Indelible pencilIn 1866, the first U.S. patent for an indelible pencil was issued to Edson P Clark of Northampton, Mass. as an "Producing Indelible Writing on Linen and other Fabrics" (No. 56,180). The pencil-lead was composed of gypsum (a hard moisture-resistance compound) and black lead (coloring agent, with optional asphaltum or lamp-black) and silver nitrate. It is the silver nitrate which blackens to make the indelible mark by the action of light or heat. The black lead and gypsum permit the pencil to be readily pointed. The patent described cementing the filling with shellac into grooved cedar wood. Clark held an earlier patent for an indelible composition, but described without the wood jacket (No. 24,195 on 31 May 1859). |
