Layups Promoting Bubbles

Intentional Bubbles

Sometimes you want bubbles. There are various ways to achieve bubble placement with certainty rather than at random.  You can use a variety of bubble powders.  There are a variety such as the UGC bubble powder – now supplemented with bubble enamels.  The use of copper oxide powder will give bubbles of varying sizes dependent upon the amount deposited. You can also use baking soda – calcium carbonate - in the same way for clear bubbles.

You can create a range of bubble textures by arranging textured glasses in various orientations.  Fine reeded glass at right angles will give a regular pattern of small bubbles.  Accordion glass will give a slightly different arrangement.  Using fluted glass at 60 degrees to one another will give you diamond shaped bubbles if you control the temperature and time.  The variety is limited only by the textures and the way you arrange the glass orientations.

Incidental Bubbles

Most inclusions – metal, mica, organic, etc. – result in bubbles to a greater or lesser extent around the objects included.  Extended bubble squeezes are required in conjunction with a sprinkling of powder or very fine frit between the inclusion and the edge of the piece.  Sometimes corner pieces can be included in the design to keep the edges open longer allowing more air to escape.

Unwanted Bubbles

These bubbles largely come from the way in which the glass is arranged. 

Single layers at full fuse will draw in at the edges and thin from the interior, allowing any air to push up and sometimes through the glass.  This is because the thicker and heavier edges resist the movement of the air from under the glass.  This resistance, added to the thinning of the interior leads to bubbles, unless the glass is fired at fire polish or lower temperatures.

This example from Danna Worley shows the effects of firing single layers

Single layers with borders compound the problems of single layers.  The borders ensure that the edges are heavier than the interior and seal air at an even earlier stage of the firing.  The bubbles will appear between the other tack fused pieces in the interior of the piece.  Again, with this kind of lay-up, the top temperature should be no more than a rounded tack fuse.

Heavy or thick borders on two-layer bases are also circumstances where bubbles can be produced.  The border on even two-layer pieces can trap air both under the whole piece and in between layers in the same way a border can on a single layer piece.  In a lay-up like this, it is best to fuse the two base layers together first and then add the decorative pieces and border in a second firing.

This example from Andy Bennett shows how, even when inducing bubbles, things can get out of hand. Here the bubbles between layers have even thinned out the bottom layer to holes to the shelf.

Encased glass pieces are a certain way to get bubbles.  If you place even a single layer of glass pieces in a pattern around the base and then cap it with a sheet of clear, bubbles will form.  This will happen even if there are clear path ways for the air to be released from the interior.  The capping glass will not conform completely to the encased glass pieces by the time the edge is sealed, no matter how long your bubble squeeze may be.  The way to avoid this is by putting the glass pieces on top of a two-layer base.  And it is better to fuse the base layer first before adding the surface glass pieces, so they do not press down unequally, leaving a thin film of air around the heavier pieces on top.

Avoidance of unwanted bubbles

There are a few ways to avoid bubbles that are not where you want them.

• ·        Avoid using single layers with pieces on top.
• ·        When using single layers fire with slow rates of advance at low as possible temperatures with a short soak at top temperature. You will need to peek at intervals to observe when the work is finished and advance to the next segment.
• ·        Non-glass inclusions should be encased with care.  They should be as flat as possible before capped.  The bubble squeeze should be long – possibly as slow as 25°C per hour between 600°C and 677°C. This is to allow the glass at the centre to settle, pushing air from the centre out. Including a sprinkle of powder or very fine frit may help reduce bubble formation, as might chads at the corners or edge of the piece.
• ·        Organic inclusions will produce large bubbles from the combustion gases.  Use a three to four-hour soak at about 540°C to allow the burnout of the organic material before proceeding to the bubble squeeze.
• ·        Avoid borders on top of the glass.  The additional weight acts to seal the glass to the shelf and between layers, leaving air underneath to rise and even break through.
• ·        Do not cap/encase glass pieces unless you have a very good reason.  The glass pieces placed on top will stick to the surface with less chance of bubble creation, and will become flat at a full fuse.
• ·        If you must have a border or encased glass pieces, consider flip and fire – fire the piece upside down to a rounded tack fuse at least, clean thoroughly, then cap the piece and fire right side up. This can reduce the bubble formation.

