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Gilding and Silvering
(From The Household Cyclopedia, 1881, This article contains antique processes, some of which are dangerous.)
Solution for Gilding.
Electro-gilding is done in like manner. The gold is dissolved in nitro-hydrochloric acid, washed with boiling nitric acid, and then digested with calcined magnesia. The gold is deposited in the form of an oxide, which after being washed in boiling nitric acid, is dissolved in cyanide of potassium, in which solution the articles to be plated with gold, after due preparation are placed. Iron, steel, lead, and some other metals that do not readily receive the gold deposit require to be first lightly plated with copper, or dipped in a solution of nitrate of silver, 1 part; nitrate of mercury, 1 part; nitric acid s.g. 1.384) 4 parts; water, 120 parts. The positive plate of the battery must be of gold, the other plate of iron or copper. The process is the same as that above described; use a feeble current.
The popular notion is, that genuine electro-gilding must necessarily add a good deal to the cost of the article plated. This is erroneous. A silver thimble may be so handsomely plated as to have the appearance of being all gold for 5 cents, a pencil-case for 20 cents, and a watchcase for 1 dollar (prices from the late 1800's, more current reference may be useful, but not as interesting. An estimate of the relative value of electrogilding, as compared with silver-plating, considering the cost of material alone, is about 15 to 1.
To Deposit Brass.
- Dissolve 5 oz. powdered acetate of copper in 1/2 gal. of water
- add 1 pt. of solution of ammonia
- dissolve 10 oz. sulphate of zinc (white vitriol) in 1 gal. of water, at 180oF
- and when cool add 1 pt. of solution of ammonia (Be very careful).
- Dissolve 4 1/2 lbs. potassa in 1 gal. of water.
- Lastly, dissolve 8 oz. cyanide of potassium in 1 gall. of hot water.
- Mix in the following order:
- add the copper solution to that of zinc
- then the potash and cyanide
- let sit for an hour or so, and add water to make up 8 gal.
- Work with a brass anode and an active battery power, occasionally adding more ammonia and cyanide.
To Copy Medals.
Casts of the medals may be made in fusible metal, plaster, wax, etc. In case of a non-metallio mould it must have its face brushed over with black lead. The metallic mould is to be coated on the back with wax or varnish. The wire is usually attached to the edge by soldering or twisting. A decomposing cell is not necessary. A watertight box is divided by a porous (plaster or leather) partition. On one side is a plate of zinc immersed in diluted, 1 to 20, sulphuric acid; on the other a solution, kept saturated, of sulphato of copper. A wire from the zinc is attached to a copper rod, from which the medals are suspended, dipping into the copper solution.
To Bronze Copper Medals.
- Brown. - Moisten the surface, well cleaned with weak nitric acid, allow it to dry, and apply a gentle heat.
- Black. - Use, instead of nitric acid, sulphydrate of ammonia or liver of sulphur.
- Green. - Expose in a close box to the fumes of chloride of lime, or to the vapor of acetic or muriatic acid.
- For bronzing all sorts of fine copper or brass work a weak solution of bichloride of platinum is used. By varying the temperature and color, between a steel gray and deep black may be obtained.
To Deposit Copper on Iron.
Prepare a solution of cyanide of copper, by dissolving oxide of copper in cyanide of potassium, or by adding cyanide of potassium to a solution of sulphate of copper, and redissolving the precipitate formed. Work with a strong battery power. The copper will not deposit unless the current be strong enough to evolve hydrogen at the cathode, which evolution should always be avoided in depositing the other metals.
Voltaic Protection of Metals.
When two metals are united and exposed to a corrosive agent, which would act unequally upon them if separate, the one which would be most acted on receives most of the force of the corrosion, while the other escapes. Thus iron coated with zinc (galvanized iron) will last for years exposed to the atmosphere. Copper points on lightning rods remain bright for a long time, when screwed into a zinc ball.
Coating Electrotype-plates with Iron.
The following has been successfully employed in coating electrotype deposits with a coating of pure iron, thereby rendering them little inferior to steel-plate engravings as regards durability:-
Dissolve 1 lb. of sal ammoniac in 1 gall. of rainwater, then add 2 lbs. of neutral acetate of iron; boil the solution in an iron-kettle for 2 hours, replacing the water lost by evaporation; when cold, filter the solution, and keep it in close-covered vats (when not in use) to prevent oxidation.
