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which the needle scratches. It is then bitten-in in the way above described. Etching on Cast Iron.-Use a solution of common salt and sulphate of copper for the biting-in.

Etching on Steel or Iron.-Take sulphate of copper, sulphate of alum, and muriate of soda, of each 2 drachms, and strong acetic acid 13 oz., mixed together. First smear the part intended to be etched with yellow soap, and write with a quill pen without a split.

ETCHING GLASS.-Glass is etched by hydrofluoric acid gas, or by liquid hydrofluoric acid. The acid for this purpose is obtained by treating fluoride of calcium, Derbyshire spar, with sulphuric acid, in a leaden vessel, as we shall presently describe. The gaseous hydrofluoric acid has the property of producing a surface which represents ground glass in its appearance; but the liquid acid produces just the contrary effect, and dissolves away the glass, leaving the surface polished and clear Etching glass, therefore, consists of two operations;-etching by the gas, and producing a dull opaque surface, and etching by the liquid, and producing a surface which is bright and clear. 1. Gaseous hydrofluoric acid is the product of the action of sulphuric acid and fluoride of calcium. Take powdered fluoride of calcium, 2 parts; sulphuric acid, 3 parts. The powdered fluoride is placed in a leaden dish or shallow box, the sulphuric acid poured upon it. By means of a stick, the acid is made into a paste with the powdered fluoride; the mixture only requires to be warmed to evolve considerable fumes of gaseous hydrofluoric acid. These fumes are disagreeable; the experiment should, therefore, be conducted in the open air or under a chimney. The glass plate to be etched is covered with wax, by gently warming the glass sufficient to melt wax, and rubbing it with a piece of white wax until it is covered by a thin layer; it is then allowed to cool, and the waxed surface is etched with a graver. The sheet of glass thus prepared is used as a cover for the leaden vessel containing the materials, with the waxed side presented to the escaping fumes. These fumes will

| attack the glass where the wax has been removed only, and produce the dull appearance desired. The entire surface of the glass can be rendered ground in its appearance by exposing it to the fumes of the acid gas in its ordinary condition, unprotected by the wax. The production of the gas is accelerated by the warmth of a hob or of a spirit lamp applied to the bottom of the leaden vessel for a few moments. 2. To obtain the liquid acid, place the mixture named above in a leaden retort, and conduct the gas from the retort into a leaden bottle containing some water, so long as the water absorbs the fumes. The water becomes thus highly charged with hydrofluoric acid, and this liquid is to be used in the second process. The glass plates are to be prepared as before, with the addition of a small wall of wax or putty, which is to be formed round its edges; the liquid acid is poured upon the etched waxed surface, and allowed to remain until a sufficient depth of etching is produced. 3. To produce a colourless pattern on a coloured glass ground, you proceed as for etching an ordinary pane of glass, but the operation is conducted upon the surface of flashed glass; that is, glass which is simply covered on one of its sides with a colour, and which is not stained throughout. This flashed glass is a cheap imitation of stained glass; the thin coating of coloured material is soon dissolved by the acid, so as to leave a transparent or ground-glass pattern on a coloured glass ground, according as the process is conducted by means of gaseous or liquid hydrofluoric acid. The acid must be carefully handled, as it attacks the skin and produces stubborn sores, which are not easily healed, and it must be kept in india-rubber bottles, as it will dissolve glass.

Engraving on Copper is performed by cutting lines representing the subject on a copper plate by means of a steel instrument, called a graver, or burin, ending in an unequal-sided pyramidal point. Besides the graver, the other instruments used in the process are a scraper, a burnisher, ar oil-stone, and a cushion for supporting the plate. In

cutting the lines on the copper, the graver is pushed forward in the direction required, being held at a small inclination to the plane of the copper. The use of the burnisher is to soften down the lines that are cut too deeply, and for burnishing out scratches in the copper; it is about 3 inches long. The scraper, like the burnisher, is of steel, with three sharp edges to it; it is about 6 inches long, tapering towards the end. Its use is to scrape off the burr raised by the action of the graver. To show the appearance of the work during its progress, and to polish off the burr, engravers use a roll of woollen, or felt, called a rubber, which is used with a little olive-oil. The cushion, which is a leather bag about 9 inches diameter filled with sand, for laying the plate upon, is now rarely used except by writing engravers. For architectural subjects, or for skies, where a series of parallel lines are wanted, a ruling machine is used, which is exceedingly accurate. This is made to act on an etching ground by a point or knife connected with the apparatus, and bit-in with aquafortis in the ordinary

way.

