Working out Glass Paint Kiln Firing Schedule with Cones
by Harry Sweezey
(Panama City Beach, FL)
I’m trying to fire glass paint, and need help with my kiln firing schedule. My glass kiln is an old one and only has three settings; low, medium, high. I step up the temperature every half hour. I kept it on high for 30 minutes and then shut the kiln down and let it cool down for 4 hours. However, the paint did not adhere to the glass; it can be scraped off with my finger nail. How long should I keep the glass in the kiln on high to have the paint adhere to the glass?
I know the glass paint is good, because I used it before, but the kiln was in the glass suppliers shop, and I paid to use the kiln each time I needed to use it. The kiln firing temperature at the store was 1200 F, (650 C) but I don't know what my kiln comes to on high.
Milly’s answer:
Hi Harry, good question. You’re right in assuming that it’s the top temperature that needs to be determined for successful glass painting. There are a couple of ways you can do this – one easy and flexible but relatively expensive, the other more tricky and limited, but cheap.
The costly option – buy a digital controller and get it fitted. That way you can enter the kiln firing schedule – for glass paint it would be 150C/270F ph to 650C / 1200F top temperature, soak for 5 mins then OFF and cool to room temperature. This option also gives you lots of flexibility to try other techniques and processes in the future.
The cheaper option – test the temperature of your kiln with a cone. These cones are temperature sensitive, and designed to fold over when the kiln reaches the temperature associated with each cone. They are available from pottery supply stores, and you will need Cone No. 019 (temperature 683C / 1281F) and Cone No. 020 (temperature 635C / 1175F). Get the self-supporting ones, then you won’t have to fiddle about with clay to get them to stand up.
Fire your glass kiln with the cones on the shelf and in the position that you will fire your glass on, and don’t put any glass in for this test firing. Try to place them so that you can peek through the spy holes with a torch to see what’s going on, but if this isn’t possible, you’ll have to open the door and peek in quickly. Use heat insulated gloves and goggles to do this, and it's always recommended that you turn your kiln off before opening.
I’d use the ‘medium’ setting to ramp the kiln firing up – and then watch and wait. When the first cone (020) collapses, you know that the kiln firing has reached 635C / 1175F, and when the second one bends your kiln temperature is 683C / 1281F. Monitor absolutely everything – the time it takes for the first cone to fold, and then the time it takes for the second one. From this information, and with a bit of maths, you can work out your glass paint kiln firing schedule for future firings.
It might also be worth learning about what happens to your glass inside the kiln – have a look at Kiln Firing Schedule.
Good luck!
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