substance, it is essential that the colouring oxide be intimately mixed with the glass, and both be brought into a state of complete fusion. But glass vessels and panes for windows are coloured, very commonly, merely on their surface, the body being an ordinary colourless glass. In stained and painted glass, the metallic oxides, mixed with vitreous bodies capable of being easily fused, and with certain fluid vehicles (very frequently oil of turpentine), are applied to the surface of the pane, commonly where a design is required, by means of a brush; but the pigments may also be applied from an engraved metallic plate or wooden block, with gentle pressure, and the pane is then exposed in an oven or muffle to a temperature sufficient to fuse the vitreous flux and dissolve the colouring matter. The glass used for this purpose should be clear, colourless, and very difficult of fusion; hence crown-glass containing a small proportion of alkali is preferred.† In another method of obtaining a sheet of glass coloured on one of its surfaces, the glass-blower first collects the proper quantity of colourless glass on the end of

* Boiled oil is said to be the proper vehicle for vitrifiable pigments when printed on the glass. This application of boiled fixed oils, and also the use of an engraved metallic plate, as a printing surface, have been patented.

+ Fluor spar (fluoride of calcium) is often applied to crown-glass in this way to produce a degree of opacity, without colour, in imitation of ground glass. The mineral is applied to the surface of a pane, in a state of most minute division, and mixed with a fluid vehicle. After the glass has been moderately heated, so as to dry the coating of fluor spar but not to fuse it, portions of the coating may be scraped off with a view of obtaining a transparent design on a translucent ground. On exposing the glass thus prepared to a high temperature in an oven, fluosilicate of lime is formed, which fuses into the substance of the glass to a small depth, thereby communicating a permanent opacity.

the blowing-iron, then dips this lump for a moment into a pot of melted coloured glass, and blows out the two together into a cylinder or globe, which is extended into a flat plate in the ordinary manner. This method has long been practised in Bohemia, and is now become very general for colouring crownglass red by means of the suboxide of copper. Ornamental vessels of flint-glass are also coloured on their outside in a similar manner, and colourless facets may be produced on such vessels by cutting through the layer of coloured glass into the substratum of colourless glass.

A patent has been recently obtained for a method of ornamenting glass, as well as earthenware and porcelain, by a process similar to that for painting or staining glass. The glass vessel or plate to be coloured is first of all covered with a thin layer of some adhesive matter, for which purpose essence of lavender is said to be preferred by the patentee; and on this coating, the colouring oxides, in a state of very fine powder, and mixed with proper fluxes, are carefully dusted. The colour is then permanently fixed by the usual process of "firing." To obtain a coloured design, the surface of the glass may be printed with the adhesive varnish by an engraved block; and the superfluous colouring oxide which does not adhere to the varnish may be removed, previous to firing, by means of a pair of bellows. Or else, the glass may be entirely covered with the varnish, and the powder be sprinkled through a perforated screen, or an open fabric, such as lace, applied to the surface of the vessel or pane.

Articles made of flint-glass are sometimes orna

mented with delicate white Argentine incrustations formed of dry porcelain clay, cemented into a solid by means of a little plaster of Paris. These figures being thoroughly dried are placed on the red-hot bulb of flint-glass, and are immediately covered with a thin layer of glass in a very liquid state, so as to completely enclose the incrustations. The polished external layer of glass gives the white figures a very agreeable silvery appearance, and they may be easily coloured, if required, by proper metallic oxides before being applied to the glass. Flint-glass vessels with coloured enamel figures on their external surface are sometimes prepared by placing the enamel figure in its proper place in the mould into which the glass is blown. The glass being very hot, the enamel figure becomes firmly cemented to the surface.*

Etching on glass.-The art of etching on glass by means of hydrofluoric acid may be practised on

