On Sun, 21 Feb 2010 13:37:07 -0800, in rec.crafts.jewelry Chilla
wrote:



Even with pure silver the metal "yucks" up pretty quickly.

Depends, perhaps, on location. I've some fine silver things I made almost 20
years ago, which are even now, mostly untarnished. Just a bit of discoloration
here and there. It might also have to do with whether the item if truely clean.
Very slight surface films of wax, oil, or whatever, might protect the surface.
If it's cast, and porous, then such materials might even sort of impregnate the
surface, making them even less likely to be cleaned off... But that's just a
guess. One thing you might try, if you've a university or other facility that
might have access to x-ray fluorescence technology, is to see if someone can
give you an analysis of the alloy. XRF is able to determine what the
composition of an alloy is, at least at the surface, with great accuracy, and is
totally non-destructive. That might tell you if there are other metals or
elements in the alloy that might have some protective effect.


I just don't understand how this ring, which would have been cast, due
to its design, in only just yellowing now. It's been in the atmosphere
for some time now.

Obviously, there is some difference between this ring, and currently made
objects. We can guess till the cows come home what that might be, but without
more data on the metalurgy of the thing, it's just guessing. There are any
number of possibilities, some of which I've mentioned.


One of the interesting things about the old casting methods, and when I
mean old I'm referring to viking dark age casting methods.

Often, work from this era and local, were poured into open stone (often
soapstone) molds. Such molds are generally poured without a lot of preheating.
The same is true of sand casting methods. Because the metal chills quickly on
contact, and because stone molds are not porous, there is no prolonged
availability of oxygen at the cooling surface to allow oxidation. Modern lost
wax casting uses investment plasters that are porous, to allow air to escape
during casting, which allows great detail easily. But it also holds a certain
amount of air at the cooling metal surface. Plus, the gypsum based investment
plasters have some sulphur content, and if overheated, can contribute to
formation of sulphides, as well as oxides.

The silver poured into the moulds would have come out fire scale free,
or so the modern experimental archaeologists have show with their work.

The interesting thing is that usually small amounts of metal were
melted, in charcoal (so a reducing atmosphere), and the lost wax moulds
used were made from clay mixed with horse dung.


This is similar to methods used even today in East African traditional casting,
where beeswax models are "invested" in a mix of clay, dung, and grasses or
straw. The organic material in the mold creates a strongly reducing atmosphere
during casting and solidification of the metal that also produces a clean
casting free of oxidation. In modern western methods, when this is desired,
there are also investments plasters available that also incorporate graphite in
the mix to also produce a reducing atmosphere in the mold. The downside to it
is that with a reducing atmosphere in the mold, complete burnout of the wax
model is more difficult, which can create resudues with some modern casting
waxes, as well as giving poorer gas permiability to the investment, so sometimes
poorer fills. But there's no denying that those traditional materials, even if
less suited to modern production methods, do indeed produce beautiful and
detailed castings if done right.

However, are you sure that viking metalwork used lost wax methods? I'd only
heard of the use of stone molds or simple sand casting methods before. I'm not
an expert in this however, and could easily have missed that. Still, I'd not
thought that lost wax methods were used that far north at that time.

I'm wondering if the use of base materials, as opposed to refined
investment plasters, would make a big difference? I haven't been
forwarded any answers yet from the archaeologists.


modern investment plasters do indeed give different results. Still, the
principal differences are in surface oxidation. After the piece is finished, a
modern cast piece should also have a clean fire scale free surface, and if done
right, free of fire stain as well. So at most, all I can envision as a
difference might be a difference in the amount of copper available at the
surface due to surface depeletion during working, as noted in my last post.
Beyond that, differences in the alloy itself would be where I'd look. Perhaps
some accidental trace element acting as a sacrificial material much as in
galvanized iron, or as something contributing tarnish resistance in the same way
some of the modern fire stain free alloys work (lead, tin, zinc, perhaps? Messes
up the workability, but might affect tarnishing too). It doesn't take much of
these elements to have an effect. Or, as I suggested, a small amount of gold in
the alloy could do that too. But don't discount as well the possibility of a
simple coating of some sort being involved. Whether intentionally applied at
some point, or simply there through the processes and history involved, such a
surface film might have a strong effect. Tenth century jewelers weren't using
modern detergents and ultrasonics to clean their jewelry. And things like
lubricating waxes or the like used in polishing or burnishing could have gotten
quite imbedded in any surface porosity. There are many modern silver polishing
agents that leave some sort of protective film, and you can't tell just by
looking, and these are merely surface films easily cleaned off. The sort of
thing I've got in mind might be a tad more durable... But that's just a guess.

Cheers

Peter Rowe