164 CARNEGIE INSTITUTION OF WASHINGTON. 



(393a) Le rioliti di Lipari. Henry S.Washington. Boll. Soc. Geol. Ital., 39, 141-159 (1920). 



This is a translation into Italian of the paper reviewed on page 173 of the 

 Annual Report for 1920 (The rhyolites of Lipari, Am. J. Sci., 50, 446-462). 



(396) The annealing of glass. Jy. H. Adams and E. D. Williamson. J. Franklin Inst., 190, 

 597-631 ; 835-870 (1920). (Papers on Optical Glass, No. 32.) 



The annealing of glass was one of the numerous problems encountered dur- 

 ing the participation of the Geophysical Laboratory in the development and 

 manufacture of optical glass. When molten glass is cooled it usually acquires 

 internal stresses and is then said to be "strained." Excessive stresses can not 

 be tolerated in any kind of glass, since they render the glass 1 iable to break 

 when handled or heated again, and in the case of optical glass even a moderate 

 amount of strain causes troublesome warping of finished lenses and prisms. 

 The prevention of internal stresses in glass (and its removal when present) 

 is a problem which requires for its complete solution a knowledge of various 

 thermal, optical, and elastic constants of the relations between such factors 

 as rate of heating, temperature gradient, and stress distribution in terms of the 

 above-mentioned constants. It is shown that the process of annealing glass 

 can best be carried out if we know, for the various glasses and for the various 

 temperatures, the rate of release of the internal stresses. The results of such 

 measurements for nine kinds of glass are here presented. The release of 

 stress at constant temperature was found to proceed usually according to the 



equation p — Tr= At, in which F is the stress at any time t, Fo is the initial 



t to 



stress, and A is a constant for the particular glass at a particular temperature 

 and is a measure of the rate at which stresses are relieved. The variation of 

 this rate with temperature follows the equation log A = Mid — M2, in which 

 Ml and M^ are constants for a particular glass. 



At any temperature a glass requires a certain annealing-time. This is 

 arbitrarily defined as the time required to reduce the stress (in optical units) 

 from 50 to 2.5/x)u per centimeter. For convenience of reference, the 150° 

 interval lying immediately below the temperature at which the annealing- 

 time is two minutes is called (also quite arbitrarily) the annealing-range. At 

 temperatures below the anneaUng-range as thus defined very little permanent 

 stress can be introduced. 



Concrete directions are given for anneaUng optical glass. The procedure 

 to be followed for other kinds of glass, such as plate-glass, bottles, chemical 

 glassware, etc., is also indicated. Mathematical analysis of the problem 

 shows that the best method for annealing requires that the glass be held at 

 constant temperature (below the customary annealing-point) for the appro- 

 priate time and then cooled at an increasing rate. It is ofi nterest to note 

 that the larger the piece of glass the lower the annealing temperature. Finally, 

 there are presented several equations which are convenient for calculating 

 the internal stresses due to heating or cooling solids of various shapes. 



While the original object of this investigation was to put on a quantitative 

 basis the operations connected with the annealing of glass, it was found that 

 many of the results have an important bearing on certain problems of geo- 

 physics. For example, the relief of internal stresses in glass probably belongs 

 in the category of elastico-viscous flow and is thus connected with such proc- 

 esses as the tidal deformation of the earth's crust. Moreover, the formulae 

 expressing the relation between temperature differences and stress distribu- 

 tion are directly applicable to the phenomenon of the "jointing" of rocks. 



Note: — Certain phases of the subject have already been covered in previous publica- 

 tions from this Laboratory, as, for example: Temperature distribution in solids during 