The iron plate used in the decomposition-cell must be of the same surface as the plate to be coated with iron; a Smee’s battery, of at least 3 cells, charged with 1 part sulphuric acid, and 60 parts water, being used for the decomposition.
To insure success the following rules must be observed: 1st. The plate must be thoroughly freed from any greasy matter by immersing in a solution of caustic soda, then rinsed in clean cold rainwater, after which dip it in dilute acetic acid, and immediately transfer it to the solution of iron; this will insure perfect adhesion between the metals. 2nd. The solution must be filtered previous to use to remove the oxide of iron formed by exposure to the atmosphere. After the plates have been coated with iron they must be well rinsed in clear warm rain-water, then in a weak alkaline solution, well dried with a piece of clean soft cotton, and slightly oiled to prevent oxidation. The coating of iron is very hard and brittle, resembling the white iron used by manufacturers of malleable iron. Should any of the surface be damaged, the whole coating of iron may be removed by immersion in dilute sulphuric acid, and re-coated again by the above process.
Copper Tubes made by Galvanic Process.
Le Genie Industrial publishes the details of a process for making copper-tubes without soldering, which consists simply in depositing copper upon lead patterns by the galvanic battery, and then melting out the lead. It is said to work perfectly, and of course tubes could be made of any desired form - straight, curved, or right-angled. This suggests the idea of forming tubes in the same manner with cores of wax or clay. The clay may be forged into the size of the pipe through a draw-plate, then allowed to harden slightly, when it may be covered with plumbago and an electrodeposit of copper made upon it with a galvanic battery. When the copper is deposited in sufficient thickness the clay may be removed from the interior by boiling the pipe in water. To conduct this manufacture it would require long depositing-troughs, and the expense would probably be too great for making straight coppertubes; but for curved tubes, such us the worms of stills, it would perhaps pay. Curved copper-tubes are commonly made by filling straight tubes with hot resin, then twisting the entire tube into its curved form. When the resin becomes cool it is driven out by striking the pipe, which breaks the resin-core into small pieces.
To Gild Glass and Porcelain.
Drinking and other glasses are sometimes gilt on their edges. This is done, either by an adhesive varnish, or by heat. The varnish is prepared by dissolving in boiled linseedoil an equal weight either of copal or amber. This is to be diluted by a proper quantity of oil of turpentine, so as to be applied as thin as possible to the parts of the glass intended to be gilt. When this is done, which will be in about 24 hours, the glass is to be placed in a stove, till it is so warm as almost to burn the fingers when handled. At this temperature the varnish will become adhesive, and a piece of leaf gold, applied in the usual way, will immediately stick. Sweep off the super-fluous portions of the leaf; and when quite cold it may be burnished, taking care to interpose a piece of very thin paper (Indian paper) between the gold And the burnisher. If the varnish is very good, this is the best method of gilding glass, as the gold is thus fixed on more evenly than in any other way.
It often happens, when the varnish is but indifferent, that by repeated washing the gold wears off; on this account the practice of burning it in is sometimes had recourse to.
For this purpose some gold powder is ground with borax, and in this state applied to the clean surface of the glass by a camel’s-hair pencil. When quite dry the glass is put into a stove heated to about the temperature of an annealing oven; the gum burns off, and the borax, by vitrifying, cements the gold with great firmness to the glass, after which it may be burnished. The gilding upon porcelain is in like manner fixed by heat and the use of borax; and this kind of ware being neither transparent nor liable to soften, and thus to be injured in its form, in a low red heat, is free from the risk and injury which the finer and more fusible kinds of glass are apt to sustain from such treatment. Porcelain and other wares may be platinized, silvered, tinned, and bronzed in a similar manner.
Preparation for Gilding Porcelain.
This preparation, the invention of the brothers Dutuste, is reported on by Salvetat. The peculiar advantage of it is, that after burning the gold is so bright as not to require polishing. Thirty-two grammes of gold are gently warmed with 128 grammes of nitric acid and the same weight of hydrochloric acid. To the solution are added 12 grammes of tin and 1.2 grammes of butter of antimony, and, when all are dissolved, the solution is diluted with 500 grammes of water.
