Kiln design
Kiln design will affect annealing & thus your annealing schedule.
The larger the kiln, greater the potential for larger temp gradient within the kiln.
Placement of heating element(s), one element at top of kiln is the worst place. 5 elements is the best - 4 sides (including door) & roof. Side element should nearly cover all the wall.
Power of heating element needs to be big enough to achieve your heating rate for your kiln size, but one too large will give excessive temp oscillations.
Thermocouple placement, at the top is bad – the rest of the kiln is much cooler, the bottom is better, but not close to heating element – oscillating temps.
Temp controller
3 common types; analogy (needle indicator) every cheap, very inaccurate – can be +/- 50C
Digital (single point) – a programable one is not much more expensive & single point usually give larger temp oscillations.
Programable – most have 30+ steps, we only need 5-8 steps, it would be very nice to have a controller that saved several schedules!! Their internal setup can be configured to reduce over shoot when set temp is reached. A badly setup controller can oscillate +/- 10C, this should be +/- 2C at max. The best programable controller will proportionally reduce power to the element as it approaches the set temp (not just switch element off/on) & reduce temp over-shoot but these are very expensive (& not what lampworkers need).
Measuring temperatures
It is difficult to accurately measure temps above 500C.
All temp controllers need to be calibrated (checked for accuracy) – the temp they display can vary from the actual (real) temp at the thermocouple by +/- 10C.
50C above annealing temp is no problem (except for some reactive glass), but 10C below annealing temp???
Simple calibration method:
Some inexpensive multi meters come with thermocouples (check its accuracy in ice then boiling water, but this doesn’t guarantee accuracy above 500C). Place this thermocouple next to the one in kiln, close door & let things stabilise. You can then take your multimeter to other lampworker’s kilns to see how measurements vary.
My kiln:
200cm wide, 300cm deep, 270cm heigh (internal), large door (no mandrel flap), heating element covering back wall 2000 watt, fire brick & thermal blanket insulation 50cm thick – it takes about 45 min to cool from 560C to 300C. Single program temp controller, actual temp is 0 – 5 C above displayed temp
Glass colors & reactive glass
Colored glass is often thicker/stiffer when lampworking, thus its working temp is higher & its annealing temp is most likely higher (many glass manufacturers/suppliers don’t give this information).
I don’t have enough experience with reactive glass & will leave this for others to comment.
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