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.
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.
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