A mixture is now prepared by heating together 80 grammes of oil of turpentine, 16 grammes of sulphur, and 16 grammes of Venice turpentine. When the sulphur is dissolved 50 grammes of oil of lavender is added.
The gold solution is now added, and the two are well stirred together, until the aqueous solution becomes decolorized, showing that all the gold has united with the balsam. The watery portion is then poured away, and the oily fluid is washed with warm water, and then heated. When the last trace of moisture has disappeared 65 grammes more of lavender oil and 100 grammes of oil of turpentine are added, and the whole warmed to insure the perfect admixture. While quite fluid 5 grammes of subnitrate of bismuth are added. Afterwards the clear part is decanted from any reduced gold and other insoluble material and the balsam is concentrated to a fit consistence for painting with. The balsam so prepared is a thick fluid, of a pale-green color, the gold being perfectly dissolved. When proper care is taken to remove all moisture this preparation never blisters in burning.
To Gild Leather.
In order to impress gilt figures, letters, and other marks upon leather, as on the covers of books, edgings for doors, etc., the leather must first be dusted over with very finely powdered yellow resin or mastic gum. The iron tools or stamps are now arranged on a rack before a clear fire, so as to be well heated, without becoming red hot. If the tools are letters, they have an alphabetical arrangement on the rack. Each letter or stamp must be tried, as to its heat, by imprinting its mark on the raw side of a piece of waste leather. A little practice will enable the workman to judge of the heat. The tool is now to be pressed downwards on the gold-leaf, which will of course be indented, and show the figure imprinted on it. The next letter or stamp is now to be taken and stamped in like manner, and so on with the others, taking care to keep the letters in an even line with each other, like those in a book. By this operation the resin is melted, consequently the gold adheres to the leather. The super-fluous gold may then be rubbed off by a cloth, the gilded impressions remaining on the leather. In this, as in every other operation, adroitness is acquired by practice. The cloth alluded to should be slightly greasy, to retain the gold wiped off (otherwise there will be great waste in a few months); the cloth will thus be soon completely saturated or loaded with the gold. When this is the case, these cloths are generally sold to the refiners, who burn them and recover the gold. Some of these afford so much gold by burning as to be worth from seven to ten dollars.
To Gild Writings, Drawings, etc. on Paper or Parchment.
Letters written on velum or paper are gilded in 3 ways: in the first, a little size is mixed with the ink and the letters are written as usual; when they are dry, a slight degree of stickiness is produced by breathing on them, upon which the gold leaf is immediatley applied, and by a little pressure may be made to adhere with suffi-cient firmness. In the second method, some white-lead or chalk is ground up with strong size, and the letters are made with this by means of a brush; when the mixture is almost dry the gold leaf may be laid on, and afterwards burnished. The last method is to mix up some gold powder with size, and to form the letters of this by means of a brush. It is supposed that this latter method was that used by the monks in illuminating their missals, psalters, and rubrics.
To Gild the Edges of Paper.
The edges of the leaves of books and letter paper are gilded whilst in a horizontal position in the bookbinder’s press, by first applying a composition formed of four parts of Armenian bole, and one of candied sugar, ground together with water to a proper consistence, and laid on by a brush, with the white of an egg. This coating, when nearly dry, is smoothed by the burnisher, which is generally a crooked piece of agate, very smooth, and fixed in a handle. It is then slightly moistened by a sponge dipped in clean water, and squeezed in the hand. The gold-leaf is now taken upon a piece of cotton from the leathern cushion and applied on the moistened surface. When dry it is to be burnished by rubbing the agate over it repeatedly from end to end, taking care not to wound the surface by the point of the burnisher. A piece of silk or India-paper is usually interposed between the gold and the burnisher
Cotton-wool is generally used by bookbinders to take the leaf up from the cushion, being the best adapted for the purpose on account of its pliability, smoothness, softness, and slight moistness.
To Gild Silk, Satin, Ivory, etc., by Hydrogen Gas.
