3620. Bird's Process for Silvering Mirrors or Specula

3620.     Bird's Process for Silvering Mirrors or Specula. The mirror or speculum to be silvered is first cleaned (see No. 3621 (To Clean the Surface of Glass for Silvering)), 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 milk sugar to 10 parts, by measure, of distilled water, must be prepared in a separate vessel, and filtered until a clear solution is obtained. Then, to 10 parts, by measure, of the silvering solution, must be added 1 part, by measure, of the milk sugar solution, and, finally, 50 ounces of the compound solution will be 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 the vessel. In 10 minutes after immersion a metallic film will be seen forming on the 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 its edge to dry. It is then ready for polishing. (See No. 3622 (To Polish a- Silvered Surface on Glass).)

3621. To Clean the Surface of Glass for Silvering

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 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.