While a student, several years ago in the laboratory of Baron von Liebig, in Munich, I assisted in conducting a series of experiments looking to the manufacture of silvered mirrors on a large commercial scale (with a view of doing away with the mercurial amalgam process, which pernicious industry was then attracting general attention in Bavaria), and by the use of the Liebig method, and what was afterwards known as the Siemens modification of the same had succeeded in making some eiquisite mirrors of a large size and almost perfect reflecting surface. Where nm-mouium aldehyde (C, H, (NH4) O) can be obtained, there is no doubt that this is the heat and most economical process, whether used on large or a small scale.
The next best process is based upon the reduction of metallic silver from its am-moniacal solution by salts of tartar. Haviag tried several formulae of this kind, all of them more or leas simple, as well as efficacious, I hare found the fol-loniog to yield the best results in the shortest time.
In 1 oz. distilled or pure rain water, dissolve 48 gr. crystallised silver nitrate. Precipitate by adding strongest water of ammonia, and contiue to add the ammonia drop by drop, stirring the solution with a glass rod, until the brown precipitate is nearly, but not quite, redissolved. Filter, and add distilled water to make 12 fl. dr.
Dissolve in 1 oz. distilled or very clean rain water, 12 gr. potassium and sodium tartrate (Rochelle or Seignette salts). Boil in a flask, and while boiling add 2 gr. crystallised silver nitrate dissolved in 1 dr. water. Con-tinue the boiling 5-6 minutes. Let cool, filter, and add distilled water to make 12 fl. dr.
For a mirror 1 1/4-1 1/2in. diameter, take an ordinary 2 oz. grado-ated glass; procure a piece of thin wood (cigar box will do) lung enough to go serosa the top of it, and through the centre of the wood thrust, as shown in Fig. 103, n wire 7-8 in. long. After cleansing the glass to be silvered by immersing it in strong nitric acid, washing in liquor potassa, and thoroughly rinsing with distilled water, with a bit of sealing was attach one end of the wire to its face, as in Fig. 103. If the glass has had mercurial amalgam on it, it will probably be necessary to clean the back with rouge. On having this surface perfectly, chemically clean, depends in a great measure the success of the operation.
Having attached the glass to the wire, lay the strip across the graduate, move the glass disc downwards until it nearly, but not quite, touches the sides of the graduate all round, taking care that its edges shall be as nearly level as possible. Having ascertained the height in the graduate at which the disc should stand, bend or clamp the wire so that it cannot slip. In the ordinary American graduate, with a mirror 1 3/8 in. diameter, this will be at the 6 dr. mark, as nearly as may be. Remove the glass and pur into the graduate enough of equal quantities of the two solutions to fill the graduate exactly to the previously ascertained level. Stir the solutions so that they will become thoroughly mixed, and replace the disc to be silvered, taking great care that the surface to be silvered shall come in contact with the silvering fluid exactly at all points. The disc should be rinsed carefully before replacing, and should be put in while wet. Great cure should be taken that no air bubbles remain on the surface of the solution, or between it and the surface to be silvered.
Now set the graduate in the sun for a few minutes, if the weather be warm, or by the fire, if it be cold, as a temperature of 113°-122° F. is most conducive to the rapid deposition of a brilliant, firm, and even film of silver. The fluid in the sunlight soon becomes inky black, gradually clearing as the silver is reduced, until when exhausted it is perfectly clear. The mirror should be removed before this point is reached, as a process of bleaching sets up if left after the fluid is exhausted. From 20-80 minutes, according to the weather, purity of chemicals, etc, is required for' the entire process.
When the mirror is removed from the bath, it should be carefully rinsed with distilled water from the wash bottle, and laid on its edge on blotting paper to dry. When perfectly dry, the pack should be varnished with some elastic varnish and allowed to dry. The wire and sealing-wax can now be removed from the face, and the glass cleaned with a little pledget of cotton and a minute drop of nitric acid, taking great care that the acid does not get to the edges or under the varnish. Rinse, dry, and the mirror is finished.
The light reflected from a mirror made thus has somewhat of a yellowish tinge, but photometric experiments show that 25-30 per cent. more light is reflected than from the old mercurial mirrors. - (Dr. James.)
(m) In Brashear's process the most important thing is the sugar solution forming the reducing agent. This greatly improves by keeping - a solution that has been made some months being much more effective than a newly-made one. It is convenient to have always some Winchester quarts of it in stock for use. I have, for convenience, varied his proportions slightly, and thus give them, as I have found them work so well. For the sugar solution I add to 10 per cent. of loaf sugar, in distilled water, 10 per cent. alcohol and 1/2 per cent. nitric acid. Solutions of 10 per cent. silver nitrate and of caustic potash are separately prepared, the latter one as wanted. These, with sufficient ammonia and a very dilute solution of silver nitrate, and also a similar vary dilute one of ammonia, are prepared, the latter in order to obtain that pale brown colour of the ammoniated solution of silver nitrate that is absolutely necessary to have before adding the reducing agent.