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Cold Silvering

Mix 1 part glass, from which the mercury was drained, chloride of silver with 3 parts pearlash, 1½ at the bottom of the frame. To convex and parts common salt, and 1 part whiting, and concave mirrors the amalgamated foil is aprub the mixture on the surface of brass or plied by means of accurately fitting plaster copper (previously well cleansed), by means moulds. The interior of globes is silvered by of a piece of soft leather or a cork moistened introducing a liquid amalgam, and turning with water and dipped into the powder. 1 about the globe till every part is covered part precipitated silver powder, mixed with 2 with it. (See Nos. 3538 and 3545.) parts each cream of tartar and common salt, 3615. To Silver Glass. An easy and may also be used in the same way. When economical process. Mix 90 parts by measure properly silvered, the metal should be well of a solution of Rochelle salts at 1.50 specific washed in hot water slightly alkalized, and gravity, with 900 parts distilled water, and then wiped dry. boil them in a flask; drop in carefully 20 parts 3612. Spencer's Method of Silvering of a solution of nitrate of silver specific grav- Wood. The first operation is to take strong ity 1.18, and boil again. This solution can alcohol or spirits of turpentine in a glass ves- be bottled and kept for any length of time. sel, and add to it a piece of phosphorus (a Another fluid has to be prepared by adding common corked phial will answer the pur- ammonia to a solution of nitrate of silver pose); the vessel must now be placed in hot until the precipitate is entirely dissolved; filwater for a few minutes, and occasionally tering and diluting 1 part of it with 100 parts shaken; by this means the alcohol will take of water. For use, put equal parts of the two about 3 per cent. of its bulk of phosphorus, preparations in a suitable vessel, clean the glass well (sce No. 3621), and immerse it in mains to polish and burnish the film of silver the mixture until sufficiently coated. The deposited, in order to make it perfectly coating of silver should be protected with a smooth, and give closeness to the grain. To coat of lac varnish. 3616. Drayton's Process for Silvering Glass. Mr. Drayton mixes 1 ounce nitrate of silver, 3 ounces water, 1 ounce liquid ammonia, and 3 ounces spirit of wine, and filters the solution after it has stood 3 or 4 hours. cover a three-feet square of glass requires 5 pints of liquor. The deposit is, therefore, about 14 drachms to every 9 square feet. To preserve the coating of silver from sulphuration and rubbing, it is covered with a paint made with 1 pound of lead pigment, 14 of turpentine. Liebig has produced the same result by depositing on the silver a coating of galvano-plastic copper, but the advantages resulting from the greatest solidity of the To every ounce of the solution he adds ounces of drying oil, and 5t ounces of spirits ounce sugar (grape sugar if possible), dissolved in equal quantities of water and alcohol. The surface to be silvered is covered with this liquid at a temperature of 160° Fahr., maintained till the deposition of silver is complete. deposit scarcely compensate for the practical When quite dry, the coated surface is covered inconveniences of the process. with mastic varnish. Other substances besides sugar occasion the deposition of silver from the ammoniacal solution; as oil of cassia, oil of cloves, and other essential oils, aldehyde, &c. Unger recommends a strong alcoholic solution of tannin. He had accident- and then filter it. Just before using, add to 3618. To Silver Specula and Other Glass Surfaces. Make a solution of anmonio-nitrate of silver, of the strength of three grains to the ounce. Render it very slightly turbid by excess of nitrate of silver, ally mixed in a dish a small quantity of a each ounce of the foregoing solution 2½ grains thick alcoholic solution of tannin with an of Rochelle salts. Having scrupulously equally small quantity of a strong solution of cleaned the glass intended to be silvered nitrate of silver; and in the course of a short (see No. 3621), place it in a convenient vessel time he found the dish coated with a thin, about one inch from the bottom, supported brilliant, and uniform layer of metallic silver. on three little cones of white wax. The He directly repeated the experiment, and met glass plate may be suspended; but in that with the same result again and again. He case there is more difficulty in avoiding vibranext proceeded to evaporate the liquid to dry- tion, the absence of which is essential to ness by placing the dish on the surface of success. Expose to a northern light, or any warm sand. As soon as it was completely other subdued light, and in about two hours dry, the coating was found to be so fast on the deposit of silver will be sufficiently the porcelain that it required the point of a thick. It must now be carefully removed, sharp penknife to scrape it off. He also succeeded in producing a brilliant metallic coating from a saturated solution of sulphate of copper by the same solution of tannin. 3617. Pettijean's Process of Silvering Glass. Two solutions are to be prepared. The first is composed of 26½ drachms nitrate of silver and 2 ounces aqua ammonia, dissolved in 1 pint of distilled water. After filtration this liquor is diluted with 16 times its volume of distilled water, and, drop by drop, a solution of 116+ grains of tartaric acid is added.

