PROCESSES FOR TIN-PLATING.
In these days of making everything look what it is not, perhaps the
best and cheapest substitute for silver as a white coating for table
ware, culinary vessels, and the many articles requiring such a
coating, is pure tin. It does not compare favourably with silver in
point of hardness or wearing qualities, but it costs very much less
than silver, is readily applied, and can be easily kept clean and
bright. In tinning hollow ware on the inside the metal article is
first thoroughly cleansed by pickling it in dilute muriatic or
sulphuric acid and then scouring it with fine sand. It is then heated
over a fire to about the melting-point of tin, sprinkled with powdered
resin, and partly filled with melted pure grain tin covered with resin
to prevent its oxidation. The vessel is then quickly turned and rolled
about in every direction, so as to bring every part of the surface to
be covered in contact with the molten metal. The greater part of the
tin is then thrown out and the surface rubbed over with a brush of tow
to equalize the coating; and if not satisfactory the operation must be
repeated. The vessels usually tinned in this manner are of copper and
brass, but with a little care in cleaning and manipulating, iron can
also be satisfactorily tinned by this means. The vessels to be tinned
must always be sufficiently hot to keep the metal contained in them
thoroughly fused. This is covering by contact with melted tin.
The amalgam process is not so much used as it was formerly. It
consists in applying to the clean and dry metallic surface a film of a
pasty amalgam of tin with mercury, and then exposing the surface to
heat, which volatilizes the latter, leaving the tin adhering to the
metal.
The immersion process is the best adapted to coating articles of brass
or copper. When immersed in a hot solution of tin properly prepared
the metal is precipitated upon their surfaces. One of the best
solutions for this purpose is the following:--
Ammonia alum 17-1/4 oz.
Boiling 12-1/2 lb.
Protochloride of tin 1 oz.
The articles to be tinned must be first thoroughly cleansed, and then
kept in the hot solution until properly whitened. A better result will
be obtained by using the following bath, and placing the pieces in
contact with a strip of clean zinc, also immersed:--
Bitartrate of potassium 14 oz.
Soft water 24 "
Protochloride of tin 1 "
It should be boiled for a few minutes before using.
The following is one of the best solutions for plating with tin by the
battery process:--
Potassium pyrophosphate 12 oz.
Protochloride of tin 4-1/2 "
Water 20 "
The anode or feeding-plate used in this bath consists of pure Banca
tin. This plate is joined to the positive (copper or carbon) pole of
the battery, while the work is suspended from a wire connected with
the negative (zinc) pole. A moderately strong battery is required, and
the work is finished by scratch-brushing.
In Weigler's process a bath is prepared by passing washed chlorine gas
into a concentrated aqueous solution of stannous chloride to
saturation, and expelling excess of gas by warming the solution, which
is then diluted with about ten volumes of water, and filtered, if
necessary. The articles to be plated are pickled in dilute sulphuric
acid, and polished with fine sand and a scratch-brush, rinsed in
water, loosely wound round with zinc wire or tape, and immersed in the
bath for ten or fifteen minutes at ordinary temperatures. The coating
is finished with the scratch-brush and whiting. By this process
cast-or wrought-iron, steel, copper, brass, and lead can be tinned
without a separate battery. The only disadvantage of the process is
that the bath soon becomes clogged up with zinc chloride, and the tin
salt must be frequently removed. In Hern's process a bath composed
of--
Tartaric acid 2 oz.
Water 100 "
Soda 3 "
Protochloride of tin 3 "
is employed instead of the preceding. It requires a somewhat longer
exposure to properly tin articles in this than in Weigler's bath.
Either of these baths may be used with a separate battery.