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Firescale after casting


#1

On this subject of firscale associated to soldering, I was
wondering if anyone has encountered grey/purple ghosts from using
an aluminum oxide buff on casted silver. I use this process
often and I’m baffled by the reoccurring blobs that appear when
the piece begins to oxidize. Can firescale occur from the
casting process? Any advise would be appreciated.

Rebecca.
San Diego CA


#2

Fire scale always happens to me when I cast silver. Usually I
just put the peice in the pickle solution for at least 30 min,
sometimes more. It’s usually gone, when I take it out, If not
just leave the peice in longer. Amanda


#3

It was said that firescale always happens after casting. I don’t
think of a blackened casting as firescale. It whitens right up
in pickle, right? It never shows up later on a polished surface
as grey shadows either does it. I think these are two different
things. Please comment. Net


#4

The blackened surface on investment cast silver is a sulfide
produced from a breakdown of the gypsum based plaster investment
will not be primarily an oxide. The black tarnish that develops
on silver is also a sulfide from atmospheric contamination. Jesse


#5
    It was said that firescale always happens after casting. I
don't think of a blackened casting as firescale. It whitens
right up in pickle, right? It never shows up later on a
polished surface as grey shadows either does it. I think these
are two different things.  

I believe this is correct. The only time that I have seen
firescale on castings is when soldering has been involved.

Timothy A. Hansen
TAH Handcrafted Jewelry
E-Mail: @Timothy_A_Hansen
Web-Site: www.home.earthlink.net/~tahhandcraft


#6

My thoughts exactly. The blackening on sterling castings fresh
out of the flask is casting and cooling oxidation. Firescale is
something much deeper than this surface oxide and can’t be
removed by simple pickling processes. As with most firescales,
it shows up after you’ve polished for a while-- often after
you’ve broken through any fine silver surface depletion guilding
resulting from repeated solderings and picklings. When this
happens, you wonder where it came from… but it was likely there
from the start. Andy Cooperman


#7

To the best of my knowledge, whenever you pickle sterling after
heating it a layer of fine silver forms on the surface because
the heat brings it up. If you have firescale on a piece you
might just be covering it up with fine silver but the scale
might still be there. I was told that if you evenly heat up the
piece (not too much)a few time and pickle and then brass brush
it minimizes the appearance of fire scale but then again I could
just be covering it up. Also there is a casting grain on the
market that claims to ve firescale proof. it might be alittle
bit more expensive but I have had good results thusfar.


#8

Hi Net, Yes the surface black is removed with pickle however,
there is a reaction with the sulfur in the investment that goes
deep into the surface molecules especially if your flask and
metal are too hot. The worst fire scale is the red kind. It is
much deeper and can be caused by “tired”, dirty, or over heated
sterling. Depending whether you are torch melting or induction
melting in an inert atmosphere, ie: Argon, Nitrogen or Forming
gas( a mixture of Hydrogen and Nitrogen) you can still get
varying degrees of the “grey ghost”. You may be aware that
sometimes the grey fire scale is easier to polish than other
times. With a torch melt it IS possible to keep the firescale to
a managable minimum. Whereas even with a Neutec, Galoni, Memco or
other induction melt machine w/Oxygen free atmosphere, you can
still get some of the “grey ghost” from the sulfur in the
investment. When torch melting use a large envelope of reduced
flame kept constantly on the crucble and sterling and use only
fresh grain from a reliable source, and will you get the best
results possible. Will you still have a chance of a little “grey
ghost”? Yes, but you will find it is the easily removeable kind.
One of the problems can be; having done the best possible you can
have some of this slight grey stuff deep witin a textured piece.
At that point if you are searching for cast-sterling-nirvana, you
add the final step of “Bombing”. John, J.A.Henkel Co.,Inc.
Moldmaking Casting & Finishing


#9

If you want castings that come out bright you should try United
Precious Metals sterling S-88 alloy. It cuts down on alot of
finishing up work and will keep some porosity down also but not
all. REally nice stuff if you cast alot of silver…Dave


#10

Hi All, If your caster is giving you castings that you are
getting firescale on, then send your castings to us… we have
not had this problem in the last 6 years of casting sterling for
a number of people… A few of our customers are Orchidites
and have seen the quality of what we do. Also, We just added a
White(pewter)metal casting section to our website. If you go to
http://www.racecarjewelry.com and click on “workshop”. You
will see some photo’s and explanations on the process we use for
whitemetal casting. You never know when you might need something
done in pewter. Daniel Grandi