Prevent aluminium vapour condensing on a surface I have a situation where aluminium vapour is created in a vacuum chamber ($10^{-6}\ \mathrm{mbar}$). I would like a window to look into the chamber; however, the aluminium vapour produced condenses on the window, clouding it up.
So I thought, why not heat up the window above the boiling point of the aluminium? I would like to use sapphire for the window, but it has a melting point of around $2000\ \mathrm{^\circ C}$, lower than the boiling point of the aluminium which is closer to $2500\ \mathrm{^\circ C}$. If the sapphire got that hot it wouldn't function very well I don't think.
However, does it need to be that hot? When you de-mist a window in a car, you heat it up, but only up to about $20\ \mathrm{^\circ C}$, nowhere near the boiling point of the water, would it be the same for aluminium? How hot does a surface need to be for condensation to cease to be noticeable (for water or Al)?
(And can you think of any other solutions to the problem of the window deposits?)
 A: This has nothing to do with the atmospheric boiling point of aluminum.  Conceptually, to prevent condensation, all that is required is for the inside surface temperature of the window to be higher than that of the aluminum within the enclosure.  To be sure, 10-20 C difference should suffice.  Of course, you need to make sure that whatever you do to heat the window doesn't also heat the aluminum inside the chamber.  .
A: Why not construct the equivalent of a windshield wiper inside of the chamber? Certainly there are materials that would function perfectly well at the temperatures in the chamber - a ceramic material for instance. You may even be ale to acquire off-the-shelf ceramic blades - in the form of household ceramic knifes etc.
You need not breach the chamber to construct a mechanism to cause the wiper to sweep across the inner surface of your window. I believe there are ceramics impregnated with magnetic particles that are commercially available. You can drive blade motion through magnetic induction. You may even be able to use a hand-held magnet to manually move the blade. 
A: I had exactly the same problem but with silver. I was making silver films on a glass substrate by evaporating silver in a vacuum, and just as you say the silver vapour would condense on everything inside the vacuum chamber.
I'm afraid I never found a good solution for this. In principle heating the window to above the boiling point of the metal would stop the condensation, but this is unlikely to be practical. In my case I eventually used a movable shield to protect the glass and only unshielded it for the brief periods where I wanted to look inside. The glass still needed to be removed for cleaning on a regular basis but at least it alleviated the problem a bit.
Putting any sort of moving mechanism inside a UHV chamber can be a pain, though I think if you're only working at $10^{-6}$ torr it shouldn't be too bad.
