Every kiln, every glass and every project is a little different, so the figures on this page are starting points, not guarantees. They are rounded, typical figures, and they are given in both degrees Celsius and Fahrenheit because makers use both. The glass maker’s own data sheet always comes first, and a small test piece in your own kiln should come before any real work. For how the processes began, see the history of fused and kiln formed glass.
How a kiln firing works
A fusing firing has the same stages whatever the glass:
- Heat up. The kiln rises slowly at first. Glass is a poor conductor, and heating it too fast makes the outside expand before the inside and crack it. Thick pieces, uneven pieces and pieces with insulating materials need a slower start.
- Bubble squeeze. A hold of about half an hour to an hour at a little under 650 to 675 degrees Celsius lets air trapped between layers escape before the edges seal.
- Process temperature. The kiln is taken quickly to the temperature at which the glass does what is wanted, whether that is sticking at the edges, rounding off or running flat. The longer the hold, the further the glass moves, so time and temperature work together.
- Crash cool. The kiln is brought down quickly to the annealing temperature, to stop the glass from changing shape and from forming a hazy crystalline surface (devitrification).
- Anneal. The glass is held at its annealing temperature long enough for the whole piece to even out, and then cooled slowly so that stress does not build up.
- Cool down. The kiln is left closed until it is close to room temperature. Opening it hot can crack the glass.
Words that matter
- COE (coefficient of expansion). The rate at which a glass expands when heated. Glasses fused together must expand and shrink at the same rate, or they crack as they cool. COE alone is not a guarantee of compatibility, so use glass that is sold as tested compatible, and test anything else.
- Strain point. Below this, stress cannot be relieved. Cooling below it is safe to do faster.
- Annealing point. The temperature at which stress relaxes in a reasonable time. The anneal soak is held at or near this.
- Softening point. Where glass begins to sag under its own weight. Slumping is done a little under it.
- Devitrification. A rough, cloudy skin of tiny crystals that can form if glass sits too long in a certain range of temperature, particularly around 700 degrees Celsius and above. It is avoided by moving quickly through that range and by not overfiring.
Glass systems at a glance
| System | COE | Notes |
|---|---|---|
| Tested-compatible 90 COE fusing glass | 90 | Sold as compatible across its range. The reference for many schedules. |
| Tested-compatible 96 COE fusing glass | 96 | Made to the 96 standard by several makers. Production of one long-established American range moved after a 2016 closure; see the history page. |
| Some stained glass ranges | 90 and 96 | A few makers sell both. Check which one before buying. |
| Other stained glass sheets | Nominally 90 to 92 | Not sold as tested compatible for fusing. Reports vary, with some devitrification and mismatches. Test before use. |
| Murano and other soft glass | 104 | A softer glass. Palette is narrower, thickness varies, and compatibility between different colours of the same make cannot be assumed. |
| Float (window) glass | About 82 | Not made for art work, but widely used. Needs hotter schedules than 90 COE fusing glass. |
Typical ranges for COE 90 fusing glass
These are rounded, typical figures. Each maker of tested-compatible glass publishes its own schedules, which should be followed in preference to this page.
- Heating. Thin, even pieces will take a fairly quick rise, roughly 170 to 220 °C an hour. Thick, uneven or insulated work needs a much gentler rise, roughly 55 to 110 °C an hour.
- Bubble squeeze. A hold of 30 minutes to an hour at about 650 to 680 °C. Some pinks, purples and strong reds and oranges do better with a shorter or lower hold, so check the maker’s notes for those colours.
- Process temperature. Tack fusing is generally done between about 690 and 790 °C, full fusing at about 800 °C, and slumping or drop-out moulds at about 650 to 700 °C. A fire polish sits somewhat below a full fuse. The exact figures depend on the kiln, the thickness and the colours.
- Annealing. A soak at about 480 to 520 °C, held for at least twice as long as the thickest area needs, then a slow, staged cool. Top-fired kilns with fibre insulation need longer.
Thicker pieces
Annealing time and cooling rate depend on thickness more than on anything else. As a general pattern, the soak lengthens from about an hour for 6 mm glass to about four hours for 25 mm glass, and the cooling rate through the annealing range falls sharply as the glass gets thicker. For thick work, follow a published annealing chart for the glass being used.
Typical ranges for COE 96 fusing glass
Schedules for 96 COE glass are similar, with top temperatures that are generally a little lower than for 90 COE: slumping at roughly 660 to 680 °C, tack fusing at roughly 730 to 760 °C and full fusing at roughly 775 to 805 °C, with annealing at about 510 °C. Small pieces, up to about 10 mm thick and 300 mm across, take the shorter holds.
Float glass (COE about 82)
Float glass generally needs about 45 °C more heat than 90 COE fusing glass to slump, fuse or anneal, and the lower expansion rate makes it a little more forgiving of heat shock. It is prone to devitrification, so avoid long holds at the top of the range. Test both faces, because the side that touched the tin bath behaves differently. For a thin piece, a full fuse in the region of 810 to 820 °C with a short hold, then an anneal at about 480 °C and a slow cool, is a reasonable starting point. Adjust the top temperature in steps of 10 to 30 °C on small test pieces until the edges are as crisp or as rounded as wanted.
Murano and other COE 104 glass
Murano glass is a soft, medium-soft soda-lime glass in a very wide range of colours, and its rods, canes and murrine are used for millefiori work. In the kiln it behaves quite differently from 90 and 96 COE glass:
- It fuses at lower temperatures, so schedules written for 90 or 96 COE glass are usually too hot. Murrine can reach a near-full fuse at well below the temperatures used for sheet glass.
- Small pieces anneal quickly, at a lower temperature than 90 COE glass, followed by a slow cool.
- Move quickly through the range around 700 °C, where devitrification starts, rather than holding there.
- Reds, oranges and yellows can change colour in the kiln and look identical beforehand. Keep scraps labelled and test them.
- Sheets sold as 104 COE are not always compatible with one another, and the thickness can vary even within a sheet. Test every combination, particularly for larger pieces.
See the history page for more on Murano millefiori.
Slumping and casting
- Slumping is done at the low end of the range: about 663 to 688 °C (1225 to 1270 °F) for typical sheet glass, held long enough for the glass to settle into the mould. Longer holds at a lower temperature give a more even result than a quick hot hold.
- Casting uses higher temperatures and long holds. Overheating can trap bubbles in the glass, so casters usually keep the final temperature as low as the glass will allow. Schedules depend on the mould, the glass and the size of the piece, so follow the mould maker’s data.
- Thick pieces need much longer anneals. See the section on thicker pieces above.
Common firing faults
- Cracks on cooling. Incompatible glass, too fast a cool through the annealing range, or a kiln that is hotter at the top than the bottom.
- Bubbles trapped between layers. No bubble squeeze, or a squeeze that was too short for the thickness.
- Cloudy, rough surface. Devitrification from an overlong hold at a high temperature, or dirt on the glass.
- Pieces will not stick. Too low a top temperature or too short a hold.
- Pieces run flat or lose detail. Too hot or too long.
- Colours change. Some reds, oranges, pinks and yellows react to heat. Test them.
Safety
Kilns reach temperatures that are dangerous to look into and to touch. Wear eye protection suitable for infrared when looking into a hot kiln, ventilate the room, keep the area clear and let the kiln cool fully before opening it. Never put wet glass or moulds in a hot kiln. See Working Safely With Glass.