Lead Borate: Faraday's Borate of Lead Glass, Boric Acid Crystals Reckoned at 42 Parts (Phil. Trans. 1830, §25)

A very heavy, very fusible lead glass that Faraday made for telescope lenses. One proportion of lead oxide to one of boric acid gives a glass of specific gravity up to 6.4, twice that of flint glass. It softens so easily that it also serves as a flux for fired paints. Source: Faraday, On the Manufacture of Glass for Optical Purposes, Phil. Trans. 1830. Faraday gives two figures for the boric acid crystals: 42 parts (§25) and 36 parts (§19). This blueprint uses 42. He writes out the melting for his silica glass; using it for plain borate of lead is ours. The nitrate gives off poisonous brown nitrous fumes: the crucible mouth must vent to a flue. Picture drawn by Youblob.