GLASS (Pr., Glace, V erre ; Ger., Glas) Ordinary glass is a fused mixture of silicates of calcium or lead with the silicates of sodium or potassium. While practically unaffected by acids, except hydrofluoric, it is attacked by strong alkalis, which dissolve out the silica. Some glasses are yellowed by prolonged exposure to light; this is frequently the case with old lenses.
Crown Glass is made usually from sand, lime, and a sodium salt, with sometimes lead oxide.
Flint Glass contains potassium carbonate, red lead, sand, and saltpetre. It is easily fusible, and is not so suitable for chemical purposes, for which a soda-lime or potash-lime glass is preferable.
Opal Glass is obtained by fusing an oxide of tin or zinc with the pot-metal ; a " flashed " opal, consisting of a thin opal layer united to plain glass, is also manufactured.
Optical Glass first received specific attention from Pierre Louis Guinand, of Les Brenets, in Switzerland (born 1748, died 1824), who obtained improved results by stirring the fused mixture with a rod of crucible clay. J. Fraunhofer (born 1787, died 1826) succeeded in avoiding striae and in procuring glass of a uniform refractive index by the simple expedient of using larger pots for melting ; he also investigated some of the optical effects resulting from variations in the ingredients and their proportions. In 1842 a son of Guinand introduced boracic acid into the glass, which, however, did not then meet with favour. In 1851 Maes, of Clichy, produced a colourless and homogeneous zinc crown glass, containing zinc oxide and boracic acid ; this was used in Charles Chevalier's photographic objectives. L. Seidel in 1856, and J. Petzval in 1857, pointed out that new glasses, having a different relation between their refractive and dispersive powers to those then in use, were required for the perfection of the photographic objective, but it was not till Otto Schott and E. Abbe began their joint work in 188r that any real progress was made. In 1886 the famous Jena works were started by Schott, Abbe, and R. Zeiss, under a liberal subvention from the Prussian Government. Since then the pro duction of optical glass of almost any required refractive or dispersive power has been rendered possible by the use of new chemical ingredients.
People often talk loosely of the Jena optical glass, as if there were only one variety, although, as a matter of fact, several hundreds of different varieties of optical glass are known, these including all the ordinary flints and crowns, besides the special glasses that have made the manufacture of the modern anastigmats possi ble ; it is these special glasses that are generally meant by the term Jena glass." With the older makes of optical glass, increased refractive power was always accompanied by a high degree of dispersion, but in the new Jena glasses a very high refractive power is obtained with a comparatively low dispersion.
(For the optical properties of glass, see "Lens.") Cements for Glass.—A cement for glass needs to be as colourless as possible, the best from this point of view being Canada balsam, which should be dried in an oven, allowed to cool, the glass gently heated, the remelted balsam thinly applied, and the surfaces brought together. This is the universal cement for lenses. Other ex cellent cements (but not suitable for lenses) are water-glass, which, however, tends to roughen the glass, and an emulsion of gelatine in suf ficient acetic acid to cover it, the process of digestion being assisted by standing the bottle in warm water.
In process work, glass is an important material. To secure good contact between the negative and metal plate in direct printing, the glass must be flat, and plate glass is preferable, though for the general run of work a good selected sheet glass is used. Plate glass of about + in. thickness is used for assembling a number of stripped films on to one plate. For process printing-frames, thick plate glass is used up to if in. thickness in the largest frames, and this glass must be well annealed in order to with stand the strain of the pressure and the heat of the arc lamps used for printing. The glass must also be free from surface scratches and other imperfections which would show in the print.