The zincs should be taken from the battery liquids when not required for use; and the porous cells should be removed every night and their contents poured into a large jug kept for the purpose. The porous cells should be placed in clean water to prevent the salts of the battery liquid from crystallizing in the pores, which would crack and spoil them for future use; the carbons should also be placed in water; and when required for use again these arrangements should be reversed, when the battery will work as well as ever.
The solution for electro-plating articles of jewellery is the next part of the subject we have to consider, of which there are many, containing various proportions of the metal employed in silver depositing. The following is one of the best we have employed: take pure silver and dissolve it in a mixture of nitric acid and water:—
| Fine silver | 5 dwts. |
| Nitric acid | 4 drms. |
| Water | 2 drms. |
The silver should be put into a small Florence flask, so as to allow the mixture of acid and water to cover it thoroughly; this mixture on being added to the metal soon promotes a chemical action, and the silver becomes gradually dissolved. If the acid employed has been weak, it will necessitate a further addition of it to complete the dissolution of the silver, or the removal of the flask to some other and warmer place, such as a sand-bath, but care should be taken not to apply too much heat. As the chemical action proceeds, red fumes are formed in the flask; and the action should be allowed to go on until these cease to rise, when the silver should by that time have become dissolved. The mixture then consists of a solution of nitrate of silver, and should be carefully poured into a suitable porcelain or Wedgwood capsule; this is then heated upon a sand-bath until a scum or pellicle appears upon the surface, when it should at once be transferred from the sand-bath to a suitable place for cooling. During the last operation the mixture begins to form itself into crystals, and the liquid which appears reluctant to crystallize should be poured away from those already formed, into another capsule, and again heated until it has sufficiently evaporated to crystallize. When the whole of the liquor has finally undergone this process, the crystals of nitrate of silver must be removed to another vessel, and about one pint of cold water added, the whole being then well stirred until they have become thoroughly dissolved. A solution of cyanide of potassium is next prepared in water, in the proportion of about one ounce of cyanide to the pint of water; some of this solution is then added to the one containing the nitrate of silver. It must, however, be added very cautiously, for precipitation soon takes place, and if too much be used, the precipitate becomes again dissolved. For this reason it is advisable to take a little of the solution from the vessel, in a wine-glass or test tube, and to add a few drops of the prepared solution of cyanide, in order to ascertain its exact state. If the application of this solution produces no effect upon the nitrate of silver, the operation of precipitation is complete.
The liquid above the sediment should next be carefully poured away to avoid any waste of silver; when this is done fresh water should be added, well stirred with the sediment, and allowed to settle; it is then again poured off, and the process repeated until the precipitate has become thoroughly washed. Now add sufficient cyanide of potassium to dissolve the precipitate and a little more, and make up two quarts of solution with fresh clean water. It is better to filter the solution before using it.
The solution may be made by means of the battery, and, if preferred, the above mode of chemically preparing it may be dispensed with. The following is the most simple method by the battery process:—Dissolve in two quarts of water about half an ounce, and no more, of best black cyanide; this should be done in an oval, or still better, oblong stoneware vessel, placed in an iron one of the same shape containing water. The stone jar should not be allowed to touch the bottom or sides of the iron one, a space being left for holding the water. When the cyanide has become dissolved, fill a porous cell with some of the solution, place this cell in the other vessel containing the cyanide; the solution should be about the same height in both vessels. Now put a piece of sheet copper, secured to the end of the wire issuing from the zinc of the battery, into the porous cell, and place in the larger vessel containing the cyanide solution about one ounce of sheet silver, properly secured to the wire issuing from the carbon of the battery. In a short time the solution in the larger vessel will have acquired the right proportion of silver (5 dwts. to the two quarts) for use; when this has been effected, the porous cell should be removed and its contents thrown away. These solutions are both worked hot, at a temperature of not exceeding 120° Fahr. with the battery we have described.
The solutions are heated by means of gas-jets, and the articles are plated by being suspended to the wire proceeding from the zinc of the battery. To the wire proceeding from the carbon is to be attached the piece of sheet silver which dissolves and keeps up the strength of the solution. The piece of silver or anode being lowered into the solution, upon the immersion of the work, an almost instantaneous deposit of fine silver takes place, the thickness of which depends entirely upon the period of immersion.
When a solution begins to plate of an inferior kind through the acquisition of organic matter, it will be better to abandon it altogether and make a new one, rather than to waste valuable time in repeated attempts at improvement, which seldom can be effected in solutions that have been employed for all kinds of work. The silver may be recovered from such solutions by means of the battery, by precipitation, and by evaporation; the first process, however, we have not always found successful, the solution in some cases refusing to give up its silver to the action of the battery. It is put into operation as follows:—The anode which supplies the solution with silver is replaced by one of platinum, on which the cyanide solution has no action whatever; a piece of clean sheet copper should be hung upon the zinc wire of the battery, and the battery kept in action until the whole of the silver held in the solution has become deposited upon it; at which stage it may be removed, and the exhausted solution thrown into the waste water tub. The piece of copper containing the silver may be used again in the place of the silver anode until it has become dissolved, or it may be removed by any other means, if preferred. In the event of the above plan failing, the process of precipitation or evaporation should be resorted to. If the former one be adopted, the solution should be poured into a large open vessel, and considerably diluted with water; sulphuric acid should then be carefully poured in, a little at a time, until it produces no effervescence. The sulphuric acid precipitates the silver, and the fumes which it creates are highly deleterious to health, therefore the process ought to be performed in the open air, and not in ill-ventilated workshops. When the sulphuric acid produces no effect upon the solution, it should be allowed to stand for a while for the precipitate to subside, when the water above (which should be clear) may be drawn off, the precipitate well washed, to free it from the acid, dried, and fused in a crucible with a little potash or soda. If the latter plan be adopted, the solution may be placed in a cast-iron kettle or saucepan, and then heated upon a gas-jet or stove, until evaporation takes place, after which the sediment should be removed and fused in a crucible, as before.