In a few minutes the solution will turn brown, then nearly black, and a silver film will be seen to form on the glass surface, and gradually spread over it. This takes place more rapidly if the solutions and mirror are all warmed to about 90° F., when the process will be completed in a few minutes; but, with cold solutions, and leaving all at rest all night, yon may get good, dense, brilliant films. When the silvering is completed, the glass is well washed with plenty of water, finishing with distilled water, and set on edge to dry, after which it is gently polished with wash-jeather, and a little very fine rouge. - (A. W. Black-lock.)
(g) Some additional hints which ex-perience has shown to be necessary for complete success are as follows: -
One of the most essential preliminaries is to see that the mirror to be silvered is chemically clean. When pitching on the block for suspension in the silver fluid, in spite of every care, the front surface of the mirror is very liable to be stained by pitch on the fingers, or particles touching the glass, and no amount of washing by water, or even nitric acid, will thoroughly remove the stains. The mirror will be spotted and speckled, when silvered, and the failure disheartening. To avoid this, first sponge the surface to be silvered, and the edges of the mirror, with turpentine, then wash well with soap and water under a flowing tap, and, finally, sponge the surface with a solution of nitric acid, washing afterwards with pure water till every trace of acid is removed. The mirror will then be chemically clean.
Another fruitful source of failure arises from water dripping from the back and sides of the mirror into the silvering solution. To prevent this, when the mirror is chemically cleaned, place it at once, face downwards, into a dish of distilled water, which contains just so much as will completely cover the surface of the mirror, and come up one-third of its depth. Here leave it till the silvering solution is mixed, and everything is ready for immersion.
By this time the back of the mirror is dry, and so much of the edge as will, when the mirror is immersed, be above the solution. Thus the dripping will be avoided, and the mirror will take the silvering, pure and clean, all round the edges, as well as in the central zones.
Transfer the mirror from the distilled water as quickly as possible, taking care that it sinks to the proper depth in the silvering solution, and equally all round.
With respect to the strength of the solution in winter, it is better to make it (as far as silver is concerned) a little stronger than that given in the formula, and also, in cold weather, it will often take a much longer time than an hour to produce a satisfactory result. If taken out too soon, in cold weather, the coating will be so thin that it cannot be polished.
Have a large jug of water ready at hand, and wash immediately on removal from the solution; then under a tap for 1/4 hour. If the process has been, as it ought to be, successful, the mirror may be polished, after 3-4 hours' drying in a warm room; but it is better to defer the polishing till the next day to secure perfect dryness and firmness. - (S. Mills.)
(h) Prof. Palmicri has devised a process for silvering glass by means of a reducing action on the salts of silver, which is said to have the advantage of producing a very brilliant metallic deposit. When into an ammoniacal solution of silver nitrate is poured, first a little caustic potash, and then a few drops of glycerine, the reduction begins at once; and this action is accelerated if ether or alcohol be added to the mixture. A moderate heat and darkness are said to increase the brilliancy of the precipitate, and darkness also favours the adhesion of the deposit to the mirror.
(i) Solution 1. Silver nitrate, I oz.; water, 10 oz. Solution 2. Caustic potash, 1 oz.; water, 10 oz. Solution 3. Glucose, 1/2 oz.; water, 10 oz.
The above quantities are those estimated for 250 sq. in. of surface. Add ammonia to solution No. 1 till the turbidity first produced is just cleared. Now add No. 2 solution, and again ammonia to clear; then a little solution, drop by drop, till the appearance is decidedly turbid again. Then add No. 3 solution, and apply to the clean glass surface. A film was obtained in 43 minutes at a temperature of 56° F. The plate of glass was rather large: 37 in. diameter and 4 1/2 in. thick, and weighed 4 cwt. - (A. A. Common.)
(A) Prepare four solutions - first, 10 grm. silver nitrate to 100 grm. water; second, an aqueous solution of ammonia of 0.984 density; third, 20 grm, caustic soda and 500 grm. water; and fourth, a solution of 25 grm. sugar in 200 grm. water, to which is added 1 cc. nitric acid at 36° B.; and let the whole boil for 20 minutes. When cold.add 50 cc. alcohol, and as much water as will make up the total quantity to 500 cc.; then take 12 parts of the first, 8 parts of the second, and 20 parts of the third solution, and 60 parts of water, and let the mixture stand for 24 hours; lastly, the solution No. 4 is added when the whole becomes of a blackish tint, in consequence of the finely-divided precipitate of silver which begins to fall.
(l) To microscopists, who, like the writer, live remote from the great commercial and industrial centres, and who are consequently frequently compelled to rely on their own mechanical skill and ingenuity, or their own technical knowledge, in emergencies where their more fortunate brethren can call to their aid the services of skilled mechanicians and instrument makers, the following will prove interesting and valuable. A short time since, minute specks began to show themselves in the mirrors, both plane and concave, of my Nachet microscope, and soon both became worthless as reflectors. Upon examination I found that the varnish protecting the amalgam had cracked into numberless little fissures, and the amalagam itself had become detached from the glass. Compelled to use my instrument almost every day, professionally, whilst it would require from two to three weeks to send off to have the damage repaired, I determined to resilver the mirrors by one of the processes now in general use in the arts and sciences, and of which to a certain extent, I had my choice.