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 as possible.
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
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
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.