Immerse a piece of white satin, silk, or ivory in a solution of chloride of gold, in the proportion of 1 part of the chloride to 3 of distilled water. Whilst the substance to be gilded is still wet, immerse it in a jar of hydrogen gas; it will soon be covered by a complete coat of gold.
The foregoing experiment may be very prettily and advantageously varied as follows: Paint flowers or other ornaments with a very fine camel-hair pencil, dipped in the above-mentioned solution of gold, on pieces of silk, satin, etc., and hold them over a Florence flask, from which hydrogen gas is evolved, during the decomposition of the water by sulphuric acid and iron filings. The painted flowers, etc., in a few minutes, will shine with all the splendor of the purest gold. A coating of this kind will not tarnish on exposure to the air or in washing.
Oil Gilding on Wood.
The wood must first be covered or primed, by 2 or 3 coatings of boiled linseed-oil and carbonate of lead, in order to fill up the pores and conceal the irregularities of the surface occasioned by the veins in the wood. When the priming is quite dry a thin coat of gold size must be laid on. This is prepared by grinding together some red oxide of lead with the thickest drying oil that can be procured, and the older the better. That it may work freely, it is to be mixed, previously to being used, with a little oil of turpentine, till it is brought to a proper consistence. If the gold-size is good it will be sufficiently dry in 12 hours, more or less, to allow the artist to proceed to the last part of the process, which is the application of the gold. For this purpose a leaf of gold is spread on a cushion (formed by a few folds of flannel secured on a piece of wood, about 8 inches square, by a tight covering of leather), and is cut into strips of a proper size by a blunt pallet-knife; each strip, being then taken upon the point of a fine brush, is applied to the part intended to be gilded, and is then gently pressed down by a ball of soft cotton. The gold immediately adheres to the sticky surface of the size, and, after a few minutes, the dextrous application of a large camel’s-hair brush sweeps away the loose particles of the gold-leaf without disturbing the rest. In a day or two the size will be completely dried, and the operation will be finished.
The advantages of this method of gilding are that it is very simple, very durable, and not readily injured by changes of weather, even when exposed to the open air, and when soiled it may be cleaned by a little warm water and a soft brush. Its chief employment is in out-door work. Its disadvantage is that it cannot be burnished, and therefore wants the high lustre produced by the following method:
To Gild by Burnishing.
This operation is chiefly performed on picture frames, mouldings, headings, and fine stucco-work. The surface to be gilt must be carefully covered with a strong size, made by boiling down pieces of white leather or clippings of parchment till they are reduced to a stiff jelly. This coating being dried, 8 or 10 more must be applied, consisting of the same size, mixed with fine Paris plaster or washed chalk. When a sufficient number of layers have been put on, varying according to the nature of the work, and the whole is become quite dry, a moderately thick layer must be applied, composed of size and Armenian bole, or yellow oxide of lead. While this last is yet moist the gold-leaf is to be put on in the usual manner. It will immediately adhere on being pressed by the cotton ball; and, before the size is become perfectly dry, those parts which are intended to be the most brilliant are to be carefully burnished by an agate or a dogs’ tooth, fixed in a handle.
In order to save the labor of burnishing, it is a common, but bad practice, slightly to burnish the brilliant parts, and to deaden the rest by drawing a brush over them dipped in size; the required contrast between the polished and the unpolished gold is indeed thus obtained; but the general effect is much inferior to that produced in the regular way, and the smallest drop of water falling on the sized part occasions a stain. This kind of gilding can only be applied on in-door work, as rain, and even a considerable degree of dampness, will occasion the gold to peel off. When dirty it may be cleaned by a soft brush, with hot spirit of wine, or oil of turpentine.
The parts to be burnished (in gilding on metals) being covered with the usual guarding, the piece is fastened by five iron wires to the end of an iron rod; it is then to be highly heated until the guarding becomes brown, when the gilding will be found to have acquired a fine gold color. It is now to be covered with a mixture of common salt, nitre, and alum, liquefied in the water of crystallization they contain; the piece to be carried again to the fire and heated until the saline coating is in a state of fusion and becomes nearly transparent, when it must be withdrawn and suddenly plunged into cold water, which removes both coating and guarding. Dip it afterwards in very weak nitric acid, and wash it repeatedly in several separate tubs of water. It may be dried either by exposure to air, or gently wiping it with clean soft, dry linen.
