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Yes Scott electron beam evaporation is far superior to just resistance heating a refractory metal boat. The substance that is evaporated is actually silicon monoxide (SiO) it has a lower boiling point than silica (SiO2).
When the bell jar is opened the the SiO slowly oxidises to SiO2. The other good thing about electron beam evaporation is that there are very few or no pinholes in the SiO coating unlike conventional coating which produces hot fast particles of SiO which then fall off and leave small patches of unprotected Al. So still treat the surface as very fragile until the SiO is fully oxidized. The other advantage is the reflectivity does not deteriorate as much, it should stay above about 95%. Fully oxidized Al will fall to about 87% or worse reflectivity depending on purity. I spent many 'happy' years making front surfaced mirrors and more for Kodak in the late 60's and early 70's. Sorry about the rave.
Bert
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