There are a number of people firing
stacked layers of glass in a pyramidical fashion to melt the layers down, in a
nod to the 1950’s.
Annealing of a full fused platter of
this nature is easier than a tack fused one. The degree of contour still in
evidence will be important in determining what the annealing schedule should
be.
Full or
Tack Fuse
In this example shown by Vicki Urbich there are at least four layers - five if the base
is two layers, although in this case there is only one. A full fuse at 800°C will not be enough to
give a flat piece for a trivet or platter.
You could extend the soak at 800°C but, it is better to go to 816°C for
ten minutes rather than an extended soak at lower temperatures to avoid
devitrification.
Damming
The
edges of this piece will be wavy, unless dammed, because of the uneven
layering. Placing dams around will give
crisp edges to the piece, even though the stacked pieces will round and spread.
Annealing
stacked pieces
Anneal this set-up for at least 9mm for a full
fused piece. The pieces will spread and attempt to fill the gaps between the
stacks. Even with an 816°C fuse, the
pieces will not be perfectly even in thickness.
To be safer, and account for the remaining unevenness, anneal as though
it were 12mm thick. Other lay-ups will have slightly different requirements.
If it is to be tack fused, you will need to
anneal considering the different thicknesses across the piece. You will have nearly 12mm thick at the
thickest and only 3mm at the thinnest.
The generally accepted recommendations are to anneal for twice the thickest part - 24mm in this
case.
The anneal is more than the length of
soak. It is a combination of the soak and the rate at which you cool to at
least 370°C. The cooling rate is directly related to the length of the
soak. If you require twice the length of
soak at the temperature equalisation soak, you will require half the speed of
anneal cool. The Bullseye Chart forAnnealing Thick Slabs will give you the relevant rates
regardless of the glass you are using. The temperature points will change for
other glasses, of course, but the rates remain the same.
Rate of
Advance
The earlier problem this lay-up gives
you is the heat up to avoid thermal shock.
The heat up of 4 layers of glass stacked on a
single or even two-layer layer base is more difficult than for even layers
across the whole piece. Each upper piece shades the heat from the lower ones,
making for cool and hot areas next to each other. With four layers, each layer needs to heat
through to transfer its heat to the one below.
This means the bottom of the stack will take a long time to become as
hot as the top layer. Meanwhile, the
uncovered glass is getting as hot as the top of the stack. This often leads to the bottom layers
breaking from the stress of the uneven heating.
Graham Stone suggests 100°C
(180°F) per hour for four, even layers. As this is four uneven layers, the rate
of advance should be at least half that. This should be used all the way up to
the bottom of the bubble squeeze to allow all the glass to heat at the same rate. Glass generally reacts better to a slow, steady contant rate of advance in heat, than faster rates with multiple soaks.
Bubble Squeeze
The bubble squeeze for this single layer base piece can be as quick as 50°C
per hour over the 50°C range. It does
not need to be slower, as the weight of the stacks pushes the air out between
layers more quickly than large, even and lighter layers. A double layer base requires a slower bubble squeeze because the weight of the stacks will push the air out to be between the two base layers. This means a rate of 30°C or even 25°C through the range.
Then you can go faster to the top
temperature.
Firing uneven layers requires extreme
care on the initial heat up to avoid thermal shock. A high fusing temperature is needed to get an
even thickness across the piece.
Annealing is easier to calculate for even pieces, but must be much more
cautious for tack fused items.
More detailed information is available in the e-book: Low Temperature Kilnforming.
