Pin
holes in screen and pot melts are the result of very small bubbles rising to
the surface. These bubbles are sometimes
within the glass melted, but more often come from small amounts of air trapped
within the flowing glass. These are
perceived to be unsightly, or make it impractical to make a functional piece
from the melt.
There
are ways to minimise bubble formation or to deal with the formed bubbles.
Bubble Formation
In
pot and screen melts, the glass spirals as it touches down onto the shelf. This
spiralling action can trap small amounts of air as each successive spiral forms
beside the previous one. Efforts at prevention of tiny bubbles in the final
piece need to concentrate on this fact.
A
preliminary element in bubble prevention is to have a long bubble squeeze to
allow the glass to settle in the pot or on the screen so that the rest of the
process can proceed with a minimum amount of air trapped within the flowing
glass.
Two-Stage Drop
In
some cases. it is possible to have the glass flow from the pot onto an angled
shelf where the spiralling glass has to flow from the initial touch down to the
edge and then flow onto the shelf. This
allows any tiny bubbles initially trapped to escape before the final drop onto
the shelf. This provides two mixing
processes and means that a lot of clear glass needs to be included to avoid a
complete mix of the colours. It requires
careful selection of the original colours to avoid a brown or black
result. It also requires a big kiln with
sufficient height for a two stage drop.
This
two-stage drop is of course, not suitable for a screen melt where you wish the
glass strands to remain. Nor is it
suitable when you wish to have many “pools” of colour mix in the final piece.
Where
the two-stage drop is not practical or suitable other methods can be used. These relate to scheduling, cold working the
surface and re-firing the piece.
Schedules
Scheduling
relates to using a soak at full fuse temperature before proceeding to the
anneal. The melt will occur at 850°C to
950°C. You can cool as fast as possible
to a full fuse temperature of about 810°C and soak there for an hour or
more. This allows the small bubbles to
surface, break and heal. Schedule the rapid
cool to the annealing soak, once the high temperature soak is complete. This will eliminate lots of the bubbles, but
not all.
A sample friring schedule from bubble squeeze upwards and then down to a high temperature bubble reduction soak |
Cold Work
Cold
working the melt is about abrading the surface to open the bubbles that are
just emerging to form a small dome at the surface. Sand blasting is a common form, as usually
kiln wash or fibre needs to be removed from the bottom of the melt, and some
devitrification from the surface. It
would be possible to continue to grind the surface of the glass to eliminate
the small depression in the glass caused by the now opened bubble, but this is likely
to expose more bubbles that were at a slightly deeper level. What next?
As
you will need to do a fire polish firing after blasting or grinding the
surface, you can use a full fuse temperature to allow the surface to become
plastic enough to fill the bubble holes.
Remember to schedule the firing as though the piece were at least 12mm
thick. You may find that more bubbles
are exposed in addition to the ones healed at the conclusion of this second
firing.
An
alternative is to fire upside down. You
will have noted that there are no bubbles on the bottom of the melt. This is because the bubbles have risen
through the heated glass. This physical
fact can be used in the second firing.
Fire with the melt surface to the shelf.
It is best to have a clean and newly kiln washed shelf, or fibre paper
(not Thinfire or Papyros) under the glass. Fire the glass to a full fuse or
high temperature tack fuse with a significant length of soak to allow the bubbles near
the original surface to move toward the interior of the glass. After firing, the glass will need thorough
cleaning before being fire polished. This should leave you with a pin hole free
piece.
Conclusion
Achieving
a pin hole free pot or screen melt requires several stages of coldworking and
firing. This makes melts inexpensive in
materials (it is scrap of course) but expensive in time and firings.
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