Bubbles in Glass

Bubbles in Glass

Bubbles are a natural occurrence in fused glass and some artists have learned to control bubbles to form geometric patterns and other effects.
If bubble appear where you don’t want them it is usually caused by firing fast and trapping air between glass layers.

Take a look at this video on troubleshooting bubbles:


How do I minimise the look of bubbles in my piece?

If you tend to get lots of little bubble trapped between layers of glass, try sifting Bullseye Clear powder between the glass layers before firing. This actually traps more bubbles, but they are much smaller and do not attract the eye.

See the Bullseye Quick Tip: Powder Power for more information.

small bubbles in glass

 How do I avoid bubbles when using frit or stringers between fused layers?

When using frit or stringers it’s always a good idea to run your design right to the edge of the glass, this will allow air to track out of the design instead of getting trapped and causing bubbles.

frit causing bubbles in glass

What is causing HUGE bubbles between the shelf and the fused glass?

Some bubbles are not between layers but come all the way through the glass and sometimes appear as a large hole in the glass. Large bubbles or holes in a piece are usually down to one of three things:

1. The main culprit is fusing thin glass too fast causing an 'apple pie' effect. This is where the glass seals around the edge, trapping air in the middle which then forms a bubble which turns into a hole when it pops.

Use the super bubble squeeze firing schedule or use 2 layers of 3mm for the base. Bubbles in designs which use a single 3mm base with frit on top are very common and difficult to resolve without using fibre on the shelf to allow the air to track out.

2. A shelf which has been primed or has not been fired for a long time often holds moisture even though it looks dry, this moisture can turn to steam during the firing which can blow bubbles. If you get these large bubbles in a 6mm thick piece, the solution is to dry your kiln-shelf by firing it to 260C for 20 minutes with the kiln fully vented.

3. A dip in the shelf can cause bubbles, if you put a straight edge across your shelf you should not see any gaps, try this on both sides of the shelf and fire on the flattest side. Ensure that your kiln shelf has no dips in it or just flip it and use the other side.

 huge bubbles in glass

Why am I getting black bubbles in my glass?

Bubbles with sooty deposits are usually caused by glue residue. The black is carbon left from the glue burning out. The bubble may not be where the glue originally was as it will track to the thinnest point. It’s best to avoid using glue or use it very sparingly. When glue is used, use it on the edge of the piece as this will allow it to burn away cleanly.

black bubbles in glass

How do I create a grid of small bubbles in my work?

You can cross-hatch stringers between layers of glass to create a pattern of small uniform bubbles. Choose a transparent glass colour, and use the same colour of stringer. Lay the stringers in a row along the base glass, and then do the same with the cap glass. Sandwich the stringers together at 90 degrees to each other before firing. This creates an elegant grid of bubbles.

grid of bubbles in glass

Bubble Pens are good for making coloured bubbles, but how can I create clear bubbles in my work?
Try mixing bicarbonate of soda with water, then spraying it onto the surface of the glass before capping and firing. This produces clear, random champagne bubbles in your work. The effect can be a little difficult to control, so experiment.

I get bubbles when using silver in my work. How do I avoid this?
This is an interesting example of how heat works in the firing.
As your piece was being fired, the silver foil on top acted as a reflector, reflecting heat away from the glass beneath it and keeping it slightly cooler (and stiffer) than the surrounding uncovered glass. As the edges of the piece heated first they sealed, trapping an air bubble in the centre. In extreme situations, a fast initial ramp rate for example, this can cause the glass to crack. Interestingly this is not specific to using silver, other reflective inclusions such as dichroic, iridised and even white or light coloured opal glass, can cause the same issue.
In order to combat this I would encourage using a bubble squeeze firing schedule, this slows down the heating process allowing extra time for the heat to get into the piece beneath the foil and melt the layers more evenly. Unfortunately this would only help in preventing the bubbles forming in the first place, now that they are trapped within the glass there's not much that can be done.

You could fire them upside-down on a tack fuse. this would flatten the bubbles so that you have a level surface to use as a coaster. However this will give you a slightly more textured surface on this side, as the glass will pick up the texture of the kiln shelf and more importantly the silver will contaminate the kiln shelf. So this would need to be done on a shelf that you don't mind contaminating.

silver causing bubbles in glass