Bubbles in glass

Why Bubbles?

There are a number of causes, in different situations, for why and how bubbles occur in glass in kilnforming processes.

These bubbles can range from small pin-head size to great eruptions destroying a work.

The causes can include:

• air trapped between layers of glass;
• air trapped in a bedding powder or underlay;
• air trapped between glass and mould;
• gasification of a bedding powder; and
• gas coming out of solution in the glass.

Air expands when it is heated and increases in volume by over 3.7 times when heated from cold to glass fusing temperature. Kilns are never completely sealed, so the air gradually escapes as the temperature rises and there will not be a build-up of pressure within.

Air trapped between layers of glass

As the glass softens, air in the crevices and undulations between layers can be trapped by the faces coming together. 

The trapped air will continue to increase in temperature but, instead of increasing in volume it will increase in pressure.

Bubble forming in glass

	A: Side view of two sheets of glass with air trapped between them
	B: Close up of trapped air between glass expanding as temperature rises in kiln
	C: Air rupturing glass
	D: Ruptured  surface of glass receding

This increasing pressure may be sufficient to force up the top layer, creating a bump as at A. The soft glass may flow off this high spot, reducing the mass of glass above the air bubble and making it easier still to push the glass upwards, as at B.

Depending on circumstance, the glass can become so thin that it ruptures and a small hole is created, as at C.

At that point the conditions of equilibrium thickness apply. Surface tension will cause the edge of the hole to recede so that it can become thicker. 

The thinner the top sheet the larger will be the hole produced under the influence of surface tension.

This is a worst case scenario.

More generally, air will be trapped between the layers and appear in the fused piece as randomly scattered oval or irregularly shaped bubbles, ranging upwards from the size of rice grains.

The amount of air which can be trapped will vary with the type of glass. Rolled glass has more uneven surfaces than float, so greater amounts of air can be trapped.   

The possibility of air being trapped can be greater when glass is side fired rather than when top fired: the greater concentration of heat on the edges of the glass can cause them to seal earlier.

There are positive steps which can be taken if problems are being experienced

• A slower rate of heating can allow more of the air to expand and escape before the edges of the pieces seal together.
• A hold can be introduced at around 720 - 730°C (1330 - 1350°F) with float, slightly lower with other glass, to allow time for the glass to soften and expel air before the edges seal.
• Fine slivers of compatible glass can be inserted at the edge between the layers to hold them apart until late in the fuse.  

Air trapped in a bedding powder or underlay

Air can be trapped below glass on a tightly packed powder bed should the edge seal to the powder before the air has escaped. Whilst a loosely packed sieved powder bed will provide more space for air to be trapped, the loose packing can also provide more avenues for pressure release.

Sometimes a powder bed will be tightly compacted to produce an almost clear fired texture and this can make it more difficult for air to escape.

Problems are more likely to occur on a slab hearth or kiln shelf than on a brick hearth, as the porous nature of the IFB hearth can allow air to escape more readily.

One solution is to place a layer of CF paper or blanket on the slab or batt to give a porous layer through which the air can escape.

CF paper is more easily obtained than thin CF blanket, as well as being more uniform in thickness.

Air trapped in moulds

Air can be trapped if the edge of a sheet of glass seals to a mould before all the air has been expelled. Unless precautions are taken, this can easily occur when slumping into a bowl type mould.

One solution is to drill one or more small holes in the mould at the point which is the last to be contacted by the glass. Holes don’t need to be large, but make sure they don’t get clogged with battwash.

Gasification of a bedding powder

Certain bedding powders can turn to gas if taken to too high a temperature.

Particularly worthy of note is whiting (calcium carbonate) which gives off carbon dioxide at around 825°C (1515°F), although there is some suspicion that it can occur at lower temperatures when the material is old. 

Organic binders in CF paper or board produce carbon dioxide when the carbon is finally oxidised. With CF paper this should occur well before the glass is soft enough to go through the bubble forming process. The better insulating properties of thicker fibreboard could delay the gasification of the binder until the glass is soft enough to be affected.

This cause should not be entirely discounted if searching for that outlandish cause of a mysterious occurrence.

Gas coming out of solution in the glass

This has been mentioned in reference to float glass, but is worthy of mention again.

In making mass produced clear glass, particular attention is not made to completely clearing it of suspended gases. To the glass maker, it is sufficient that gas bubbles are not visible so as to affect the appearance of the product.

When re-fired, these small bubbles can migrate and join up to make larger bubbles which can be visible to the eye. The likelihood of this occurring increases with the number of re-fires. The glass must be fluid for the bubbles to be able to move, so, the higher the firing temperature the greater the possibility of the problem appearing.

It can occur with any glass, but is more likely with float and commercial rolled products. An obvious solution is to restrict the number of re-fires to those absolutely necessary. Makes sense from a cost perspective also; firings cost money.

Bubbles to order!

There are times when one wants bubbles to appear.

The use of two layers of fine reeded sheet glass is worth repeating. The pieces are placed with reeded faces touching but with the reeds at right angles to one another. A regular and reasonably uniform series of small bubbles will result.

A number of glass artists have made the arrangement of bubbles a feature of their work, either by careful cutting and arrangement of glass segments or by the use of chemicals to induce gasification. 

Drilling holes part way through a sheet in selected positions in the top sheet of a fuse  so that the holes trap air between the sheets can give a random bubble effect.   

Like so much in kilnforming, one can let the imagination run riot.