For the alkaline platinate bath, the following directions may suffice: - Platinic hydrate, 2 oz.; caustic potassa (or soda), 8oz.; distilled water, 1 gal. Dissolve one half of the caustic potassa in 1 qt. distilled water; add to this the platinic hydrate in small quantity at a time, facilitating solution by stirring with a glass rod. When solution is effected, stir in the other half of alkali dissolved in 1 qt. water; then dilute with enough distilled water to form 1 gal. of solution. To hasten solution, the caustic alkali may be gently heated; but this is not necessary, as the platinic hydrate dissolves very freely. This solution should be worked with a current of about two volts, and will yield metal of an almost silvery whiteness upon polished surfaces of copper and brass, and quite freely. There should be slight, if any, perceptible evolution of hydrogen at the cathode, but a liberal evolution of oxygen at the anode. The addition of a small portion of acetic acid to this bath improves its operation where a heavy deposit is desired. The anode may be of platinum or carbon, and owing to the readiness with which the metal is deposited, an excess of anode surface is to be avoided.
Articles of steel, nickel, tin, zinc, or German silver will be coated with black herent, on copper and brass. Other metallic objects must receive a preliminary coppering as above. The deposited metal is dense, with a steely appearance, and can be obtained of any desired thickness.
The deposit obtained in the oxalate baths is sensibly harder than that from the alkaline platinate bath, and will bear buffing tolerably well.
The phosphate bath may be prepared by the following formula: and more or less non-adherent platinum; but by giving objects of these metals a preliminary thin electro-deposit of copper in the hot cyanide bath, they may be electro-platinised in the alkaline platinate bath equally as well as copper. The bath may be worked hot or cold; but it is recommended to work it at a temperature not exceeding 100° F. It may be diluted to one-half the strength indicated in the formula, and still yield excellent results. The surface of the objects should be highly polished by buffing or otherwise prior to their introduction in the bath, if the resulting deposit is designed to be brilliant.
The deposition of platinum takes place promptly. In 5 minutes a sufficiently heavy coating will be obtained for most purposes. The deposited metal is so soft, however, that it requires to be buffed very lightly. A heavier deposit will appear grey in colour, but will accept the characteristic lustre of platinum beneath the burnisher.
The oxalate solution is prepared by dissolving 1 oz. platinic hydrate in 4 oz. oxalic acid, and dilutiug the solution to the volume of 1 gal. with distilled water. The solution should be kept acidified by the occasional addition of some oxalic acid. The simplest plan of using this bath, and which requires no attention to proportions, is simply to work with a saturated solution of the oxalate, keeping an undissolved excess always present at the bottom of the vessel. An addition of a small quantity of oxalic acid now and again will be found advantageous. The double salts of oxalic acid with platinum and the alkalies may be formed by saturating the oxalate of the desired alkali with platinic hydrate, and maintaining the bath in normal metallic strength by the presence of an undissolved residuum of platinous oxalate.
The double oxalates are not so soluble n water as the simple salt. The oxalate baths, both of simple and double salts, may be worked cold or hot (though not to exceed 150° F.), with a current of comparatively low pressure. The metal will deposit bright, reguline, and ad- Phosphoric acid, syrupy (sp.gr. 1.7).. .. 8 oz. Platinic hydrate . 1-1 1/2 oz. Distilled water 1 gal.
The acid should be moderately diluted with distilled water, and the solution of the hydrate effected at the boiling temperature. Water should be added cautiously from time to time, to supply that lost by evaporation. When solution has taken place, the same should be diluted with sufficient water to make the volume 1 gal. The solution may be worked cold or warm to 100° F., and with a current much stronger than that required for the platinates and oxalates. The ammonio-(and sodio-) platinic phosphates may be formed from the simple phosphate by carefully neutralising the solution of the phosphate with ammonia (or soda); then adding an excess of phosphoric acid, or enough to dissolve the precipitate formed, and an additional quantity to ensure a moderate amount of free phosphoric acid in the bath. The phosphate baths will be maintained of normal strength by additions of platinic hydrate, the solutions of which will have to be assisted by heating the bath, preferably at the close of each day's work. The metal yielded by the electrolysis of these phosphate solutions is brilliant and adherent. It has the same steely appearance as that exhibited by the oxalate solutions, but to a less pronounced degree.
The physical properties of the deposited metal are in other respects like those described in connect tion with that obtained from the oxalate baths.
The following method of depositing brilliant films of platinum on glass was described recently at a meeting of the London and Provincial Photographic Association, when samples were shown. The method is that devised by Prof. Bottger. In order to succeed in coating porcelain or glass with a perfectly faultless film of platinum, of the brilliancy of silver, it is indispensably requisite to make use of perfectly dry platinum chloride, which must be as free from acid as possible. To that end pour some oil of rosemary over the perfectly dry platinum chloride in a small porcelain mortar, and knead it up with the pestle, renewing the oil about three times, and continue this operation until at length there is produced from the brownish-red chloride a soft plaster-like mass, the colour of which is as black as pitch, and wherein no particles of undecomposed platinum chloride are discoverable. The oil of rosemary assumes hereby a more or less yellow colour, in consequence of its partially taking up chlorine from the platinum chloride.
When at length we have arrived at converting the whole of the platinum chloride into the black plaster-looking mass spoken of, rub it well up with the pestle, after pouring the oil of rosemary off, with about five times its weight of oil of lavender, and continue to do so until it has become a perfectly homogeneous thin fluid. It must then be left to stand for 1/2 hour or so, for it is not until after that interval that it can be used with advantage for platinising. For the production of the brilliant platinum film, all that is now required is to apply the mass as uniformly as may be, and in the thinnest possible coat, to the objects of porcelain, earthenware, or glass by means of a soft delicate brush. The thinner the coat of the above described preparation, the more brilliant the film of platinum subsequently proves. When, in fine, the articles have been gone over as thinly as possible with the fluid conformably with these instructions, all that is required further is to subject them for a few minutes to a very low, scarcely-perceptible red heat, either in a muffle, or in the flame of a Bunsen's gas blow-pipe used with caution.
The articles receive from this baking (supposing always that the temperature described has not been exceeded), without requiring any subsequent treatment, an incomparably beautiful lustre, as brilliant as silver. If, by any oversight, the coating of platinum upon the articles has turned out faulty, or in the case of breakages occurring during the baking, every trace of the metal may be recovered with facility, from the objects that have suffered, by means of the following very simple galvanic process, without being obliged to have recourse to the use of aqua regia. Nothing more is required than to pour common hydrochloric acid over them, and then touch them with a zinc rod. On doing this, as quick as lightning, in consequence of the hydrogen evelved both at the upper and lower surface of the film of platinum, which acts as the negative pole, we see the shining metallic coating peel off in the form of infinitely thin leaves from the base of porcelain or glass, and, notwithstanding the specific gravity of the metal, ascend partially and float on the surface of the acid. On separating the hydrochloric acid by the use of a filter, the whole of the platinum, which would be otherwise lost, is recovered, so that no complaint arises as to the waste of any of the metal in question.
Prepare at once only as much of the platinising fluid as is required for the day's use, inasmuch as it loses in efficiency by keeping. That which forms the active principle in the fluid, which results from treating platinum chloride with oil of lavender, as above described, is an organic platinum salt, which, in point of fact, one can obtain, after some time, in the form of small elongated octohedral crystals, of a pale yellowish colour, by washing out carefully with alcohol a tolerable quantity of the fluid. The crystals have the property of taking fire with a brilliant flame on being brought near a lighted candle, leaving a residue of compact platinum of dazzling whiteness.