Glass Annealing Temperatures: Why You See 900°F, 950°F and 960°F (2026)

Updated October 2026 · By the Peach State Pottery team

Ask what temperature to anneal fused glass at and you will get three answers: 900°F, 950°F and 960°F. People will argue about it in forums. All three numbers are correct, and the reason is the thing nobody says first:

The annealing temperature is a property of your glass, not of annealing. Different manufacturers formulate different glass, so each publishes its own anneal soak. Use the one for the glass in your hands and ignore the argument.

Then there is the part that actually breaks pieces, which is not the temperature at all — it is the hold time and the cooling rate, both of which are set by the thickness of the work.

Quick picks

What annealing actually does

Glass is not a crystal with a melting point. It is a supercooled liquid that stiffens gradually, and on the way down through that stiffening range it passes two temperatures that matter:

• The annealing point — warm enough that the glass can still flow very slightly, so internal stress relaxes away on its own.
• The strain point, lower down — below which the glass is rigid and any stress you put into it is locked in permanently.

Annealing is two moves. First you hold the piece at the annealing point long enough for existing stress to relax out of it. Then you cool it slowly through the range down to the strain point, slowly enough that the outside and the inside stay close in temperature, so no new stress gets built in as it rigidifies.

Skip either move and the piece does not necessarily break in the kiln. It breaks three days later on a shelf, or the first time it is set down hard, or when sunlight warms one side of it. That delayed failure is the signature of an under-annealed piece, and it is why annealing is the one part of a glass schedule you cannot eyeball.

Bullseye's published annealing cycles by thickness

Bullseye publishes the most complete thickness-based annealing chart in the industry, which is why it has become the reference even for people firing other glass. These figures are Bullseye's, for Bullseye glass, for flat slabs of uniform thickness. Rates are degrees Fahrenheit per hour; holds are hours:minutes.

Source: Bullseye Glass, "Annealing Thick Slabs." Bullseye notes the chart applies only to flat slabs of uniform thickness that cool evenly from top and bottom; for other shapes it says to use the cycle for a piece at least twice the thickness of the thickest area.

Read that table once and three things become obvious

The temperature never changes. The time changes enormously. Every row anneals at 900°F. What scales with thickness is the hold and the cooling rate — and not linearly. Double the thickness from 1″ to 2″ and the firing goes from about 14 hours to about 47.

Standard fusing work is cheap to anneal. Two layers of 3 mm Bullseye is 6 mm — one hour at 900°F and about three hours total. This is why most people never think about annealing until they make something thick.

Cast and thick-slab work is a different hobby. A 4″ cast piece is a seven-day firing. If that is where you are heading, kiln capability stops being about maximum temperature and becomes entirely about whether the kiln can hold a stable, even, very slow cool for days — which is a controller and insulation question.

Why your kiln, not your schedule, is often the problem

Bullseye puts the warning plainly: even the most conservative annealing cycle can fail if the kiln cannot cool the work uniformly. That is worth sitting with, because it means a correct schedule in the wrong kiln still cracks glass.

Two things determine whether a kiln can anneal evenly:

Element placement. Glass kilns put the elements in the lid, firing straight down onto a flat shelf, so the whole surface sees the same radiant heat. A pottery kiln's side-wall elements heat the perimeter of a flat piece before the centre, and in the annealing range that gradient is exactly what you are trying to avoid. We go into the physics in Pottery Kiln vs Glass Kiln.

Thermal mass. A kiln lined entirely in light fiber heats and cools fast, which is good on the way up and unhelpful on a long slow anneal — it wants to shed heat faster than your schedule says. Firebrick stores heat and releases it gradually, which smooths the cool. Several glass kilns are deliberately hybrid for this reason: the Evenheat GTS 17 uses a lightweight fiber lid over firebrick walls and floor — fast on the climb, steady on the anneal — and fits a solid-state relay as standard, because tight temperature control is the whole game below 1,000°F.

And a practical one: count your controller's segments. A Bullseye cycle for thick work needs a ramp up, a process hold, a drop to the anneal, the anneal hold, and three cooling segments — seven or eight stages. Controllers vary; Paragon's SENTINEL Touch Xpress, fitted on the F130, runs programs up to 32 segments and stores 12 custom profiles. If you are casting, ask us how many segments the controller on a given kiln actually has before you order it.

Annealing lampwork and beads is a different job

If you are a lampworker or beadmaker, almost none of the above applies to you. You are not cooling a flat slab from a fuse — you are taking hot work off the torch and bringing it down through the annealing range so it does not crack days later. Thin, small, and repetitive.

