In response to my suggestion to use the traditional word
‘blanching’, Mark Nelson of Rio Grande Technical Support wrote:
This is wonderful Janet. Just the term we're perhaps looking for.
Do you all think it's worth Rio changing the wording on the Aura
22 instructions to reflect that? It seams as though Depletion
Gilding is inaccurate. Or would it only cause more confusion? I
would like us to be accurate.
Mark,
I just looked at your instructions for Aura 22
( http://www.ganoksin.com/gnkurl/ep7zn2 ) and it is most definitely
incorrect…:-)…! The process you are describing is not depletion
gilding (it is not any sort of gilding!), and the acid is in no sense
“the depletion”. The process is traditionally called blanching, and
uses the same principle as depletion gilding, namely, removing
undesirable metal(s) from the surface. In the case of ‘depletion
gilding’, copper and silver are depleted from the surface of a gold
alloy; in the case of ‘blanching’, copper is depleted from the
surface of a silver alloy (usually sterling). It would NOT be
INcorrect to refer to blanching as ‘depletion silvering’, but that
would be coining a new (accurate) term. People would have to get used
to it, but I guess it would help clarify the process, as people are
already familiar with the term ‘depletion gilding’. I prefer the term
‘blanching’, but ‘depletion silvering’ clearly does give more
It would NOT be INcorrect to refer to blanching as 'depletion
silvering', but that would be coining a new (accurate) term.
But a good term for communication. I understand ‘depletion silvering’
immediately, but when I hear ‘blanching’, I expect broccoli. Anyway,
I wouldn’t call it a new term. I’ve seen it used much more often than
‘blanching.’
Coper is oxidized by heating in the presence of air, copper oxides
are soluble in an acid aka pickle, copper oxides dissolved silver
left behind.
James Binnion and others who so generously share your knowledge – I
can’t thank you enough for being there! I don’t post often, but read
daily and enjoy and appreciate being educated by you all.
So if I want to build up the fine silver layer, I know the process
is to heat the metal in an oxygen atmosphere, pickle the metal until
it is white and frosty, burnish it with a brass brush and repeat the
process.
If I want to build up the fine silver faster and/or make a thicker
layer, am I better off heating longer and creating more of the
copper oxides or am I better off leaving the metal in the pickle
longer to etch the copper deeper? How much does the burnishing
matter for the copper oxidation of the next heating cycle?
If I want to build up the fine silver faster and/or make a thicker
layer, am I better off heating longer and creating more of the
copper oxides or am I better off leaving the metal in the pickle
longer to etch the copper deeper? How much does the burnishing
matter for the copper oxidation of the next heating cycle?
Repeating the short steps will help you more then a longer heating
time cycle. When the copper oxide layer is dissolved, it’s gone. The
silver turns white, showing you a nice clean silver surface.
Keep in mind that longer heating cycles will create the building of
higher copper oxide. These higher copper oxides (cupric oxides) are
NOT soluble in pickle. They appear as a grayisch layer in your silver
surface which is not wanted (amongst the most of us).
Remelting the used silver will not clean it from the higher copper
oxide.
So if I want to build up the fine silver layer, I know the process
is to heat the metal in an oxygen atmosphere, pickle the metal
until it is white and frosty, burnish it with a brass brush and
repeat the process. If I want to build up the fine silver faster
and/or make a thicker layer, am I better off heating longer and
creating more of the copper oxides or am I better off leaving the
metal in the pickle longer to etch the copper deeper? How much does
the burnishing matter for the copper oxidation of the next heating
cycle?
You cannot really do too much to speed it up, and the thickness is
going to be something you cannot really do much to increase. While
the oxygen will penetrate the sterling matrix to some degree the
pickle will not. It it really only a very thin surface layer that is
affected by the process. Wish it were otherwise but. The wire
brushing is to burnish the surface to smooth over the roughness from
the copper oxide being dissolved from the surface matrix, after a
couple of cycles you will have removed all the copper from the
surface and further cycles will not do much to change the results.
Keep in mind that longer heating cycles will create the building
of higher copper oxide. These higher copper oxides (cupric oxides)
are NOT soluble in pickle.
There are two copper oxides CuO and Cu2O both are soluble in acids
like pickle.
For example with sulfuric acid:
CuO + H2SO4 → CuSO4 + H2O, cupric sulfate and water solution
Cu2O + 2 H2SO4 → CuSO4 + H2O + H2, cupric sulfate and water
solution
They appear as a grayisch layer in your silver surface which is not
wanted (amongst the most of us).
I think you are referring to firescale, if so the reason it forms is
that readily oxygen readily penetrates the sterling crystal matrix
and reacts with the copper present in the matrix making CuO2 which
is visible in the upper surface of the metal. Hiding this is the
whole reason for “raising” the fine silver on the surface.
Remelting the used silver will not clean it from the higher copper
oxide.
Again I must disagree, the copper oxides will react with fluxes like
borax and be reduced back to its metallic state. Copper oxides are
also reduced by charcoal, carbon, monoxide, hydrogen and hydrocarbon
gases at the elevated temperatures of the molten metal. So as long
as your melting practice is good you will end up with relatively
little in the way of copper oxide left in the metal.
-To deplete a thicker fine silver layer on sterling silver, it is
helpful to heat longer and/or more times. Tim McCreight’s basics
book talks about heating in a kiln for a length of time, rather than
heating with a torch. A student of mine likes the kiln method, so
you might try that.
-Pickling longer after the metal is white is not helpful.
-Burnishing with soapy water and a fine brass brush bends over the
the little spikes of fine silver left behind after the copper is
removed by the pickle. Therefore, I believe in brushing in one
direction, rather than back and forth.