The second is prepared in the same manner, but with a double quantity of tartaric acid.

As these solutions are rapidly reduced, prewashed, and dried. When the surface next the glass is to be used as the reflector, the glass side should be cleaned by nitric acid if the state of its surface, after the silvering, so require; and the silvered side should receive a protecting coating of a good tough black varnish.

3619. Liebig's Process for Silvering Glass Mirrors. The process of silvering glass generally rests on the reduction of metallic silver from a solution by means_of glucose or some other organic substance. By Liebig's method the deposit of silver is produced by the action of a mixture consisting of 50 parts by measure of a silver solution, and 10 parts of a reducing solution, this latter pare in the morning the liquors to be used previously diluted with 250 to 300 parts waduring the day. Before silvering, the glass is ter. The components of the silver solution perfectly cleaned, first with chalk and a fine arc: 140 parts of a solution containing 10 per cloth, then with a bung and a little of the cent. of nitrate of silver; 100 parts of a solufirst solution. It is then rubbed dry with a tion of nitrate of ammonia (free from chlorine) piece of chamois leather. (Sce No. 3621.) of 1.115 specific gravity (or a solution of sul- The glass, laid horizontally upon a table of phate of ammonia of specific gravity 1.105cast iron, at a perfect level, is heated (by 1.106;) lastly, 750 parts of caustic soda lye of means of a cast iron water-bath beneath) to specific gravity 1.050. In case sulphate of 113° Fahr., an India-rubber roller dipped in ammonia is used, its solution must be added distilled water is next passed over its surface, to the silver solution, not as in the case of and then its surface is covered with No. 1 nitrate. The reducing solution consists of 1 solution. The deposit of silver commences in about 10 minutes, and is completed in about 15 minutes afterwards. The glass is then tilted up so as to allow the liquor to run off, and rinsed with water rather more than lukewarm to carry away the non-adherent powder. It is then restored to its horizontal position and covered with solution No. 2.

In a quarter of an hour the deposit is completed. The next thing is to wash the plate as before, and dry it, after which it only repart by measure of sugar liquor and 1 part of copper liquor.

The sugar liquor is prepared by dissolving 50 grammes (771) grains) white sugar in water to a thin syrup, kept for 1 hour at a boiling heat with 36 grammes (48 grains) tartaric acid; the solution is then diluted to measure 500 cubic centimeters (16 pints).

The copper liquor consists of a solution of. 2 grammes (44 grains) dry tartrate of copper in water, by the aid of a caustic soda solution added by drops until the blue salt is dissolved; the whole is then diluted with water to measure 500 cubic centimeters (1To pints).

