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is straw yellow, and another which is of a more or less dark-red colour. The white acid, without nitrous gas, does not cleanse well, especially when freshly used. The red acid acts too powerfully and pits the copper. The straw-yellow acid is preferred to the others. Nitric acid at 40° is too energetic and costly; however, certain operators who have cleanse large quantities of copper wares prefer it on account of the rapidity of the operation. The acid is spent when its action on copper goods becomes too slow, and when the objects removed from the bath are covered with a kind of bluish-white film. Such acid is preserved for the preceding operation, namely, dipping in old aquafortis; or for dipping in the whitening bath. Very good aquafortis may cleanse imperfectly when the temperature is too low or too high. This accounts for the difficulty of cleansing in frosty weather, or during the great heat of summer.

Aquafortis for Bright Lustre.-There is an excellent way of obtaining a bright lustre for any pieces, the surfaces of which have been dulled or slightly pitted by a defective cleansing, or by their passage through the acids for removing gold or silver. Place them for a few minutes in a bath composed of old aquafortis, nearly spent, 1 part; hydrochloric acid, 6 parts; water, 2 parts. The pieces, when removed from the bath, are entirely black, and must be thoroughly rinsed in water to remove the kind of black mud which covers them. They are then cleansed and dipped again. This bath will be found useful by electro-gilders. It is also convenient for removing the nd adhering to the castings of copper alloys. Large pieces may remain in the bath for 20 or 30 minutes, as this mixture acts very slowly on copper and its alloys.

Dipping in Compound Acids for a Bright Lustre.-These acids are of two kinds, according to the object in view. If the pieces are to have a bright lustre, they are stirred for 1 or 2 seconds in a liquid, prepared the day before, and cold, made of nitric acid at 36°, 100 parts; sulphuric acid at 66°, 100 parts; com

mon salt, 1 part. In preparing this bath, nitric acid is first put into the vessel, and then sulphuric acid, which is much denser, and would not mix readily if it were put in first. At the time of mixing, especially when the salt is added, considerable heat and a quantity of acid and injurious fumes are produced, so that it is prudent to operate in the open air, or under a good chimney-hood with a movable glass sash. As these acids must be employed cold, it is necessary to prepare them in advance. Copper articles, after this dipping, are lighter coloured and much brighter than after the passage through aquafortis. They may then be considered as completely cleansed, and must be immediately rinsed in plenty of clean water. The above acids are too energetic for small articles, such as pins or hooks, which are generally cleansed in stoneware colanders. As the number of small articles stop up the perforations, the acid cannot run out so quickly as desired, and begins to heat and give off fumes, and the pieces blacken before they can be rinsed. Therefore, for small pieces, add to the above mixture one-eighth of its volume of water. Place the articles in a stoneware pot; stir rapidly with a small quantity of bitters, as the last mixture is termed, and then the whole is plunged into a quantity of fresh water as soon as the acid has sufficiently acted. This method is not economical, as the acid is lost; but the dipping liquors do not become heated.

Whitening Bath consists of old aquafortis, sulphuric acid, common salt, and uncalcined soot. Pour into a large stoneware vessel a certain quantity of old aquafortis from previous dippings, and then add twice the volume of sulphuric acid at 66°. The mixture is allowed to cool off until the next day. The nitrate of copper of the old aquafortis becomes converted into sulphate of copper, which, by cooling, crystallizes against the sides of the vessel. Decant the liquid portion into another vessel, and then add 2 or 3 per cent. of common salt, and as much of calcined soot. This mixture is much less energetic than the compound acids for a bright lustre, and often re

places them advantageously. The crys- | 2 seconds, and a last rinsing, it becomes tallized sulphate of copper is collected clear enough. and sold. This bath is strengthened, when necessary, by the addition of stronger aquafortis and oil of vitriol. To replace the portion used up during the day, equal quantities of old aquafortis and oil of vitriol are added at the end of the day. The next morning the liquors are decanted, and the sulphate of copper is gathered. Soot and common salt in sufficient proportions are then added. In this manner a perpetual and cheap whitening bath is prepared.

