July 15, 1942
Dr. T. Dunham, Jr.
Mt. Wilson Observatory
813 Santa Barbara Street
I have had only one idea in connection with the problem of the chemical changes
on the surface of polished glass. This is that it might be possible to apply a protective
coating of silica or alumina or a mixture of the two by the following technique. You
know that substances such as alkyl orthosilicates have been used to form protective
coatings on porous sandstones and similar materials. The alkyl orthosilicates hydrolyze
with water, giving a deposit of silica. Similarly alkyl aluminates hydrolyze to deposit
alumins. This is the basis of the method which Dr. Williams has devised to put a protective
coating on plastics. The plastic is soaked in water, some of which it takes up, and
then is put in a solution of alkyl silicate or aluminate.
It occurs to me that it might be possible to soak a polished lens in acid,
say one normal nitric acid, until the soluble constituents have been removed from
a surface layer, leaving a porous silica framework. The lens would then be washed
thoroughly with water, and, without thorough drying, that is, with only wiping off,
transferred to a non-aqueous solution of alkyl silicate or aluminate or a mixture
of the two. The water in the porous surface layer combines with this substance or
these substances to deposit silica or alumina. On removal from this solution and dried,
the lens should perhaps be baked in order to produce as hard and porous surface layer
In view of the results which the people at Corning have obtained with their
new high silica glass, I think it might be possible that a protective coating on class
could be made simply by putting the lens in acid for a while, then washing it thoroughly
with water, and then heating to a pretty high temperature, at which the porous silica
framework would collapse into a dense silica surface layer. I am afraid however that
the trouble with this is that the glass itself would soften before a sufficiently
high temperature were reached, or that the alkali and alkaline earth in the glass
would migrate into the surface layer.
Tests of the first method suggested above could be carried out pretty easily.
The durability of the product could best be tested by the method described by Frank
L. Jones in the Journal of the American Ceramic Society, 24, 119 )1941). This would not involve a great amount of equipment, and the whole job
should not take very much time.
I am afraid that Badger has so much to do already that he could not take
this work on. A possibility is Dr. L. Reed Brantley, one of our own Ph.D.'s, who teaches
chemistry over at Occidental. He has asked me to be on the lookout for something for
him to do this summer, and he might be able to continue on a part-time basis after
the school term begins in the fall. He is industrious, and is a reasonably able man.
I think too that we could scrape up some graduate student to work as assistant to
Another possibility is Dr. J. Norton Wilson, Instructor in Applied Chemistry
in our Department. He is now doing some preliminary work on another possible project,
but it may well fall through, leaving him available for full-time work during the
rest of the summer and for part-time work thereafter.
Another possibility which occurs to me is that of leaching the glass in acid
to remove everything except silica and then subjecting it to very high temperature
for a very short time, so that only the surface layer is brought to a high temperature,
in order to produce a compact silica layer over the surface.
I hope that these ideas may have some use.