Wednesday, 5 February 2020

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.

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