Compound Acids for a Dead Lustre. If it is desired to give the objects a dead lustre, they are, after dipping in aquafortis and rinsing, plunged into a bath, prepared previously, composed of nitric acid at 360, 200 parts; sulphuric acid at 66°, 100 parts; common salt, 1 part; sulphate of zinc, 1 to 5 parts. Copper articles may remain from 5 to 20 minutes in the cold bath, and the dead lustre will be the more apparent, the longer the immersion has been. From this bath, after a long rinsing, the objects have an earthy appearance. This dulness is removed by a rapid passage of the pieces through the compound acids for a bright lustre, and by an immediate rinsing. If they remain too long in the latter acids, the dead lustre will disappear, and the operation for dead lustre will have to be repeated. If a bath for the bright lustre is not at hand, the objects, after rinsing, may be rapidly passed through the deadlustre bath, which will remove the dulness of the lustre caused by too long immersion. After long use, the compound acids for a bright lustre may be employed in a certain measure for a deadlustre bath. The mode of operation remains the same. For large embossings for furniture, or for some clocks, a hot bath for dead lustre is used, composed as follows;-Old aquafortis, about 4 to 5 parts; sulpauric acid, 1 part; sulphate of zinc, 8 to 10 per cent. The sulphate of zinc is gradually added when required, for increasing the deadness of the lustre. The lustre thus obtained appears dull and yellowish; after a thorough rinsing, a passage through the same bath for 1 or

Dipping in Nitrate of Binoxide of Mercury.-This operation consists in plunging the cleansed articles for 1 or 2 seconds into a solution of water, 24 gallons; nitrate of binoxide of mercury, a third of an ounce; nitric acid or, preferably, sulphuric acid, two-thirds of an ounce. When nitrate of binoxide of mercury is poured into the water, a thick cloud is formed, of a yellowish-white colour, which subsequently disappears. Stir the mixture before using it. The proportion of mercury salt above-named must be modified, according to the size of the pieces, and the nature of the alloy. Thus less mercury will be used for light pieces of jewellery which need a very thin deposit. On the other hand, more mercury is required for heavy objects, such as 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, whilst the colour of the light articles is scarcely changed. After a perfect cleansing, the pieces will, after passing through a strong mercurial solution, be perfectly white and bright. But there will be a cloudy appearance, or various shades of colour, if the cleansing has not been properly done. The amalgamating bath becomes spent by use; it may be revived by the addition of a few drops of nitrate of mercury; but it is better to prepare a fresh one. No intervals must be allowed between the various operations of cleansing. The dipping baths are ordinarily held in vessels of glass, stoneware, porcelain, or of any other material which resists the corrosion of acids. Common earthenware and that with a lead glaze must be carefully avoided. The dipping pots must be rather high, and be furnished with a cover, in order to prevent evaporation. Those with ground edges may be covered with a pane of glass. Wide open-mouthed earthen pans are very good for rinsing. A large hood, communicating with a chimney, and closed with a sliding glass sash, should contain the following apparatus for complete

cleansing operations; A furnace and | separate pans for first dipping, old aquafortis, aquafortis and soot, compound acids for dead lustre, compound acids for bright lustre, solution of nitrate of mercury, acids to dissolve gold from old pieces, acids to dissolve silver from old pieces; with two large pans for rinsing with a constant flow of water. If the draught of the chimney is not sufficient, a small fire may be kindled under the hood. A gasburner is often sufficient. The pot of nitrate of mercury, with two rinsing pans, may be placed near the electroplating bath.

SILVER.

CLEANSING Mechanical agents will not, like acids, act simultaneously on every part of the object, and it is impossible to entirely prevent the action of the air, steam, gases, and acid fumes. Heat the object to a dull red heat upon a slow fire. If the silver is pure, it becomes covered with a thin bluish film; but if, as is nearly always the case, the silver is alloyed with a variable proportion of copper, the latter becomes oxidized, and covers the piece with a greyish-black coating. While the piece is still hot, plunge it into a boiling pickle of water and sulphuric acid, which dissolves the oxide. If the heat has been sufficiently protracted for oxidizing all the copper on the surface, the object, when removed from the pickle, is of a perfectly dead white. It is greyish if the heating has been too short, and the operation must be repeated as many times as are needed for a perfect lustre. Or the silver may be placed in sheet-iron boxes filled with a mixture of powdered borax, lime, and charcoal dust. The borax dissolves the oxide of copper as soon as formed. If the objects to be cleansed are hollow, it is necessary, before heating, to make a small hole which will allow of the escape of the air expanded by the fire. Without this precaution, the piece will burst open. When the piece is put into the pickle, the acid liquor enters through the hole, and takes the place of the air between the shells, and is difficult to remove. In order to prevent the spotting of the piece by this liquor, it is dipped for a few minutes into a very dilute solution of ammonia or of

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soda crystals, which prevents the action of the acid upon the silver. Then place the article between layers of dry and warm fir wood saw-dust, which will absorb the saline solution. Nitric, instead of sulphuric, acid may be used for the pickle bath. In this case, the water must be distilled, and the acid free from chlorine or hydrochloric acid, otherwise the silverware will be covered with a bluishwhite film of chloride of silver This method will not suit articles in which iron or zinc may be present. In such cases, employ alkalies, and polish afterwards with very fine sand or pumice-dust, with the aid of a stiff and short brush, or with a scratch-brush alone. Perfectly cleansed silver may directly receive a metallic deposit which will have the same dead lustre as the object itself, but it is customary, before introducing the articles into the electroplating bath, to scratchbrush them.