To Gild Copper, etc., by Amalgam.
Immerse a very clean bright piece of copper in a diluted solution of nitrate of mercury. By the affinity of copper for nitric acid, the mercury will be precipitated; now spread the amalgam of gold rather thinly over the coat of mercury just given to the copper. This coat unites with the amalgam, but of course will remain on the copper. Now place the piece or pieces so operated on in a clean oven or furnace, where there is no smoke. If the heat is a little greater than 660o, the mercury of the amalgam will be volatilized, and the copper will be beautifully gilt. In the large way of gilding, the furnaces are so contrived that the volatilized mercury is again condensed and preserved for further use, so that there is no loss in the operation. There is also a contrivance by which the volatile particles of mercury are prevented from injuring the gilders.
To Gild Steel.
Pour some of the ethereal solution of chloride of gold into a wineglass, and dip therein the blade of a new penknife, lancet, or razor; withdraw the instrument and allow the ether to evaporate. The blade will be found to be covered by a very beautiful coat of gold. A clean rag, or small piece of very dry sponge, may be dipped in the ether and used to moisten the blade with the same result. In this case there is no occasion to pour the liquid into a glass, which must undoubtedly lose by evaporation; but the rag or sponge may be moistened by it by applying ether to the mouth of the phial. This coating of gold will remain on the steel for a great length of time, and will preserve it from rusting.
This is the way in which swords and other cutlery are ornamented. Lancets too are in this way gilded with great advantage to secure them from rust.
Gold Powder for Gilding.
Gold powder may be prepared in three different ways: Put into an earthen mortar some gold-leaf with a little honey or thick gum-water, and grind the mixture till the gold is reduced to extremely minute particles. When this is done, a little warm water will wash out the honey or gum, leaving the gold behind in a pulverulent state.
Another. - Another way is, to dissolve pure gold (or the leaf) in nitro-muriatic acid, and then to precipitate it by a piece of copper or by a solution of sulphate of iron. The precipitate (if by copper, must be digested in distilled vinegar and then washed by pouring water over it repeatedly) and dried. This precipitate will be in the form of a very fine powder; it works better and is more easily burnished than gold-leaf ground with honey as above.
Another. - The best method of preparing gold powder is by heating a prepared amalgam of gold in an open clean crucible, and continuing the strong heat until the whole of the mercury is evaporated; at the same time constantly stirring the amalgam with a glass rod. When the mercury has completely left the gold, the remaining powder is to be ground in a Wedgwood mortar, with a little water, and afterwards dried. It is then fit for use. Although the last mode of operating has been here given, the operator cannot be too much reminded of the danger attending the sublimation of mercury. In the small way here described, it is impossible to operate without danger; it is therefore better to prepare it according to the former directions, than to risk the health by the latter.
To Cover Bars of Copper, etc. with Gold, so as to be Rolled out into Sheets.
This method of gilding was invented by Mr. Turner of Birmingham. Mr. Turner first prepares ingots or pieces of copper or brass, in convenient lengths and sizes. He then cleans them from impurity, and makes their surfaces level, and prepares plates of pure gold, or gold mixed with a portion of alloy, of the same size as the ingots of metal, and of suitable thickness. Having placed a piece of gold upon an ingot intended to be plated, he hammers and compresses them both together so that they may have their surfaces as nearly equal to each other as possible; and then binds them together with wire, in order to keep them in the same position during the process required to attach them. Afterwards he takes silver-filings which he mixes with borax to assist the fusion of the silver. This mixture he lays upon the edge of the plate of gold, and next to the ingot of metal.
Having thus prepared the two bodies, he places them on a fire in a stove or furnace, where they remain until the silver and borax placed along the edges of the metals melt, and until the adhesion of the gold with the metal is perfect. He then takes the ingot carefully out of the stove. By this process the ingot is plated with gold, and prepared ready for rolling into sheets.
To Silver Copper Ingots.
