are you posting a reply? Is it just to tell us again how great your kiln and process are?

I have no interest in nor do I ever use float glass. So you will never see me posting an opinion on threads that are about working with float.

Jim

The theory that you can devise a heatup schedule based on an anneal cool schedule is a flawed notion. Different glass configurations along with different kiln configurations combine to make widely different heatup requirements. All the while, all of these different situations, if the same thickness, could well want the same anneal schedule timetables. I didn't have to study the relationship between the heatup and anneal schedules to know that. This is true regardless of what glass you are using.

The set of schedules I refer to in Stone are all written for float glass and temperature variations are given for a variety of other glasses. This is a big part of my point. Annealing is all about thickness requiring time. I never even look at the heatup times because I believe that nobody can tell you what your ideal heatup rate is because there are too many variables. If you get slow enough, it will work for everybody, but why would I want to do that? The only time I would advise somebody about their heatup schedule is after they write, "I have been using this schedule for a long time and nothing ever broke before" The answer at that point, is always slow down some more.

[Lauri Levanto]

So Lauri, you suggest a rule of thumb of twice as fast a heat up as the initial rate of cooling until the Annealing temp is reached. Then double the initial re-heat rate.
Do I understand you correctly?
Stephen

Pretty much that Stephen.

The Delta=10 C seems to be a safe rule downwards. A bit on the safe side. The final cooling rate tells it is possible to go higher. Better to err on the safe side, however. It must be quite right on the way up, too.

The essential point is how the heat creeps in.
Over the annealing range, the glass is pliable. I cannot visualize a thermal shock over 500 C. There you can go AFAP, If you remember that in the example of the obelisk, fast heating means that the thin top softens and deforms earlier than the thick base.

-lauri

[Bert Weiss]

If we had the luxury of test firings with embedded thermocouples and a graph recorder, we would really know the Delta variations, and devising a firing schedule would take a few tests to find the edges of what works. But as soon as there is a change in any configuration, you would have to start from scratch. The pyramid or obelisk are certainly in the most challenging shape category.

But, we don't have that luxury (at least most of us). What we do have is practice. The trick is to guess a conservative enough schedule, that doesn't make us go crazy, waiting. Then adjust, if it is still too fast. The big deal is to understand that what works for one person, may not work the same for you. The variables of kiln configuration and glass configuration are pretty wide.

Step one in designing a heatup schedule is understanding that the goal is to keep the temperature variation, within the mass of glass, within a range that will not result in cracking. The good news is that glass is a fairly good conductor of heat.