The glasses to be silvered, if for mirrors, are placed upright on their edge in the silvering tank and held together in pairs by clamps; when for optical purposes, they are held in a horizontal position, just touching the surface of the fluid. In cold seasons the temperature must be kept at 68° to 84° Fahr. The quantity of silver necessary for a square yard of surface is from 46 to 54 grains. 3620. Bird's Process for Silvering Mirrors or Specula. The mirror or speculum to be silvered is first cleaned (see No. 3621), and then suspended, face downwards, in a silver bath prepared thus: A large flat shallow vessel of glass or porcelain is provided, to contain the solution. 750 grains nitrate of silver are dissolved in 6 ounces distilled water, and to this is added pure liquid ammonia, drop by drop, until the precipitate which is thrown down is redissolved. 2 ounces caustic potash are dissolved in 50 ounces, by measure, of rain water; and 15 ounces of this solution are added to the ammoniacal solution, when a brown-black precipitate will be produced.

Ammonia is again added, drop by drop, until this precipitate is just redissolved; and 29 ounces of distilled water are then added to the whole. To this mixture is again added, drop by drop, stirring with a glass rod, a strong solution of nitrate of silver, until a precipitate, which does not redissolve, begins to be formed. Previous to immersing the speculum, 1 part, by weight, of powdered with the purest cotton wool. The surface is then wetted entirely with dilute nitric acid, and afterwards thoroughly washed with distilled water poured over it; and, last of all, the piece of coated glass is suspended in a flat vessel containing alcohol, where it remains until the bath is ready to receive it. 3622. Το Polish a Silvered Surface on Glass. To accomplish this, rub the surface gently, first with a clean pad of fine cotton wool, and afterwards with a similar pad covered over with cotton velvet, which has been charged with fine rouge. The surface will, under this treatment, acquire a polish of intense brilliancy, quite free from any scratches. 3623. To Silver Glass for the Reflectors of Telescopes. The solutions employed are four in number, and they require some care in their first preparation; but once made they are always ready, and can be used with great rapidity and certainty for depositing a lustrous, mirror-like surface of silver on a piece of glass of any desired shape or curvature :-

Solution No. 1 is prepared by dissolving 1 part, by weight, of nitrate of silver, in 10 parts of distilled water.

Solution No. 2 consists of an aqueous solution of pure ammonia, having a density of 13.3° Baumé.

Solution No. 3 consists of 4 parts of pure caustic soda in 100 of distilled water.

Solution No. 4 is made by dissolving 12 parts of the best white loaf sugar in 100 parts distilled water. To this add 1 part, by measure, of nitric acid, boil for 20 minutes, in order to invert or alter the molecular arrangemilk sugar to 10 parts, by measure, of distilled ment of the particles of the sugar, and then water, must be prepared in a separate vessel, add water to increase the volume to 500 parts and filtered until a clear solution is obtained. by measure, and finally add 50 parts alcohol. Then, to 10 parts, by measure, of the silvering These solutions will remain unchanged for solution, must be added 1 part, by measure, of a long time. When required for use, prepare the milk sugar solution, and, finally, 50 a silvering liquid by pouring into a flask 12 ounces of the compound solution will be parts, by measure, of the silver solution, No. sufficient to silver a speculum 9 inches in diameter. To facilitate the suspending, a circular block of wood is very firmly cemented to the back of the speculum with marine glue or pitch, and three pins inserted at equal distances round the margin, to which strings may be fastened. On lowering it into the bath, care must be taken that no air bubbles intervene, that the speculum be not deeper in the liquid than half its thickness, and that a depth of 2 inches, at least, intervene between the face of the speculum and the bottom of 1; 8 parts, by measure, of the ammoniacal solution, No. 2; then 20 parts of the soda solution, No. 3; and, lastly, add 60 parts of distilled water, in order to make up the volume to 100. If the proportions have been properly observed, the liquid so prepared will be perfectly clear, but will be rendered turbid by the smallest addition of nitrate of silver solution.

It must be allowed to remain without disturbance for 24 hours, to permit the floating particles to settle. The clear liquid decanted from the sediment will then be ready for use. the vessel. In 10 minutes after immersion a The surface of the glass which has to be metallic film will be seen forming on the silvered must be well cleaned with a tuft of glass, and in an hour or two a compact silver coating will be laid over the whole surface.

