In a process of gaseous opalization a film of silica is formed by an oxidation reaction with a dry gas containing oxygen. Pure oxygen or oxygen diluted in an inert gas, other than nitrogen, may be used. An electric arc is generated by an alternating voltage between 5,000 and 10,000 volts and is directed...http://www.google.fr/patents/US4438152?utm_source=gb-gplus-shareBrevet US4438152 - Process of gaseous opalization of light bulbs