SCRATCH-BRUSHING.-Scratch-brushing is to remove the dead lustre on an object by the frequently-repeated friction of the points of many stiff and straight metallic wires, called a scratch-brush or wire-brush. Its shape varies with the articles to be operated upon. A hand scratch-brush is made of numerous wires, stiff and straight, taken from a bundle or coil of large diameter, so that the wires have little tendency to curve. For delicate objects, scratch-brushes are made of spun glass, the fibres of which are very thin and elastic. For making a good hand scratch-brush, choose a bundle or coil of brass wire of the proper thickness, and wrap a good string tightly round it for about two-thirds of the intended length of the instrument, usually about 8 inches. Then, with a cold chisel, cut the bundle of wire close to the string at one end, and at 2 inches from the other end of the string wrapping. Dip the end closed by the string into a neutral solution of chloride of zinc, and plunge into melted tin, which solders all the wires, and prevents their separation and injury to the hand of the operator. The tool is then fixed to a thin wooden handle which projects above the soldered end. Very small scratch-brushes are necessary for

reaching small holes and corners. An old scratch-brush, the wires of which nave been bent in every direction, and fixed to a long handle, is useful for rubbing the insides of certain pieces, such as Etruscan vases. Scratch-brushing is seldom done dry; the tool and pieces must be constantly wetted with a stream of water, which carries away the impurities. Good metallic deposits are only polished by the friction of the scratchbrush; bad ones scale off from the defective adhesion. A large tub, with a board placed across it, on which to rest the pieces, may be used; and various solutions are employed to assist the brushing, such as water and vinegar, or sour wine, or solutions of cream of tartar or alum, when it is desired to brighten a gold deposit which is too dark; but generally a decoction of liquorice-root, horse-chestnut, marsh mallow, or bark of Panama wood, all of which allow of a gentle rubbing with the scratch-brush, with the production of an abundant scum. Every 5 or 6 days the old liquid is carefully decanted, so as not to carry away the deposits at the bottom, which always contain some of the precious metals, which are collected to be afterwards treated. For small objects and articles of jewellery, hold the scratch-brush as a writing pen, and the motior is imparted by the wrist only, the forearm resting on the edge of the tub. For larger articles of bronze, hold the fingers extended close to the fore part of the scratch-brush, so as to maintain the wires, and, with raised elbow, strike the piece repeatedly with a sliding motion at the same time. When

a hollow is met which cannot be rubbed lengthways, a twisting motion is given to the tool. Circular wire-brushes, fixed on the spindle of a lathe, and the wires of which move all in the same direction, have been constructed for certain pieces of silversmith work, such as forks and

spoons.

Lathe for Scratch-brush.-An ordinary lathe is used for scratch-brushing, upon the spindle of which is fixed a circular brush of brass wires. A wooden frame covers the wire brush; it is open in front; the top supports a small reservoir from

which a slender jet of water runs upon the brush. A board receives the projected water, and lets it fall into a zinc pan resting on the bottom of the box.

Scratch-brushes.-The brass wire used for the manufacture of hand or circular scratch-brushes is of various strengths. Thick wires are employed for bronzes, and thinner wires for lighter articles. The wires must be preserved stiff and straight. When a hand scratch-brush becomes too short, cut the twisted ends with a cold chisel, and a new portion of wire is uncovered by removing part of the string wrapping. To remove the twisted wire ends, rest the scratch-brush upon a lead block, and cut them with a sharp cold chisel, with one stroke of a hammer if possible. When they begin to curl, they are now and then beaten with a mallet of boxwood, upon a small block kept between the knees, so as not to produce a dead stroke. Scratch-brushes if kept too long in water become hard; when greasy, they are cleansed in caustic potash; oxide is removed by the compound acids. This last operation, and even dipping in aquafortis, are sometimes resorted to for diminishing the size of the wires, and making them smoother. The circular brush is occasionally resorted to for diminishing the size of the wires, and making them smoother. The circular brush is occasionally reversed, in order to change the direction of the wires.

Bright Lustre for small Articles.-Very small articles, which cannot be scratchbrushed, receive a bright lustre by mutual friction. The operation is generally performed with the hands. The articles to be brightened are introduced, together with boxwood saw-dust, bran, or sand, into a bag; the ends of the bag being gathered into the hands with the thumbs inwards, the bag is shaken to and fro. As this operation becomes very fatiguing, mechanical means may be employed to effect the shaking.

CLEANSING ZINC.-Zinc is cleansed by being passed through a boiling solution of caustic lye, without remaining too long in it, because it may be corroded, and even dissolved; after rinsing, it is plunged for a few minutes into water contain

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