Hello,
During the production of quartz products for the lamp industry a genome called quartz fogging occurs. It may be seen on some arc tubes of modern lamps. I think one of the best examples is the Osram Powerstar excellence.

For lamps with higher optical demands, i.e. optical lamps like HMI/XBO or HBO this phenomena seem to be removed by heating with with a gas burner.

I suspect that the quartz fogging phenomena is due to crystallisation on the surface of the quartz but I am very unsure about that. I hope that some of the members here with more experience in lamp manufacturing may be able to answer me that question.



Best regards,

Alex

I think you are talking about silica smoke.  Since quartz is a very poor conductor of heat and also requires extremely high temperatures to become soft and workable, enormous quantities of heat have to be delivered before it is pressed or blown to the required shape.  This is usually provided either by oxy-hydrogen or oxy-propane gas fires directed at the outside surface of the quartz, or by igniting an argon plasma discharge inside the tube, or by a carbon dioxide laser that is scanned over the area to be softened.  The result is that in order to get the whole wall thickness to heat up and soften as quickly as possible, the surface to which the heat is applied reaches extremely high temperatures.  So high that the quartz sublimes directly from solid into vapour phase without even melting.  During heating, the quartz parts begin smoking due to the fumes of silica molecules boiling away from the heated surfaces.  These then condense on the nearest cold areas.  The result is that the machines used for quartz lampmaking quickly become caked in thick crusts of condensed white silica, and some also condenses on the colder areas of the quartz just above the heated area.  This leaves an unsightly greyish band with a rough surface.  On the higher quality lamps where this might be optically undesirable, a second heating is usually provided to boil away the condensed smoke and render the lamp crystal clear again.

That was a good question but also a very nice answer. I also wondered myself how and why these white shadows around the formed areas of the Quartz.

Thank’s