COPPER PLATE.-The plate must be perfectly polished, very level, and free from any imperfection; to this must be transferred an exact copy of the outlines of the drawing. To do this the plate is uniformly heated in an oven or otherwise till it is sufficiently hot to melt white wax, a piece of which is then rubbed over it and allowed to spread, so as to form a thin coat over the whole surface, after which it is left in a horizontal position till the wax and plate are cold. A tracing having been taken of the original design with a black-lead pencil on a piece of thin tracing paper, it is spread over the face of the prepared plate, with the lead lines downwards, and, being secured from slipping, a strong pressure is applied, by which operation the lead lines are nearly removed from the paper, being transferred to the white wax on the plate. The pencil marks on the wax are now traced with a fine steel point, so as just to touch the copper; the wax is then melted off, and a perfect outline will be found on

the copper, on which the engraver proceeds to execute his work.

Engraving on Silver or Gold. -1. The engraving is first exposed to the vapour of iodine, which deposits upon the black parts only. The iodized engraving is then applied, with slight pressure, to a plate of silver, or silvered copper, polished in the same manner as daguerreotype plates. The black parts of the engraving which have taken up the iodine part with it to the silver, which is converted into an iodide at those parts opposite to the black parts of the design. The plate is then put in communication with the negative pole of a small battery, and immersed in a saturated solution of sulphate of copper, connected with the positive pole by means of a rod of platinum. Copper will be deposited on the non-iodized parts, corresponding to the white parts of the engraving, of which a perfect representation will thus be obtained; the copper representing the white parts, and the iodized silver the black parts. The plate must be allowed to remain in the bath for only a very short time, for, if left too long, the whole plate would become covered with copper. The plate, after having received the deposit of copper, must be carefully washed, and afterwards immersed in a solution of hyposulphite of soda to dissolve the iodide of silver, which represents the black parts; it is then well washed in distilled water, and dried. 2. Heat a silver plate, previously coated with copper, to a temperature sufficient to oxidize the surface on the copper, which successively assumes different tints, the heating being stopped when a dark-brown colour is obtained. It is then allowed to cool, and the exposed silver is amalgamated-the plate being slightly heated, to facilitate the operation. As the mercury will not combine with the oxide of copper, a design is produced, of which the amalgamated parts represent the black, and the parts of the plate covered with oxide of copper represent the white parts. The amalgamation being complete, the plate is to be covered with three or four thicknesses of gold leaf, and the mercury is evaporated

by heat, the gold only adhering to the black parts. The superfluous gold must then be cleared off with the scratchbrush; after which the oxide of copper is dissolved by a solution of nitrate of silver; and the silver and copper underneath are attacked with dilute nitric acid. Those parts of the design which are protected by the gold, not being attacked, correspond to the black parts of the plate; the other parts, corresponding to the white parts of the engraving, may be sunk to any required depth. When this operation is completed the plate is finished, and may be printed from in the ordinary method of printing from woodcuts.

Line Engraving on Gold. To obtain from the same prints plates with sunk lines, similar to the ordinary engraved copper-plates, a plate of copper, | covered with gold, is operated upon. On immersion in the sulphate of copper solution, the parts corresponding to the white parts of the engraving will become covered with copper. The iodine, or compound of iodine, formed, is then to be removed by the hyposulphite; the layer of deposited copper is oxidized, and the gold amalgamated, which may be removed by means of nitric acid, the oxide of copper being dissolved at the same time. In this instance the original surface of the plate corresponds to the white parts of the print, and the sunk, or engraved, portions to the black parts, as in ordinary copper-plate engravings.

Electro-metallurgy.

The first and most important operation in all branches of the electro-deposition of one metal upon another, is to effect a thorough and chemical cleansing of the surface of the metal upon which the coating is to be deposited.

