A question about why a tack fused 6mm/0.25” piece of combined dense white and black in a slump firing broke has been raised. Other pieces of black and other whites also tack fused in the same firing did not break.
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| "Living in the Grey" Stephen Richard |
Contrasting colours
Combining the most
viscous and the least viscous of bullseye glasses - dense white and black - is
a challenge. The survival of other pieces
in the firing with slightly less viscous white give an indication. Their survival shows that the anneal and
cooling conditions were too short and fast for the broken piece.
It may be worth
checking how much stress is in the surviving pieces. It may not be possible directly on these fired
pieces. There is a way. Mock up the
black and white in the same way as the surviving pieces. Put this on a larger clear piece and fire in the
normal way. This enables you to see stress in opalescent layups. If there is any,
it is revealed on the clear by using polarising filters.
The usual
recommendation is to anneal and cool as for twice the thickness was followed in
this firing. It is important to anneal
and cool more conservatively in cases of contrasting colours. Strongly
contrasting colours and styles (low viscosity transparent and high viscosity
white opalescent) require more time at annealing and need slower cooling. I do that by using a schedule for one layer
thicker than calculated. In this case,
as for 15mm/0.61” (two tack layers needs firing as for four tack layers, plus one
extra for the high and low viscosity combination).
Viscosity
The reasons for
this are viscosity:
·
Annealing is done at a temperature that achieves
a viscosity of around 1013.4 poise. It can be done in a range from
there toward the strain point of 1014.5 poise. Below the strain point temperature (which is determined
by the viscosity), no annealing can occur.
The glass is too stiff. The
closer to the strain point that the annealing is done, the more time is
required at the annealing temperature.
·
The annealing of Bullseye is already being done
in the lower range of viscosities. It is possible the viscosity of the white is
so high as to be difficult to anneal with the usual length of soak.
·
Although I do not know the exact viscosities of
dense white and soft black at the annealing temperature, it is known white has
a higher viscosity than the black. The
means to achieve less stress in the glass is to hold at the annealing
temperature longer than normal. A cooling
schedule related to the length of the anneal hold is needed. This information can be obtained from the
Bullseye chart for annealing thick glass. The rates and times apply to all soda
lime glass, which is what fusing glass is. Only the temperatures need to be
changed to suit the characteristics of your glass.
Slumping
The slumping of
this combination of high and low viscosity glasses requires more care too. My research has shown that the most stress-free
result in slumping is achieved by firing as for one layer thicker than that
used for the fuse firing. For a tack fuse,
this means firing for twice the thickness, plus one more layer for contrasting
colour and style. Then schedule the
slump by adding another layer to the thickness. This means scheduling as for 19mm/0.75"instead
of as for 12mm/0.5”. This is to account
for profile, contrasting colours, and stress from slumping. This is about three times the actual height
of the piece.
Slumping tack fused
pieces of contrasting colours requires very cautious firing schedules. These longer schedules need to have a
justification. It is not enough to add
more time or slow the cooling just in case.
Excessively long anneal soaks, and slow cools can create another set of
problems.
More viscous glass
needs more time at the annealing soak to an even distribution of temperature between the more and less viscous glasses.
More information about other low temperature processes can be found in my eBook Low Temperature Kilnforming. Available from Bullseye and Etsy
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