Glass Fusing Temperature Guide: Slump, Tack & Full Fuse Schedules (2026)

Every fused glass failure has a temperature behind it. A piece that cracked on the shelf, a bubble that wouldn't go away, edges that stayed sharp when you wanted them rounded — each of those traces back to a number in your firing schedule, or a number that was missing.

This guide collects the published firing temperatures from the two glass lines most American fusers use: Bullseye and Oceanside (the line most people still call System 96). Every figure here comes from the manufacturers' own published schedules, not from forum consensus. Where a manufacturer doesn't publish a number, we say so instead of inventing one.

Quick answer: the numbers you came for

First, let's kill the 960°F myth

Search for glass annealing temperatures and you will eventually be told that System 96 anneals at 960°F. You may also be told Bullseye anneals at 960°F. Both claims are wrong, and the second one is wrong twice over.

Here is what the manufacturers actually publish:

• Bullseye anneals at 900°F. This appears consistently across their technical notes, tipsheets and their current Annealing Thick Slabs chart.
• Oceanside anneals flat fused work at 950°F. Every current Oceanside flat-glass schedule uses 950°F.
• 960°F is real, but it belongs to torchwork. Oceanside's torchwork and bead annealing guide specifies a 960°F soak. Somewhere along the way that figure got transplanted onto sheet glass fusing, where it does not belong.

Fifty degrees sounds like a rounding error. It is not. The anneal soak is the step where stress either leaves the glass or gets locked into it permanently, and Oceanside publishes an anneal point of 955°F (±10°F) with a strain point of 890°F (±10°F). Soaking at the wrong end of that window does not do the job you think it is doing.

The five stages of a fusing firing

Whatever glass you use, a fusing firing does the same five jobs in the same order.

1. Initial heat — the thermal shock zone

Cold glass is brittle and cracks if you heat it unevenly. Bullseye publishes initial heat rates of 300–400°F per hour for uniform work under 11mm, dropping to 100–200°F per hour for uneven, reflective or insulating pieces, and 200°F per hour for work 12–20mm thick.

Worth knowing: Bullseye's own documents are slightly inconsistent about when the risk ends. One says thermal shock risk passes around 850°F; another says 1000°F. Treat anything below 1000°F as the cautious zone and you will not go wrong.

Oceanside builds the same protection into their published schedules differently — their standard schedules open with a 250°F per hour ramp to 250°F with a 15–30 minute hold, explicitly to prevent thermal shock.

2. Bubble squeeze

As layers soften they trap air between them. The bubble squeeze is a slow passage through the temperature band where the glass is soft enough to settle but not yet sealed, giving trapped air time to escape at the edges.

Bullseye defines this as a soak in the 1150–1250°F range. For transparent pinks, purples and corals specifically they call for a two hour hold at 1200–1225°F.

Oceanside spreads the same work across several segments rather than labelling one: a 30–60 minute hold at 1050°F, then a slow climb to 1250°F with a 10 minute hold, then a 20 minute settling hold at 1350–1370°F.

3. Process temperature — how fused do you want it?

This is the step that decides what your piece looks like, and it is the one number people adjust most.

Result	Bullseye	Oceanside	What it looks like
Slump	1160–1265°F	1225–1250°F	Flat glass takes the shape of a mold
Tack fuse	1275–1450°F	1350–1370°F	Pieces stick together, keep their edges and dimension
Contour fuse	within the tack range	1400–1440°F	Edges soften and round, some relief remains
Full fuse	~1490°F (worked example)	1460–1475°F	Layers become one smooth flat sheet
Combing	not published	1660–1700°F	Glass is fluid enough to rake and pattern

One honest caveat. Bullseye does not publish a full fuse temperature range. Their tipsheets give worked examples — 1490°F with a 10 minute hold for platemaking, and various tile examples between 1315°F and 1400°F. Anyone quoting you a precise "Bullseye full fuse range" is quoting a community average, not the manufacturer.

4. Rapid cool to anneal

Once the glass has done what you wanted, you drop it to the anneal temperature as fast as the kiln will go. Bullseye calls for cooling as fast as possible — kiln closed, natural cooling rate — and elsewhere caps that at 500°F per hour. Oceanside writes this as a 9999°F per hour segment, which is controller shorthand for "full speed."

