Single Layer Circle with Decorative Rim

A question arose:

If you fuse a single 20cm diameter sheet of 3mm glass to full fuse, [with a decorative rim] what happens? … Would the lack of two layers in the centre be a problem for the 6mm rule?

This layup risks trapped air and a large central bubble.  The explanation involves the combination of volume control and weight.

Volume control

The volume control relates to the single 3mm layer in the centre.  The glass will thin in the centre and thicken at the perimeter.  This leads to the risk of thinning to the degree that bubbles are created in the centre.  The edges will also draw in as the viscosity - surface tension - of the glass pulls the glass toward a 6mm thickness.

Weight

The explanation is also about weight.  The decorative rim adds weight to the outside of the piece.  This weight will “seal” the rim of the glass to the shelf, reducing the possibility of air escaping from under the central portion of the piece.  This weight effect on the rim increases the risk of a large central bubble.

Profile

Another influence on the result of the fuse is the degree of fuse.  At full fuse the viscosity of the glass is less and so resists the force of expanding air much less than when cooler. Even at rounded tack fuse, the glass will be unable to resist the formation of bubbles. As the glass thins and viscosity decreases, any air at all will cause a bubble.

Changes for the future

Avoidance of bubbles in this piece relate to design, scheduling and technique.

Design

It is possible to design a piece of this nature to avoid the volume control issue.  The base piece could have a smaller circle or rectangle centralised on top inside the proposed perimeter.  The rim can then have the decorative elements placed.  If they are spaced widely, frit can be used to fill significant gaps.  The piece can then be placed in the kiln for a full fuse.

Scheduling

You can also fire the piece as originally described very slowly to a low temperature.  This uses the concept of heat work. By applying the heat over a long period, you can achieve the same effect as would be achieved by a faster rate of advance to a higher temperature. 

There are at least two ways to increase the heat work.  You can use a very slow rate of advance to a point slightly above the softening point of the glass.  This will be the lower end of the slumping temperature range of your glass.  The soak may be for hours.  You will need to observe when the effect you want is achieved.

You also can choose the same lower slumping temperature and reach it in your standard fashion.  This will require an even longer soak time to achieve the same result.

In both these low firing approaches, you will need to observe to determine when the piece is finished.

Technique

The “flip and fire” technique may also work on the single layer with an added rim.  To do this you build the piece upside down on the shelf.  It helps to draw an outline of design on Thinfire, or Papyros.  Place the decorative elements and cap them with the clear.  Take the whole to a rounded tack fuse.  When cool, clean well and fire to a tack fuse again.  This will give something less than a full fuse, but it will be more than a tack, as the heat work is cumulative.

Further information is available in the ebook Low Temperature Kiln Forming.

Summary

A single layer piece with a decorative rim is most likely to produce bubbles in the centre.  There are some ways to overcome this: design, scheduling, and technique. Design is the most likely to be successful.

Broken base glass

Firing a piece with a partially covered base layer requires more care than two even layers to avoid the fracture of the glass during the heat up stage of a firing.  Slower rates of advance need to be used.

Glass is a poor conductor of heat and electricity. This can be good in certain circumstances but is usually one of the limitations in kilnforming.  The poor conductivity of glass means the top layer of glass will need to be heated before it begins to transmit heat to the glass below. 

A while back an example was shown that is a special case, but also illustrates the general principle (apologies to the poster, as I didn’t take down the name at the time and can’t find the original post now).

This sheet of clear glass was covered by an arrangement of stringers, with a border of clear exposed.  I don’t know positively, but I presume this was done in the knowledge that the single sheet of clear glass would become smaller, and the border would be cut down to the appropriate size.

Be that as it may, the exposure of the clear allowed the edges of the clear to heat up faster than the covered part of the sheet.  The stress of the temperature differential between the centre and the edges led to the fracture of the glass during the heat up.  This can be confirmed by the rounding of the broken edges.  It is further confirmed, by observing the relative straightness of the stringers, that the break occurred before the stringers became sticky enough to even laminate to the base glass - the clear glass broke underneath, leaving the stringers relatively undisturbed. It is also an indication that the glass broke earlier than the slumping temperature, as the stringers would have been sticky enough to break with the clear otherwise.

One speculation given for the break was that it was affected by the size.  You can see the size is relatively large for the kiln.  This may have had some influence on the fracture as well.  But it is not so much the size as the shielding of the heat from above for a large part of the base sheet. We don’t know if this was a side fired kiln, but if it was, there would be an increased exposure of the edges of the glass to the heat and so increase the likelihood of temperature differentials leading to too much stress for the base glass.

The rate of advance for partially covered sheets needs to be reduced to be slower than for evenly covered base sheets.  Even on evenly covered base sheets, there is a risk of breakage of the bottom sheet, if the rate of advance is too quick.  Slower heating reduces the temperature differentials, as the gradual rise in heat allows the glass to be closer in temperature from top to bottom.

Single Layer Slumping

Almost all glass can be slumped as a single layer, whether produced for kiln working or not.  A few are extra sensitive at even slumping temperature and change character at around 630C-650°C, but all others can be slumped.  This posts concentrates on slumping of single layers of non-fusing compatible glass, but most of these elements can be applied to fusing compatible glass too.

The things you need to take care about are:

• Temperature
• Soak Times
• Edges
• Devitrification
• Annealing
• Testing

It certainly is possible to slump single layers. The resulting glass will be slightly less robust than two or more layers of glass, but simply because it is thinner.

Temperature

The temperature that you use needs to be high enough to allow the glass to take the shape of the mould, but low enough that the glass does not distort or stretch and thin.  This is a balance that you can achieve through observation of the firing. 

It most often is best to use the lowest practical forming temperature that you can.  Practicality here is about how long you want to wait for the glass to conform to the mould.  It is possible to take the glass to about 580°C and soak for multiple hours, but not very practical.  It does depend on the glass as to the temperature to be used for the slump.  There are two sources here that can help: the slump point test  and this table of glass characteristics. 

Soak times

A practical soak time will be 30 – 90 minutes, which will avoid marking the underside of the glass.  This means that the temperature will need to be lower than the softening (or slump) point of the glass. Your slump point test will tell you the temperature at which the glass begins to deform.  That is the best temperature to use.  If it is taking too long, advance the temperature by about 10°C.  If you used the table of glass characteristics to find a softening point, reduce that temperature by about 30°C as a starting point.

Edges

The temperature that you will choose to use is not high enough to allow the edges to change as they would in a fuse.   This means that you need to have the edges exactly as you want them in the finished project.  This will require cold working by hand or machine.  Neither will take a long time, but require the correct tools. This post gives you the comparison of fused and cold working methods.

Devitrification

While most glass can be slumped you need to be careful with opalescent glass, as it can devitrify easily.  Most wispy glasses are fine, but the more opalescent wisps they have, the more difficult there may be.  Streaky and single colour glasses are usually fine. 

Annealing

Another element in slumping glass not formulated for kiln working is the annealing of the glass after the slumping.  The annealing temperature can be estimated as 40C below a low temperature slump of a 280mm span of glass. The slump point test mentioned earlier will help determine the annealing point. You need to soak for a time - maybe 30 minutes - at the estimated annealing temperature and then cool slowly in case you have miscalculated on the annealing temperature.  In any case, a long slow anneal cool will pay dividends in a more robust glass.

Testing

You will find some manufacturers’ glasses are less adaptable to kiln forming than others.  So, it is best to run tests on the glass before committing to larger projects.

Remember TADSET - temperature, annealing, devitrification, soak, edges, test.