I have not needed to deplete sterling since 1999, when I started
using Argentium Sterling. My standard procedure used to be to torch
heat, pickle, and brass brush at least 6 times. Those 35 year old
pieces still look pretty good. On the other hand, the Paul Revere
sterling at the Museum of Fine Arts Boston has been polished enough
times that the fine silver layer is gone, and the purplish
firestains/fire scale show clearly. That is why I changed my method
(around 1985) to preventing firescale, rather than depletion
silvering. Later, I fell in love with Argentium Silver.
My knowledge of chemistry is of a low level. Nice from you how you
explaine the formula but that is far behond my knowledge of
chemistry.
I believe what you write James but in this case the proof is in the
pudding. I’ve used sparex (sodium bisulfate), vitrex (sulphuric acid
for a far as I know). Just to be very specific, I didn’t use them
together. I can lay the contaminated silver in the warm solution and
forget about it for day’s. I must be doing something wrong James
because the firestain will not come of. Starting with a new fresh
solution doesn’t help.
What does help is the remelting of the silver with melting powder
containing potassiumnitrate. The kind of melting powder I’ve being
talking about in other articles. The use of phoshorized copper could
be another solution. However, measuring the level of contamination in
order to use the correct amount of CuP is the problem for the little
I know of this matter. Using too much CuP gets you more (other)
trouble. According books that I have, lithium (Li) could be another
help which I don’t know of, never tried thatone
Again I must disagree, the copper oxides will react with fluxes
like borax and be reduced back to its metallic state. Copper
oxides are also reduced by charcoal, carbon, monoxide, hydrogen and
hydrocarbon gases at the elevated temperatures of the molten
metal. So as long as your melting practice is good you will end up
with relatively little in the way of copper oxide left in the
metal.
Copper oxide (CuO) do get drapped in borax, I agree 1oo% with that
statement.
I also agree with you that copper oxides are reduced by charcoal
etc. Problems with the buidling of Cu2O are history to me. I do see
many novice fighting this phenomena by performing prolonged, to
careful heating processes and lack of anti firescale or coating flux.
Another way to remove this Cu2O (i.e. firestain) -to a certain
degree of contamination- is to sand or file that layer down to clean
sterling silver surface.
Thank you for your reaction James, I’m still learning.
I believe what you write James but in this case the proof is in
the pudding. I've used sparex (sodium bisulfate), vitrex (sulphuric
acid for a far as I know). Just to be very specific, I didn't use
them together. I can lay the contaminated silver in the warm
solution and forget about it for day's. I must be doing something
wrong James because the firestain will not come of. Starting with a
new fresh solution doesn't help.
Of course not, the oxide is actually within the metal matrix, the
acid solutions cannot reach it. That is the problem with firestain
you cannot remove it except by grinding it away.
What does help is the remelting of the silver with melting powder
containing potassiumnitrate. The kind of melting powder I've being
talking about in other articles.
Potassium nitrate will not remove the copper oxide, actually it
makes more of it. This is how it is acts as a “refining flux”. The
copper oxides or other base metal oxides are less dense than the
metal so they will float on the surface and be either carried away
by flux glass or reduced by it.
The use of phoshorized copper could be another solution. However,
measuring the level of contamination in order to use the correct
amount of CuP is the problem for the little I know of this matter.
Using too much CuP gets you more (other) trouble. According books
that I have, lithium (Li) could be another help which I don't know
of, never tried thatone
These would reduce the copper oxide but as you noted they do require
a little more about oxide levels for proper use so there
is no excess deoxidizer left in the metal.
If you want to know the quick and easy way of removing copper oxide
fire stain from silver, you do as follows.
you need to buy Johnson Mathhey’s stainless steel brazing flux. This
contains fluorides and is highly reactive when heated to a runny
molten state.
Place your silver item on a stainless mesh support with say 1in of
clearance under it.
Apply the flux and heat with a large neutral flame till it runs then
apply more to keep it flowing and dripping off through the mesh. you
will see it dissolving the copper oxide out of the silver. Keep doing
this till it runs clear.
I thought I would comment on the use of either copper-phosphorous or
lithium when melting silver alloys.
When molten and if unprotected (i. e. no gas cover or protective
layer of flux or graphite/charcoal) silver and silver alloys have a
great affinity for oxygen and can absorb about 10 times its own
volume of oxygen when just above their melting point.Commercial
manufacturers of sheet, wire and grain have no way of knowing the
amount of oxygen present in the fine silver that they purchase from
different sources (for example re-cycled/refined fine silver and
fine silver ‘good delivery’ bars). To reduce the oxygen content of
this source silver small additions of either copper-phosphorus or
lithium can be made to de-oxidise the molten silver. If too much
copper-phosphorus is added then this can lead to low melting point
copper-phosphides being present in the cast products and this will
result in a hot-short product. Lithium is a very reactive metal and
although a small addition does give effective deoxidation I would
not advise its use for anything other than a commercial melting
process as it does need proper handling and packaging before being
inserted into molten silver or its alloys.
Where you have bought commercially produced sheet/wire or casting
grain and are re-cycling your own scrap the amount of
’oxides/oxygen’ you will have contaminated the material with will be
relatively slight and will be easily removed using either a borax
based flux or a graphite/charcoal mix as recommended by Jim
Binnion.Commercial casting grain is also generally manufactured to
very low oxygen levels so provided you melt it with a proper melt
cover you should not have any issues with excess vie oxygen content
on re-melting.
In general; in my experience, the use of a good charcoal/graphite
cover and stirring the molten silver or silver alloy with a graphite
rod is easily the best way of reducing/controlling oxygen contents
for small and medium scale casting operations. Unless you have
access to very good analytical equipment to allow analysis of the
metal before casting I would not recommend the use of either
copper-phosphorus or lithium.