This step surprises people. After all that careful heating, you crash the temperature. The reason is that the glass is above its anneal point and still relaxed — the stress-free window is the one you want to arrive at quickly and then hold.

5. Anneal soak, then controlled cool

This is where pieces are saved or lost. Annealing is a controlled cool that lets internal stress equalise before the glass becomes rigid. Get it wrong and the piece either breaks on the shelf or, worse, carries invisible stress that fails weeks later on someone's table.

Oceanside's warning is blunt and worth repeating: never open your kiln during any anneal phase.

The annealing table: how thickness changes everything

The single most useful thing in Bullseye's documentation is their Annealing Thick Slabs chart. The pattern inside it is simple enough to memorise:

Anneal hold doubles every time thickness doubles — and the cooling rates fall roughly with the square of thickness.

All Bullseye anneal cycles soak at 900°F, then step down through 800°F and 700°F before the final cool to room temperature.

Thickness	Hold at 900°F	900→800°F	800→700°F	700°F→room	Total cycle
1/4" (6mm)	1 hour	150°/hr	270°/hr	900°/hr	~3 hours
1/2" (12mm)	2 hours	100°/hr	180°/hr	600°/hr	~5 hours
3/4" (19mm)	3 hours	45°/hr	81°/hr	270°/hr	~9 hours
1" (25mm)	4 hours	27°/hr	49°/hr	162°/hr	~14 hours
1.5" (38mm)	6 hours	12°/hr	22°/hr	72°/hr	~28 hours
2" (50mm)	8 hours	6.8°/hr	12°/hr	41°/hr	~47 hours
3" (75mm)	12 hours	3°/hr	5.4°/hr	18°/hr	~99 hours

Read that last row again. A three inch slab needs a four day cool. This is why serious casting and slab work almost always means a dedicated annealer rather than tying up your fusing kiln.

Oceanside publishes a shorter version of the same idea, all soaking at 950°F: 60 minutes at 1/4", 90 minutes at 1/2", 120 minutes at 1", 180 minutes at 1.5", and 240 minutes at 2".

Two caveats Bullseye attaches to their chart, both important:

• The chart assumes flat slabs of uniform thickness, positioned to cool evenly.
• For tack-fused or uneven work, use the cycle for at least twice your piece's thickest area. Some kilns — top-fired fiber kilns in particular — may need three times.

Neither Bullseye nor Oceanside publishes a 9mm (three layer) anneal row, despite that being an extremely common thickness. If your work is 9mm, use the 12mm cycle. Over-annealing costs you time; under-annealing costs you the piece.

Why "COE 90 vs COE 96" is the wrong question

This one surprises people, so here it is plainly: Bullseye does not publish a COE number for their glass at all. Their own FAQ states they do not test for COE or rate their glasses with COE numbers. "Bullseye COE 90" is trade shorthand that the manufacturer itself does not use.

Bullseye's technical notes go further, arguing that matching COE numbers is simply not an accurate measure of compatibility, partly because the standard measurement range ignores the temperature band where expansion behaviour actually changes.

Oceanside takes a parallel position. They test their own certified products to fit together, and explicitly decline to speak to the compatibility of other manufacturers' products labelled "COE 96."

So the real rule is not about numbers. Stay within one manufacturer's tested-compatible line. That is what both companies actually guarantee. Mixing across lines because two numbers look similar is exactly the reasoning both manufacturers warn against.

One genuinely useful practical difference Oceanside does publish: their glass is a lower temperature glass, reaching a given viscosity with less heat than common COE 90 products. That is why the Oceanside full fuse figure sits below Bullseye's worked examples.

What this means for the kiln you buy

Fusing glass makes different demands on a kiln than firing pottery, and the differences are not the ones people expect.

You do not need high temperature. A full fuse happens around 1475–1490°F. Combing tops out near 1700°F. A cone 10 pottery kiln reaching 2345°F has roughly 600°F of capability you will never use for glass.

You do need precise control and even heat. Ten minutes at 1465°F versus twenty minutes is the difference between a contour fuse and a full fuse. That means a programmable controller that holds a soak accurately, and heat that arrives evenly across the shelf.

Element placement is the real difference. Pottery kilns put elements in the walls, which heats a stacked load evenly. Glass sits flat on a single shelf, so glass kilns put elements in the lid to radiate down onto the work. A wall-element kiln fusing flat glass tends to fuse the edges before the centre.

