Glass for coloured windows is made either as crown- or as sheet-glass. The circles and sheets, however, are thicker in substance and smaller in dimensions than those used for ordinary glazing. The manipulation of coloured circles is the same as that of crown-glass. In the manipulation of the coloured sheet-glass, so soon as one end of the cylinder is opened, a flattened circular mass of glass, attached to the solid working-rod, and larger in diameter than the cylinder, is made to adhere to the opened end of the cylinder, the closed end being at the same time detached by a sharp blow from the end of the blow-pipe. The cylinder is now manipulated by the working-rod. The small broken end is heated and gradually opened, and the finished cylinder is detached from the circular mass of glass, and sent to the annealiug-kiln. The size of the cylinder is usually only about 14 in. x 7 in.; the size of the sheet, 14 in. x 21 in. The attempted process of ripping up the cylinder as soon as made, by means of a large pair of shears, iu order that the cylinder may be flattened without being previously annealed, is too dilatory to be practically useful ; the coloured cylinders are annealed, cut by a diamond, and flattened, in the same way as the thick cylinders of ordinary sheet-glass.
The metallic oxides, necessary for the production of colours, are introduced into the crucibles with the raw materials. Sheets or circles may be entirely gathered from one crucible, or from more than one, eo ae 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 hydrofluorio 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. Sheets of glass marked with irregular squares or oblongs are often used for domestic glazing, in which translucency without transparency is desired. This effect is produced by inserting the partially expanded bulb, which is formed in making sheet-glass, into a mould marked by deeply indented ribs, and by forcing the glass by the pressure of the breath to adapt itself to the form of the ribs. By twisting the hollow ribbed mass, the lines are made to cross, and the cylinder is finished in the ordinary way. The bulls'-eyes so largely used now for domestic glazing, in imitation of the centres of old crown-glass, are formed like crown-glass on a very minute scale.
Hartley's rolled coloured plate is used in considerable quantities for domestic glazing. Glass stamped with patterns, and shaped into quarries, is produced in the same manner as pressed-glass.
These quarries, when part of the stamped pattern is picked out with yellow stain, are very effective and cheap.
The first step in the manufacture of a mosaic window is the drawing of a small sketch to scale. According to the general features of the sketch, a full-sized drawing or cartoon is formed; a tracing on transparent cloth is then taken from the cartoon. Upon this cloth, are marked the shapes in which the pieces of glass must be cut, also numbers, indicating which colours are to be used. The cutter selects the glasses, and places them over the working drawing, which is raised upon e, transparent desk, and marks the shapes with a diamond. The glass is severed by a sharp blow, and minute alterations of curvature or size are effected by means of clippers. If the window is merely to be filled with glass ornamentally arranged, it is only necessary to unite the different pieces in a panel; this is usually accomplished by surrounding and binding together the pieces with doubly-grooved strips of lead. These are first roughly cast in lengths of about 1 ft. These lengths are separately placed in a mill, and compressed between two wheels, revolving in a perpendicular plane and an inward direction. On either side of the space between the edges of the two wheels, is a cheek, which forms the double leaf of the leaden strip. When the end of a length of lead is introduced between the wheels, and these are caused to revolve, the lead is drawn inwards, and at the same time compressed and elongated. The lead issuing from between the wheels is usually more than, six times longer than when it was inserted. When the pieces of glass have been united, and a panel has been formed, and bound round with a leaden grooved strip similar to, but stronger than, that used for uniting the small pieces of glass, the joints of the different pieces of lead are made good by solder ; and finally, the union of glass and lead is rendered impervious to moisture, by temporarily raising the leaf of the lead, and rubbing in a cement made of boiled oil and white-lead.
Stains.—There are only two transparent glass-stains at present known : a yellow stain, produced by oxide of silver ; and a ruby, by cuprous oxide. The latter is very rarely used. A stain may be roughly described as a transparent effect of colour obtained by applying oertain metallic oxides to the surface of 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 should 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 peroxide of iron, and kaolin.