Optical Glass: Borosilicate Crown Melted, Stirred and Cooled in the Pot

Optical glass must be free of streaks (striae), bubbles and strain, so it is melted in a single clay pot, stirred for hours and cooled slowly in the pot. This is the American wartime practice that F. E. Wright wrote down for the US Ordnance Department in 1921, with his batch for a borosilicate crown (refractive index 1.511). Batch figures are parts by weight (pounds or kilograms). The picture: breaking apart pots of optical glass at Bausch & Lomb (Wright 1921, Fig. 48, public domain).