PRECIOUS STONES. 59
and
the whole gem polished ; this is done by the polisher. But first it
must be mounted on a suitable holder by the solderer ; small brass
cups, known as " dops," of one or two inches in diameter, and having a
short stalk attached, are heated in a Bunsen flame and filled with a
solder composed of one part of tin to two parts of lead, similar to the
solder plumbers use for wiped joints in lead pipes. This alloy melts at
about 441° F. or 227° C, and has the property of being plastic at
considerably below its melting point. When the solder has attained the
plastic state by sufficient heating, and has been worked up into a cone
projecting from the hollow of the cup, the stone is embedded in the
metal at the apex of the cone in such a way that only the part of the
gem to be immediately worked on is exposed ; on cooling down the solder
contracts considerably and grips the stone very firmly. Soldering, like
other operations in gem cutting, requires great dexterity and
experience, as the stone must be fixed at the correct angle, so that
its position can be known by the position of the holder, and further,
in the case of the Diamond (or other cleavable gems), the alternate
contraction and expansion from heating and cooling may cause flaws if
the operation is not done very skilfully.
In
some cases a split clamp, tightened by a thumb-screw, is used to hold
the stones, instead of their being soldered into the dops.
The
dop, or its substitute, is now mounted in a clamp which in its simple
form consists of a bar rather less than a foot in length, having a slot
at one end; the parts of the clamp on either side of the slot can be
brought together by a bolt and nut so that the peg of the dop may be
firmly