Clam shell designs — where the lid and base separate completely — make loading flat work and mold work much easier than reaching down into a top-loader.

Which glass kiln should you buy?

Prices below are our price as of September 2026. Browse the full range at glass kilns.

Jewelry, pendants and small test pieces

Small 120V kilns that plug into a normal outlet. The Evenheat Studio Pro STP at $1,183.99 and the Hot Shot HS-7G at $1,511.10 are both built for this. If you are working in metal clay as well as glass, the Paragon SC2 at $1,440 is the long-standing choice for both.

Your first fused plates and bowls

You want enough shelf to make something worth slumping. The Olympic Champ XL at $1,305.15 and the Evenheat Studio Pro 14 at $2,023.99 are both 120V. The Paragon Fusion 7 at $2,070 runs on either 120V or 240V, which is useful if you expect to move it.

Larger flat work

Once you are past about 14 inches you are into 240V. The Evenheat GTS 17 at $2,584 and the Paragon Fusion 19 at $3,690 both give you real shelf space with proper glass element placement.

Clam shell loading

If most of your work is flat and mold-based, a clam shell earns its keep every firing. The Paragon CS14S at $2,160 is 120V; Hot Shot's clam shells run from the HS-12G at $1,852 up to the HS-24G at $5,264.

Thick slabs and casting

If you read the annealing table above and realised your work needs a multi-day cool, you want a dedicated annealer so your fusing kiln stays free. Paragon's F-series is built for exactly this, from the F120 at $3,150 through to the F500 Pro at $7,200. The Hot Shot 18A Annealing Cabinet at $4,310 is a 120V alternative.

Production studios

The Evenheat Studio Pro 41 at $7,663.99 and the Olympic GF15E at $10,989.30 are the largest glass-specific kilns we carry.

Frequently asked questions

What temperature do you fuse glass at?

A full fuse happens at roughly 1460–1490°F depending on your glass. Oceanside publishes 1460–1475°F; Bullseye's worked examples use about 1490°F. A tack fuse, where pieces stick but keep their shape, happens lower — 1350–1370°F for Oceanside, 1275–1450°F for Bullseye.

Why did my fused glass crack?

Four usual suspects, in rough order of likelihood. Inadequate annealing, which locks stress into the piece. Heating too fast through the thermal shock zone below 1000°F. Opening the kiln during the anneal phase. Or mixing glass from different compatibility lines. If the crack appeared days or weeks after firing rather than in the kiln, suspect annealing first.

How long does annealing take?

For quarter inch work, about three hours from anneal soak to room temperature. For one inch work, about fourteen. For three inch work, about ninety-nine hours. The hold doubles every time thickness doubles, and the cooling rates slow far faster than that.

Can you fuse glass in a pottery kiln?

Sometimes, badly. The temperature is not the problem — any pottery kiln reaches fusing temperature easily. The problem is that pottery kilns put their elements in the walls, so flat glass on a shelf heats from the edges inward. You will get uneven fusing and more failures. Some top-loading ceramic kilns are sold as dual purpose, and we tag those accordingly, but a lid-element glass kiln will give you better results with less fighting.

Can you mix COE 90 and COE 96 glass?

No — but the reason matters more than the answer. Neither manufacturer frames this as a COE problem. Bullseye does not publish COE figures at all and argues matching COE is not a valid compatibility test. Oceanside tests its own certified products together and declines to vouch for anything outside that line. Stay inside one manufacturer's tested line, rather than trusting a number.

Do I need a separate kiln for slumping?

No. Slumping happens at 1160–1265°F, comfortably below fusing temperature, so any glass kiln handles both. What you do need is mold clearance — check your interior height against the mold you want to use, not just the shelf size.

Talk to someone who fires these

Kiln shopping goes wrong when specs are compared on paper without anyone asking what you actually make. If you tell us the largest piece you want to fire, the thickest work you plan to do, and what power you have available, we will tell you which kilns fit — including when the cheaper one is the right answer.

Every kiln ships flat-rate freight with liftgate service, so it comes off the truck without you arranging it. We price match. And if you find a better price after you order, tell us.

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Related reading: What size kiln do I need? · Best 120V kilns

Firing temperatures in this guide come from Bullseye Glass and Oceanside Glass published technical documentation, September 2026. Manufacturers describe their schedules as starting points, not rules — your kiln, shelf and piece will need adjustment. Prices are our price as of September 2026.