Effect of Combustion Gasses

Some materials will partially or completely combust at fusing temperatures. This gives off gasses which expand and blow big bubbles from under the glass.

Some kiln washes, especially for ceramics, give this problem. If you believe this is the cause, try a different brand of kiln wash or pre-fire the kiln shelf.

Sometimes organic materials have been introduced accidentally or purposely onto the shelf. Either clean the shelf of the old kiln wash, or support the glass on beads, or frit to allow the gasses to burn out before the glass slumps to trap the gasses.

Damaged Shelves

Shelves that have gouges or pits can give rise to bubbles from trapped air. Since air expands much more than glass, it will force its way out through the most plastic material. At fusing temperatures, this is the glass.

To determine if this is the problem, note where the bubbles form in relation to the shelf. If it is always in the same area, there is reason to believe it is related to the shelf. By noting the location you now have an area to inspect for damage.

If you can see no damage, it may be that the shelf is warped, or has a low spot. These can trap air, just as the pits and gouges can. But these are difficult to determine by direct visual inspection. You can place a straight edge on the shelf and look for any gap as you move the edge along the shelf.

Possible solutions are:
- Avoid fusing over the shelf "pits".
- Fill shelf scratches and nicks with kiln-wash.
- Mend the shelf with cement fondue or other refractory materials.
- Fire on fibre paper - this will provide an escape path for the air.
- Flip warped shelves, as the opposite side is likely to be equivalently bowed, but in the opposite direction. The degree of bowing is imperceptible, so will not affect the appearance of the fused result.
-Grind the shelves flat. This can be done commercially with a milling machine, or you can do it manually. Place two shelves with their concave faces together with some sandblast grit between. Rub the shelves together and this will reduce the convex areas on each to flat.

Bubbles Between the Glass and Shelf

Eliminating the bubbles that can occur between your kiln shelf and the glass is important because these are the bubbles that can rise up through your work, blowing a large hole through the entire piece – Australians call these space helmets.

Common causes relate to damaged shelves, firing too rapidly, uneven heating, and combustion gasses.

Bubbles Between Layers – “Flip and Fire”

Another approach to avoiding bubbles is to plan on two firings. This works well for pieces that have multiple layers, with glass or other inclusion in the middle.

For the first firing, put the middle pieces flat on the kiln shelf with one layer of glass on top. Take this to at least a tack fuse, although full fuse temperature is better as there should be no remaining gaps for air to be trapped within. Now turn this over and clean it well. Place this part in the kiln with the middle layer up. Place the top layer over this piece – now right side up – and take to the full fuse. Remember that now you are firing a thicker piece than in the first firing so take the temperature up more slowly.

This is most often applied to three layer pieces, but in principle can be applied to any number of layers.

Using baffles
Supporting the edges
Design elements
Arrangement of glass sheets

Bubbles Between Layers – Baffles

Bubbles are often caused by the edges of the layers sealing before the air can escape from between. This frequently happens in side fired kilns, and top and side fired kilns.

Set up heat baffles around the edges of the sheets being fused to decrease the chance of the edges getting more heat than the centre and trapping air between layers. The baffles can be made from kiln furniture, strips of fibre board, cut pieces of old kiln shelves, etc. - anything that will witstand the top temperature.

Arrangement of glass sheets
Designing for fewer bubbles
Edge supports to reduce bubbles

Bubbles Between Layers - Supports

A common way to reduce bubbles that appear between layers of glass is to support the edges of the glass allowing the middle of the top sheet to sag before the edges so pushing the air in front of the collapsing glass.

You can do this with small beads - especially useful for large glass sheets. These beads are prepared in advance by firing small pieces of glass during a previous fuse firing. The glass draws up into a bead-like structure. You place these beads around the edge of the glass sheets. Use glass that is the same colour as the base glass to avoid strong colour spots in the finished work.

Make sure you advance the temperature slowly enough to allow the glass to slump from the middle outwards, allowing the air to escape. Note that even clear beads will leave a trace, so design your work to take advantage of these faint marks.


Another method is to put small pieces of frit every few centimetres around the edge of the bottom piece of glass. Place the top piece of glass on top of these spacers. When fired, the middle of the top sheet will sag first and the area of contact between the two sheets will spread from the middle pushing the air out as it goes, just as with the beads. But the evidence is not so marked as with the use of beads. However the frit is not so useful on large pieces.

Design factors
Arrangement of layers
Using baffles

Fibre Papers

As there always is concern about the health effects of ceramic fibre paper, the report I prepared for a supplier may be of interest.  It can be found here.

Bubbles Between Layers - Design

Design your work to minimise the possibilities of trapped air.

One way to do this is to use strips. Lay thin strips of glass on edge and fuse these together, instead of layers stacked on each other.

Another is to design work with many smaller pieces, rather than large ones. These create more pathways for air to escape.

Some advocate cutting the bottom layer in several strips to allow the air exit spaces from between the glass layers.

Note that all these methods leave marks of where the edges of the cut glass was, so they need to be planned to fit with the design.

In general terms, you need to think about how the air will move out of the piece. Are there places where there is no escape for the air? Allow a channel for the air to move from the centre to the outside.

Glass arrangement considerations
Supports
Using baffles