How to Work Out a Kiln Firing Schedule

To put together a kiln firing schedule you need understand the various states the glass goes through during a firing cycle. It’s a complicated business! There are so many variables:

the size of your kiln
where the elements are situated
how well insulated it is
hot and cold spots

And that’s before you even start on:

different types of glass
thicknesses
colours and suchlike…argh!

Be warned: because of all these variables, you can only ever use other peoples’ kiln firing schedule as a guide.

So what happens to glass inside a kiln?

1. Going up: Primal heat up - 0-540 C

glass kiln This is the thermal shock range – don’t heat your glass too quickly or open the kiln when the glass too hot. A heating rate of 150 C per hour is a conservative bet for this stage. Vent a kiln during this phase to allow fumes to burn off.

2. Going up: Forming stage – 540- 710 C

The glass changes from light to bright honey colour at this temperature.
Stain (or silver nitrate) fires at 540-50 C, glass paint 650 and low-firing enamels 550-650 C. A suitable kiln firing schedule for these techniques only is to take them to top temperature, soak (or hold) for five minutes, and then simply turn the kiln off to cool to ambient temperature at its own rate. Firing these surface techniques are the simplest types of kiln firing schedule you can do.
Most glass will slump within this range. Glass will stick together if touching, and bubbling between layers can occur if the glass is heated too rapidly at this stage.

3. Going up: Stretching stage - 710-820 C

kiln firing schedule Now it’s getting exciting. Glass looks bright red, and has a putty-like consistency. The dreaded devitrification – unsightly crystals forming on the surface of the glass – could happen if glass heated too slowly through this range. Most glass will be fully fused at the top of this stage. High firing enamels fire in this range.

4. Going up: Liquid stage - 820-1000 C

Glass glows red at this heat. Don’t soak at this temperature – it’s too hot and the glass may become milky. If you want to comb your glass, around 890 C is the right top temperature for your kiln firing schedule. Don’t forget, if you’re doing that, TURN THE KILN OFF otherwise you are at risk of getting an ELECTRIC SHOCK! Glass firing kiln

5. Going down: Quick descent - 1000-600 C

You need to get from your top temperature to the annealing (cooling down) stage as quickly as possible by programming ‘Full’ or ‘999’ into the controller. You can open the kiln door at this point to crash cool. It’s very exciting (and hot!), although Bullseye Glass don’t recommend it for their glass.

6. Going down: Annealing range - 600-420 C

Stress is relieved from the glass during this stage – but probably not from you! Annealing can take ages, and it’s a nerve-racking wait when you have a project you’ve spent hours on in a glass kiln. When glass is heated above its Strain Point (around 460 C) the molecules are rearranged, and need to be given time to rearrange during cooling, but before the glass hardens. Soaking (holding glass at one temperature) spreads the heat evenly. It’s very important to give the glass enough time to realign its molecular structure at this stage.
Remember: you can never over anneal glass. But it’s likely to crack if you under anneal it

7. Going down: Cooling – 425-100 C

Once below Strain Point, glass can be cooled more rapidly. I have read of glass artists who say you can crack kiln open at any point in this range, but I certainly wouldn’t recommend it. If you can bear it, it’s best to wait until room temperature or at least 100 C – and even then only open it a tiny bit.

Find out more:

Contemporary Fused Glass