FWD: Re: [ATM] Does glass retain moisture?

["john sherman" <atm@mail.johnspics.com>]

I tried to send this to the list last week, but it didn't make it thru. So I'll try again. I'm sorry for the delay.

>when Corning was working with formulating Pyrex, one of the formulae
>seemed very promising, and a large ribbed disc was made of it, but 
>then it was found that this particular formula was very 
>hygroscopic (if that is the correct term); 

Hi Davey,

Yes, that is the correct term for a material which absorbs more than its fair share from the atmosphere. It looks like most hygroscopic materials are salts. They do actually bind with the water. Maybe there is another term for materials which can absorb or retain water without actually having molecular bonding? 

Thanks to Richard Schwartz who not only understood the question, but who was able to provide a second, very  reasonable, possible cause which might explain the observations. He also asks:

>That is another damn good question.   And if the water can get in,
>can it also get out, or is it trapped forever?   

In the scenario I imagined, the water can migrate out just as easily as it got in. Just as soon as the atmosphere has a lower humidity than the mirror. In the scenario where the water actually bonds to the glass, then yes, it might be trapped forever.

>(If water was expanding the glass and it 
>penetrated perfectly through the whole piece, 
>the expansion would be 
>uniform and the figure would not change.) 

Deepens the curve, but also increases the diameter a proportional amount? That's what I was asking about. So I guess I'll forget it.

Thanks,

John








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