The principal difficulties in plating copper ingots are, to bring the surfaces of the copper and silver into fusion at the same time; and to prevent the copper from sealing; for which purposes fluxes are used. The surface of the copper on which the silver is to be fixed must be made flat by filing and should be left rough. The silver is first annealed, and afterwards pickled in weak muriatic acid: it is planished, and then scraped on the surface to be fitted on the copper. These prepared surfaces are anointed with a solution of borax, or strewed with fine powdered borax itself, and then confined in contact with each other, by binding wire. When they are exposed to a sufficient degree of heat, the flux causes the surfaces to fuse at the same time, and after they become cold they are found firmly united.
Copper may likewise be plated by heating it and burnishing leaf-silver upon it; so may iron and brass. This process is called French-plating.
Equal parts of sal-ammoniac and corrosive sublimate, are dissolved in spirit of nitre, and a solution of gold made with this menstruum. The silver is brushed over with it, which is turned black, but on exposure to a red heat, it assumes the color of gold.
To Dissolve Gold in Aqua Regia.
Take an aqua regia, composed of 2 parts of nitric acid and 1 of muriatic acid; let the gold be granulated, put into a sufficient quantity of this menstruum, and exposed to a moderate degree of heat. During the solution an effervescence takes place, and it acquires a beautiful yellow color which becomes more and more intense, till it has a golden or even orange color. When the menstruum is saturated, it is very clear and transparent.
To Gild, by Dissolving Gold in Aqua Regia.
Fine linen rags are soaked in a saturated solution of gold in aqua regia, gently dried, and afterwards burnt to tinder. The substance to be gilt must be well polished, a piece of cork is first dipped into a solution of common salt in water, and afterwards into the tinder, which is well rubbed on the surface of the metal to be gilt, and the gold appears in all its metallic lustre.
Amalgam of Gold in the large way.
A quantity of quicksilver is put into a crucible or iron ladle, which is lined with clay and exposed to heat till it begins to smoke. The gold to be mixed should be previously granulated, and heated red hot, when it should be added to the quicksilver, and stirred about with an iron rod till it is perfectly dissolved. If there should be any superfluous mercury, it may be separated by passing it through clean soft leather, and the remaining amalgam will have the consistence of butter, and contain about 3 parts of mercury to 1 of gold.
To Gild by Amalgamation.
The metal to be gilt is previously well cleaned on its surface, by boiling it in a weak pickle, which is a very dilute nitrous acid. A quantity of aqua-fortis is poured into an earthen vessel, and quicksilver put therein; when a suf-ficient quantity of mercury is dissolved, the articles to be gilt are put into the solution, and stirred about with a brush till they become white. This is called quickening. But, as during quicking by this mode, a noxious vapor continually arises, which proves very injurious to the health of the workman, they have adopted another method, by which they, in a great measure, avoid that danger. They now dissolve the quicksilver in a bottle containing aqua-fortis, and leave it in the open air during the solution, so that the noxious vapor escapes into the air. Then a little of this solution is poured into a basin, and with a brush dipped therein they stroke over the surface of the metal to be gilt, which immediately becomes quicked. The amalgum is now applied by one of the following methods, viz:
1st. By proportioning it to the quantity of articles to be gilt, and putting them into a white hat together, working them about with a soft brush till the amalgam is uniformly spread.
Or, 2dly. By applying a portion of the amalgam upon one part, and spreading it on the surface, if flat, by working it about with a harder brush.
The work thus managed is put into a pan, and exposed to a gentle degree of heat; when it becomes hot, it is frequently put into a hat, and worked about with a painter’s large brush, to prevent an irregular dissipation of the mercury, till at last the quicksilver is entirely dissipated by a repetition of the heat, and the gold is attached to the surface of the metal. This gilt surface is well cleaned by a wire brush, and then artists heighten the color of the gold by the application of various compositions, this part of the process is called coloring.
For silvering copper, covering the worn parts of plated goods, etc.
1. Nitrate of silver, common salt, each 30 grs.; cream of tartar, 3 1/2 drs. Mix. Moisten with cold water and rub on the article to be silvered.
2. Pure silver (precipitated from the nitrate by copper), 20 grs.; alum 30 grs.; cream of tartar, 2 drs.; salt, 2 drs.
3. Precipitated silver, 1/2 oz.; common salt, sal ammoniac, each 2 oz.; corrosive sublimate, 1 dr. Make into a paste with water. Copper utensils are previously boiled with cream of tartar and alum, rubbed with this paste made red hot and afterwards polished.
