Old formulas with Magnesium Oxide or Hydroxide
I've been reading over some old formulas, both published and in the FIAT reports. They discuss "alkalinity factor" or "acidity factor" or whatever else they call it depending on source.
What I have been able to find and deduce is that old gelatin, prior to about 1940, varied widely in pH and had to be adjusted by use of Magnesium or even Calcium salts before or during use. This varied from supplier to supplier and from batch to batch sufficiently that there are notes to this effect in several old formulas to beware of the "magnesia effect" etc.
Today, we don't have to worry about this. Most all gelatin used in photography is deionized and oxidized. Food grade gelatin need not be deionized nor oxidized. I have found that this oxidation is not as good in all cases though and I reported on this elsewhere.
In any event, if you run across formulas that rely on a Magnesia effect, make sure that this is not just to compensate for old style gelatin, otherwise you can fog your emulsion due to excessive alkalinity. It could form silver hydroxide or silver oxide if the pH became too high.
IN one of my new photo tehcnology books (Neblette's 8th ed, I think) I saw recently that they mentioned how calcium has a restraining effect on emulsions. I don't remember if magnesium did as well.
Do you have any info on what this "magnesia effect" is? Simply pH being higher than expected? I suspect this is only an issue with paper emulsions, and that ammoniacal emulsions would not suffer?
And did you have a link for these FIAT reports? The only FIAT report I've ever gotten was "Fix It Again, Tony"...
(Off Topic - I USED to own a 1969 FIAT 124 spyder. Fun car, but not as reliable as my VWs of the same and older vintage were. Especially after the rubber donut at the end of the driveshaft broke on the Fiat and it whacked off the end of the gearbox as I was coming down a rather steep hill, knocking out the brakes, and breaking the fuel line... I still remember wondering why the windshield was covered in oil and why was my knee getting really hot, and then I looked over and saw the back of the gearnbox gone with the gears all still spinning away, and the end of the driveshaft whacking about and making a big hole in the floor.)
The FIAT reports are available at GEH, Library of Congress and EK Research Library. Sorry. I cheated. They are not on-line.
IDK what other photographic effects there were from either salt but I do know that Kodak gelatins are Calcium free even though we used to wash in "sweet water" which contained Calcium Sulfate to reduce gelatin swell. Go figure.
The effect, as far as I can deduce, was pH only, and alkali in the absence of a silver halide solvent acts entirely different than alkali alone or silver halide solvent alone.