244 



GLASS 



very carefully selected, as the glass must be of 

 great purity ; the greenish tint in sheet or plate, 

 (lue to the soda, would be very objectionable in 

 flint glass. Its brilliant flashing appearance, when 

 cut into suitable patterns, is owing to its high 

 power of refracting and dispersing light, a property 

 arising from its comparatively high density. The 

 working of flint-glass resembles in a general way 

 that of the other kinds ; and, as we have not room 

 for details, we note here a list of the stages in 

 making a wine-glass, to give an idea of the process 

 up to the point where the manipulation of the 

 metal in a welding state finishes : ( 1 ) Gathering of 

 metal; (2) same marvered, and bowl formed; (3) 

 glass with metal for stem dropped on; (4) same 

 with stem formed; (5) same witn foot stuck on; 

 ( 6 ) same with foot heated and half opened ; ( 7 ) 

 same with foot opened, bowl cracked oft', heated, 

 and sheared ; ( 8 ) same finished. It is then an- 

 nealed. Crystal is a name loosely used for superior 

 kinds of glass. 



Optical Glass. This is made both of flint and 

 crown glass. In the case of lenses for a telescope, 

 for example, a combination of the two kinds is 

 necessary to make it achromatic their unequal 

 densities conferring upon them different refractive 

 powers. Good flint-glass for optical purposes is 

 extremely difficult to make, especially when the 

 required slabs or discs are of large si/e. It must be 

 perfectly homogeneous and free from striae, and it 

 will be deficient in refractive power if it does not 

 contain a very large proportion of lead, which, from 

 its weight, has a strong tendency to settle at the 

 bottom of the crucible, and so destroy the homo- 

 geneity of the glass. ' The fused glass is therefore 

 continually stirred until it has cooled to a consist- 

 ency sufficiently thick to prevent the lead settling, 

 and is then left still in the crucible to complete the 

 cooling. When cold, the crucible is broken away, 

 and the result is a cake of immensely heavy glass, 

 of which it is not yet known whether the value is 

 to be calculated in pounds or pence.' It is after- 

 wards reheated, brought to the required disc-like 

 shape, and then tested for flaws. If these are 

 numerous, as many smaller discs or slabs are taken 

 from it as possible. Messrs Chance of Birmingham 

 supplied in 1871 a pair of discs 26 inches in dia- 

 meter for the telescope of the Washington Naval 

 Observatory. The LicK Observatory and other large 

 discs will be mentioned under TELESCOPE. The 

 hard crown made by the above firm has a density 

 of 2-485 ; soft crown, 2'55 ; light flint, 3'21 ; and 

 double extra dense flint, 4 '45. A great many 

 experiments in connection with optical glass have 

 been tried of late years with chemical substances 

 other than those we have named, the results of 

 which will be found in the Reports of the British 

 Association for the Advancement of Science. 



Slag-glass. The slag from iron blast-furnaces is 

 itself a coarse glass, but, until lately, it has been a 

 waste product in the fullest sense. Bricks, however, 

 have been successfully made from it of late years ; 

 and still more lately, under a patent obtained by 

 Mr Bashley Britten, glass bottles are being made 

 from it by a company in Northamptonshire. The 

 slag is used in the molten state as it runs from the 

 iron-furnaces, which, of course, so far saves fuel ; but 

 it requires to be mixed to the extent of nearly one- 

 half its bulk with other materials. The process is 

 said to be successful ; yet we fancy there must be 

 great difficulty in procuring, for any length of time, 

 slag of nearly the same composition. Slag-wool is 

 a name for the same iron-slag when blown into 

 glass threads of a hair-like fineness, in which state 

 it somewhat resembles wool, and is now much used 

 for covering steam-boilers, it being, like all glass, a 

 powerful non-conductor of heat. 



Toughened Glass. Much curiosity was excited 



when, in 1875, M. de la Bastie, a French engineer, 

 announced that he had succeeded, after many 

 experiments, in making glass so ' tough ' that it 

 could scarcely be broken. So great was the value 

 which the inventor attached to his process, that he 

 demanded no less than one million sterling for the 

 English patent right, and abroad it Avas proposed 

 that the purchaser of the patent should pay so much 

 per head of the population. His original process 

 consists in heating any piece or pieces of glass till 

 they are about to soften, and then plunging them 

 into a bath of oil at a greatly lower temperature. 

 Usually, however, a mixture chiefly of oily sub- 

 stances, such as oils, tallow, wax, rosin, &c., is put 

 in the bath ; and some manufacturers, who worked 

 the process for a time, dropped the newly made 

 glass vessels while still hot into the oleaginous 

 mixture, by which plan neither reheating nor 

 annealing by the ordinary process is required. 

 After the articles acquire the temperature of the 

 bat4i, they are removed. Either from the want of 

 care or from some other cause, the results of the 

 treatment of glass by De la Bastie's process are not 

 uniform, because many samples of his toughened, 

 or, as it should rather be called, hardened glass, are 

 almost as easily broken as ordinary glass. Objects 

 such as tumblers, when allowed to fall, nearly always 

 break if they strike the floor on the lip or mouth. 

 Still, there is no doubt that most glass treated by 

 this process will stand a great deal of rough usage, 

 and that some examples are practically unbreakable. 

 In the case of window-glass, there is the disad- 

 vantage that a diamond will not cut it, and no 

 variety of glass so hardened can be safely engraved 

 or 'cut,' because when the tool penetrates much 

 below the skin the glass falls to pieces almost 

 to dust. This is a difficulty which has baffled not 

 only M. de la Bastie, but all other producers of the 

 hardened article. These defects, as well as the 

 high price of toughened glass, have as yet prevented 

 its corning into extensive use. 



In 1885 Mr Frederick Siemens produced three 

 kinds of 'tempered glass,' of very homogeneous 

 character and of great strength and hardness, by 

 means of his regenerative gas-radiating furnace. 

 ' Press- hardened glass ' is that which, after being 

 cut into the proposed shape, is softened in the 

 radiation furnace, and then placed between cold 

 metal plates. It may thus be so rapidly cooled 

 that the diamond will not,touch it. 



Colouring of Glass. Any kind of glass can be 

 coloured by metallic oxides, and the chief colours 

 given by these are noted in the following list. 

 Crimson of various shades, from gold, ' Purple of 

 Cassius' (a compound of gold and tin) being the 

 compound generally used. So small a quantity as 

 nnnyTnjth part of gold imparts a rose colour to glass. 

 A red colour is also got from protoxide of copper. 

 Purple or violet-red is obtained from peroxide of 

 manganese. Blue from oxide of cobalt or oxide of 

 copper, but chiefly from the former. Green from 

 the same oxides, together with sesquioxide of iron ; 

 a fine green is likewise got from sesquioxide of 

 chromium. Yellow from oxide of antimony or 

 sesquioxide of iron ; sometimes from carbon. Ses- 

 quioxide of uranium gives a beautiful opalescent- 

 yellow with a greenish cast. Chloride or silver is 

 used to stain glass yellow. Arsenious acid produces 

 an opaque white ; so also does the mineral Cryolite 

 (q.v.), as well as aluminate of soda. Aventurine 

 glass is a beautiful material of a brownish-red 

 colour, with gold-like spangles, in imitation of 

 Aventurine (q.v.) quartz. It is largely used in the 

 ornamental glass made at Venice. 



Coloured glass is made in several ways. When 

 the colour is all through the body, the glass consists 

 of pot-metal ; but for some purposes, and especially 

 when the colouring material is expensive, it ia 



