I found this in a book named ABC Guide to Photography - no
date, but it looks like it was published about the time dry plates became
popular as it mentions the 'old "wet plate" days'.

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Brass - to Blacken

The amateur often requires to re-blacken stops or other portions of the
brass works of his apparatus, which have seen considerable use. The
old black should first be cleaned off with a piece of fine emery cloth, and
the metal should then be dipped in a mixture of equal parts of the
following solution:-

No. I. Solution.
Silver nitrate..... 40 grains
Water..... 100 minims

No. II. Solution.
Copper nitrate..... 40 grains
Water..... 100 minims

When the stops are removed from the above solution they should be
allowed to dry, and should then be uniformly and gradually heated until they
assume the desired black colour.
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1 gram = 15.4323584 grains.
Looks like a 'minims' is a drop.

Phil