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I've collected data from the Skutt Pinto for time/temperature graphs. They're handy for enabling you to see, when you're at a certain temperature, how long it will take to get to a desired temperature at a given setting.
They're also handy if you want to raise the temperature at a specific rate.
I originally thought that these time/temp charts should be useful to others with Pintos, but recently I have come to question that assumption. When a complaint was made to Skutt that one of their Pintos took over 90 minutes to reach 1200F, on HI, with the lid shut, (after the new kiln owner saw the chart showing mine hitting that temperature in half the time) they answered that even 90 minutes was quicker than design time. This is all second hand, so I don't know who the Skutt rep was, or how familiar he actually is with design specifications.
That being the case, these charts may not really be that useful to
anyone other than myself and Boyce Lundstrom. (After receiving the
above mentioned information, I looked in his book, and his Pinto hits
1200 after 67 minutes with the lid propped open 1/2 inch. That would
very closely match my experience.
One other factor that can effect heating rates is widely divergent amounts of thermal mass (which was not the case in the above complaint) - in other words, a kiln with 10 lbs of glass, and a 5 lb shelf in it will take longer to reach temperature than one with a half-pound of glass and a half-pound shelf.
I think these probably pretty much cover the rates we use on real-life pieces.
Here are some (more or less) constant rate graphs:
And some constant dial curves:The maximum recommended rate of heating for single sheet projects is 15 degrees per minute. You can push that to 20 degrees/min with some glass, but I did have one piece "go into thermal shock" and develop a crack half way across the piece. (I think I may have even been pushing 22 or 23 deg/min on that one). Max heating rates apply only up to the strain point of the particular glass. For most soft glass, that's around 900F. After that, the glass is fluid enough to withstand just about anything you can throw at it. (In sagging and slumping, though, you don't want to exceed 5 deg/min above 1200, or you can lose control of the process.)
Annealing is one of the most important things you can do for any fused glass project, whether it's beads, or multiple sheet laminations. Annealing allows the glass to cool slowly and evenly enough to eliminate internal stresses which occur at higher cooling rates. Each glass formulation has its own anneal point. At the exact anneal point, glass can have all internal stresses eliminated in 4 minutes. This point is determined not by temperature, but by viscosity of the glass. The anneal point is reached when the viscosity is 10^12[*] poise. For Moretti's soft rod, this falls at 968 deg F., while Pyrex is at 1022F. Most glass you'll ever run across will fall somewhere between these two.
The strain point (also known as the yield point) is at 10^13[*] poise. At this temperature, it takes glass 4 hours to completely anneal. The difference betwwen the strain point and the anneal point is generally about 70 deg F. The (calculated) strain point for Moretti would then be around 898F, while the (actual) strain point for Pyrex is 937F.
[*] These viscosities are taken from an older physics book. After having had an earlier version of this article published here, I was informed that the actual viscosities should be 10^13 and 10^14.5, respectively. Not having any idea how either of these sets of numbers were arrived at, I'll give both, and let the physicists sort it all out.
It is between those two points that the cooling rate is critical. Even though a piece might soak at the anneal point for the required 4 minutes, if cooling between the anneal point and strain point is too rapid, stresses can be re-introduced.
There is some math involved in determining the proper cooling rates for allowable stresses, but for projects which are 1/2 inch thick, a 20 minute soak at the anneal point, and then a 90 minute ramp down to the strain point is sufficient to keep new stresses practically nonexistent.
For a two layer (1/4 inch) fused project, the times can be halved to a 10 minute soak at anneal point, and a 45 minute ramp time down to the strain point.
Here's the anneal cycle I use on Moretti
beads up to an inch in diameter.
Warm Glass FAQ Forming and Fusing Glass in a kiln
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validated 17 Nov '98
