jgourlay
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Help me deplate something
Gents, I have a metal fist made of a copper alloy containing aluminum, zinc, and possibly lead. During the cast, some or all of those "white metals"
migrated to the surface, creating a very thing layer of ugly.
Is there some chemical method I can use to etch off the outside layer and get to the brass/bronze underneath? Btw, I know what the inside looks like
because I cut an ingot taken from this same heat.
This part has very fine detail (you can see fingerprints!) which is why I don't just wire brush it.
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hissingnoise
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It must give you a knock-out punch!
It's obviously a case of ugly being skin deep. . .
Ugly is in the eye of the beholder.
Seriously though, chemical etching might soften fine detail.
Is bronze-plating electrolytically an option?
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jgourlay
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LOL!! All true.
I don't mind losing some detail. The plating thing will entail workspace issues I can't sustain right now. I'm hoping there is an answer along the
lines of "dunk it in muriatic acid for while".
If not, I'll just wire brush it. And if I don't like that, I'll melt it down and do it again.
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hissingnoise
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If the layer of white metal isn't of uniform thickness etching will be very difficult.
OTOH, if you look at it long enough its beauty will come through?
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jgourlay
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Well, actually this one has a very "artistic" look. I'm betting someone would kill to learn how to get this finish.
OTOH, it's not what I intended. So I'm going down two roads. First, how to fix it (here) and second how to prevent it (over on
backyardmetalcasting.com).
In the future, I'll be doing this for friends and family and others where it has to be right (ie, bronzy shiny) the first time because I won't able to
get the wax positive twice.
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watson.fawkes
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Lead acetate is soluble, so a soaking in white vinegar is a mild first step to try. Since lead is sticky, it's possible that it's acting as the binder
of a coating. If so, taking out the lead may convert the rest of the coating to powder.
For mechanical measures, you might consider buffing with a soft cloth wheel charged with a fine abrasive (i.e. non-grinding) such as red rouge. A
cloth wheel, not sewn near the edge, won't change the shape as a felt wheel or a hard cloth would.
While HCl would certainly dissolve the zinc, it will also etch zinc out of the alloy, creating pitting. If you try this, dilute heavily at first and
ramp up concentration slowly until you get a concentration that takes off the zinc with minimal surface disturbance.
Digressing to casting technique now. As for having a unique positive, you can make it non-unique by using a rubber glove mold (for the surface) and a
plaster mother mold (for mechanical support of the glove mold). With such a system you can cast multiple wax positives and invest each one separately.
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jgourlay
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Watson, thanks. Can you explain the rubber glove mold to me more detail? Realize I am taking the impressions of the hands of various folks using
Alginate "rubber". Are you saying make an investment negative of the wax positive, and then use the investment negative to make rubber positives
which can then be used to make additional investment negatives for the actual pour?
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Paddywhacker
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You need a test piece to experiment on.
Aluminium and zinc will both dissolve in 10% caustic soda solution, wheras copper will not.
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jgourlay
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Sweet! Now I have a program. 1. vinegar 2. caustic soda.
Thanks!
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watson.fawkes
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Quote: Originally posted by jgourlay  | | Can you explain the rubber glove mold to me more detail? Realize I am taking the impressions of the hands of various folks using Alginate "rubber".
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Here's the process in summary. Realize that there are books on the subject and you'll want to read one.Alginate negative. You're doing this now. Alginate, incidentally, is a polysaccharide.
Plaster positive. You can use other materials, such as moulage, but plaster is easy and works great. Use a more permanent plaster like Hydrocal
and you can reuse the positive years later. Removal generally means destroying the alginate.
Glove mold negative, in rubber. Natural latex and urethane are both used for this. If you need lots and lots, spring for silicone. Removal is by
peeling off the layer of rubber like a glove. The plaster positive remains untouched.Plus Mother mold, in plaster. The glove mold is
flexible, so you'll need to support it. The mother mold is a negative of the outside (positive) of the glove-mold on the plaster positive. It has at
least two pieces and at least one parting plane. Removal is in pieces.
Wax positive. Cast in the negative mold of the glove-inside-mother. Removal is by disassembling the mother and peeling off the glove.
