Glass
always wants to be 6mm thick
This is true
only at some temperatures.
The surface
tension or viscosity of the glass, together with gravity determines the extent
to which the glass will thicken or thin.
The viscosity of glass is such that at high temperature tack and full
fusing heats, the glass does tend to become 6mm - 7mm thick. This is taken
advantage of in kiln forming to obtain rounded edges, and in making frit balls.
A single layer of frit up to about 10mm
will become a round dome due the action of the viscosity and weakness of
gravitational forces acting on a small mass.
Larger pieces
of single layer glass begin to shrink as the viscosity is great enough to overcome
gravitational forces to allow thickening at the edges. This causes
dog-boning. At the same time the glass is thickening at the edges, it is thinning in the interior allowing large bubble formation on thin pieces. It also is
the cause of the needle points on thinner pieces at higher temperatures. The glass is soft enough to conform to any
imperfections in the surface and so be stretched thin as the main mass of the
glass contracts.
This contraction also applies
to low mass items such as frit in casting moulds. The glass particles contract to form a single
mass of material, leaving some stuck to the mould. These pieces may be
completely separate as tiny frit balls, or if attached to the main mass, a
series of needle points on the edge of the finished piece.
However, the
viscosity at full fuse temperatures is not great enough to keep thicker glass
in its original shape. So the effect of
gravity on glass of 9mm or thicker overcomes the weakening viscosity force and the stack
begins to expand. The extent of the expansion is the result of both viscosity
(heat dependent) and gravity (mass dependent).
At lower
temperatures, the viscosity is much greater.
This can be used for low temperature tack or laminating temperatures.
The glass can be adhered with heat without distortion of the single layer, as
the viscosity is so high the glass does not change shape, even retaining sharp
edges, although stuck together.
At temperatures
above full fuse the viscosity decreases further allowing the glass to flow. This is used in casting, blowing, and various
higher temperature processes, such as aperture melts and stringer formation. Here the viscosity is low enough to allow
gravity to make thin and elongated shapes.
There is a
range of temperature above which glass will thin more than the 6mm – 7mm “rule”. I do not know the exact correlation between
temperature and thickness, but at around 1150°C the glass will become only a
little under one mm thick. This can be
seen from the results of kiln runaways. The glass that is melted onto the
surface of the shelf is extremely thin, showing that the viscosity was so low
that gravity was able to thin it to a fraction of what we think of as normal thicknesses.
More information is in the e-book: Low Temperature Kilnforming.
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