I checked through my books and they don't even mention
cadmium in solders; Sharr Choate gives recipes but simply uses
varying quantities of brass for different melting solders
without mentioning the composition of the brass. An alloying
metal used in gold and silver solders to reduce melting
temperatures is usually zinc, but I do remember reading
somewhere about cadmium being used also for that purpose.
Others of my books do give zinc as a melting-point-reducing
metal but don't mention cadmium in that context. I took a
piece of my own (Johnson Matthey) solder, dissolved it in
nitric acid and tested for the presence of cadmium but
couldn't find any. However, I would suggest that the
percentage of cadmium in a solder would be low, and if one
didn't heat too strongly or for too long, and didn't breathe
in the fumes (if any), the effect of cadmium in solder
wouldn't be all that bad.
All silver-copper-alloys between silver 80/- and silver 912/-
have the same solidus temperature - 779 C. Alloys over 912/-
have solidus points between 779 C and 961.9 C. So, to get solders
for silver alloys, you need to add another metal to bring the
melting range down as the liquidus of the solder should be lower
than the solidus of the silver alloy (another aim is to keep the
melting range, i.e. the difference between solidus and liquidus
as narrow as possible). This can be done with zinc and cadmium,
and when they are used together, the melting range is brought
down even more than when using one of them alone. I have never
come across a silver solder with more than about 680 thousandths
of silver. Therefore, you shouldn’t use a lot of solder with
your pieces, or you will bring down silver content, and perhaps
violate law (they sell silver 935/- here for use as sterling to
make up for this).
Something you might have observed is that, when you heat up
solder with a sharp, oxidizing flame, there are very small
fuzzes, like miniature aspen seeds, flying around. These are
brown pieces of cadmium oxide or white zinc oxide.
Regarding health hazards, I’ve never heard of a bench goldsmith
with cadmium intoxication, and I think the cause for it being
banned from use by dentists here has more to do with what
happens in the mouth of the patient. On the other hand, zinc
isn’t too healthy, too, and I suspect that any substitute for
cadmium is equally noxious, only not yet researched as well.
I’ve got some recipes for silver solder without cadmium which I
give here, but I didn’t try them myself.
very hard: 601.6 g silver, 267.4 g copper, 30 g zinc, makes
hard: 601.6 g silver, 267.4 g copper, 60 g zinc, makes silver
middle: 601.6 g silver, 267.4 g copper, 90 g zinc, makes silver
easy: 601.6 g silver, 267.4 g copper, 120 g zinc, makes silver
hard: 680 thou silver, 265 thou copper, 55 thou zinc;
middle: 640 thou silver, 250 thou copper, 110 thou zinc;
easy: 600 thou silver, 235 thou copper, 165 thou zinc;
very easy: 560 thou silver, 220 copper, 220 thou zinc;
Be careful when using brass for alloying, I wouldn’t do. Brass
for turning has lead (!) in it to harden it.
Hope this is of use for you, Markus