In 1890 physicist and later chief developer Dr. Richard Küch (1860-1915) joined the
company led by his schoolmates Heinrich and Wilhelm Heraeus (1860-1948). In 1899
Küch succeeded in melting large quantities of rock crystal in an oxyhydrogen
compressor, forming pure silicon dioxide (SiO2) or quartz glass. A number of qualities
made quartz suited to all kinds of laboratory equipment: it was transparent to
ultraviolet rays, had a high temperature resistance, could withstand sudden
temperature changes and was resistant to most acids. In 1904 Küch invented the
sunray lamp, made of mercury and quartz glass. Mercury vapour arc discharge lamps
were initially used for lighting large spaces, such as streets and halls, until more
powerful metal-wire lamps replaced them. Küch had noticed that close proximity to the
discharge lamp caused burns to appear on his face and hands, an effect attributable
to the ultraviolet light that penetrated the quartz glass. The permeability for ultraviolet
radiation and a temperature resistance as high as 1000 degrees Celsius enabled
mercury vapour arc discharge lamps made from quartz glass to produce a much
higher yield of medically effective light in the wavelength range between 400 and 280