Lenses and ‘Glass Disease’

I was also told, in relation to microscopes, to be wary of using old lenses since over time the glass deteriorates. For photographic lenses, telescopes and binoculars that problem is seldom mentioned these days. The other problems that befall old lenses such as fungus, dust, separation of elements because of balsam deterioration, and condensation of lubricants as well as mechanical damage to the surfaces all get covered in guides to buying and repairing but actual deterioration of the glass itself is only rarely mentioned. I supposed the writers and repairers are often not dealing with really old lenses, merely ones that are not current models. For users and potential purchasers of old film cameras, particularly those made before the widespread adoption of coated lenses, binoculars, telescopes and microscopes, the problem can be real and insoluble.

The warning about using old lenses for microscopy came in what I can only describe as the most long-winded and boring course (apart from botany) it was my misfortune to attend. Robert Barer* was the new Professor of Anatomy in Sheffield and he gave his now infamous snail-pace course on the use of the microscope he had started at Oxford to those of us who were honours students in zoology. Before deserting most of the sessions, I did pick up that snippet on old lenses. In order to check my memory I bought a copy of his book, Lecture Notes on the Use of the Microscope, first published in 1953. There, he wrote:

Do microscope lenses deteriorate with age?

They may do so. The surfaces of some glasses tend to become cloudy with age, particularly in the presence of moisture. The balsam used to cement lenses together may dry up irregularly, or may retract, leaving an air space. Moulds sometimes grow in the space between lenses. Many old lenses are still perfectly good, but nevertheless caution is advised when purchasing any objective more than about twenty years old.

Since then I have seen a number of lenses affected by what is called ‘glass disease’ in lenses of all sorts. Glass Disease is a major problem in museum collections of glass objects. Its appearance depends on the chemical composition of the glass and the humidity of the surrounding air. It shows first as a cloudiness on the surface of the glass and soon becomes irreversible; the chemistry involved is explained in an article on Wikipedia. Because of the importance of chemical composition, some glasses show relatively rapid deterioration when in high humidity or submerged; others show no deterioration whatsoever.

The nature of glass disease was brought home to me during covid-lockdown. In 1964, my wife, before she achieved that status, spotted on old tip while cycling to work during the summer vacation. Investigating further, she found a number of old medicine bottles and the like. They were left uncleaned with her parents who simply moved the whole lot with them each time they moved house. They emerged when we cleared their house and were transferred to a shed in our garden. Re-emerging as the shed was tidied I was volunteered to clean them up. I have to say with due modesty that I did a very good job. Several, however, had large patches, inside and out, of surface cloudiness. That was resistant to every chemical treatment I could devise, from strong acid to organic solvent. In short they had glass disease and I read that the only way it could be removed was by gentle but prolonged mechanical abrasion.

A long-buried bottle with 'glass disease'

I have, over the years, seen examples of glass disease in microscope lenses, old binoculars and photographic lenses. Since high humidity is needed it is not surprising that fungal growth and glass disease sometimes go together, as in lenses kept without desiccant in the tropics. In the three years before he died my father acquired a large collection of classic cameras. Some of those had cloudy, uncleanable lens surfaces.

Regardless of chemical composition, the coatings applied to modern lenses prevent glass disease. Indeed I was surprised to read that they were invented for that purpose by Lord Raleigh in 1886. Only afterwards did he discover that they decreased the reflectivity and therefore increased the transmittance of light. Some coatings though peeled off in patches or were worn away by zealous cleaning, leaving the surface open to glass disease.

Difficult to see but this 1950s lens has signs of
cloudiness on the front surface

The only ‘remedy’ for lenses with glass disease I have seen is to polish them with jewellers’ rouge, a mild abrasive. The effect on the optical properties of the lens was not stated.

I, needless to say, avoid really old or decrepit lenses. They, like many of the cameras they were made for, are simply beyond redemption and are best relegated to the display shelf.

*Although he bored us rigid, we did have great respect and sympathy for him. Word had got out that he was the first to have entered the typhus-ridden Sandbostel Concentration Camp in April 1945; that experience affected him for the rest of his life. He was medical officer in the Guard’s Armoured Division in the advance across Europe. He was awarded the Military Cross for his actions, including rescuing the crews of three burning tanks while under accurate shellfire. Robert Barer (1916-1989) wrote One Young Man and Total War, Portland Press, 1998.