Example used with the maker’s permission
Piece in kiln ready to fire
Description of the piece.
A commissioned piece made up of Bullseye glass, 38cm/15” diameter, 3mm base with 2 and 3mm strips laid on top to a 6mm maximum depth, fired on a “standard” full fuse at 795°C/1463°F, annealed for 3.5 hours. The piece took up the whole of a newly primed shelf.
The fired piece developed an off centre bubble, and when the piece was cooled it rocked. The shelf was checked and it was level.
Previous firings of similar projects were mostly successful, but one was not, although felt to be interesting:
The piece bubbled and various sized holes were randomly drilled to determine the effect.
Other successful pieces were only slightly smaller:
The question was what to do to recover the piece, and why did it happen. Previous bowl blanks on the same program were ok, although this one was larger.
My Response:
The layup is the problem. A thinner (single?) layer in centre surrounded by radiating strips will trap air in the interior and so create bubbles. This kind of layup needs to have 1 - 3 mm fibre paper topped with shelf paper, under this kind of piece to allow air out.
This is a commission, so a repair is not acceptable. A new one needs to be created, because there would be reputational damage by passing off a repair that will inevitably show evidence of the fault. In general, repairs are unsuccessful. Repurposing the glass is a better solution.
Other possible causes in addition to the lay up are:
When a piece rocks on a flat surface it has become bowed, and it is evidence of stress developing during the annealing cool.
The annealing and cooling were likely to be inadequate – too short a soak, too fast a cool, or both.
In this case, the anneal soak was certainly long enough, so too fast a cool is the likely cause of the bowing.
The nearness of the glass to the kiln walls is often a cause of uneven heating, although in this case it did not become a problem.
It is also possible that the additional diameter was enough to push a barely adequate bubble squeeze beyond the possibility of full elimination of air from under the glass.
As a result of the experience of the “moonscape”, variously sized holes were randomly randomly around the centre of the bubbled piece to determine if that would have been an acceptable fix. The thicker ring around the burst large bubble remained in the attempt at a fix.
Holes
drilled randomly and extra
clear dots
were
added.
The
client was contacted to explain the accident and to confirm the
making of another.
