Fine silver surface

I am going to be casting sterling silver rings for enameling. In doing
the homework for this I have found a source from one book that says the
surface of the model exits the cast with a layer of fine silver without
copper. If this is true, it would eliminate the need of further
preparation of the surface to accept the enamel. Anyone have
experience with this or know the mechanism envolved?

On Tue, 29 Mar 2005 17:40:04 -0800, in Tõ wrote:

I am going to be casting sterling silver rings for enameling. In doing
the homework for this I have found a source from one book that says the
surface of the model exits the cast with a layer of fine silver without
copper. If this is true, it would eliminate the need of further
preparation of the surface to accept the enamel. Anyone have
experience with this or know the mechanism envolved?


Using standard sterling silver alloy, and standard commonly practiced casting methods,
sterling silver will come out of the casting process with the surface layer have had
it's copper content oxidized to a black oxide layer. This is a somewhat thin layer, but
underneath, the oxidation of the copper to a lesser degree, penetrates to a short
distance into the surface of the metal. Under that, the remainder of the metal is
clean sterling silver. Normally, after casting, one cleans the raw castings in an acid
"pickle" solution, usually either dilute sulphuric acid, or these days, a sodium
bisulphate (a sulphuric acid salt) solution. The acid cleaning dissolves the surface
black oxide, leaving that thin surface layer a clean matte white color. That surface
layer is fine silver, without copper. But underneath that thin layer, there remains the
deeper imbedded oxide layer. Known to silversmiths as "fire stain", it shows up when
you go to polish the item, and if you polish a bit into the metal, you find that what
at first appeared bright and shiny, has a slightly hazy, lighter, and faintly pinkish
color than the metal you encounter as you buff down into the surface. The color
difference is not obvious until you've cut through that surface layer, and have both the
surface layer's color and the color of the deeper metal to compare it to. At that
point, it becomes very annoying...

But I digress. You're wondering whether the initial fine silver layer left by initial
pickling of the silver will allow enameling. Here's the problem. At enameling
temperatures, the copper in the metal is actually somewhat mobile, and can migrate
towards the surface. So in order to keep the surface layer free of copper, it needs to
be thick enough to allow you to enamel without the copper deeper down in the metal
making up into the surface you're enameling. Whether your initial castings will have a
thusly thick enough silver layer may depend on details of your casting method, etc, but
I'd expect that you'd need to take additional steps to increase the thickness of the
surface silver layer.

Methods that suggest themselves to me a

Electroplating with silver (easy, but needs additional equipment, and the best silver
plating solutions are cyanide based, so you need some familiarity with such chemicals
and the knowledge and ability to use them safely.

Depletion gilding. This is simply a continuation of the process by which the surface of
your silver casting is clean and white in the first place. What you do is simply heat
the casting again, to a good annealing temp, And allow copper to migrate to the
surface as it will, turning the piece black. You then pickle the piece till it's white,
and repeat this process until heating no longer discolors the metal. At that point,
you've depleted most of the copper near enough to the surface to be able to trust the
thickness of the fine silver layer to be able to withstand your process. Note that
you will, in this process, also made the depth to which the still underlying fire stain
extends into the silver, much greater. So you should perhaps be prepared to finish the
final items with a so-called fire stain finish. That simply amounts to polishing the
item so as to not cut through the fire stain layer. So long as you've only got that
color visible, and not both it and the underlying true sterling color, it will look
just fine.

You might also wish to do some inquiries of your metal supplier. There are now a number
of variations on the traditional silver/copper alloy called sterling. These have the
same percentage of silver, but some or all of the copper has be replaced by other
alloying metals, in order to give an alloy that does not develop that fire scale and
fire stain in casting. As these alloys avoid the formation of copper oxides at and
penetrating into the surface, it seems possible to me that some of them may be directly
compatible with enamels, without the need to treat the surface in any way. However,
I'm not certain of this. I know the alloys are out there, but am unfamiliar with
whether any of them are enameled easily.

Hope that helps.

Peter Rowe

As always, good information. Thanks Peter. Don Pardoe

vj found this in rec.crafts.jewelry, from :

]I have found a source from one book that says the
]surface of the model exits the cast with a layer of fine silver without
]copper. If this is true, it would eliminate the need of further
]preparation of the surface to accept the enamel. Anyone have
]experience with this or know the mechanism envolved?

yes, that's basically correct.


