Working Out Firing Schedule Using Cones

Help with Firing Schedule

I’m trying to fire paint, and need help with my firing schedule. My 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 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 firing temperature at the store was 1200 F, (650 C) but I don’t know what my kiln comes to on high. Harry Sweezey

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 program the firing schedule – for 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 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 firing schedule for future firings.

It might also be worth learning about what happens to your glass inside the kiln – have a look at my page that explains that here.

Good luck!

Flat Bed Kiln Question

Hi wonder if you can help me . I am interested in buying a reconditioned flat bed kiln for glass slumping but dont know where to look …I have looked on various web sites but have not really found much . could you recommed a site to go to ?
thanks

Milly says: Second hand flat beds are notoriously difficult to get hold of! You could try letting local stained glass suppliers know that you’re looking, but be prepared for a long wait! Sorry, I don’t know any websites specialising in this, think they would go out of business…

kiln firing with cones

A potter-friend lent me an old kiln for a while, and I was able to use the cone technique above very successfully. I began by doing a series of 5 test-firings with small identical paintings of a little acorn motif. The motif had thin and thick lines and flooded areas, so I could see how variations in timing and temperature affected these. I also painted the number of the firing on each piece.

I made a log of the firings, recording the exact timings, notes on the results and sketches of the cone positions as they looked when I turned the kiln off. I also penned the firing numbers on the cones themselves and put them in a box with the acorn paintings. That way, I knew precisely how to operate the kiln to get matt paint, shiny paint, and also how to fire polish.

It was a good exercise, and got me into the habit of keeping a thorough kiln log, which I still find really useful.

Milly’s comment: Brilliant advice! Thanks for that – it’s so much easier to progress when you have notes.

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