So you do not need a fusing kiln, and buying one is overpaying. A dedicated annealer is cheaper and better suited:

Model	What it is	Our price (Oct 2026)
Jen-Ken Chilipepper	120V / 8A, 16″ × 6″ × 6″, 1,100°F ceiling on purpose, 19 lbs, flip door on the long side. Plugs into a wall outlet; usually ships same or next business day.	$1,074.00
Paragon F120	Compact front-loading lampworking annealer.	$3,307.50
Paragon F130	11″ × 13″ × 11″, 1700°F, 120V, spring-loaded vertical-drop door with a 4″ × 8″ punty port, five-slot stainless rod warmer, 3″ firebrick.	$4,063.50
Paragon BlueBird XL	Bead annealer built around mandrel work.	$2,646.00
Hot Shot 18A Annealing Cabinet	Larger cabinet-style annealer for batches and bigger sculptural work.	$4,525.50
Paragon F500 Pro	Top of the line for production lampworking studios.	$7,560.00

The Chilipepper's 1,100°F ceiling is the design, not a limitation — it will not fuse or slump glass, and it is not supposed to. If you want one kiln that both anneals beads and fuses, say so when you call and we will spec a glass kiln instead, because the two jobs pull in different directions on door design and chamber shape. Full range: Glass Annealing Kilns.

How to not get this wrong

Use your glass manufacturer's number. Bullseye says 900°F for Bullseye. System 96 publishes its own, around 950°F. Those are not opinions to average.

Measure the thickest point, not the average. The schedule is set by the thickest part of the piece. A mostly-6 mm panel with a 20 mm bump anneals on the 20 mm cycle, and Bullseye's own guidance for non-flat shapes is to go further still — use the cycle for twice the thickest area.

When in doubt, go slower. Over-annealing costs you electricity and patience. Under-annealing costs you the piece, usually after you have already given it away.

Do not mix COEs. Two glasses with different expansion coefficients fused together will stress against each other no matter how well you anneal, because the mismatch is permanent. No schedule fixes it.

Do not crack the lid to "help it cool." The whole point of the cooling segments is that the piece cools evenly and on a known curve. Opening the kiln puts a gradient straight across it at exactly the temperature where that gradient gets locked in.

Buy a polariscope if you sell your work. Stress in annealed glass is invisible to the eye and plainly visible under crossed polarizing filters. It is the only way to know your schedule works rather than hope it does.

FAQ

What temperature do you anneal fused glass at?

It depends on the glass. Bullseye (COE 90) specifies 900°F. System 96 (COE 96) is usually given as about 950°F, sometimes 960°F. Older references often say 960°F for Bullseye too, because that was Bullseye's recommendation before 2009. Follow the figure your glass manufacturer publishes.

Why do I see 900°F and 960°F for the same glass?

Bullseye lowered its recommended anneal soak from 960°F to 900°F in 2009. Books, class handouts and forum posts written before that still carry the old number, and they get quoted forever. For current Bullseye glass, use 900°F.

How long do I hold at the annealing temperature?

By thickness. For Bullseye glass: one hour at ¼″, two hours at ½″, four hours at 1″, eight hours at 2″, 16 hours at 4″. The hold is only the first half — the controlled cool afterwards is longer and matters just as much.

What happens if glass is not annealed properly?

It carries locked-in internal stress. Sometimes it cracks in the kiln, but more often it survives the firing and fails later — days or weeks on, under a small knock, a temperature change, or no obvious trigger at all. That delayed failure is the classic sign of under-annealing.

Can I anneal glass in a pottery kiln?

The temperature is easy; the uniformity is not. A pottery kiln's side-wall elements heat the edges of a flat piece before the centre, and an uneven cool through the annealing range is precisely what creates stress. Small pieces on a high shelf with slow rates can work. Large flat panels are unreliable. See Pottery Kiln vs Glass Kiln.

Do I need a separate kiln just for annealing?

For fusing, no — the anneal is the tail end of the same firing, in the same kiln. For lampworking, yes, effectively: you need an annealer running alongside the torch so work goes straight in hot, and a dedicated annealer like the Chilipepper at $1,074.00 is far cheaper than a fusing kiln.

What is the difference between the annealing point and the strain point?

At the annealing point the glass can still move enough for stress to relax out of it. Below the strain point it is rigid, and any stress present is permanent. Annealing means holding at the top of that window and then cooling slowly through it so nothing new is introduced on the way down.

Can I anneal float glass or bottle glass?

Yes, but you have to find its annealing range yourself, and it is not the same as fusing glass. Float glass behaves differently and bottle glass varies by manufacturer and even by batch. Test with scrap and a polariscope before you put work into it, and never combine it with COE 90 or 96 glass in one piece.

Still not sure? Ask us.

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More from The Kiln Room: Glass Fusing Temperature Guide · Best Glass Fusing Kilns · Pottery Kiln vs Glass Kiln · Orton Cone Chart

Annealing figures are taken from each glass manufacturer's published data and apply to that manufacturer's glass. Prices are our price as of October 2026 and are subject to change. Test any schedule on scrap before committing finished work.