ax cannot be filed off, a very convenient instrument for its
removal by scraping is the watchmaker's graver, a square rod of hard
steel ground to a bevelled point (Fig. 80).

Fig. 80.

Several precautions require to be mentioned. In the first place,
spelter is merely rather soft brass, and consequently it often cannot
be fused without endangering the rest of the work. A good protection
is a layer of fireclay laid upon the more delicate parts, such for
instance as any screwed part.

Gun-metal and tap-metal do not lend themselves to brazing so readily
as iron or yellow brass, and are usually more conveniently treated by
means of silver solder.

Spelter tends to run very freely when it melts, and if the brass
surface in the neighbourhood of the joint is at all clean, may run
where it is not wanted. Of course some control may be exercised by
"soiling" with fireclay or using an oxidising flame; but the erratic
behaviour of spelter in this respect is the greatest drawback to its
use in apparatus construction. The secret of success in brazing lies
in properly cleaning up the work to begin with, and in disposing the
borax so as to prevent subsequent oxidation.

Sec. 100. Silver Soldering.

This process resembles that last described, but instead of spelter an
alloy of silver, copper, and zinc is employed. The solder, as
prepared by jewellers to meet special cases, varies a good deal in
composition, but for the laboratory the usual proportions are:

For soft silver solder

Fine silver 2 parts
Brass wire 1 part

For hard silver solder

Sterling silver 3 parts
Brass wire 1 part

The latter is, perhaps, generally the more convenient.

Silver solders may, of course, be purchased at watchmakers' supply
shops, and as thus obtained, are generally in thin sheet. This is
snipped fine with a pair of shears preparatory to use.

As odds and ends of silver (from old anodes and silver residues)
generally accumulate in the laboratory, it is often more convenient to
make the solder one's self. In this case it must be remembered in
making hard solder by the second receipt that standard silver contains
about one-twelfth of its weight of copper--exactly 18 parts copper to
220 silver.

The silver is first melted in a plumbago crucible in a small furnace
together with a little borax; if any copper is required this is then
added, and finally the brass is introduced. When fusion is complete,
the