Monday, 15 April 2013

Initial Heat Up Rates

Example of a graph of a heat up for industrial purposes

There is quite a lot of information on the annealing cool rates, but not so much on the initial heat up rates. This is probably because the cooling rates are more critical than the heat up. But everyone knows that you can heat the glass up too quickly for its thickness.

My experience leads me to suggest some heat up rates to 50C above the annealing temperature for circular and nearly square full fused pieces. These have worked for me, but of course, may not work in all kilns.

6mm heat up at 160ºC/hr
12mm heat up at 110ºC/hr
19mm heat up at 50ºC/hr
25mm heat up at 30ºC/hr

In general, these heat up rates are no more than twice the initial annealing rate for the relevant thickness.  That is, the initial anneal cool for 6mm is 80C/hr; and for 12mm is 55C/hr.  When you get to 25mm, my initial anneal cooling rate is only 15C/hr.  So you can see how the doubling of the initial anneal cool works.

Tack fused and pointed pieces require much more careful heating because of the differing thicknesses within the piece, or the relative narrowness of one end or area in comparison to other areas. The suggestion is that the heat up for these should be at the rates suitable for items at least twice as thick as the thickest part of the tack fused or tapered piece.