(a) Electro-metallurgists do not write in favour of the electro-deposition of lead, viewing it from a commercial point of view. But some persons view the operation with interest, as a means of coating iron with lead to resist the attacks of acid. Following are the results of some experiments with the only promising solution of lead at command, and the course of reasoning which led up to them: -
Reviewing the scanty information obtainable on the subject, and the solutions used by other experimentalists, we were led to abandon all thought of using an acid solution, because the free acid liberated at the cathode would have a tendency to attack both the deposited metal and the metal on which it was deposited, and in this way cause a loose deposit. A similar reason led us to reject a solution of the double cyanide of lead and potassium. Knowing that caustic soda has no action on iron, but will readily dissolve an anode of lead, we decided to use a solution of soda plumbite prepared as follows: Dissolve 1/4 lb. best caustic soda in 1/2 gal. distilled water made to boil in an iron saucepan; when the caustic alkali is all dissolved, add 1/2 oz. litharge, and stir well until all the litharge is dissolved. Allow the liquid to cool; carefully decant it from the saucepan into a stoneware vessel, and add 1 qt. more distilled water, when the solution will be fit for use. The iron to be coated with lead must be freed from rust and scale by filing, and then scouring with sand and water until bright. It must then be put at once into the depositing solution.
Attached to the wire leading from the zinc element of the battery, a sheet of lead, slightly larger than the article to receive the deposit, must be suspended in the solution from the wire leading from the copper element of the battery, and care must be taken to have the battery elements larger in surface exposed to the liquid than that of the lead anode.
Deposition must be allowed to proceed slowly, and it will be found that an E.M.F. of 1 volt, or the current from one Daniell cell alone, will be amply sufficient to decompose the solution, and throw down a reguline adherent coat of lead on the iron.
On raising the E.M.F. to 1 1/2 volts a slightly porous imperfectly-adherent deposit was obtained, whilst 2 volts gave a crystalline non-adherent deposit. The crystals formed with great rapidity on increasing the force of the battery, and quickly resolved themselves into large -granular lumps when the force had been raised to 12 volts, whilst at the same time hydrogen was given off in abundance from the anode in a mass of frothy foam. It will thus be seen that the necessary conditions to ensure a reguline deposit of lead upon iron may be obtained from a constant battery of low E.M.F., such as the Daniell, operating on a solution of soda plumbite with an anode of sheet-lead. Should the deposit show a tendency to form in crystals, it will be well to take the articles out of the solution, rinse them in water, and burnish down the crystals with a burnisher of polished steel, after which the articles may be replaced in the solution to receive a thicker coat. A similar operation must be performed when the required coat has been deposited, and thus a smooth surface secured before the lead becomes oxidised.
(6) After the soda plumbite bath has cooled off, the metallic or metallised article, connected with the positive pole, is dipped into it. Then the platinum wire, communicating with the negative pole, is gradually introduced into the liquor without touching the article, which is immediately coloured in various shades; too much intensity in the current will hide all the various tinges under a uniform dark brown coat. When an article is unsatisfactory in its colouration, dip it rapidly into aquafortis, to dissolve the lead oxide, and restore the metallic surface to its primitive state. This process may be used for the decoration of stoneware and porcelain previously coated with platinum.
(c) Prof. Emerson Reynolds thus describes one of the best methods of applying his new process of galenising, or covering with lead various substances. Take 16 grm. solid sodic hydrate (NaOH), dissolve it in 1.75 litres water and add to the liquid 17 grm. lead nitrate (Pb2NO3) with 250 cc. water; raise the temperature of the mixture to 194° F. If sufficient lead salt has been added, the liquid will remain somewhat turbid after heating, and must then be rapidly strained or filtered through asbestos or glass-wool, into a convenient vessel. The filtered liquid is then well mixed with 100 cc. hot water containing in solution 4 grm. sulpho-urea or thio-carbamide. If the temperature of the mixture be maintained at about 168° F., deposition of galena in the form of a fine adherent film or layer quickly takes place on any subject immersed in or covered with the liquid, provided the object be in a perfectly clean condition and suitable for the purpose. When the operation is properly conducted, a layer of galena is obtained, which is so strongly adherent that it can be easily polished by means of the usual leather polisher.
It js not necessary to deposit the galena from hot liquids, but the deposition is more rapid than from cold solutions.
The most convenient solution for deposition on brass is thus prepared. Take a quantity of soda lye containing 1 1/2 oz. real soda (NaOH); dissolve in this, with the aid of heat, 3 oz. leal tartrate, and just before diluting the solution to 1 gal. cold water, add 5 dr. sulpho-urea previously dissolved in a small quantity of hot water. The articles are to be immediately immersed in this bath, and the temperature raised to boiling. When the desired tint is obtained, the articles are to be removed, washed, and polished. The above solution can be used for glass or porcelain, hot or cold, if the proportion of alkali bo reduced one-third or thereabouts.