The metallic oxides necessary for the production of coloured glass, are introduced into the crucibles with the raw materials. Sheets or circles may be entirely gathered from one crucible, or from more than one, so as to produce a glass composed of two differently coloured layers. Copper - ruby and gold - ruby glasses are always treated in this manner, on account of the great strength of the colours. Ruby is often cased upon blue, green, and yellow, as well as upon white; and blue upon white and green. These cased glasses are particularly useful for representing heraldry, as the casing can be removed by abrasion, or by hydrofluoric acid, and the subjacent ground discovered. Splashed or sprinkled glass is produced by rolling the gathered mass of molten glass in small fragments of differently - coloured glasses; the fragments become incorporated in the molten glass, and expand together with it.

Only 2 transparent glass stains are at present known: a yellow stain, produced by silver oxide; and a ruby, by cuprous oxide, the latter being very rarely used. A stain may be roughly described as a transparent effect of colour obtained by applying certain metallic oxides to the surface of the glass, in the same manner as pigments are applied to canvas or paper, and by subjecting the glass to heat. The stain should be incorporated in the glass, and be as durable as the glass itself.. For yellow, either oxide or nitrate of silver is used; the latter is preferable by reason of its solubility and easy manipulation. In either case, it is necessary to employ some finely - divided infusible medium, moistened with water or tar - oil. The media generally used are iron peroxide and kaolin. (Powell.)

(2) The articles made at Venice of black glass are distinguished above all other productions of that sort by their deep black colour, and on this account meet with much favour. Dr. Kayser, of Nurnberg, investigated the composition of a frit from a Venetian glass factory, and also some black glass wares made there. The following were the results: -

(a) The glass frit had the ordinary appearance of a frit, and showed under the microscope isolated small pieces of black glass. In hot water, 29.7 Per cent. were soluble, consisting of alkalies and salts of magnesia. These were in combination with carbonic acid, chlorine, and sulphuric acid.

The frit contained -

(b) A rod of black glass, such as is used for the manufacture of glass pearls, contained: -

From the analyses it is probable that the black colour of the glass is caused by the large proportion of manganese in it. To certify this conclusion, a mixture of sand, soda, and powdered manganese ore (to the extent of 15 per cent.) was melted in a Perrot's glass furnace. The glass substance obtained was deep black while in very thin layers, and dark violet when drawn out in very thin threads, as well as in thin splints. Hence, in point of colour, it was exactly like the Venetian black glass. (Gewerbehalle)

(3) Glass may be stained by painting its surface with a fusible coloured glass ground to a fine powder and mixed up with gum - water or turpentine, and, after drying, then heating the painted glass in a furnace until the coating fuses. Collodion, shellac, or spirit - copal varnishes, coloured with one of the coal - tar dyes, can in some cases be advantageously resorted to as a coloured wash for white glass.

(4) The pigments commonly employed for decorating glass and porcelain have hitherto been prepared either by melting the metallic salt, which is generally the nitrate, in rosin (colopho - nium), or by decomposing soluble rosin soaps with the solutions of these salts, whereby an insoluble resinate is formed, which is first dried and then dissolved, just as that formed by fusion is in oil of turpentine, lavender, nitro - benzol, or some similar solvent. Both these methods of preparation have their, disadvantages, the principal being that a considerable quantity of the metallic salt remains undissolved, and when the resinous mass is dissolved, it is precipitated and lost, or at best is only - recovered by a tedious operation. With the help of carbolic acid, these pigments can be prepared without difficulty, and without any insoluble metallic compounds separating, worth mentioning.

Bismuth

2} dr. metallic bismuth are dissolved in aqua - regia, and evaporated in a porcelain dish to a thin syrup. When cold, 121/2 dr. carbolic acid liquefied by gently warming in hot water are added. It is left standing a few hours, for, if warmed and stirred at once, an energetic reaction takes place, with violent foaming. At the end of this time, it is well stirred with a glass rod and heated awhile in a steam bath, when there will be an evolution of hydrochloric acid vapours. It is taken out of the steam bath as soon as a drop removed on a glass rod will dissolve clear in nitre-benzol. When this point is reached, the mass is dissolved in nitro-benzol or a mixture of nitro-benzol and oil of spike, when the preparation will be ready to use.

Tin

2 1/2 dr. pure tin are dissolved in aqua - regia, and the solution is evaporated to a thin syrup, then mixed with 12 1/2 dr. carbolic acid in the manner above described. The remainder of the operation is the same as for bismuth.

Uranium

3 3/4 dr. uranium nitrate are mixed with 10 dr. hydrochloric acid and dissolved. This solution is also mixed with 12 1/2 dr. carbolic acid, as before, and treated as already described.

Iron

3 3/4 dr. iron perchloride are dissolved in pure hydrochloric acid, and any excess is removed by evaporation, so that the solution when cold will have the consistence of a thin syrup. To this are added 12} dr. carbolic acid; and it is then treated as described under bismuth.

A managanese pigment can be made from manganese chloride, and nickel and cobalt pigments from their chlorides, in precisely the same manner as that of iron is made from its chloride. The finished preparation can be diluted to any desired extent, as the concentration of the original preparation leaves plenty of play for the dilution. The different pigments above described may be mixed with each other to form all kinds of combinations. (Dr. R. Kayser, Deut. Ind. Zeit.)