--
@vicki [SnuggleWench]
(Books) http://www.booksnbytes.com
(Jewelry) http://www.vickijean.com
(Metalsmithing) http://www.snugglewench.com
[it's a Callahan's thing]
-----------
vj -- pounds metal, mother of three "Js", lives in Paradise,*
and is generally considered a smarta$$.
Yahoo ID: *vjean95967

wrote:
I am going to be casting sterling silver rings for enameling. In doing
the homework for this I have found a source from one book that says the
surface of the model exits the cast with a layer of fine silver without
copper. If this is true, it would eliminate the need of further
preparation of the surface to accept the enamel. Anyone have
experience with this or know the mechanism envolved?


Peters reply has covered the metalurgical side of your question.
there are other considerations you need to take into account with
your enamelling plans.
taking the broad picture first, enamel on silver is relatively easy
especially if your enamelling into cloisonne cells that are formed
during the initial casting.
however you will find that sterling will still suffer from fire stain
during the firing process, you cant do it in a reducing atmosphere as
this will change the enamel surface, it has to be an oxidsing kiln
atmosphere, ie normal air even if you follow the procedure advised by
Peter to remove the copper from the surface of the item. The reason for
this is that some enamels are not very resistant to acid pickles so you
need to plan your process very carefully to get it all to work out right.
Now to the particular.
What type of enamels are you thinking about?
translucent? or opaque?
the former are much more difficult to use . these require you to have
a highly reflective metal surface under the enamel to achieve the best
brilliance of colour. How you plan to do this is another hurdle for you
to overcome. Also Reds on silver are notoriously diffult to achieve
without a hard enamel clear underlay or the use of fine gold foil.
Blues are the easiest.
Opaques are the easiest to use of all enamels as the surface of the
metal under the enamel only needs to be clean and etched.
also you need to use enamels that have the same coefficient of
expansion and contraction as the silver alloy you plan to use. otherwise
the enamel will crack sooner or later and spall off. So you cant use for
example enamels for iron on silver without the risk of failure.
where do you plan to get your enamels? and who is the maker? the best
translucent enamels were obtained from Schauer & Co in Atzgersdorf in
Vienna, but they have stopped production and retail sales of these
products. I used a particular range of their high lead ,ie very soft
low temp melting enamels on fine silver and gold and had a product range
that sold well through Andrew Grima in London in the 1970's.. these
particular enamels needed a high firing temp to get the best colour
brilliance.
If you can find one of their full colour catalogues from the 1960/70's
its a most useful guide.
You will need to run very careful trials with your enamelleing
techniques BEFORE you actually put enamel onto your rings. there are
very many variables that can change the outcome and you therefore need
to be able to replicate the process accurately to make it a viable
proposition.
Enamelling onto a flat surface in cloisonne cells is the easiest
technique, however enamelling in the round ie all around a ring is very
difficult.
Do keep us posted as to your progress, preferrably with pictures.
good luck,
ted Frater.

Ted,

I have read about the problems with the transparent enamels and with
the reds and decided to begin with Thompson opaque enamels, medium
temperature and medium expansion. There is an excellent article at
www.glass-on-metal.com by Woodrow Carpenter : Metals Suitable for
Enameling. (Peter: forgive me if it is inappropriate to list sites. If
so, I hope you will suggest an alternate way to suss out this paper.)
There is a lot of useful information in this article, written, I'm
guessing, by metallurgists at Thompson. Most interesting to me was the
recommendation that enamel on sterling should be fired in a very tight
window between 1400 and 1450. This obviously requires one to pick
enamels with the firing temperatures and expansions that fit. The
reasons for firing 200 degrees below what has in the past been
considered the critical temperature are explained in detail.

I have decided to stay away from leaded enamels. I suspect one can get
intense and beautiful colors this way, but considering the health
hazards and the perception of health hazards on rings in constant
contact with the finger I chose to start out with Thompson enamels.
Another and not insignificant reason is that there's a shop within
walking distance of me that sells all the colors and has gone to the
trouble of providing fired samples, which I have photographed and
loaded on my computer.

I am glad to hear some feedback on the difficulty of firing emamels in
the round. I suspected it wasn't going to be easy. I am hoping
klyr-fire or something of the sort will make it possible.

Thanks again for the information. Don

In response to Peter's comment on the possibility of other silver
alloys working with enamel, I e-mailed Rio Grande's customer service
department asking about their sterling silver anti-firescale casting
grain. Their reply was that it does accept enamel without problems.
Don.