Capping with a clear
or tinted top layer is necessary in many cases of inclusions, or desirable when
looking for depth or distortion in flat fused work.
Capping inherently
has bubble creation potential. The
development of a bubble squeeze helps prevent the largest of bubbles. It cannot eliminate all the trapped air that
then turns into small bubbles around the inclusions or multiple pieces when
covered by a sheet of glass.
An alternative is to
do away with the sheet glass capping and instead use enough frit to provide the
desired depth, or the necessary material to cover the inclusion. In fusing with two large sheets, a fine
covering of powder between the layers will help to eliminate bubbles. However, this will not be enough to
successfully cover metal or other inclusions, or provide the amount of glass to
give an appearance of depth.
The size of frit to
use in a given application can be determined from other styles of glass
working. It is known from glass casting that the smaller the frit the greater
number of small bubbles will appear in the fired piece. This means that you need to use medium sized
frit for cast work. Fine frit is likely
to produce many very small bubbles across the whole piece in fusing
applications. Large frit is likely to
produce larger bubbles, as the pieces themselves trap air as they deform. This means that medium frit is a good
compromise between large and small bubbles in capping.
The layer of frit
should be at least 2mm thick. This means
a lot of frit is required to do the job.
To judge the amount, you can measure the area of a rectangle or circle
in square centimetres and multiply that by 0.2 to give you the volume (in cubic
centimetres) of frit required.
Multiplying the volume by 2.5 (the approximate specific gravity of soda
lime glass) will give you the weight of frit needed to cover the area.
Alternatively, if
the piece is irregular, you can weigh the base and add the appropriate weight
of frit on the top. If the base is 2mm,
no further work is required to determine the weight. Weigh the 2mm sheet and use
the weight of frit to equal the base. If
the glass is 3mm, you need two thirds of the weight in frit, and so on for
thicker glass.
Using frit to cap is
unlikely to eliminate all bubbles, but it will reduce them to a minimum.
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