l8o REPORTS ON INVESTIGATIONS AND PROJECTS. 



(4) Quartz glass. Arthur L. Day and E. S. Shepherd. Science, n. s., 23, 670-672, 

 April 27, 1906. 



The peculiar properties of quartz glass, so called, which have made it a 

 very important material for lenses, laboratory vessels, etc., made it seem that 

 any investigation of quartz would be incomplete which did not include a 

 careful study of the conditions of formation of this glass. From the investi- 

 gation of silica (see No. 8, "Lima-silica series of minerals") we learned that 

 if quartz is heated to 800° it becomes unstable, and will change its crystal 

 form if given time enough to do so, going over into tridymite. This change 

 is somewhat sluggish, and may be delayed far beyond that temperature. If 

 the heating is then continued, the tridymite melts slowly, beginning at a tem- 

 perature of about 1,600° C. The temperature is so high and the material 

 itself so inert that the melting usually proves to be a totally different phenom- 

 enon from the melting of most other solids. It is still further peculiar in the 

 fact that the liquid silica, after melting, is so viscous that it can hardly be 

 made to run at all unless heated far above this temperature. With such a 

 peculiar material, therefore, it is not surprising that great difficulty has been 

 encountered in melting quartz down to a clear glass. Small single crystals 

 have, to be sure, been melted in the oxyhydrogen flame to a clear liquid which 

 cooled down without crystallization, but when the attempt is made to melt 

 any considerable quantity of quartz it invariably comes out a spongy, white 

 mass, which resembles common pumice stone, and is, of course, useless for 

 any of the purposes to which glass is commonly applied. The viscosity is 

 such that the air included between the solid fragments before melting is 

 firmly held in small bubbles, which stoutly resist the usual methods of dis- 

 lodging them. 



It was found in the laboratory that quartz would not run together into 

 a homogeneous mass after melting, without superheating, but the super- 

 heating of quartz in an electric furnace, with carbon electrodes, appears to 

 cause a reduction of the silica and the formation of gas in sufficient quantities 

 to cause more and larger bubbles. The problem which confronted us was, 

 therefore, to get the quartz sufficiently fluid without its undergoing chemical 

 change. We accomplished this by applying an air-pressure of about 500 

 pounds to the square inch, which proved sufficient to allow of considerable 

 superheating without the usual decomposition. By this means clear quartz 

 glass could be melted down from random fragments of pure material into 

 clear, transparent blocks of a size suitable for many technical applications. 

 It is probable that greater pressure would operate to still further hinder the 

 appearance of gas and increase the certainty of a clear product, but the labo- 

 ratory has no bomb strong enough to test this hypothesis. 



The possibility of preparing this glass with reasonable facility in consider- 

 able quantities is extremely important, for when made it can not be melted 

 before reaching a white heat, and its expansion coefficient, which is a fair 

 measure of its tendency to break when heated, is insignificant. 



