Experiments related to slumping show that shaped items such
as slumped, textured and kiln carved glass need annealing for at least one
layer thicker than they are. The annealing for one layer thicker than the
calculated thickness provides the most stress-free result for the finished
product. Annealing for the calculated thickness does not always produce a
stress-free result.
Full Fuse
This indicates that an evenly thick 6mm thick piece will get
the best result from an anneal as for 9mm.
Texture Moulds
A piece of glass on a texture mould with 3mm or more
differences in height requires careful annealing. The more defined/sharper the
texture, the greater care will be required. A 6mm blank on a mould with 3mm
variation taken to a well-defined texture needs to be annealed as though it
were 18mm thick. A sharp tack requires annealing as for a piece 2.5 times its
actual thickness plus another 3mm. This
gives the 18mm/0.75” thickness annealing requirement for the 6mm thick piece.
Kiln Carvings
The same kind of calculation applies to kiln carved items as
for sharply textured pieces. Pieces with less sharply defined profiles can be
treated as one of the more common tack fused profiles.
Credit: Vitreus-art.co.uk
Tack Fusings
A rounded tack fused piece of a 6mm base with 3mm tack elements that is being slumped will
need annealing as for 21mm. Twice the
actual thickness plus 3mm giving the annealing requirement as for 21mm/0.827”.
A contour tack of the same dimensions as given in the first
example will require annealing as for 19mm/0.75”. The annealing requirement when slumping is
for 1.5 times the thickness plus another 3mm.
In General
The general approach to annealing shaped pieces is to calculate the thickness for the
anneal and add one layer more to get a good anneal for slumped and other formed
pieces.
The research and the reasoning behind this approach is given in LowTemperature Kilnforming, An Evidenced-Based Guide to Scheduling available from the Etsy shop VerrierStudio and from Bullseye.
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