Why are some old brass lenses so corroded, and others are not?

[Ted R]

Brass is not a pure metal it is a mixture (alloy) of copper and zinc. In addition there may be small amounts of other constituents for special purposes.

During the period that vintage lenses were being made (>100y ago) the varieties of brass available were not under strict control and the makers of lens barrels may have been vague in the specification of the material. Over time the various batches of brass used by a single maker may have had different corrosion resistance properties.

All brass corrodes, especially when in the presence of atmospheric contaminants (think coal burning Victorian city air)

Finger marks on the freshly cleaned surface if allowed to persist will become permanent stains.

To remain shiny brass requires constant removal of the surface dullness by polishing.

To eliminate the need for polishing the clean surface may be given a clear coating, one type probably was shellac laquer.

Wear and damage may penetrate the coating and permit corrosion to occur.

[Richard Wasserman]

Jac, I used to buy lacquer (in 55 gallon drums) just for this purpose. It works well and is easy to apply, or remove if you need to.