* A very singular method of forming pictures of coloured glasses was practised by the ancients, which seems to have consisted in laying together fibres of glass of various colours, so adapted to each other that a section across the fibres afforded a representation of the object to be painted, and then cementing the bundle into a homogeneous mass. The following interesting account of two of these productions is given by Winkelmann in his "Annotations on the History of the Arts among the Ancients." "Each of them is not quite one inch long and one third of an inch broad. One plate exhibits, on a dark ground of variegated colours, a bird, representing a duck of various very lively colours, more suitable to the Chinese arbitrary taste, than adapted to shew the true tints of nature. The outlines are well decided and sharp; the colours beautiful and pure, and have a very striking and brilliant effect, because the artist, according to the nature of the parts, has in some employed an opaque, and in others a transparent glass. The most delicate pencil of the miniature painter could not have traced more accurately and distinctly either the circle of the pupil of the eye, or the apparently scaly

all kinds of glass, but the most proper description is good crown-glass. The facts on which this art is founded are, that glass becomes powerfully corroded by exposure to hydrofluoric acid, and that certain parts of the glass may be easily protected by a resist varnish, on which the acid exerts no action except at a high temperature. The varnish usually employed by artists for this purpose is either common turpentine varnish mixed with a little white lead, or a strong aqueous solution of isinglass. In

feathers on the breast and wings, behind the beginning of which this piece had been broken. But the admiration of the beholder is at the highest pitch when, by turning the glass, he sees the same bird on the reverse, without perceiving any difference in the smallest points; whence we could not but conclude that this picture is continued through the whole thickness of the specimen, and that if the glass were cut transversely, the same picture of the duck would be found repeated in the several slabs; a conclusion which was still further confirmed by the transparent places of some beautiful colours upon the eye and breast that were observed. The painting has on both sides a granular appearance, and seems to have been formed in the manner of mosaic works, of single pieces, but so accurately united, that a powerful magnifying glass was unable to discover any junctures. This circumstance and the continuation of the picture throughout the whole substance, rendered it extremely difficult to form any direct notion of the process or manner of forming such a work; and the conception of it might have long continued enigmatical, were it not that, in the section of the fracture mentioned, lines are observ able, of the same colours which appear on the upper surface, that pervade the whole mass from one side to the other; whence it became a rational conclusion, that this kind of painting must have been executed by joining variously coloured filaments of glass, and subsequently fusing the same into one coherent body. The other specimen is of almost the same size, and made in the same manner. It exhibits ornamental drawings of white, green, and yellow colours, which are traced on a blue ground, and represent volutes, beads, and flowers, resting on pyramidally converging lines. All these are very distinct and separate, but so extremely small, that even a keen eye finds it difficult to perceive the subtle endings; those, in particular, in which the volutes terminate; notwithstanding which, these ornaments pass uninterruptedly through the whole thickness of the piece."

performing the process on a small scale, purified bees'-wax will be found to form a convenient varnish.

The piece of glass to be etched is first of all warmed in any convenient manner, and one of its surfaces is then rubbed over with the wax, the temperature of the glass being high enough to cause the wax to melt and be distributed uniformly over the entire surface. The glass is then set aside to cool; and when the wax is become quite solid, the design may be traced with a pointed, but not very fine, instrument, such as a bodkin. A carpenter's bradawl is a convenient tool for this operation, since, from being flattened at the end in one direction, it may be made to trace lines of different degrees of fineness, according to the position in which it is held. Care must be taken to cut through the entire thickness of the wax, so as to lay the glass quite bare through the whole length of the line.

The next part of the process consists in the application of the hydrofluoric acid. The vessel employed for this purpose is a shallow basin, either of lead or of Wedgwood's ware, (no glazed vessel should be employed,) large enough to include within its area every part of the design, when the prepared glass is placed upon its edge. The materials for generating hydrofluoric acid, consisting of one part of powdered fluor-spar, and about two parts of highly concentrated oil of vitriol, are introduced into the basin and well mixed; the glass plate is then laid on the edge of the basin with the waxed side undermost, and a moderate heat is applied to disengage the vapour of hydrofluoric acid. A spirit-lamp will be found a convenient source of heat, from the facility