Wednesday, 22 April 2020

Bubble Squeeze for Multiple Layers

Difficulties often occur with bubble formation within pieces composed of several layers. There are a couple of factors in addition to the number of layers that have an influence - temperature and rate of advance to the bubble squeeze temperature.

The top temperature for the bubble squeeze does not need to change with multiple layers. It is the advance to the bubble squeeze that needs to change in relation to the number of layers.

Rate of Advance
It would  be suitable to reduce the rate of advance to about three-quarters of the two-layer schedule to account for three layers.  And a reduction to one half of the two-layer schedule for a four-layer piece would be appropriate. The reasons for these slower rates of advance follow.

A normal rate of advance for two even layers would be about 200°C per hour to the bubble squeeze temperature.  Sometimes a very slow rate of advance is used from 50°C below the top of the bubble squeeze.  This strategy can continue to be used for thicker pieces made up of many layers with some modifications.

Multiple Layers
But for a three-layer piece, slowing the rate of advance to about 150°C is important to assist in a good bubble squeeze.  This helps get all the glass at the same temperature by the time the bubble squeeze is approached. Glass is a good insulator, and also a poor conductor of heat. This slower advance allows the bottom layer to be at the same temperature as the top piece.

For a four-layer piece, a rate of about 100°C would be suitable.  When the lower point of the bubble squeeze is reached (about 50C below the upper soak), the slow rate of advance can be used to go to the upper end of the squeeze, using the normal soak length.  

This illustrates that the more layers of glass in the stack, the slower the rate of rise must be in the bubble squeeze range.

Five Layers and Beyond
For pieces made up of more than four layers, a different strategy is needed to ensure the heat reaches the bottom layer of glass.  Graham Stone* calls this the “catch-up” schedule. It is essentially an overnight schedule with temperature equalisation soaks of 20 minutes at 125°C intervals all the way to the bubble squeeze. At each stage the rate is increased by 10°C.

This means that with a first segment rise of 20°C per hour, the second from 125°C to 250°C is at 30°C with a 20 minute soak, then 40°C to 375°C soak for 20 minutes, 50°C to 500°C and soak for 20 minutes, and finally 60°C to 625 for a final 20 minutes with 70°C to your normal bubble squeeze temperature.  This will take about 17 hours before you go on to the forming temperature.

This long heat up schedule illustrates the problem of getting the heat to the bottom layers of the stack, and the need to squeeze the air from between the layers.

Thicker pieces apply more weight to press out bubbles from lower layers, but only if the lower layers are equally as hot as the top.  This requires long schedules.

An alternative approach to this bubble squeeze problem is to fuse two layer pieces of the appropriate number to achieve the thickness required.  If these are fired with good bubble squeezes there will be a minimum of bubbles.  Combining these 6mm blanks will give fewer bubbles with a proper bubble squeeze.

Another approach is to start with 6mm glass as it comes from the maker.  This is not always possible, because it is not common for 6mm fusing glass to be made in anything but clear.  It may be possible to incorporate the clear within the stack, if it is not appropriate on the bottom.  These thicker sheets have fewer bubbles proportionally than 2mm or 3mm sheets.  So there are fewer bubbles in the final piece.

Of course, placing shards of glass at the corners, or sprinkling a very thin even layer of powder between multiple sheets will also help reduce bubbles between layers, but it is the slow rate of advance to the bubble squeeze that is the important element.

*Firing Schedules for Glass; the Kiln Companion, by Graham Stone, 2000. ISBN 0-646-39733-8

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