Peter W. Rowe wrote:
On Mon, 15 Dec 2003 21:34:53 -0800, in rec.crafts.jewelry Marion
Margoshes wrote:
Heinrich Butschal wrote:
Marion Margoshes wrote:
I cast two pieces , an orchid and a rose, in fine silver, since I
enamel these silver flowers. Using centifugal casting, melting
the metal with a torch --- they were cast separately, in two
flasks. The rose metal looked normal, silver, while the orchid
metal was dark,
evenly black. I have been trying to remove the black surface with
pickle and an hour in my tumbler ,with little improvement. Any
help
and an idea of why this happened would be appreciated.
Souds like sulphuric silver at the surface. Without destroing the
surface
I could remove it only by throwing the silver hot in
cyandisolution. However I donīt remmoned it, for cyanid is very
poisonous. :-(
--
Heinrich Butschal
casting technologies
http://butschal.de/werkstatt
What do you think made this happen? Why would the button be pure s
ilver color while the orchid is black?
Marion,
I think what Heinrich means is that the surface is converted to
silver sulphide.
Thanks for adjustment. Cause Iīm German I have to look first for
translation.
This time I took the wrong term from http://dict.leo.org/ ;-)
This is the same stuff of which ordinary "tarnish"
is composed. While fine silver tarnishes much more slowly than does
sterling, it does still tarnish, and since silver sulphide is not
affected by pickling, this would explain why it didn't come off
there. Also, steel shot tumbling will only give the color a nice
blue/black gunmetal tone, from burnishing, without removing it,
unless you tumble in an abrasive medium.
What causes this to happen is when either the investment mold is
overheated, causing the gypsum therein to break down, or the metal
was too hot in melting, and this then caused the same thing to happen
when it hit the investment. If you're torch melting with a
centrifuge, it's also possible that the torch flame, going through
the hole in the back of the crucible, and playing on the sprue
opening, overheated the outer portions of the investment mold prior
to the metal being thrown in by the centrifuge. In all these cases,
the overheating means that sulphur compounds, including some gasses,
are then available to the hot metal surface as it cools, forming the
black. This is different from the black color you get with normal
casting of sterling silver, since those are just copper oxides, which
easily pickle off. The higher melting point of fine silver makes it
more possible to have this problem, but the usual cause is that the
burnout itself got too hot, with the investment getting over 1400
degrees F. But as I said, melting technique can also do it. It's not
common in any case, but it can happen. If the surfaces of the metal
are smooth and fine, even though black, including the sprue and
button surfaces that were in contact with the investment, then it's
likely just the melting technique. If the surfaces are rougher, then
it may well have been a burnout problem.
As Heinrich says, one way to remove this black is with cyanides. But
you can also remove it with common commercial tarnish removing
liquids, such as Tarn-X. Use newly purchases solution, and the tarn-X
brand is mixed rather dilute, and it has a distinct shelf life. The
stuff is based on Thiurea, and smells like rotton eggs, but it works.
If you have frequent need of the stuff, look up the Vin-Rok company
in Cleveland (no web site that I know of, but you can get contact
info at the Thomas Register site), and order a pound of their CQ-2
silver tarnish remover. That is the dry chemical mix, which you mix
with water, to get the same end product as Tarn-X, and as far as I
know, Tarn-X is made from that actual commercial product. It will
cost you about ten or fifteen dollars, if I recall, for a pound of
the powder, and the dry powder last indefinately, and will make up
several gallons of the fully concentrated mix, not the overly diluted
stuff Tarn-X charges so much for. it works a lot faster and better
when mixed full strength like that.
The other traditional way to chemically remove tarnish might also be
worth a shot. Washing soda mixed with water, in an aluminum (not
anodized) pan, with the silver resting on the bottom, sets up an
electrolytic cell that slowly reduces the silver sulphide back to
silver. I'm not sure if it will work well for you, since youv'e got
a probably fairly thick layer of the black, and have now burnished it
down pretty well too...
You could also take it off with abrasive media in your tumbler.
And for the next time you cast, try putting something between the
crucible and the flask while you melt the metal, to prevent the flame
that comes through from overheating the flask, as well as making sure
your burnout temperature does not exceed 1350F.
Hope that helps.
Peter Rowe.
Great explanation. Full Ack
If there is a thin layer only, somtime it helps to put some ureae powder
into solution and boil it. However this was working very slow.
--
Heinrich Butschal
casting technologies
http://butschal.de/werkstatt
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