Emiko
2007-08-07, 7:53am
I found that it says the annealing temperature is 470 C (870 F) in Effetre's web site. I also remembered that Lucio said that he anneals his pieces at a temperature lower than 900 F. So I wondered why much higher temperature range is recommended in the US and asked Mike Frantz in the Kiosk. He didn't reply but I talked with Craig of Arrow Springs, and he explained it well. As per Judy B.'s suggestion, I'll post it here so that more people see it.
Glass can be annealed at the straining point or higher temperature so long as you don’t go too high to distort it. If the temperature is higher, the amount of time that takes to anneal is shorter. So theoretically Effetre can be annealed at 840 F which is the strain point. But what pyrometer reads has errors in the range of + or - 0.3 % of the real temperature even if it is not faulty. So when it says it is 840 F the real temperature could be 837.5 F and it doesn’t anneal because it is lower than the strain point. Also the strain point vary according to colors so 840 F is not a rigid figure. So what they do is to anneal at 50 F higher than the strain point to be safe. Then why do they give higher number than 890 F? That comes from the experience. When pieces go into the kiln, the surface is hot but the core is cooled. So if the temperature of the kiln is cooler, the glass reaches too quickly to the strain point and it causes cracks. 940 F was the temperature that the most people had the least cracks. Also going higher to 940 F to 968 F range gives a benefit of Bullseye and Effetre being annealed toghther. So the safest and the most convenient range is listed as recommendation. But if you are making something large rather than beads it is better to use lower temperature because the weight of the glass can cause distortion even under the softening point.
Glass can be annealed at the straining point or higher temperature so long as you don’t go too high to distort it. If the temperature is higher, the amount of time that takes to anneal is shorter. So theoretically Effetre can be annealed at 840 F which is the strain point. But what pyrometer reads has errors in the range of + or - 0.3 % of the real temperature even if it is not faulty. So when it says it is 840 F the real temperature could be 837.5 F and it doesn’t anneal because it is lower than the strain point. Also the strain point vary according to colors so 840 F is not a rigid figure. So what they do is to anneal at 50 F higher than the strain point to be safe. Then why do they give higher number than 890 F? That comes from the experience. When pieces go into the kiln, the surface is hot but the core is cooled. So if the temperature of the kiln is cooler, the glass reaches too quickly to the strain point and it causes cracks. 940 F was the temperature that the most people had the least cracks. Also going higher to 940 F to 968 F range gives a benefit of Bullseye and Effetre being annealed toghther. So the safest and the most convenient range is listed as recommendation. But if you are making something large rather than beads it is better to use lower temperature because the weight of the glass can cause distortion even under the softening point.