Investment plaster negative. Remove the wax by steaming and then bake out.
Metal positive. Remove by destroying the investment.
The main reason to deal with a glove mold is because of undercuts. If you have no undercuts (not the case for human hands, as a rule) there are
simpler ways of making the mold.
Another useful detail, if you're doing sculptural work, is that the positives you cast in the glove mold can be worked (both additively and
subtractively) before investing. Rodin, for example, reused foot casts within his sculpture The Burghers of Calais.
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Paddywhacker
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With the vinegar and caustic you might get surface leeching of everything except copper, and then when you buff you'll get a copper shine instead of
brass. The effect might be patchy. Could be an 'artistic' touch.
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jgourlay
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Paddywhacker: It's art now....so I'll try it. But thanks for the warning!
Watson: You know, there are 4 boards on the net, of which this is one, that is just full to the brim of smart, helpful people. Your post is a
perfect example of a piece of information that one would never expect to find here, and which "the experts" appear to be reluctant to give up on their
discussion boards. Thanks!
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chloric1
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Certain metal salt solutions are causitc and discolor metal surfaces. I know Antimony chloride is for "bronzing iron but the presense of antimony on
the workpiece is probably objectional.
Copper salts might be usefull though. For instance a copper sulfate solution mixed with enough ammonia to redissolve the copper hydroxide would be
interesting. This alkaline solution would dissolve the zinc and the aluminum to some extent. The white metal that remains would reduce the copper
ions into cuprous oxide or even copper metal. A gentle buffing would reveal a copper finish. You might get robin eggs blue patches if the ammonia
fumes are concentrated enough. Maybe do this inside a covered trash barrel over a saucer of ammonia.
Fellow molecular manipulator
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watson.fawkes
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| Quote: Originally posted by chloric1 | | Certain metal salt solutions are causitc and discolor metal surfaces. I know Antimony chloride is for "bronzing iron but the presense of antimony on
the workpiece is probably objectional. |
See The Colouring, Bronzing, and Patination of Metals: A Manual
for the Fine Metalworker and Sculptor, by Richard Hughes, for a whole recipe book of this sort of thing. It's a luscious book, full of color
plates and all the recipes they used to make them. I used to have a copy, but I lent it to a friend who destroyed it out of carelessness.
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chloric1
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Yes watson, I have a couple patina books. This is where I drew my ideas from. I paid out $85 for my copy of "Coloring and Patination of metals".
Lending out prized possesions to friends is almost always a recipe for disaster. Its like having a relative manage your personal savings.
Personnally, it really is too sad to be selfish but I would have photocopied the book for the friend. So, if you don't mind me asking, how did he
destroy the book? Did he accidently spray it with hot ferric nitrate?
Fellow molecular manipulator
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watson.fawkes
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| Quote: Originally posted by chloric1 | | So, if you don't mind me asking, how did he destroy the book? Did he accidently spray it with hot ferric nitrate? |
What I heard, only heard, is that he left
it in the back of a very hot car and it warped all the pages to hell.
The benefit of this conversation is that I now have a tickler to myself to get it replaced (finally).
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chloric1
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What really turns me on is the book writen by the owner of Sculpt Nouveau. I like the fuming method in a large closed container. He suggest vineger
for verdigris and ammonia for blue patina.
Hughes book is the only mention of iridescent heavy metal sulfide films on the base alloy. Everything thiosulfate based. Pure genius. Would love to
try this on silver! The lead based one scares me but the copper one might be nice. This woud be a nice experiment for nickel and cobalt in
thiosulfate.
Fellow molecular manipulator
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UnintentionalChaos
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| Quote: Originally posted by chloric1 | What really turns me on is the book writen by the owner of Sculpt Nouveau. I like the fuming method in a large closed container. He suggest vineger
for verdigris and ammonia for blue patina.
