(D) Large Mirrors For Photography

Dissolve 150 gr. silver nitrate in 6 oz. distilled water, and to this add ammonia, drop by drop, until the precipitate at first thrown down is redissolved. Now, ! having made a solution of caustic potash, in the proportion of 2 1/4 oz. to 50 oz. water, add 15 oz. of this to the above solution of silver; and add ammonia as before, until the deep-brown precipitate again thrown down is redissolved. Now add 29 oz. distilled water, after which drop in some solution of silver nitrate, gently stirring all the while with a glass rod, until a precipitate begins to be formed. Previous to the immersion of the glass to be silvered, dissolve 1 oz. sugar of milk in 10 oz. water. This must be filtere4 and kept in a separate bottle. Have ready a clean glass vessel of a size sufficient to contain the glass plate to be silvered; when everything is ready, mix together the silver solution with that of the sugar of milk, in the proportion of 10 of the former to 1 of , the latter. Lower the glass down in the i solution until it is a little distance from the bottom, and allow it to remain there for a period varying from 15 minutes to 4 hours, according to the thickness of the coating of silver desired.

After removing it from the bath, wash with distilled water, and, when dry, polish by means of a soft pad of cotton-velvet charged with rouge. An intensely brilliant surface may be thus obtained on both sides of the glass plate. Make a 3-gr. solution of ammonio-nitrate of silver. Render it slightly turbid by excess of silver nitrate, and then filter it. Just before using it, add to each ounce of the foregoing solution 2 1/2 gr. Rochelle salt, immerse the glass as before, and expose to a subdued light while it remains in the bath. In about 2 hours the deposit of silver will be sufficiently thick.

(E) Specula

Prepare three standard solutions. Solution A - Crystals of silver nitrate, 90 gr.; distilled water, 4 oz.; dissolve. Solution B - Potassa, pure by alcohol, 1 oz.; distilled water, 25 oz.; dissolve. Solution C - Milk-sugar, in powder, 1/2 oz.; distilled water, 5 oz. Solutions A and B will keep in stoppered bottles for any length of time; solution C must be fresh.

To prepare sufficient for silvering an 8-in. speculum, pour 2 oz. solution A into a glass vessel capable of holding: 35 oz. Add, drop by drop, stirring all the time with a glass rod, as much liquid ammonia as is just necessary to obtain a clear solution of the grey precipitate first thrown down. Add 4 oz. solution B. The brown-black precipitate formed must be just redissolved by the addition of more ammonia, as before. Add distilled water, until the bulk reaches 15 oz.', and add, drop by drop, some of solution A, until a grey precipitate, which does not redissolve after ' stirring for three minuses, is obtained; then add 15 oz. more distilled water. Set this solution aside to settle. Do not filter. When all is ready for immersing the mirror, add to the silvering solution 2 oz. solution C, and stir gently and thoroughly. Solution C may be filtered.

Procure a circular block of wood, 2 in. thick, and 2 in. less in diameter than the speculum. Into this should be screwed three eye-pins, at equal distances. To these pins fasten stout whipcord, making a secure loop at the top. Melt some pitch in any convenient vessel, and, having placed the wooden block, face upwards, on a level table, pour on it the fluid pitch, and on the pitch place the back of the speculum, having previously moistened it with a little spirits of turpentine, to secure adhesion. Let the whole rest until the pitch is cold.

Place the speculum, cemented to the circular block, face upwards, on a level table; pour on it a small quantity of strong nitric acid, and rub it gently all over the surface with a brush made by plugging a glass tube with pure cottonwool. Having perfectly cleaned the surface and sides, wash well with common water, and finally with distilled water. Place the speculum, face downwards, in a dish containing a little rectified spirits of wine, until the silvering fluid is ready.

(f) On the whole, the Rochelle salt process is the most certain, as well as the simplest. The two solutions are made thus: - Solution A - Silver nitrate in crystals, 10 gr.; distilled water, 1 oz. Dissolve the crystals in the water, then add liquid ammonia, drop by drop, until the grey precipitate is just redissolved. A few drops more of the silver solution are added, until there is a slight permanent precipitate, which does not re-dissolve. This solution is now filtered, and, if not required for immediate use, will keep for years. Perfect films have been produced with solution made nearly twenty years before. Solution B - Rochelle salt (potassio-tartrate of soda) dissolved in distilled water. In the original formula it was specified that crystals, must be used, and the strength was 10 gr. to the oz.; but crystals of Rochelle salt are not easy to get, and I used to dissolve some of the ordinary powdered variety in water and re-crystallise it; but 1 am now using 25 gr. of the powder instead, with perfect success and much less trouble. This solution does not keep more than a few days.

The glass to be silvered is cleaned in the usual way with pure nitric acid - some of the acid being poured on its surface and well spread with a brush or mop made by tying some calico round the end of a rod or strip of glass, using plenty of acid, and washing it off with a good stream of water from a tap; finishing with distilled water, and laying the glass, face downwards, in a dish of distilled water until everything else is ready. The dish in which the glass is to be silvered should be thoroughly cleaned and rinsed with distilled water, and should allow the mirror to lie face downwards with a depth of at least 1/4 in. between it and the bottom of the dish. The mirror may be supported on two small blocks of glass cemented by shellac to the bottom of the dish at the extremities of diameter, or the mirror may have a block of wood fixed on its back by pitch, and be suspended by strings. This should be done before cleaning, and the necessary quantity of bath solution ascertained. When all is ready, equal parts of silver ammonio-nitrate (solution A), of Rochelle salt (solution B), and of distilled water are taken and mixed together, and the cleaned mirror is immersed in it.