CHAPTER VII. CLEANSING ZINC-LEAD-TIN-CAST IRON-WROUGHT IRON -STEEL-ETC. Cleansing Zinc. ZINC, next to copper and silver, is the metal that the electroplater has most frequently to deal with. This substance, which the ancient chemists had ranged among half metals (i. e. those whose brittleness unfitted them for many uses in the arts), has for some years past received so many improvements in its preparation that, at the present day, there are few metals applied to more various uses than zinc. It may be drawn out into quite tenacious medium-sized wire, which takes the place of lead and brass wire for many uses, and into wire so fine that it is possible to sew and embroider with it; it is rolled into plates whose properties of resistance and lightness render them often preferable to sheet lead, iron, or copper; it may be cast into all kinds of shapes and patterns, and the castings are sufficiently sharp and economical to permit the production of a great number of ornamental objects, such as were formerly made of copper, bronze, or brass. The clocks, cups, candelabras, chandeliers, statues, and ornaments of all kinds cheaply obtained with this metal have made these useful and artistic objects very common. It is sufficiently strong and sonorous for such uses, but its appearance is not prepossessing, and we cannot do better than to study the processes by which its surface is modified and rendered similar to that of more precious metals. Objects of zinc are occasionally coated with gold or silver; but the common practice is to give them a coating of copper or of brass, which, after certain chemical manipulations to be described further on, may be made to present the various shades of bronze known as verd antique, Florentine, medallion, artistic, etc. But before undertaking any of these operations, the zinc, like all other metals, must first be cleansed of all its surface impurities. The cleansing of zinc is performed as follows: it is dipped into a boiling solution of caustic potash, in which it should only remain a short time, since this liquid readily attacks the metal. It is then rinsed in fresh water; then immersed for a few minutes in water acidulated with from one-tenth to one-twentieth of sulphuric acid; then thoroughly swilled (in hot water if possible); and if it should be necessary, scoured with a stiff brush and pumice powder, or vigorously scratch-brushed. This last operation is especially useful when parts have been united with tin solder, which becomes black and dull by the action of the alkaline and acid baths. These processes never give entirely satisfactory results, and the cleansing is much more thorough when the object is rapidly dipped into a cold mixture of Sulphuric acid Nitric acid Common salt. 100 volumes. 100 66 1 per cent. and then still more rapidly rinsed in water perfectly free from copper salt, which will blacken the zinc. If, instead of this quick cleansing of the zinc, we allow it to remain a little longer in the above mixture of acids, it acquires a fine "dead" lustre, which, in some cases, may be utilized for producing agreeable contrasts between the various parts of the same piece. The "dead" lustre disappears and gives place to a 6 bright" one, when the piece is replunged a number of times in the acid mixture, and rinsed rapidly after each immersion. When, as frequently happens, the different members of which a zinc article is made up, are united by tin or plumbiferous solder, these portions of the surface will come out of the cleansing liquor blackened; but the inconvenience is trifling, as a few strokes of the scratchbrush before immersion in the depositing vat suffice to remove it. Zinc should be lightly amalgamated by passing through the "quicking" solution (nitrate of mercury), previously indicated for copper. This treatment insures greater density to the electro-deposits. When, as often happens, in consequence of careless or defective cleansing of the pieces, the copper or brass deposit is unsatisfactory, having only a slight adherence at many points of the surface, it becomes necessary to repeat the operation of brassing or coppering. In such cases it is always prudent to completely "strip" the zinc of its cupreous coating. This is best accomplished by immersing it in strong nitric acid until on removal it appears completely black, which indicates the complete solution of the copper. An immersion, then, into the acid mixture above named will render the surface perfectly white, and in condition to receive a new deposit of copper. But, if the defective deposit was of considerable thickness, the "stripping" will be attended with much difficulty, and it rarely happens that we can succeed in saving the piece, for the reason that the zinc will be rapidly corroded in those places that are first stripped of their copper coating. Cleansing Lead, Tin, etc. Tin, lead, the alloy of these metals called solder, Algiers metal, Boulogne silver, Britannia metal, etc., are much more difficult to cleanse than zinc. The usual process is a rapid cleansing in potash lye and subsequent friction with a hard substance. The objects are sometimes plunged into diluted hydrochloric acid, but the first operation is nearly always necessary. Cleansing Cast Iron. Cast iron is cleansed by immersion for two or three hours in water containing one-hundredth part of sulphuric acid. After this bath, the metal is rinsed in cold water, and scoured with fine sand with the aid of a brush or a coarse rag, then again immersed in the acid pickle, . rinsed, and plunged into the depositing vat. If more than one per cent. of sulphuric acid be added to the water, the length of the immersion must be shortened, otherwise the cast iron will be deeply corroded, and the carbon of the metal, which is insoluble in the pickle, can only with great difficulty be removed by the friction of the sand. In fact, cast iron that has been injured by too prolonged an immersion in the pickle, or by the use of a pickle unnecessarily strong, cannot be made to yield good results in the depositing vat. For cleansing cast iron some operators prefer to use hydrochloric acid (muriatic acid) in place of sulphuric. A solution containing 5 per cent. of muriatic acid will be found to answer best; but the operation is more costly and more uncertain. A one per cent. solution of sulphuric acid will last for a week, and at the end of that time. can be revived again by the addition of a fresh hundredth of acid. In fact, we prefer a pickle thus revived, and consequently charged somewhat with sulphate of iron, to an entirely fresh one, for the former has the advantage over the latter of attacking the cast iron more uniformly. The action of hydrochloric acid, on the other hand, is much more energetic, and a portion of it is volatilized with the hydrogen gas disengaged during its action. If it be desired to keep cast iron thus cleansed for some time before submitting it to the depositing bath, it is necessary to keep it immersed in a water rendered alkaline by the addition of caustic lime, potassa, or soda, or their carbonates; but caustic lime-water is the cheapest and simplest method, and cast iron which has remained in it for a few hours will not rust after quite a long exposure to a damp atmosphere. Cleansing Wrought Iron. The cleansing of wrought iron is effected in the same manner as that of cast iron, with the difference that it will bear a stronger pickle and a longer immersion. Reference is made here to ordinary wrought iron covered with a film of black magnetic scale or of red rust. Whitened, filed, or pelished irons should be treated like steel. Cleansing Steel. Articles of polished iron or steel are usually cleansed by boiling in caustic Iye (soda or potash), and afterwards gently scouring with pumice powder to slightly roughen the surface, and thus provide points of attachment for the metal to be subsequently deposited. They are then passed rapidly through a solution composed of— |