These are produced either by simple dipping or by galvanic methods.
Copper deposits by dipping are seldom practised except upon iron, and are generally wanting in lasting qualities, since, from the thinness of the deposit, the iron is not protected against atmospheric influences.
(a) If the iron is steeped in a solution of copper sulphate, 3 1/2 oz.; sulphuric acid, 3 1/2 oz.; water, 1-2 gal., for a short time, it becomes covered with a coating of pure copper, having a certain adhesion; but should it remain there for a few minutes, the deposit of copper is thicker and muddy, and does not stand any rubbing. In this case, compress it by means of rollers or a draw plate, in order to impart a certain cohesion to the particles of copper.
(b) Small articles, such as hooks, pins, or nails, are coppered by jerking them about for a certain time in sand, bran, or sawdust impregnated with the Above solution, diluted with three or four times its volume of water.
(c) Clean the iron by rubbing with cream of tartar, and afterwards with charcoal powder; place it in a bath of spirits of salts, diluted with 4 times its volume of water, in which a few drops of bluestone have been placed. After a few minutes have expired, withdraw the iron and rub it with a piece of cloth, then replace it in the solution, adding another portion of bluestone. By following out this plan, and adding at each immersion a new supply of blue-stone,, the layer of copper may be increased at pleasure. Lastly, introduce the coated iron into a solution of soda, then wipe clean and polish with chalk. The coating thus obtained will be firm and durable.
(d) Iron can be coppered by dipping it into melted copper, the surface of which is protected by a melted layer of cryolite and phosphoric acid. The articles to be coppered must be heated to the same temperature as the melted copper.
(e) Dip into a melted mixture of 1 part chloride or fluoride of copper, and 5 or 6 parts cryolite, and a little barium chloride. If the article when immersed is connected with the negative pole of a battery, it hastens the process.
(f) Dip in a solution of copper oxalate and soda bicarbonate, dissolved in 10-15 parts of water, acidified with some organic acid.
(g) The surfaces are cleaned with a brush and hydrochloric acid,, and the objects are then left in water slightly acidified; they are afterwards placed in a bath composed of 25 grm. copper oxide, 170 grm. hydrochloric acid, 1/4 litre of water, and 1/2 litre of alcohol. A regular deposit of copper takes place on the surface of the iron, the rapidity depending upon the proportion of the of copper sulphate, the zinc or the iron is dissolved, and the copper is deposited upon the silver. When the article is intended to be gilded or silvered, it is immediately passed through the solution of nitrate of binoxide of mercury, rinsed in cold water, and placed in the electro-baths, without drying or scratch-brushing.
The method of coating iron with copper in such a manner that the film of deposited metal shall give the appearance of iridescence is thus described by Dr. Weil: - 35 parts crystallised cupric sulphate, or an equivalent amount of any other salt of copper, are precipitated as hydrated oxide' by means of caustic soda; the oxide is added to a solution of 150 parts Rochelle salt, and dissolved in 1000 parts water. To this 60 parts best caustic soda, containing about 70 per cent. NaO, is added, when a clear solution of copper will be formed. Other alkaline tartrates may be substituted for the Rochelle salt above mentioned, or even tartaric acid may be employed, but in the case of tartaric acid or acid tartrates a small additional quantity of caustic alkali must be added, sufficient to saturate the tartaric acid or acid tartrate. Oxide of copper may also be employed precipitated by means of hypochlorite, but in all cases the proportions between the copper and the tartaric acid should be maintained as above, and it is advantageous not to increase to any notable extent the proportion of the caustic soda.
The object to be coppered is to be cleaned with a scratch brush in an alkalino-organic bath, attached to the cathode, immersed in the coppering bath, and treated with the usual precautions, when it will become rapidly coated with an adherent film of metallic copper. As the bath gradually loses its copper, copper oxide as above prepared should be added to maintain it in a condition of activity, but the quantity of copper introduced should never exceed that above prescribed as compared with the quantity of tartaric acid the bath may contain. If the quantity of copper notably exceeds this proportion, certain metallic irisations are produced on the surface of the object. These effects may be employed for ornamental and artistic purposes. According to the time of the immersion, the strength of the current, and the proportion of copper to the tartaric acid, these iridescences may be produced of different shades and tints, which may be varied or intermingled by shielding certain parts of the object by an impermeable coating of paratfin or varnish, while the iridescent effect is being produced on the parts left exposed.
The use of cyanide baths for plating on zinc has the double disadvantage of being poisonous and expensive. Hess overcomes the objections by rendering the cyanide bath unnecessary. This he accomplishes by the use of an organic salt of copper, for instance a tartrate. Dissolve 126 grm. copper sulphate (blue vitriol) in 2 litres water; also 227 grm. potash tartrate and 286 grm. crystallised soda carbonate in 2 litres water. On mixing the two solutions a light blueish-green precipitate of copper tartrate is formed. It is thrown on a linen filter, and afterwards dissolved in 1/2 litre caustic soda solution of 16° B., when it is ready for use. The coating obtained from this solution is very pliable, smooth, and coherent, with a fine surface, and acquires any desired thickness if left long enough in the bath. Other metals can also be employed for plating in the form of tartrates. Instead of tartrates, phosphates, oxalates, citrates, acetates, and borates of metals, can be used, so that it seems possible to entirely dispense with the use of cyanide baths.