Hot Spots in the Kiln

You may suspect you have hot spots in your kiln because of bubbles or one side of the piece being more fully fused than another. A good method for determining the temperature distribution across the kiln is given on the Bullseye site.  It does not require any sophisticated equipment – just supports equal distances apart and strips of glass equally wide and long – to be witnesses for the hotter and cooler parts of the kiln.  You fire slowly to a very low slump temperature – ca. 620C - for only 5 minutes.  Go as fast as possible to the annealing point and soak for 15mins. Then you can turn the kiln off, and let it cool as fast as the kiln can.

This test will show where the hotter areas are.  You will see from the test results that there is a gradual change of temperature across the shelf, rather than small hot areas that would be required for localised large bubbles originating from under the glass.  It will tell you where the cooler areas are, so you can avoid placing pieces in that area when you need precise profiles on the finished piece.

There is little to no relation between hotter areas of the kiln and localised bubbles.  Do not think hot spots are the cause of large bubbles.

Bubbles more often relate to:

Bubble squeeze

Alack of an adequate bubble squeeze, as described here.

Avoidance is the best approach.

Do not be lead into the idea that mistakes are automatically art, or that all of them can be rescued.

Rapid firing rates

Firing rates need to be adjusted to the materials you are firing.

As fast as possible firing rates can cause problems.

High temperature rapid firings can also cause problems.

Rapid firings are more likely to harm the glass than the kiln.

Damaged shelves

Distortions or damage to shelves can trap air and so cause bubbles to form between the shelf and the bottom of the glass.

Checking for flat shelves and fixing.

There are some remedies.

Volume control

Varying volumes within the piece can give problems.

There are a variety of related things that can cause large bubbles.

Glues

Glues and adhesives have a variety of effects and dangers, especially if generous amounts are used:

There are a variety of glues each with their own characteristics.

Uneven layers/layup

You must think of ways for the air to escape from the interior of the glass and from under the glass.  Most often we set up things in a way that creates bubbles. There are two main ways that we do this.

Encased items

When we put glass or other materials between an upper and lower sheet of glass we are creating conditions for bubbles to form.  The encased items hold the upper glass above the lower glass by an amount related to the thickness of the inclusion.  Routes for the air to escape must be planned. 

One of the ways to reduce the height of the space taken up by the enclosures, is to fire upside down with the inclusions on the shelf. This allows the glass -which will be the bottom layer - to form around the materials, reducing the air space between the bottom and capping layer.  This is known as flip and fire.

You then clean the face which will be capped very thoroughly.  Place the capped piece on fiber paper – which can have Thinfire placed over it, or coat with kiln wash.  This is to allow the air in the uneven bottom surface to escape from underneath through the fibre paper.

Weight

Even when there is no encased material, the weight of the glass pieces on top can create areas where the air can be trapped.  On a single layer the arrangement of pieces can create areas where the glass cannot resist the air pressure that cannot disperse from the pockets caused by the glass on top.  Very clear and generous exits for the air are required.

This can happen with two layers as well, although usually a higher temperature is required.  A means of avoiding large bubbles when there is glass – powders, frits or pieces of glass – placed on top is a two-stage firing of the piece.  First fire the base layers together at full fuse so they become one whole.  Then add the decorative elements on top and fire.  Remember to fire more slowly than for two unfired layers.  The main piece is now 6mm thick and needs a slower rise in temperature.  The additional heat work this entails may mean that a lower top temperature, or a shorter soak will be required than normal.  You will need to peek at intervals to check on the progress of the firing.

There is a multiplicity of ways that bubbles large and small can be created.  Careful layups, bubble squeezes, slower rates of advance and lower top temperatures can minimise, but not always eliminate, bubbles.

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.

Encapsulation of Fused Glass Panels

You can encapsulate both leaded and fused glass panels into double glazed units.

Leaded glass panels have the outer came built in “Y” shaped came.  The tail of the “Y” is held between the spacer bars at the edge of the double glazed unit. You normally need to leave 25mm space on each side.  It is calculated as the reduction from the glazing size of the opening.  This is required to accommodate the width of the heart of the came, the spacer bars and sealant of the double glazing assembly.

The same sizing guide is required for fused glass panels.  Usually the “Y” came is designed for 3 mm glass, as it has a 5mm high heart.  However, the “Y” is broad enough that the leaves can be opened to accept the thicker 6mm fused glass.  It is also possible to grind a bevel on the back of the fused glass to make it easier to slip into the came.

It is best to find the person or company which will be making the double glazing unit before starting.  Discussion with them at the start will enable you to determine how much allowance is needed for the spacer bars and the sealant. This will assure you in setting the dimensions for the panel you will make.