Unwanted Bubbles in Fused Glass? Check Your Layers, Shelf, and Firing Schedule

A few small bubbles can be a normal feature of kilnformed glass. A large pocket that distorts your design, raises a dome, or breaks through the surface is a different problem. Before changing your firing temperature, work out where the air was trapped and review how the project was assembled.

1. Locate the bubble before choosing a fix

Wait until the kiln and glass are fully cool according to the equipment and material instructions. Inspect from the top, edge, and underside without forcing or flexing the piece. Photograph the bubble and note whether it appears between glass layers or is associated with a cavity underneath.

A bubble between layers points your investigation toward the layup and heating cycle. A raised area with a cavity underneath also calls for checking the shelf and the project base. These clues guide your checks; they are not a certain diagnosis from appearance alone.

2. Review your layer arrangement

Lay out a paper sketch of the original stack. Mark large cap pieces, scattered decorations, uneven overlaps, and gaps between cut pieces. Component placement can create pockets where air becomes enclosed as the glass softens.

For your next test, simplify the arrangement and compare how neighboring pieces fit. Do not assume that adding another large cap automatically removes trapped air. Keep to a construction method recommended for your particular glass system and intended firing.

3. Check how early the edges may seal

If edges soften and fuse before air can escape from the interior, a bubble can remain between the layers. Heating rate and the kiln's heat distribution can affect that sequence. Record whether the project was close to heating elements and whether the problem repeats in one part of the kiln.

Do not improvise shields, move elements, or obstruct the kiln. Review positioning and any approved heat-management methods with the kiln manufacturer or an experienced instructor.

4. Understand a bubble squeeze

A bubble-squeeze stage is intended to let layers soften and settle while air can still escape before the edges seal completely. Its settings are not universal. The glass system, thickness, dimensions, layup, and kiln all matter.

Use a manufacturer-supported schedule appropriate to your project. Do not copy a temperature and hold from a different glass system or a much smaller sample. Keep the annealing and cooling stages suitable for the actual thickness; surface appearance is not a reason to shorten them.

5. Inspect the shelf and its preparation

A distorted shelf can create a pocket below the glass. With the shelf cool and safe to handle, check its condition and flatness using the manufacturer's inspection guidance. Also review how the separator was applied and whether the shelf preparation was fully dried as required.

Moisture in a shelf can turn to steam during heating and contribute to bubbles. Follow the shelf, separator, and kiln instructions for preparation and drying rather than assuming that a surface that looks dry is ready. Do not dry-sand dusty kiln materials; follow their dust-control and handling requirements.

6. Compare construction and volume

A thin base with scattered decoration behaves differently from an even multilayer layup. At a full fuse, glass can change its footprint as it seeks its characteristic thickness. Do not treat a single-layer base and a thicker, evenly stacked project as interchangeable when choosing a schedule.

Record the actual number of layers and their arrangement, not simply the total amount of glass used. Ask an instructor to review an unfamiliar construction before committing an important project.

7. Run one controlled test

Use identified, compatible kilnforming materials and a manufacturer-supported test setup. Photograph the stack before firing. Keep a record of dimensions, thickness, shelf preparation, placement, and the complete firing program.

Change one relevant variable at a time. For example, compare a simplified layup with the previous arrangement while keeping the other suitable conditions consistent. If every setting and material changes together, you will not know what improved the result.

What not to do with a failed piece

Do not assume that firing hotter will flatten every bubble. Do not puncture a bubble or coldwork a damaged piece without appropriate instruction, equipment, and dust controls. A burst bubble can leave sharp edges, and a distorted or cracked piece needs assessment before display or use.

For persistent problems, show an experienced kilnforming instructor your firing log, stack photographs, shelf condition, and the cooled result. Those details are much more useful than the instruction to simply 'fire it again.'

Cover image: an illustrative example of bubbles in glass, not a photograph of a documented firing test.

Compare a shelf-preparation supply

If your process calls for shelf paper, compare the Bullseye ThinFire Paper Roll. Follow the current manufacturer instructions for suitability, handling, firing, cleanup and dust control. Shelf paper does not correct every bubble problem and cannot replace a suitable layup, sound shelf or firing schedule.

Buy now — Bullseye ThinFire Paper Roll