NOTE ON THE BREAKING OF PYREX FLASKS1
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W O L D A. WOOSTER University of Wisconsin, Madison, Wisconsin
SEVERAL 2-liter pyrex Florence flasks were broken while preparing COz-free distilled water by boiling with a Meker-type burner. Since the,donble precautions of a heavy wire gauze and boiling chips were used, these breakages were theoretically, as well as practically, annoying. An attempt to break by drastic heating of 300-ml. ppex Florence flasks, filled with water, was then made. The inner cone of an M.I.T. blast-lamp, played on the unprotected bottom of these small flasks, would boil the water contained therein, with no fractures. The same flasks, placed on a wire gauze and heated with the same flame, would break after about G e e minutes of heating. A disappearing-filament optical pyrometer was used to determine,'the temperatures of wire gauzes heated with several different types of burners. (This iustrument i s calibrated for black-body radiation. Since the emissivjt? of iron oxide is 0.98, the error of the instrum$nr ;from this source is small.) The.fbhowing values; which are the temperatures of agauze heated in the flame, and not necessarily flame t&m$eratds; were obtained: . .
... . ?This work was dine while the author was holding a scholarship from the Wisconsin Alumni Research Foundation.
Blast-lamp Fisher burner Meker burner Bunsen burner
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1300'C. 108o0C. 1O8O0C. - 970°C.
The International Critical Tables give the "softening temperature" of pyrex glass as 850°C. I t would seem probable that the wire gauze presses several points a t an even higher temperature against the glass, causing cracking in the same way that a piece of hot glass may be used to crack tubing. This was checked by using the end of a small file, curved and clamped against the bottom of the water-filled flask so that only the end of the file pressed against the glass. When this flask was then heated by a Fisher burner, i t would crack. A control flask, heated with the same burner, but without the point of hot iron pressed against it, would boil the water without breaking the flask. By supporting the flasks to be heated about 2 cm. above the gauze, so that i t serves merely as a source of radiant heat, i t is possible to use the full flame of a Meker-type burner without cracking the flask. When this is impossible, i t is desirable to use a gauze with an asbestos center.
