NEW! CURING, FIRING & ANNEALING GRAPH (pdf) (click to view; right-click + “save link as” to download) has been added to Tech Downloads page. The new graph finally changes the annealing times & temperatures to more closely match that of Bullseye’s Annealing Thick Slabs chart.
The new chart also takes into account info about quartz inversion, which has been referenced in several glass casting publications—for example, Angela Thwaits: Mould Making for Glass and Bullseye’s Tech Notes: Basic Lost Wax Casting. Quartz inversion is the phenomenon of the silica in standard plaster/silica mould mix suddenly expanding in the 1100ºF range. If we follow the standard practice of “spiking” the kiln from annealing temperature (or 1000ºF) to casting temperature—which was taught in many kiln-casting publications as a way to avoid devitrification—then we might push through the quartz inversion stage too quickly, causing the outside of the mould to expand before the interior. This uneven expansion can create cracks in the mould, leading to mould failure at casting temperature.
This new Firing Chart takes that into account and recommends a slower ramp from 1000ºF to 1200ºF before spiking to casting temperature. At this writing I feel it’s still important to spike from 1200º to casting temp as fast as possible to avoid prolonged exposure in the devitrification range, especially when firing open-faced moulds where glass billets have been loaded directly into a mould.