As it is frequently convenient to be able to silver a piece of glass for a special purpose, we quote from Faraday's work on Chemical Manipulation, the following directions for performing this operation:
A piece of clean, smooth tinfoil, free from holes, is to be cut to the same size as the glass and laid upon a couple of sheets of filtering or blotting paper folded into quarters. A little mercury is to be placed on the foil, and rubbed over it with a hare's foot, or with a ball of cotton slightly greased with tallow, until the whole of the upper surface of the leaf be amalgamated and bright. More mercury is then to be added, until the quantity is such as to float over the unfoil. A piece of clean writing paper, with smooth edges, is to be laid upon the mercury, and then the glass surface, previously well cleaned, is to be applied to the paper. The paper is to be drawn out from between the mercury and the glass, while a slight but steady pressure is to be applied to the latter. As the paper recedes it carries all air and dirt with it from between the glass and the metal, which come into perfect contact.
The mirror is now made, and may be used for an experiment; but there is still much more mercury present than is required to make the definite and hard amalgam of tin constituting the usual reflecting surface. If it be desired to remove this excess, the newly-formed mirror must be put under the pressure of a flat board, in a slightly-inclined position, and loaded with weights.
The success of this operation will be found to depend chiefly upon the care exercised in cleaning the glass.
This is best effected by depositing pure silver on the glass. The light reflected from a mirror made thus has somewhat of a yellowish tinge, but photometric experiments show that from 25 to 30 per cent, more light is reflected than from the old mercurial mirrors.
Where ammonium aldehyde can be obtained, there is no doubt that this is the best and most economical process, whether used on a large or a small scale. But those who have not had considerable experience in the laboratory cannot always prepare this compound.
The next best process is based upon the reduction of metallic silver from its ammoniacal solution by salts of tartar. After a trial of several formulae of this kind, all of them more careless simple, as well as efficacious, the following has been fa and to yield the best results in the shortest time.
In 1 ounce of distilled or pure rain water, dissolve 48 grains of crystalized nitrate of silver. Precipitate by adding strongest water of ammonia, and continue 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 fluid drachms.
Dissolve in 1 ounce of distilled or very clean rain water, 12 grains of potassium and sodium tartrate (Bochelle or Seignette salts). Boil, in a flask, and while boiling add 2 grains crystalized nitrate of silver dissolved in 1 drachm of water. Continue the boiling five or six minutes. Let cool, filter, and add distilled water to make 12 fluid drachms.