This
term most often refers to placing a single piece of glass over the
whole of the project. The decisions relate to whether to do it at
all, in what circumstances and in what order. Whatever you place on
top of the project is what the eye will first see. A tinted top
layer will give that tint to all the pieces making up the object. So
most often the top is a piece of clear glass.
Many
times the purpose of capping is to give the volume of glass required
to keep the piece contracting as a result of the surface tension of
the glass trying to pull itself up to 6mm thickness.
When
using opalescent glass as the main component in the work, you should
consider capping with clear. Opalescent glass is slightly more prone
to devitrification than transparent glasses, so any work to be fired
a number of times might be best fired with a clear cap. It also
protects against any bubble formed between the other glass and the
cap showing as a clear spot within the opalescent as it pushes the
colour aside and reveals the clear below.
There
are some times when you should consider placing the clear on the
bottom. If your design layer is made up of lots of pieces where air
might be trapped, but is uneven enough to be the likely cause of
bubbles, then the clear should go on the bottom to ensure there is
sufficient volume. An alternative is to do a high tack or full fuse
of the whole upside down on fibre paper, then clean up and fire right
side up with the capping glass.
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