Fused Glass Cracks

Fused glass cracks or breaks can happen during or after the glass fusing process .

It is sometimes possible to determine the cause of the crack or break from the appearance of the glass.

You can also see if a break occurred at which stage of the firing schedule.

If a crack or break has rounded edges then the fault happened during the heating cycle, during this phase the edges will soften and become ro

Thermal Shock

Thermal shock may happen if the glass is heated too quickly.

The glass can break into a number of pie shape wedges, scattered across the glass kiln, the pieces will probably have rounded edges as the break occurs early in the heating cycle.

Revise your heating schedule and fire more slowly in your glass kiln to overcome the problem.

Using smaller pieces of fusing glass in the design may also help.


Faulty annealing may show up as curved cracks across the fusing glass, with a sharper curve towards the glass edge.

The glass may break into two or three pieces due to stress.

Solve this problem by raising your annealing temperature slightly and annealing for longer.

All things being equal it is virtually impossible to anneal for too long.

Refiring a previously kiln formed glass piece that has been incorrectly annealed may also result in a cracked or broken item.

Spiders Web

Normally seen on the underside of the glass, these fine cracks usually stem from a central spot.

Close inspection will usually show glass sticking to the kiln shelf with some kiln wash sticking to the glass piece.

Clean the glass and shelf and apply more kiln wash.

Post Firing Cracks

Some times glass that has been fired some time previously will develop fused glass cracks or breaks for no apparent reason.

This is generally caused by a build up of stress in the glass.

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