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'.
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
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.
1 gram = 15.4323584 grains.
Looks like a 'minims' is a drop.
A minim is a small volume measurement, 1/60 fluid dram, or just about a drop...
40 grains is 2.6 gram,
100 minims is 6.16 ml
"1 gram = 15.4323584 grains.
Looks like a 'minims' is a drop."
60 minims = 1 dram
8 drams = 1 fl. oz.
I think a drop is too large. Thanks for the formula.
One thought, if you are going to paint brass, "sweat" the moisture out of the metal first with a propane tourch and have the metal hotter than room temp. before painting but below 95*F. You will be suprised how much water will come out of the metal, it stops the paint from sticking over time.
Good luck with it.
Actually, with a minim being just about 17 to the ml, a drop is just barely on the small side. A dropper tip drop of water (other liquids vary due to viscosity and surface tension differences) runs between 20 and 30 drops to the ml, though this also varies somewhat by the size of the tip the drop is falling from.
That's a pretty stiff solution, with something over 5 grams of salts in 12+ ml of water. I'd guess the exact strength isn't critical, though, and suggest you might want to just use 3 grams of each salt in 15 ml water (divided for mixing, though it's not clear to me why you'd need to do so) and call it good. I'd also suggest you'll need to multiply the batch a bit if you need to blacken anything bigger than a Waterhouse stop.