The speculum should remain in the bath for 4 hours, by which time the process is completed; it is then carefully removed, copiously washed with distilled water, and placed on or its volume of the sugar solution, cotton and a few drops of nitric acid, and then washed with distilled water. (See No. 3621.) Drain it, and support it on the surface of the silvering bath, which is composed of the above described silvering liquid, with the addition of its edge to dry. It is then ready for polishing. (See No. 3622.) 3621. To Clean the Surface of Glass for Silvering. As the success of the silvering process depends greatly on the glass surface being made chemically clean previous to immersion in the bath, the utmost pains must be taken to accomplish this object. The surface is first covered with thick whiting cream. free from grit, which, when dry, is rubbed off 1 No. 4. The surface to be silvered, should, by preference, be at the upper part of the liquid, so that the silver may be deposited on it from below upward. There are two advantages in this-first, the deposit is finer and more even: and, second, there is no danger of floating particles of dust settling on the surface. It is, however, scarcely necessary to say that silver will be deposited upon every part of the glass which is under the surface of the liquid, as well as upon the sides and bottom of the put in a closely stoppered bottle until the vessel; so that, as a matter of economy, as little as possible of the back of the glass Liquor No. 2. -Dissolve 20 parts nitrate of moment of using. should be exposed to the action of the liquid.

The action seems to be more rapid in the light than in darkness. Under the influence of diffused light the liquid becomes yellow, then brown, and in a few minutes the whole silver in 20 parts solution of ammonia, and add to this solution 650 parts distilled water.

When it is intended to operate, the two preceding liquors are mixed in equal quantities, and, after having been well agitated, filtered. of the exposed surface of the glass will be As the solution of ammonia of commerce has covered with a fine deposit of silver. In not always the same degree of concentration, about a quarter of an hour the thickness of it would be better, perhaps, to dissolve the nesses of blotting-paper to dry spontaneously. the mixture. The deposition of silver can be the metallic coating will be sufficient to bear the subsequent operations without injury; it must then be washed with plenty of water, and rested by one corner on several thicknitrate of silver destined for the liquor No. 2, first in distilled water, then mix the solution with liquor No. 1, and then add ammonia in quantity only just sufficient to entirely clear accelerated by the employment of heat; in this case, the temperature depends upon the nature of the objects to be submitted to the operation. The method of employing the above liquors in silvering the surfaces of different materials is given in the following six The surface will now be covered with a thin whitish veil, which may be readily removed by gentle friction with chamois leather; it may afterwards be polished with jewelers' rouge, when a perfectly brilliant surface will be produced. (See No. 3622.) 3624. To Repair the Silvering of receipts:

Looking-Glasses. The repairing of the sil- 3627. Το Silver Silk, Woolen, Cotton, vering on the backs of looking-glasses has Etc. When it is intended to silver silk, hitherto been considered a very difficult woolen, cotton, etc., commence by washing operation. A new and very simple method, the substance clean; this done, immerse it for however, has been described before the Poly- a moment in the saturated solution of gallic technic Society of Leipsic. It is as follows: acid; then withdraw it to plunge it for a Clean the bare portion of the glass by rubbing second in another solution composed of 20 it gently with fine cotton, taking care to re- parts nitrate of silver to 1000 parts distilled move any trace of dust and grease. If this water. These alternate immersions are concleaning be not done very carefully, defects tinued, until the substance from being dark will appear around the place repaired. With the point of your knife cut upon the back of another looking-glass around a portion of the silvering of the required form, but a little larger. Upon it place a small drop of mercury; a drop the size of a pin's head will be sufficient for a surface equal to the size of the nail. The mercury spreads immediately, penetrates the amalgam to where it was cut off with the knife, and the required piece may now be lifted and removed to the place to be repaired. This is the most difficult part of the operation. Then press lightly the renewed portion with cotton; it hardens almost immediately, and the glass presents the same appearance as a new one.