CLEANSING COPPER AND ITS ALLOYS. -This is done in six operations. 1. Cleansing by fire, or by alkalies. 2. Dipping. 3. Dipping in old aquafortis. 4. Dipping in new aquafortis and soot. 5. Dipping in compound acids for a bright or dead lustre. 6. Dipping in nitrate of binoxide of mercury.

Cleansing by Fire, or by Alkalies.This is to remove any foreign substances, especially those of a fatty nature, which

are destroyed by heating the pieces in every direction over a gentle fire of charcoal, breeze, or spent tan. A muffle furnace, heated up to a dull red heat, is preferred; but small articles may be cleansed in a hot revolving cylinder. This operation is not adapted to very delicate articles, or for table-forks and spoons, which must keep their toughness, or to those pieces in which the different parts are united by soft solders. Boil such articles in a solution of potash or soda, which renders the fatty substances soluble in water. This is done in a cast-iron kettle, provided with a cover, where there is a boiling concentrated solution of carbonate of potash, or soda, or of American potash. The caustic potash or soda must be dissolved in ten times its weight of water. This solution lasts a long time; when it has lost part of its power, it may be revived by a few fragments of caustic alkali. At the boiling point it will cleanse copper in a few seconds. If the articles to be scoured are joined with tin solder, they must not be allowed to remain too long in the caustic liquor, which would dissolve the solder and blacken the copper.

Dipping. The pieces are then dipped in a mixture of from 5 to 20 parts in weight of sulphuric acid at 66° Baume for 100 parts of water. Most of the pieces to be cleansed may be dipped hot in this mixture; but certain alloys, in which tin, zinc, or antimony predominate, such as cast bronze, must not be so treated, as the sudden cooling will occasion cracks and flaws. Copper articles may remain any length of time in the dipping bath; they should not be removed before the black coat of binoxide of copper, caused by the heating, is entirely dissolved. The remaining coat of red protoxide of copper is unacted upon by the sulphuric acid. Articles having parts made of iron or zinc must not be submitted to the action of dilute sulphuric acid, or they will be entirely dissolved; therefore avoid the use of implements or wires of iron, zinc, or steel. A dipping bath which contains copper in solution from previous operations will not suit for articles which may contain iron, tin, tin solder, antimony,

bismuth, or lead. In such a case, use a Dipping in Aquafortis and Soot. newly-made dipping bath and a small After rinsing in fresh water, the articles proportion of acid. Articles which are well shaken and drained, and then have been cleansed by alkalies must be plunged into a bath composed of nitric washed before being put into the dipping acid at 36° Baumé, 100 parts; common bath, or pickle. Thoroughly and rapidly salt, 1 part; calcined soot, 1 part. This rinsing in fresh water all the articles, mixture attacks the metal with the before and after each of the following greatest energy, and the pieces should operations, must be strictly attended to. therefore not remain in it more than a The various manipulations which com- few seconds. The volume of acid should plete the cleansing succeed each other be about 30 times that of the articles to without interruption; and the articles be cleaned, in order to prevent too great must be stirred as well as possible an elevation of temperature due to the in the acid baths, and in the rinsing chemical reaction, which would result in water. After dipping and rinsing, the the rapid weakening of the acid. After various pieces are fixed to a brass wire, this bath, and rapid rinsing, in order to or hooked upon brass or copper hooks. prevent the production of nitrous vapours Small articles of jewellery are suspended the pieces present a fine red lustre, gold to a stout copper wire. These hooks are yellow or greenish yellow, according to better if made of pure copper than of the alloy employed, and such as to make brass, and it is still better to use glass one believe that they are entirely cleansed hooks, which are cheap and are not cor- of foreign matter; yet if the pieces in roded by the acids. Such nooks or sup- this state are plunged into a gilding or ports can be made by bending glass rods, silvering bath, they become entirely black, by the heat of a charcoal fire, or of a gas and without any metallic lustre. If the burner, to the desired shape. Those ob- pieces are put aside without rinsing, there jects which cannot be suspended or at-rises on their surface a green froth and tached to hooks, are put into perforated nitrous vapour, which indicate the decomladles of porcelain or stoneware. It is position of the acid with which they are less economical, but sometimes absolutely contaminated. When the vapours have necessary, to use baskets of brass or copper disappeared, the pieces, even after washwire cloth. Those who frequently have ing, remain of a dull black, on account of to cleanse, very small articles will find it the formation of a basic copper salt which advantageous to employ a basket of pla- is not soluble in water. This last mode tinum wire cloth, which, although ex- of operating, called blacking by aquapensive in the first cost, will be found fortis, is preferred by a few gilders, varcheaper in the end, as it is almost inde- nishers, and colour fixers, who find it structible. economical to allow the production of nitrous vapours while the pieces are draining on top of the vessel which contains the acids. Any subsequent operation is to be prefaced by a rinsing in fresh water. When small objects, such as pins, caps, or eyelets, are to be dipped, they are put into a stoneware pot, with a small quantity of aquafortis, and then rapidly shaken and stirred. In this case the acid is entirely used up with the production of abundant vapours, and the objects remain blackened, and ready for a further cleansing. Care must be taken in the choice of aquafortis. Three kinds of nitric acid at 36° are to be found in the trade;-One is perfectly white, another