4. Nitrate of silver, 1 part; cyanide of potassium, 3 parts; water enough to make a paste.
Removing Silver from Injured Plated Ware.
Among the many branches of manufacturing at Nuremberg, in Germany, that of metals into various articles has obtained considerable importance. They include silverplated ware of different styles and quality; which necessarily produce large quantities of spoiled materials and clippings, the recovery of which has hitherto been very imperfectly accomplished; thus causing annually a considerable loss. The reason of it was, the want of a method by which the silver might be removed without much expense, and the copper thus forged from its coating used again.
Repeted experiments have led to a very simple process, by the action of concentrated nitric acid on silver and copper when present together. If these metals are placed into common commercial acid, (sp. gr. 1.47) they will both be strongly acted on; but a separation of the two is unattainable, since the copper, so long as any remains undissolved, will precipitate the silver from its solution by galvanic action. Nitric acid of the highest specific gravity (1.5), however, acts on the silver, but not on the copper: it renders the copper more electro-negative than before, less oxidizable, and deprives it of the property of decomposing the acid, and precipitating the silver.
To produce this passive condition of copper, it is not absolutely necessary to employ directly acid of that specific gravity; for any concentrated nitric acid can be made to answer the purpose by the addition of a sufficient quantity of the oil of vitriol, which deprives it of a portion of its water and thus contributes to make it stronger. A mixture of one volume of nitric acid (sp. gr. 1.47) and six of vitriol does not dissolve copper at the temperature of boiling water; but with a smaller proportion of sulphuric acid, evolution of nitrous acid takes place. The same end and much cheaper, is obtained by employing a mixture of oil of vitriol and nitrate of soda, which are the materials used in the practice. The following is the method now generally employed: Oil of vitriol, together with five per cent. of nitrate of soda, is heated in a cast-iron boiler; or better, a stoneware pan, to 212o Fahr. The silver-plated clippings are placed in a sheetiron bucket or colander, which is fastened to a pulley that may be moved about in the acid. As soon as the silver is removed, the colander is raised, allowed to drain, then immersed in cold water and emptied, to be again used in the same manner.
When the acid-bath is fresh, the desilvering proceeds very rapidly, and even with heavy plated ware takes but a few minutes; with the gradual saturation of the bath more time is required, and it is readily perceived when the acid must be renewed. The small amount of acid solution adhering to the copper, precipitates its silver when brought into the water. To obtain its complete removal, the clippings, when raised from the desilvering bath and before immersion in water, may be dipped into a second bath prepared in the same manner, which is afterwards to be used in place of the first.
The saturated bath, on cooling, congeals to a crystalline semi-fluid mass of sulphate of copper and of soda. The silver is removed by chloride of sodium, which is added in small portions at a time, while the solution is yet warm. The chloride of silver separates readily, and is washed and reduced in the usual manner. The acid solution contains but a very small portion of copper, hardly enough to pay for recovering.
This process is applied to recover the silver from the plated metal, which has been rolled down for buttons, toys, etc., without destroying any large portion of the copper. For this purpose, a menstruum is composed of 3 lbs. of oil of vitriol, 1 1/2 oz. of nitre, and 1 lb. of water. The plated metal is boiled in it till the silver is dissolved, and then the silver is recovered by throwing common salt into the solution.
To Plate Iron.
Iron may be plated by three different modes.
1. By polishing the surface very clean and level with a burnisher, and afterwards by exposing it to a blueing heat, a leaf of silver is properly placed and carefully burnished down. This is repeated till a sufficient number of leaves are applied, to give the silver a proper body.
2. By the use of a solder; slips of thin solder are placed between the iron and silver, with a little flux, and secured together by binding wire. It is then placed in a clear fire, and continued in it till the solder melts, when it is taken out, and on cooling is found to adhere firmly.
3. By tinning the iron first, and uniting the silver by the intermedia of slips of rolled tin, brought into fusion in a gentle heat.
To Heighten the Color of Yellow Gold.
Take of saltpetre, 6 oz.; green copperas, 2 oz.; white vitriol and alum, of each, 1 oz.
