I'm new here, and something of a beginner to pottery. I recently bought a old small electric kiln --- basic thing controlled by two dials: power 0% to 100% and target temperature up to 1300*C, and a flick switch for 'hold' or 'power off' once above the target temperature --- and am trying to work out some basic glazes.
I've just fired two glazes based on Glossy Base 1 and High Calcium Semi-Matt 2 from Hesselberth and Roy, and am baffled by the results. The glaze based on High Calcium Semi-matt has come out rather glossy, and the glaze based Glossy Base 1 is matt --- very very matt.
Can anyone shed any light on what might be going on?
I fired up to Cone 6 (cone bent over until the tip is touching the base) over 9 hours. The last 100*C took an hour and half, the cone was over at a little over 1200*C. (I can roughly check the temperature according to the pyrometer by turning the target temperature dial and seeing when the relay kicks on/off.)
Fired down as follows:
- Natural drop to 1000*C (1830*F) took around 35 minutes
- Hold 1000*C (1830*F) for 30 minutes
- Hold above 930*C (1700*F) for 30 minutes
- Hold above 900*C (1650*F) for 1 hour
- hold above 870*C (1600*F) for 1 hour
- hold above 840*C (1545*F) for one hour
- kiln off
The only thought I have is that Glossy Base 1 has MgO 0.279 in the unity formula where as High Calcium Semi-matte has only an incidental amount (0.04) and maybe something in the firing cycle is causing the magnesium to give matt but not the calcium in the high calcium semi-matte. The overall rate of cooling is not so different to that recommended in H&R, but obviously they assume a continuous ramp down, but because of how my kiln works I've used a series of holds -- could that cause some effect?
Any thoughts gratefully received!