Dipping in old Aquafortis.-If there is any aquafortis, nitric acid, already weakened by preceding dippings, plunge into it the articles which have passed through the sulphuric acid pickle bath, and have been rinsed. They may remain there until the red coat of protoxide of copper has entirely disappeared, leaving, after rinsing, a uniform metallic lustre. The dipping in old aquafortis, though not absolutely necessary, is recommended for two reasons; it economizes the cost of fresh acids; and, as its action is slow, it prevents the too rapid corrosion of the cleansed copper during the time of the solution of the protoxide.

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The paint work of all is partis inglish weights. Were the and the plate is plunged into water, the dite of potash in eighth of the total time them theil Sketch the subject have had the darkest with some pale but decided water line, whole time required, the red or yellow fe emple, wing the the time, the pale lions point of a small camel-bir ir. This visible whilst the plate is teal the palest lions co-aghth will chiting-in has the being etched age of the which preceded t

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the work is distinctly seen costly tools. The hath should be formed ption; black on a white or in an oblong square piece of light wood, , without any deceptive about 14 inch thick, and larger than tly as it is to be seen in the well, which must be a square hole, Ce the copper plate, and a little larger than the plate, and mag and a little of about an inch deep. Cover the board ide of silver. and well with about six couts of japa with a which protects the wood from the action plate will be properly of the acid, and the dark colour makes as too thick, add the plate look whiter from the contrast It is wished A thin piece of wood, stained black, mast dead white, be used as a hand-rest. Before using a of the copper new bath or well dissolve a small piece my paper, each of copper and of zinc in it with or from left to acid. Lay the plate in the desired posi ded to work tion, and fix it by pressing small pieces awlegd made by of modelling wax at the corners against the sate the plate and the board. Etch with an Dette a few days; ordinary strong sewing needle inserted the milky in a holder. It must be sharp enough to red scratch well through the silver, otherwise

the silver sile; more tin in thickness from the first to the gatly but firmly from side to last lines; as the time of exposure that the solation may to and diminishes, this property must be care then pour all the superfous ground fully attended to. Thus, if the subject kate the bottia. In finishing more requires only about 2 hours' work in plate more rapidly. Let the ground etching, this must be spread over 5 3 days Apply a second cost hours' exposure in the bath, which is the same manner, and let it dry for time necessary to produce the darkest aa quiet room, where it will lines; other work can be carried e any dust. If the plate is dried simultaneously, but this process cat eat of a spirit lamp, the ground be hurried. If however, the subject is transparent, but not of the dead elaborate, and requires more ting lour which is desirable. Paint than can be finished in 5 bus select varnish to protect them in the the whole plate; clean and re-grand the kk and edges of the plate with for the first sitting various parts or which must be composed as fol- plate; at the second sitting add work to Chlorate of potash, 20 grammes; that previously done, and so mil the ter, 880 grammes or the same pro-as to work always at the same period of hydrochloric acid, 100 grammes: plate is finished, so arranging the fis

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