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Black Blobs in my Sterling


#1

Can anyone identify my blobs?

I have a small line of cast rings called Claw Baby and I do three
finishes: depletion silvering matt white, matt sulphured black, and
polished stg.

I heat by putting the rings on a st/steel mesh and hold the
mesg+rings over a stationary bushy propane burner.

Today I was working on 5 at a time, depletion silvering by heating
to just under annealling temp and quenching hot in pickle. After
three such cycles I was heating and two sings looked slightly pink,
like they were a little past the annealing temp I was aiming for, so
I stopped heating.

Then I saw these blobs grow out of the metal. Crazy! Black
secretions from inside the metal.

Image page here, with thumbnail: http://www.adam.co.nz/blobs

I guess the casting company uses a casting sterling alloy with some
additive that is reracting to being heated.

Any ideas?
Brian
B r i a n A d a m
e y e g l a s s e s j e w e l l e r y
Auckland NEW ZEALAND
www.adam.co.nz


#2
   Then I saw these blobs grow out of the metal. Crazy! Black
secretions from inside the metal. 

Very strange indeed, Brian.

I’d guess that it’s flux (some casting fluxes are black), or a
combination of flux and graphite from crucible breakdown, or some
such goop, and the casting process was such that the stuff got mixed
with and carried along with the metal during casting, forming flux
inclusions within the metal. I’d also guess that, as with many
fluxes, it’s at least somewhat water soluable, so that with the first
pickling or two, moisture was able to penetrate the metal’s porosity
(all castings, after all, have some degree of porosity, and your’s
with those inclusions, might have more) and be absorbed by the
included flux, so then the next heating created enough steam pressure
within the metal to force heat softened flux out of the castings?
Just a guess. Perhaps it’s not directly too much flux added during
casting, but perhaps use of a melting crucible that was deteriorating
or already had too much flux in it, or some such.

I’m recalling occasional castings I’ve seen that were porous but
with only small surface pores but apparently significant internal
cavities/shrinkage voids, etc, that after cleaning off polishing
compound in an ultrasonic, or other such soaks in liquid, subsequent
heating would cause liquid to bubble up out of seemingly solid metal.
Very freaky surprise indeed. In the case I’m thinking of, the
situation only became apparent after totally finishing the casting
and polishing it, , as the initial cast surface seemed solid, and the
castings looked and acted just fine. Finishing off the casting took
off a surface skin, exposing slightly increased porosity, but even
then it didn’t look bad. I later cut cross sections of it, and it
turned out most of the center areas of the heaviest parts of that
ring shank were pretty much filled with shrinkage cracks and voids.
Enough to have absorbed a significant amount of liquid…

Peter


#3

I think that Peter is on the right track.

I too have cut open castings to discover included flux, investment
and horrid porosity that was not very evident from the surface.

There is also a possibility that the casts have a great deal of
internal porosity that sucks pickle into it when the ring is
quenched. When heated it would bubble out as melted salts that make
up the pickle. This would be black as it picked up oxides from the
porosity. The streaky like spots under the claws may suggest a
porosity. (by the way the photos are great, they are worth thousands
of words)

If it is this type of porosity, I have seen something similar. It
was sterling silver that was contaminated by lead and or tin. This
metal did strange things. One was to increase it’s porosity as it
was reheated. Some of the metal melted before the rest leaving a
sponge like structure of solid metal and molten metal. When cooled
it was very porous.

In any case I feel that this metal in your cast is ruined. From the
variety of spots I do not think that the contamination came from
you. I would ask the caster to recast using only new metal. You do
not want part of that old button to be included in the new cast.

This should be done at no cost to you as a good customer service. Let
us know how this plays out it will be interesting.

Franklin


#4

I think you need to talk to your caster. Those black blobs could be
deoxidizer he added to the melt. While I advocate this for bronze,
it’s not intended for silver. Is your caster strictly a jewelry
caster, or is a foundry caster? If the latter, he might have added a
degasser to rehabilitate old metal. This doesn’t work for silver or
gold.

Ciao
Gail


#5

It’s solved … it was my fault.

I talked to the jewellery casting company (a major precious metals
supplier and casting house) and a man there analysed the blob
material. I might have mis-heard but I thought he said it was a high
proportion of copper (21.5%) in the form of cupric oxide (CuO).
Possibly the result of uncontrolled heating during annealing.

