The art of uniting metals by another metal or alloy, is called
soldering (which includes “hard and soft soldering,” and “brazing”).
If any metal be applied in a melted state to the surface of a piece
of cold metal, under ordinary circumstances it will not adhere, but
runs off in globules, this is owing to the surface being covered with
“oxide” or rust, but if the surface be scraped or filed bright and
some substance applied which will defend it from the air, and at the
same time become fluid at the heat of the melted metal, then it will
adhere. For this purpose borax is used in hard soldering and brazing,
that is in soldering with metals which require a considerable amount
of heat to melt them; and sal ammoniac, rosin, oil, &c., in cases of
soldering with “soft solder,” or solder that will readily melt. This
soft solder is made of a mixture of lead and tin, and if required
to melt very easily (as in soldering pewter), then some “bismuth”
is added. Bismuth itself does not melt more readily than lead, but
it has the property of causing other metals to melt more readily.
If the edges of two pieces of tin, for example, have to be soldered
together, an iron with a wooden handle and a piece of copper joined
to the other end is used. This is made red-hot, and the pieces of
tin being placed smoothly together and their edges sprinkled with
rosin or sal ammoniac, the hot iron (first touched on a piece of
rosin to clean it) is then applied to the joint, a piece of soft
solder being applied at the same time, and as this melts it is drawn
in a melted state by means of the hot iron (to which it adheres)
down the joint. An excellent substance for soldering: all sorts of
small work, such as pieces of brass, copper, or tin, is chloride of
zinc-this may easily be made by putting pieces of zinc into spirit
of salt, (hydrochloric acid), and allowing them to remain as long as
any effervescence continues; this solution may be kept in a bottle
and applied to any edge to be soldered, by means of a small brush
or feather. When iron and copper have to be “brazed,” the joints
are made bright, and then coated with borax ground into a paste
with water. A mixture of brass and zinc (called spelter) in small
grains is sprinkled on the joint and it is then put into the hollow
of a bright fire which is urged by bellows till the spelter melts.
Silver is joined by hard or “silver” solder, which is a mixture of
silver, zinc, and copper, and the fusion is generally effected by a
blow-pipe, (see “Blow-pipe”); gold is soldered by a mixture of gold
and copper. Leaden pipes are joined by having the ends to be united
scraped bright and introduced a short way one within the other, some
melted solder is then poured from a small iron ladle on the joint at
the same time that it is rubbed round with a piece of folded cloth
greased on the surface. Joints in cisterns, etc., are generally made
by scraping the edges clean with a steel scraper, and applying some
lamp-black and size by means of a brush to the parts beyond, leaving
a bright space of an inch or so on each side of the joint, a ladle of
melted solder is then gradually poured on the joint and rubbed down
with a piece of greased cloth, the lamp-black and size preventing the
solder adhering to any part but that left bright, and in this way a
straight neat joint is produced.