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Rant about cooling.

Posted: Sat Apr 12, 2014 3:58 am
by Lauri Levanto
I have understood that theory of cooling, or more properly tradition of cooling comes from some charts Corning has published.
Those were based on experience with glass blowing. A difference is made between single side cooling (bottles) and double side cooling (bowls and othe open forms). Corning published values for double side cooling.

That seems to be a tradition that tables for fusing also declare double side cooling although the normal situation is that the blank lies on a shelf with great thermal mass.

Even the latest BE tables speak of double side cooling although the measurements appear fo be made in single side situation.

There are two difference between double side and single side
cooling. 1. Double side have double so much cooling surface..
2. In double sided the distance heat must cover is only one half of the thickness. As the cooling slows down in the square of distance the single side values must be four times longer.

I have seen here that people here take the face value of BE tables for single side work and succeed. Why the words speak of double side?

In casting the situation is different. The cooling surface is often small. A thick plaster silica mold is great insulator.
Then one must really estimate the distances heat must travel
in the glass.

Just now I have no access to kiln. A planned test to measure the heat transverse rates of different materials is currently not possible. Is Morganica or someone else interested to set equal rods of glass, mullite, plaster-silica and ceramics in test. Heat one end and measure the temperature rise at the other end.

Re: Rant about cooling.

Posted: Sat Apr 12, 2014 7:34 am
by Don Burt
I understand that glass slab on a thick mullite shelf will have a complex cooling life, where most of the heat will necessarily move through the upper face. And that will be different than a slab that is standing on end and cooling more rapidly from both faces, if such a thing were contrived. Their heat-up history would also be different and complex. Do you expect to be able to chart a cooling profile that will be useful? What would you do with the information? Seems like there are too many variables to try to chart, even if you established/fixed the exact glass piece, shelf, schedule, and kiln ( are you going to measure from the dead center of the piece....what about the corners?). Don't most people just monitor one place inside the kiln atmosphere, and hope that soak period and slow descent through the strain point does the job?

I don't understand the problem statement.

Re: Rant about cooling.

Posted: Sat Apr 12, 2014 9:05 am
by Bert Weiss
Bullseye ran tests for thick slabs of glass cooling on a shelf. They placed a thermocouple under the glass, one embedded in the core of the glass, one on top of the glass and 3 in the air. Then they recorded the graphs of where each thermocouple was during a firing cycle. Annealing takes place when the entire mass of glass sits within 5ÂșC, inside the annealing range, for around 15 seconds. The idea here was to observe, in real time, how long it takes the core, top, and bottom thermocouples to equalize with one another, and compare that with the air temperature. We control the air with the expectation that the glass follows along.

One could actually run firings using the all thermocouples if a controller was devised to do that. There are industrial controllers that can accomplish that.

It has long fascinated me that holding too long in an unevenly heated box can introduce uneven heating profiles in to the glass. These can be avoided in a quicker cool down. You can anneal soak or ramp too long, if your kiln is not even enough. In any given kiln space, the kiln walls effect the temperature near them. In a brick lined kiln, the walls suck heat during the heatup and emit heat during the cool down. In my rectangular fiber kilns, the middle always wants to be hotter than the ends. If you divide the kiln space in to 3 rectangular areas, the outside zones have 3 cold faces and 1 hot face. While the middle zone has 2 hot and 2 cold.

Everybody's kiln is a bit different. Every load inside the kiln can be different. So, while physics do rule the roost, it is nearly impossible to have enough information to scientifically precisely devise a firing schedule. So, we do it by the seat of our pants. There is not one way that works or one way that fails. Our first goal is to find a way that works. Our second goal is to get the job done in as little time as is necessary. Our task is to choose a schedule that gets the job done without wasting too much time and energy.

The toughest judgement is for a tack fused piece with odd 3D shapes. The challenge being to not let the thin parts cool down too much while the thicker parts are still hot.

Re: Rant about cooling.

Posted: Sat Apr 12, 2014 4:15 pm
by Tom Fuhrman
no one's mentioned the work that Libensky did on annealing and cooling castings. He put out a lot of charts on this subject.