That sounds right, as Ruth reminded me about what happened earlier
in the day. I was batch-heating 5 of these rings using a stationary
propane burner and sitting the rings on a tripod+mesh. I left them
there for a short time leaving the gas on low! Ruth came by a while
later to discover them, looking rather reddish. I was certain I’d
turned the gas off! Now I’m rather pinkish with embarrassment.

Thanks for your replies.

Brian

Brian Adam
Auckland NEW ZEALAND
www.adam.co.nz


#6
   It's solved ... it was my fault. I talked to the jewellery
casting company (a major precious metals supplier and casting
house) and a man there analysed the blob material. I might have
mis-heard but I thought he said it was a high proportion of copper
(21.5%) in the form of cupric oxide (CuO). Possibly the result of
uncontrolled heating during annealing. 

Brian, I’m not sure I believe this, if you were using a standard
sterling silver, ie 7.5 percent copper and the balance silver. I’ve
many times heated silver to a nice orangy glow, sometimes for
extended times, playing with, among other things, the grain growth one
gets with overheating during annealing, as well as various variations
on depletion gilding, and in a few cases, intentionally trying to
damage the metal in various ways to just see what happens. If simply
overheating sterling silver, even for extended times, could create
islands of cupric oxide that would ooze out, we’d all have seen it.
While it’s likely that your unintended extended heating brought out
the problem which you’d not seen in other batches not treated that
way, I suggest that the extended heating was not the actual cause,
merely the condition that made the problem evident.

While extended heating might oxidize the copper, certainly, the
normal mechanism is for the copper to migrate towards the surface as
it oxidizes, forming our familiar nemesis, fire stain and it’s black
surface cousin, fire scale. Neither of these have ever in my memory
even remotely resembled the odd blobs you experienced. While your
refiner may well have found that the gunk was high in cupric oxide,
something that might make sense given the basic copper content of the
metal, the form it was in suggests something much more fundamentally
wrong than just overheating or overannealing, since that does not
produce islands of oozing goo in small spots. it produces a fairly
uniform series of surface and subsurface oxide layers, none of which
seem prone to ooze out.

I remain convinced that the problem is related to the casting
process, such as porosity problems, inclusions from the crucible,
inclusions of flux, or some other similarly related problem, and not
to what you did after the pieces were cast. You did not heat them
enough to melt them, after all. A pinkish cast after extended
annealing isn’t a surprise, just a confirmation of my statement to
expect fire stain and fire scale, which could easily do exactly
that. The ooze you saw is something different.

Just because the guy works for a major supplier, and knows metals,
does not automatically mean he’s got his guess as to the cause
correct. If what he’s done is determine that it’s high in copper
oxide, well that’s good info, but not yet an explanation. And keep in
mind that there is a clear conflict of interest. He doesn’t really
wish to admit that it’s the fault of his company, after all, so
deciding this was your fault is clearly in his favor. Try consulting
an independent matalurgist, and don’t be so quick to assume the
caster is right and you’re at fault. He may well not know what caused
this, in spite of his facilities and knowledge. I’ve known several
large respectable companies who’s knowledge of their own product’s
engineering and performace was surprisingly lacking, such as the long
time manufacturer of a platinum investment who’d done many
engineering tests of the material’s set up time response to burnout,
etc, all nice and scientific, but who’d never actually cast platinum
in it, and were thus unaware of a metal to mold reaction that
produced rougher than necessary surfaces.

Peter Rowe


#7

I Agree with Peter Row that the metal Blob problem is not caused by
Standard (copper/silver) sterling alloy.

I’m 99% sure of is that it is caused by Some form of DeOx sterling…
There are many deox sterling alloys on the market and having tested
about 8 varieties, I can remember seeing some black blobs on
castings … It was caused by overheating the metal and /or using a
high oxygen concentration in the flame. It is definitely possible to
bring out some black ooze on certain deox materials when it is heated
too far…

Some of these alloys contain metals like Tin and other low melt
materials that can come out of the alloy when over heated. Hope this
is helpful.

Daniel Grandi
Racecar Jewelry Co. Inc
Tel: 401-461-7803