If it be wanted redder, a small portion of blue vitriol must be added. These are to be well mixed, and dissolved in water as the color is wanted.
To Heighten the Color of Green Gold.
Take of saltpetre, 1 oz. 10 dwts.: sal ammoniac, 1 oz. 4 dwts.; Roman vitriol, 1 oz. 4 dwts.; verdigris, 18 dwts. Mix them well together and dissolve a portion in water, as occasion requires.
The work must be dipped in these compositions, applied to a proper heat to burn them off, and then quenched in water or vinegar.
To Heighten the Color of Red Gold.
To 4 oz. of melted yellow wax, add, in fine powder, 1 1/2 oz. of red ochre, 1 1/2 oz. of verdigris, calcined till it yield no fumes, and 1/2 oz. of calcined borax; mix them well together. It is necessary to calcine the verdigris, or else by the heat applied in burning the wax, the vinegar becomes so concentrated as to corrode the surface, and make it appear speckled.
To Separate Gold from Gilt, Copper and Silver.
Apply a solution of borax, in water, to the gilt surface, with a fine brush, and sprinkle over it some fine powdered sulphur. Make the piece red-hot and quench it in water. The gold may be easily wiped off with a scratchbrush, and recovered by cupellation.
Gold is taken from the surface of silver by spreading over it a paste made of powdered sal ammoniac, with aquafortis, and heating it till the matter smokes, and is nearly dry, when the gold may be separated by rubbing it with a scratch-brush.
To Tin Copper and Brass.
Boil 6 lbs. of cream of tartar, 4 galls. of water, and 8 lbs. of grain-tin, or tin shavings. After the materials have boiled a sufficient time, the substance to be tinned is put therein and the boiling continued, when the tin is precipitated in its metallic form.
To Tin Iron or Copper-plate.
Iron which is to be tinned is first steeped in acid materials, such as sour whey, distillers’ wash, etc., then scoured and dipped in melted tin, having been first rubbed over with a solution of sal ammoniac. The surface of the tin is prevented from calcining by covering it with a coat of fat. Copper vessels must be well cleansed, and then a suf-ficient quantity of tin with sal ammoniac is put therein and brought into fusion, and the copper vessel moved about. A little resin is sometimes added. The sal ammoniac prevents the copper from scaling, and causes the tin to be fixed wherever it touches.
To prepare the Leaden Tree.
Put 1/2 oz. of the sugar of lead, in powder, into a clear glass globe or wine decanter, filled to the bottom of the neck with distilled water and 10 drops of nitric acid, and shake the mixture well.
Prepare a rod of zinc with a hammer and file, so that it may be a quarter of an inch thick and 1 inch long, at the same time form notches in each side for a thread, by which it is to be suspended, and tie the thread so that the knot shall be uppermost when the metal hangs quite perpendicular. When it is tied, pass the two ends of the thread through a perforation in the cork, and let them be again tied over a small splinter of wood which may pass between them and the cork. When the string is tied, let the length between the cork and the zinc be such that the precipitant (the zinc) may be at equal distances from the sides, bottom and top of the vessel, when immersed in it. When all things are thus prepared, place the vessel in a place where it may not be disturbed and introduce the zinc, at the same time fitting in the cork. The metal will very soon be covered with the lead, which it precipitates from the solution, and this will continue to take place until the whole be precipitated upon the zinc, which will assume the form of a tree or bush, the leaves and branches of which are laminal, or plates of a metallic lustre.
To prepare the Tin Tree.
Into the same, or a similar vessel to that used in the last experiment, pour distilled water as before, and put in 3 drs. of chloride of tin, adding 10 drops of nitric acid, and shake the vessel until the salt is completely dissolved. Replace the zinc (which must be cleared from the effects of the former experiment) as before, and set the whole aside to precipitate without disturbance. In a few hours the effect will be similar to the last, only that the tree of tin will have more lustre.
To prepare the Silver Tree.
Pour into a glass globe or decanter 4 drs. of nitrate of silver dissolved in a lb. or more of distilled water, and lay the vessel on the chimney piece, or in some place where it may not be disturbed. Now pour in 4 drs. of mercury. In a short time the silver will be precipitated in the most beautiful arborescent form, resembling real vegetation. This has been termed the Arbor Diana.