Peter W. Rowe wrote:
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.

You know that's an extremely good idea, I will pass that on to my friend.


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.

I think I'll be looking at graphite based investment plasters, the wax I
currently use is a high detail gravity wax, so it liquefies very. As
with anything the down side is that wax models are very brittle, so you
can't drop them (repairs to this wax are never very good).


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.

Absolutely, there are finds of moulds that have been broken open, some
of the jewelry pieces have a linen material imprint in the back. We
learn a lot from actual finds as well as the broken moulds.

Viking Artefacts is a great reference for pieces from this period.

Could you contact me via email, I think that you would be an asset to a
few groups I belong to, I can discuss it there.



Regards Charles