The
desire is to have the same degree of fusing on both sides of the glass. An example is where a person wants to have
their colourline paints equally matured on both sides of the glass in one
firing. This is difficult and requires a
different strategy than normal fusing.
Background
A
bit of background first. Glass is a very good insulator. This means that heat
travels slowly through the glass. Its practical effect is that we have wavy
lines on the top and very crisp lines on the bottom. This results from the temperature
differential between the two surfaces.
This can be many degrees different during the plastic phase of the
glass. It is dependent on how fast the
temperature rise is. The faster the rise
in temperature, the greater the difference as the glass transmits the heat from
top to bottom so slowly. The problem is
how to keep the temperature differential as small as possible.
Heat Work
The
concept of heat work relates to the way heat is put into the glass. It can be done quickly to a high temperature,
or slowly to a low temperature and still get the same effect. This shows glass reacts to the combination of
temperature and time. Putting heat into the glass slowly allows lower
temperatures to be used to achieve the desired effect, than fast rises in
temperature.
The
insulating properties of glass means that the heat work needs to be applied
slowly to achieve similar temperatures on both sides of the glass. The thicker the glass the longer it will take
to temperature equalisation.
The
mass of materials also needs to be considered.
The glass will normally be on a ceramic shelf of 15mm to 19mm. This mass also needs to heat up to the
temperature of the top of the glass. Until it does, it will draw heat from the
glass. This also points to the need for
slow heat input.
The
question that prompted this note was how to get glass strainers paints to have
the same degree of maturation on both sides at the same time. The maturation temperature of Reusche tracing
paints is around 650°C. If you use a
normal rate of advance – say, 200°C – the bottom of the glass will be
considerably cooler than the top. This
is both because of the insulating properties of the glass and the mass of the
shelf.
Methods to achieve the effect.
Some
methods are worthy of consideration separately or in combination.
Use refractory fibre board as shelf. This dramatically reduces the mass of the
shelf to be heated up. This kind of
shelf requires more care to avoid damage than a ceramic shelf. It would be possible to place smaller fibre
shelves on top of the standard ceramic shelf rather than having one large fibre
board shelf. This will not be so
efficient an insulating mass as fibre board on its own. Also, it will not be sufficient on its own to
obtain equal temperatures on both sides of the glass.
Use 3-6mm refractory fibre paper between
shelf and glass. This
again reduces the heat sink effect of the ceramic shelf, but not as much as a
fibre shelf on its own. Again, the fibre
paper on its own is not enough. The scheduling is important.
Use very slow rates of advance. A slow rate of advance in temperature is
important to achieving equal temperatures throughout the glass. Even using 3mm glass, the rate of advance
might need to be as slow as 50°C per hour.
The corollary of this is that you will not need to use as high a
temperature to achieve the effect. Heat
work means that it is not an absolute temperature that will achieve the
effect. The slower you put the heat into
the glass the lower temperature required.
The understanding of this relationship will require experimentation to
establish the relationship to the rate of advance and the top temperature
required. For example, a satin polish of
a sandblasted surface can occur at 650°C, if held there for 90 minutes.
In
this case, a 50°C rate of advance will probably not require more than 600°C – and
probably less - to achieve the shiny surface normally achieved at 660°C with a
200°C rate of advance. At 50°C per hour,
it will take 12 hours to reach 600°C, although a little more than 3.25
hours at an advance of 200°C to reach 660°C. The input
of heat acts upon the glass throughout the process, making lower working
temperatures possible. The reduction in
temperature required is not directly related to the reduction in the rate of
advance. You will have to observe during
the experimental phase of this kind of process.
If
it was desired to fire enamels that mature at 520°C to 550°C you could put the
sheets in vertical racks to allow the heat to get to both sides equally as Jeff
Zimmer does. But this will only work
for very low temperatures and for quick firings, otherwise the glass will begin
to bend.
There
are limits to this strategy of getting upper and lower surfaces to the same
temperature, both in terms of physics and practicality. There are temperatures below which no amount
of slow heat input will have a practical effect, for example, due to the brittle
nature of the glass. Even where it is
possible, it can take too long to be practical.
For example, I can bend float glass at 590°C in 20 minutes into a 1/3
cylinder. I could also bend it at 550°C (just
10°C above the annealing point), but it would take more than 10 hours – not
practical.
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