View Single Post
  #23  
Old 04-11-2009, 08:36 PM
rat156's Avatar
rat156
Registered User

rat156 is offline
 
Join Date: Aug 2005
Location: Melbourne
Posts: 1,653
Quote:
Originally Posted by marki View Post
Yes but the Al coating is applied in a vacuum so oxidation will not be a major problem particularly if the protective coating was also applied before the mirror was exposed to air. My concern is simply based on the integrity of the protective coating considering their micron thickness. Mirrors that are exposed to the environment such as the case in open tube and truss frame designs have to have their mirrors recoated periodically as the Al coating some how comes in contact with oxygen (eg the coatings fail) and dulls due to oxidation and the formation of AlO3. Could it not then be possible that 2-propanol used as a cleaner might also reach the surface Al ? I imagine any scope mirror that did not have protection against oxidation would have very little usefulness beyond 48Hrs as the mirrors would dull rapidly. Just as an aside, I did a little experiment today. Freshly cut Al does indeed react with with 2-propanol but very very slowly. It took about ten minutes for the surface to visibly dull.

Mark
I doubt that it was the IPA dulling your Aluminium. Freshly cut Sodium reacts slowly with IPA, that's what we use to get rid of excess Sodium without too much of an exotherm. Probably dissolved Oxygen in the solvent. AlOxide coatings on fresh aluminium are essentially impervious to liquids, though gas molecules are small enough to permeate slowly. Alkaline or acidic solutions will slowly dissolve the AlOxide and dull the Aluminium surface, which is why you shouldn't leave your mirrors wet. Also remember that most aluminium available to us is an alloy, not pure aluminium as would be vacuum deposited.

Quote:
Originally Posted by RobF View Post
All this talk of chemistry and labs stimulated my safety gene. Be careful with isopropanol:

- extremely flammable
- nasty in your eyes
- don't want to breathing it in too much while you're agonising over your optics

Probably why the chemist was worried about selling. Really just requires the care and respect you'd give while using any solvent.

Safety rant over now....
Geez, I'd hate to take you to a petrol station...

Petrol
- extremely flammable, forms explosive mixtures with air readily
- nasty in your eyes, on your skin (especially in those sensitive areas)
- don't want to breath in too much
- until recently actually contained a heavy metal poison (Lead)
- until recently contained a confirmed carcinogen (benzene)
- contains CNS toxins (Xylene and Toluene)

Amounts in use, IPA about 125 ml, Petrol usually over 40 litres...

Cheers
Stuart
Reply With Quote