I have been looking for a different way than flows or melts to mix
colours and thought glass pressing might be a promising way to achieve what I
wanted.
Weight vs Temperature
I conducted some experiments attempting to thin 1.25 kg/2.75 pounds of
glass to 3-4mm. One and then two 40x40cmx15mm
thick shelves were placed on top of the glass cullet with 3mm spacers at the
corners. The glass was fired at 220ºC/396ºF to 825ºC/1517ºF and initially held
for 30 minutes, later extended to 90 minutes.
The thickness stubbornly remained between 5 and 7mm.
A few other attempts with different times and temperatures gave
inconsistent results. Perhaps the uneven
piling of cullet had an influence on the outcomes, but I was still looking for
a flow and mixing of colours different to that obtained by melts.
Other experiments were being conducted in parallel, relating to
viscosity. These indicated that glass became thinner than 6-7mm at higher
temperatures without pressing. These
experiments lead me to think there are four elements controllable by
kilnformers in pressing: size, weight, time, temperature.
The same weight of press with the
same temperature and time will make small amounts thinner than large amounts,
and this is not surprising. More time with
the same temperature, weight, and amount allows some slight decrease in
thickness.
Higher temperatures with the same
weight, and time will allow thinner pressings of the same amount of glass. Viscosity decreases with temperature, so higher
temperatures make glass easier to thin.
More weight is required get the
same thickness when pressing a greater volume of glass. Of course, more time and temperature can be
added to increase the effect of the weight.
However, the main factor in pressing large amounts of glass
is higher temperatures, which results in reducing the viscosity and the resistance to thinning.
Annealing and Cooling
An important aspect of pressing is the annealing requirements. It is sensible to anneal for a longer time
than normal for thick glass, because of the heat retention of the pressing
weights.
This image shows the stress in an 8mm/0.3” (or 5/16”) after annealing as
for 16mm/0.63” (5/8”). There is widespread
low level stress with 30mm thick pressing weight.
Indications are that extending the annealing to at least 3 times the
target thickness is a minimum annealing soak requirement. Alternatively, if it is possible to remove
some, or all, of the weight from the glass at the beginning of the anneal soak, the annealing time can be reduced.
Veiling
The stress picture above shows there is visual element too. This veiling is most apparent in clear glass, and less obvious in coloured and opalescent glass. Small volume stacks, which are pressed thin will exhibit less of the veiling.
Four factors that kilnformers can control in pressing glass to less than
6mm are weight, size, time, and temperature.
The main one is temperature.
