GLASS. 



405 



loose, or removable by the finger, the glass 

 should be rejected. If no defect is thus made 

 evident, the second method may be resorted to ; 

 and if at the same time a blunt knife-blade be 

 drawn across the surface viewed, the slightest 

 efflorescence that may be upon it will be ren- 

 dered manifest. "With the best qualities of glass 

 no efflorescence is discoverable. The applica- 

 tion does not unfit good glass for use or sale ; 

 and the author declares that by it he is able to 

 distinguish with certainty a good and durable 

 from a poor glass. The method can be applied 

 to colored glass, except when, as in intensely 

 red glass, the coloring material forms a large 

 part of the entire weight. Still a good flint- 

 glass (containing lead) is not appreciably cor- 

 roded by the application. The author proposes 

 further to investigate the subject, especially as 

 to how large a proportion of alkali can be in- 

 troduced into glass without detriment to its 

 durability and value. 



Coloration of Glass, as ordinarily manufac- 

 tured. M. J. Pelouze has been during some 

 time past engaged in a series of experiments on 

 the manufacture of glass, his papers some at 

 least of which had been presented to the 

 Academy of Sciences being published suc- 

 cessively in the Comptes Rendus. Most of the 

 experiments were performed in Siemens' fur- 

 naces, used in glass-making, at Saint Gobaiu. 

 The crucibles used were generally of a very re- 

 fractory fire-clay, of a composition for all prac- 

 tical purposes the same with the materials 

 ordinarily contained in glass ; though, to avoid 

 every source of error, platinum crucibles pro- 

 tected by tire-clay were sometimes employed. 



Observing that the presence of certain admix- 

 tures in the materials gave a decidedly yellow- 

 ish tinge to the glass produced, the author ex- 

 perimented with a view to determine the cause 

 of this result. He proved that the yellow color 

 was not directly due to free silicium; and 

 that it is imparted to glass alike by addition of 

 carbon, sulphur, silicium, toron, phosphorus 

 (introduced in phosphate of lime), or alumin- 

 ium, and at a red heat even by introduc- 

 tion of hydrogen, however rigidly purified. 

 Neither zinc nor arsenic produced the color. 

 The production of the same tinge by addition 

 of substances so unlike, led to the supposition 

 that the coloration must be due in all the 

 cases to some single cause. Finding, further- 

 more, that all commercial glasses window, 

 plate, table, bottle, and Bohemian contain 

 sulphates, as of soda, potash, and lime, M. Pe- 

 louze inferred that the cause sought must be the 

 liberation, by reaction of one or other of the 

 elements above named, of an alkaline sulphuret 

 from sulphates present in the molten glass ; and 

 this view a variety of examinations and experi- 

 ments served to establish. The sulphates in 

 glass can como directly from use of the salts as 

 a flux, or from presence of sulphate of soda as 

 an impurity iu the commercial carbonate. A 

 glass from Pompeii was found to contain two 

 per cent, of sulphate of soda. When hydrogen 



was passed over plate glass containing much of 

 this salt, and at a red heat, it produced at once 

 a very deep yellow color. But the final proofs 

 were that when the glass materials were first 

 purified from sulphates, the addition of any of 

 the elements already referred to failed to give 

 the yellow color ; and that this was again pro- 

 duced when into the pure materials an alkaline 

 sulphuret was introduced. 



M. Pelouze also alludes to the fact that the 

 surface of window-glass, for many years ex- 

 posed to the atmosphere, is frequently found 

 corroded, and in part opaque, while examina- 

 tion with a magnifier shows it to be covered with 

 a crystalline deposit; and he attributes the 

 liability to such change largely to presence of 

 the sulphates named, in the glass. He thinks a 

 glass containing no sulphates would prove more 

 homogeneous and less changeable than any at 

 present known, and especially valuable for opti- 

 cal purposes. He accordingly prepared, by re- 

 peated crystallizations, a flux of carbonate of 

 soda freed from all traces of the sulphate, and 

 with 100 parts of this (dried), mixed 250 parts 

 of white sand, 50 of pure carbonate of lime, 

 and 2 of charcoal. The glass obtained by fusion 

 of this mixture was perfectly colorless; and 

 other samples were still so, in which boron, 

 silicium, or hydrogen was substituted for the 

 charcoal. 



Coloration of Glass ly Selenium. Upon mix- 

 ing selenium with the ordinary glass composi- 

 tion, but prepared with the purified carbonate 

 of soda, M. Pelouze obtained a perfectly transpar- 

 ent glass, of a beautiful orange color inclining 

 to red, and resembling certain varieties of topaz, 

 essonite garnet, and hyacinth zircon (its hue 

 the 5th orange-red -$, 9th of Chevreul's scale). 

 Upon varying the proportion of selenium from 1 

 to 3 per cent, of the mixture, the hue obtained was 

 still always the same and of the same intensity. 

 Thus, while it is proved that sulphur and sele- 

 nium both directly color glass, it is also shown 

 that the well-known analogies between the two 

 extend to their reactions with alkaline and 

 earthy silicates. 



A New, or Chrome Aventurine. The prop- 

 erty in virtue of which the sesquioxide of chrome 

 (chromium) imparts to fluxes, and especially to 

 glass a green color, is under heat exhibited also 

 by the bichromate of potash: this salt is, on 

 heating, decomposed into oxide of chrome 

 and neutral chromate of potash ; while, in the 

 fused glass materials, the latter is by reaction 

 with silica decomposed into silicate of potash, 

 oxygen, and oxide of chrome ; so that under 

 these circumstances, in the form of such oxide, 

 all the chrome of the bichromate enters into the 



Using the glass mixture already mentioned, 

 of sand 250 parts, carbonate of soda 100 do., 

 and calcareous spar 50 do., M. Pelouze fused 

 along with this in different trials, different pro- 

 portions of the bichromate of potash, and with 

 the following results: adding 10 parts of the 

 bichromate, the glass melts and anneals well, 



