Saturday, 3 January 2015

Strain Points

A critical range is the temperature around the annealing point. The upper and lower limits of this range are known as the strain points. The higher one is the highest temperature at which annealing can begin. The lower one is the lowest point at which annealing can be done. Soaking at any lower temperature will not anneal the glass at all. This temperature range is a little arbitrary, but it is generally considered to be 55C above and below the annealing point. The ideal point to anneal would be at the annealing temperature, as annealing occurs most rapidly at this temperature.

However, glass kiln pyrometers are not accurate in recording the temperature within the glass, only within the kiln. The glass on the way down in temperature is hotter than the recorded kiln atmosphere temperature. So a soak at the annealing temperature is required. If you do a soak at 515°C for example, the glass is actually hotter, and is cooling and equalising throughout to the 515° point during the soak. The slow cool to below the lower strain point constitutes the annealing, the soak at the annealing point is to ensure that the glass is at the same temperature throughout, before the annealing begins.

It is still possible to give the glass a thermal shock at temperatures below the lower strain point, so care needs to be taken. But no further annealing will take place. If you do not anneal properly the glass will break either in the kiln or later no matter how carefully you cool the glass after annealing.

The glass is brittle below the upper strain point temperature, although it is less and less likely to be subject to thermal shock as it nears that point.  It is after the upper strain point that you can advance the temperature as fast as you like without shocking the glass.  So, if you have a glass that gives its annealing temperature as 515C, you can safely advance the temperature quickly after 570C (being 55C above the annealing point).