and blackens the articles before it is possible to rinse them. To obviate this inconvenience, the above acid mixture should be diluted with one-eighth of its volume of water. This mixture bears the name of "bitter water for bright dipping." Frequently, in place of dipping the objects in the "bitter water," they are put in a stoneware pot, covered with a small quantity of the cleansing liquid, briskly agitated, and the whole plunged into a large volume of fresh water, as soon as the acid has acted sufficiently, and threatens to fume. This method is not economical, since the acid is lost, but it has the advantage that the entire mass of the dipping liquid is not heated. Whitening Bath. Varnishers and the majority of gold and silver platers, in place of frequently preparing fresh baths of the above mixed acids for bright dipping, use in place of it a mixture composed of variable proportions of Old aqua fortis, Common salt, and Soot,—which is called the "whitening bath. It is prepared about as follows: Into a large stoneware vessel, a certain quantity of old aqua fortis from previous dippings ("strong pickle") is poured, to which is added twice its volume of sulphuric acid of 66°. The mixture becomes heated, and is allowed to cool off until the next day. The nitrate of copper contained in the old aqua fortis is converted into sulphate of copper, which is much less soluble, and which in cooling crystallizes upon the sides of the vessel. The liquid portion is decanted into another vessel, and then about two or three per cent. (by volume) of sea salt, and as much of calcined soot, are added. This mixture is much less active than the above described mixed acids for bright dipping, and may often replace them advantageously. The crystallized sulphate of copper may be preserved for use or sold. This whitening bath is now and then strengthened, as the operator may find it desirable, by the addition of aqua fortis or of oil of vitriol. To replace the portion used up during the day, equal volumes of old aqua fortis and oil of vitriol are added at the end of the day. The next morning the liquor is decanted, and a new crop of sulphate of copper crystals is gathered. Soot and salt in sufficient proportions are also added. In this manner a perpetual and cheap whitening bath is prepared. Some operators heat this bath, and replace the salt of the formula with hydrochloric acid. Acid Mixture for Dead Dipping., If, instead of a bright appearance, it is desired to impart to the objects a dead lustre, they are, after dipping in aqua fortis and rinsing, plunged into a bath prepared the day previous and composed of— Nitric acid of 36° (yellow aqua fortis) Sulphate of zinc (white copperas) 200 volumes. 100 1 volume. 1 to 5 volumes. It will be noticed that in this mixture the proportion of nitric acid is double that of sulphuric acid. The copper articles may remain from five to twenty minutes in this cold mixture, and the dead lustre will be the more decided as the immersion has been more protracted. When they come from this bath, and after a prolonged and thorough swilling in water, the objects present an earthy appearance which is disagreeable to the eye. This objectionable dulness may be cleared up without causing the “dead” luster to disappear, by passing the pieces rapidly through the acid mixture for bright dipping previously described, followed by an immediate rinsing. If they are permitted to remain too long in the bright lustre liquid, the dead lustre will disappear, and the operation must be begun anew. If the bath for bright dipping be not conveniently at hand, it will suffice to rapidly pass the objects again, after rinsing, through the bath for dead dipping, which will clear up the dull appearance caused by their first immersion in the same bath. The acid mixture for bright dipping may, after a long use, be employed, to a certain extent, for a dead lustre bath. The mode of operation remains the same. A few varnishers employ a hot bath for dead lustre, prepared as follows: The last substance being gradually added when needed for increasing the deadness of the lustre. The lustre thus obtained at first appears dull, ochreous, and yellowish; but after a thorough rinsing, a passage through the same bath for one or two seconds, and a final rinsing, it becomes clear enough. The proportions which we have indicated for this bath need not be rigorously adhered to. They may be made to vary with the fancy and the habits of the operator, and will depend somewhat on the degree of energy of the old aqua fortis employed. Dipping in Nitrate of Mercury ("Quicking" Solution). In order to cause the deposited silver, etc. to adhere firmly, electroplaters have borrowed from the fire-gilders an operation which will be found of some importance in practice. This operation consists in plunging the cleansed articles for a few seconds into a solution composed of In dissolving the nitrate of mercury in water, a heavy, yellowish-white precipitate ensues, from the formation of a basic salt. By the addition of the acid, the precipitate disappears, and the solution becomes clear. It is well to stir the mixture before using it. Certain authors recommend the nitrate of the sub-oxide of mercury (mercurous nitrate); but experience has shown that it is inferior to the nitrate.* The proportion of mercury salt above indicated will be modified with the size of the pieces and the nature of the alloy. For instance, less mercury will be needed for light pieces of jewelry which need a very thin deposit. On the other hand, more mercury is advisable for heavy objects, such as spoons, forks, and table ornaments, which should receive a thick deposit of gold or silver. The latter must come from the mercurial solution with a perfectly white and bright appearance, looking like silver, while the color of the former should show but little change. * In this case theory is confirmed by experience, since the exchange of bases takes place between oxides, analogously constituted (HgO and CuO). It may be laid down as a rule that the behavior of the articles in the "quicking" solution is the test of the thoroughness of the preceding cleansing operations. If the cleansing has been properly done, the articles will come out of the "quicking" solution perfectly white and bright like silver. Where the cleansing has not been thorough, on the contrary, they will come out blackened or colored of various shades and generally without metallic lustre. When the "quicking" solution becomes exhausted by use, it may be revived by the addition of a few drops of pernitrate of mercury; but it is more prudent to discard it and prepare a fresh solution. We cannot insist too strongly on the necessity, especially when gilding or silvering by dipping, of allowing no intervals between the various operations of cleansing, which, excepting the time necessary for cleansing by fire or by alkalies, ought to take place in less time than is needed to read the following résumé :— 1. Cleansing by fire or by alkalies. 2. Dipping and rinsing in water. 3. Dipping in old aqua fortis and rinsing. 4. Dipping in strong aqua fortis and rinsing. 5. Dipping in the liquid for a bright lustre, or in the whitening bath, and rinsing. 6. Dipping in nitrate of mercury, and rinsing. And, lastly, immersing the articles in the depositing vat. This treatment, preparatory to obtaining the desired metallic deposit, answers equally well for copper, and all the alloys rich in this metal. German silver, however, on account of the nickel it contains, will sometimes be a little difficult to manage, and may require freshly prepared dipping baths. But in this, as in many other cases, the operator will quickly learn by practice all the slight |