Seedy base glass

Sometimes your clear base has bubbles, or as the trade calls it, seeds.  When capped with opalescent glass, in certain circumstances, these tiny bubbles can become larger and rise to the surface, pushing the opalescent aside as it rises.  This leaves a clear spot in the midst of the opalescence.

Clear cap

One way of reducing this problem, is to avoid it altogether.  This can be done by placing the clear on top of the opalescent as a cap.  This way the bubbles, if any, are rising through the clear.

Flip and Fire

If you can’t, for one reason or another, cap the piece with clear, you can fire upside down. Again, the bubbles are rising through the clear.  When the firing is complete, you can flip it over to the right side.  You will need to clean thoroughly and take to a fire polish temperature to get the shiny surface back.

Heat Work

Another way is to fire to a lower top temperature with a longer soak.  This means the glass can take up the profile you want without becoming so soft that the bubbles can rise through the glass.  You will need to observe to determine when the glass has the right profile, and then advance to the cooling and anneal phases.

Low and Slow

This last way of reducing the possibilities of bubbles rising through toward the top is based on the characteristics of glass.  As glass becomes hotter, it becomes less viscous and so allows the air to rise toward the top of the glass surface.  Using a low temperature gives a more viscous glass to resist the bubble movement.  The long soak at the chosen lower temperature allows the surface of the glass to take up the profile you want, as the surface is hotter than the bottom of the glass, therefore reducing the possibilities of bubbles rising.  It does take a longer soak at the top temperature, but it also reduces the marking on the bottom of the piece.

This low temperature process is using the principles of heat work.  The effect on the glass is a combination of temperature and time.  The higher the temperature, the less time is required.  The longer the time, the less heat is required.  The heat work put into the glass to achieve the effect you desire is determined by the combination of temperature and time used in firing the glass.  This principle of heat work is why you can achieve the same effect at very different temperatures, depending on the length of time a piece is soaked.

Capping with Frit

Capping with a clear or tinted top layer is necessary in many cases of inclusions, or desirable when looking for depth or distortion in flat fused work.

Capping inherently has bubble creation potential.  The development of a bubble squeeze helps prevent the largest of bubbles.  It cannot eliminate all the trapped air that then turns into small bubbles around the inclusions or multiple pieces when covered by a sheet of glass.

An alternative is to do away with the sheet glass capping and instead use enough frit to provide the desired depth, or the necessary material to cover the inclusion.  In fusing with two large sheets, a fine covering of powder between the layers will help to eliminate bubbles.  However, this will not be enough to successfully cover metal or other inclusions, or provide the amount of glass to give an appearance of depth.

The size of frit to use in a given application can be determined from other styles of glass working. It is known from glass casting that the smaller the frit the greater number of small bubbles will appear in the fired piece.  This means that you need to use medium sized frit for cast work.  Fine frit is likely to produce many very small bubbles across the whole piece in fusing applications.  Large frit is likely to produce larger bubbles, as the pieces themselves trap air as they deform.  This means that medium frit is a good compromise between large and small bubbles in capping. 

The layer of frit should be at least 2mm thick.  This means a lot of frit is required to do the job.  To judge the amount, you can measure the area of a rectangle or circle in square centimetres and multiply that by 0.2 to give you the volume (in cubic centimetres) of frit required.  Multiplying the volume by 2.5 (the approximate specific gravity of soda lime glass) will give you the weight of frit needed to cover the area. 

Alternatively, if the piece is irregular, you can weigh the base and add the appropriate weight of frit on the top.  If the base is 2mm, no further work is required to determine the weight. Weigh the 2mm sheet and use the weight of frit to equal the base.  If the glass is 3mm, you need two thirds of the weight in frit, and so on for thicker glass.

Using frit to cap is unlikely to eliminate all bubbles, but it will reduce them to a minimum.