Hughes book is the only mention of iridescent heavy metal sulfide films on the base alloy. Everything thiosulfate based. Pure genius. Would love to
try this on silver! The lead based one scares me but the copper one might be nice. This woud be a nice experiment for nickel and cobalt in
thiosulfate. |
Traditionally, you'd use liver of sulfur, a crude mix of sodium sulfide, polysulfide, and thiosulfate made by dissolving sulfur powder in concentrated
aqueous KOH. I've made it before with sodium hydroxide instead. You need to add a small amount of isopropanol or other alcohol at the beginning so
that the solution wets the sulfur, followed by extended heating.
Very dilute solutions can be used to generate multicolored thin films on a polished sterling silver surface. Pure silver is significantly more
resilient to this treatment and you may need to use concentrated solutions or ones that have had the pH dropped so there is some free H2S in solution.
These films are not terribly wear resistant and normal "tarnishing" will ruin the effect.
[Edited on 6-20-09 by UnintentionalChaos]
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'In organic synthesis, we call decomposition products "crap", however this is not a IUPAC approved nomenclature.' -Nicodem
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watson.fawkes
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| Quote: Originally posted by chloric1 | | What really turns me on is the book writen by the owner of Sculpt Nouveau. I like the fuming method in a large closed container. He suggest vineger
for verdigris and ammonia for blue patina. |
I see two books by Ron Young, Methods for Modern
Sculptors and Contemporary Patination. Can you recommend one over the other?
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chloric1
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IRC the book even mentions that the sulfide films are delicate. I would think several layers of spray laquer would be in order.
The solution proposed by hughes are fairly strong solutions of neutral sodium thiosulfate to which a metal salt is added. Examples are ferrous
sulfate, lead acetate, and copper sulfate/acetate. These for thiosulfato complexes which are unstable during the heating and deposit the sulfides on
the workpiece. Being that the metallic salts have acidic dispositions, I am sure there is H2S along with the polythionic acids present. It is
amazing how unstable thiosulfate is to low pH values. Even adding a trifle of bromine to sodium thiosulfate solution immediately turns it into
opaque milk because of the small amounts of HBr formed.
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UnintentionalChaos
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| Quote: Originally posted by chloric1 | | IRC the book even mentions that the sulfide films are delicate. I would think several layers of spray laquer would be in order.
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The colors are from thin-film diffraction though, and adding another thin layer over the top will kill the iridesence or change the color.
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'In organic synthesis, we call decomposition products "crap", however this is not a IUPAC approved nomenclature.' -Nicodem
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watson.fawkes
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Well, maybe.
The thickness of even a thin lacquer is quite large with respect to the wavelength of light. If it were otherwise, lacquer itself would form
iridescence. So the lacquer should be considered thick here, insofar as its optical properties are concerned.
As for killing the iridescence, that will only happen if the index of refraction is the same as (or close enough) to that of the sulfide layer. Now
the closer it gets in index the less light is reflected back off the outer boundary layer, so a material that's close will cause a dimming of the
effect.
It won't, however, change its color. The color is an interference between reflections off the top and bottom layers of the sulfide. As long as those
reflections still happen (i.e. no match for refractive index) that distance doesn't change, the wavelengths of reinforcement don't change, and thus
the color doesn't change. A different second layer, one that's thin enough to diffract, would alter the apparent color by adding its own color
contribution, but that's not the same as changing the color of the first layer.
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jgourlay
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| Quote: Originally posted by watson.fawkes | | Quote: Originally posted by chloric1 | | Certain metal salt solutions are causitc and discolor metal surfaces. I know Antimony chloride is for "bronzing iron but the presense of antimony on
the workpiece is probably objectional. |
See The Colouring, Bronzing, and Patination of Metals: A Manual
for the Fine Metalworker and Sculptor, by Richard Hughes, for a whole recipe book of this sort of thing. It's a luscious book, full of color
plates and all the recipes they used to make them. I used to have a copy, but I lent it to a friend who destroyed it out of carelessness.
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AND HE DIDN"T REPLACE IT?!?!? WOW!!! I think that would have taught me more about the fellow than I would have cared to have known.
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watson.fawkes
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| Quote: Originally posted by jgourlay | | AND HE DIDN"T REPLACE IT?!?!? WOW!!! I think that would have taught me more about the fellow than I would have cared to have known.
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I don't speak with him at all any more. And not just from this. I like to think I've become a much better
judge of character in the interim.
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