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Investment and castings are so yellow


#1

Hi Orchid,

I am still having the problem of a yellowish cloudy layer on my
castings resembling firescale but yellow instead of red or purple. I
tried using rio’s anti-firescale sterling and had the same result. I
noticed that the investment around the castings was quite yellow as
well, and the silver was very black before pickling.

Is it possible that I’m overheating the flasks in the burnout and
causing release of sulfur? I set my kiln for 1350 F at the hot
stage, that’s what R&R recommends I think, but the thermocouple sits
between the top of two flasks where it may heat more slowly than the
flasks.

anyway, if anybody knows why my investment and castings are so
yellow, please give me a clue.

Thanks,
Douglas


#2

Hi dear Douglas,

I think the problem is with the burn-out cycle, minimum two hours at
730 C, and then drop down the temp to the casting temp of the flask
and hold it for one hour at the final casting temp and then cast.

Also check the circulation of air in your burn-out oven, is there
sufficient space for air to circulate around your flask, so that all
the gases can easily escape out of the oven, an extractor fan is
highly recommended to remove all the harmful gases and fumes during
the burn-out cycle.

If your burn-out oven is small then try to put only one flask at a
time, this will help the air to circulate around the flask and
elimination of gases will be easier and the flask will be at uniform
temp before casting. I hope this should solve your problem.

May all be Peaceful Healthy and Happy.

Umesh


#3

It sounds to me as though you’ve hit the nail on the head. The yellow
left on the castings and the black on the silver prior to pickling
sound typical of sulphur compounds. The blackened silver is very
"liver of sulphur" in behaviour. I don’t know much about casting so
someone else can advice about kiln temperatures and time but my
instinct says sulphur is your problem.

Helen
UK


#4
anyway, if anybody knows why my investment and castings are so
yellow, please give me a clue. 

Overheating would be tending to break down the investment, which
you’d see most prominantly as a rougher surface, not necessarily a
yellowish color. Silver can easily be quite black when first removed
from the investment, so that doesn’t by itself, suggest anything.

You say the investment looks yellow before casting? That suggests
incomplete burnout. What sort of wax are you using? Some plastics,
though usually not standard casting waxes, might do that. And if the
kiln doesn’t have sufficient air flow through it, you might be
getting incomplete oxidation of burn out products, leaving surface
discoloration. That would certainly affect the surface of the
casting.

You might still be overheating. It’s possible for controllers to be
miscalibrated. An easy check would be to set the max temperature a
bit lower. There’s not a tremendous difference in burnout ability
between 1300 and 1350, so if your system is accurate, at 1300 you’d
likely get as good a burnout, but if your system isn’t calibrated
right, it might be getting hotter than 1350, which might cause
problems. I’d try reducing the max temp by fifty degrees to 1300, as
well as extending the time spent at the max temp. Be sure also that
the kiln you’re using was intended for casting burnout. Some kilns
sold for enamelling, or for PMC, are not well enough ventillated to
give a proper burnout. Some of them might require something like
additional holes drilled, perhaps in the top of the kiln into the
heating chamber (be sure to avoid heating elements!), to give better
exhaust of burnout products. A check for this would be to try a test
burnout with the door of the kiln not completely closed. Leave it
just cracked slightly open, just a couple millimeters, enough so a
very little bit more air gets around the door seal, rather than
actually swung open, since then it would never heat up all the way.
But very slightly ajar, maybe just with the latch not quite closed,
should let a little more air circulation happen without preventing
proper heat up. See if that makes a difference.

Peter Rowe


#5
It sounds to me as though you've hit the nail on the head. The
yellow left on the castings and the black on the silver prior to
pickling sound typical of sulphur compounds. The blackened silver
is very "liver of sulphur" in behaviour. I don't know much about
casting so someone else can advice about kiln temperatures and time
but my instinct says sulphur is your problem. 

I’m not so sure. While overheating the investment can cause the
gypsum componants to break down, giving rise to sulphur compounds
that would indeed increase the black color of the surface, these
compounds are not likely to be actually turning the investment
yellow, especially not at the outer surfaces, since one would expect,
at these temps, any actual released sulphur would oxidize to sulphur
dioxide gas, rather than remaining yellow in the investment. While I
wouldn’t rule out sulphur compounds as an issue, the yellow color of
the investment is, I suspect, due to something else. Liver of sulphur
does not generally get used to blacken silver while heated to 1300
degrees, so the obvious comparison isn’t as valid as it may seem at
first glance. If it IS sulphur, forming typical liver of sulphur type
black sulphides on the silver surface, this is easy to diagnose,
since sulphides like this are not easily dissolved in the usual
pickle compounds. Usually, the black colors on silver castings are
from copper oxide, not silver sulphide. The oxides normally pickle
off quickly enough to leave a white or near white casting. If
instead, they don’t pickle off, or do so incompletely, then perhaps
you do have sulphides forming. (Or your pickle might be exhausted.
Have you tried changing out the pickle?)

Peter Rowe


#6

Peter,

You are almost certainly right. Everything you have said about the
temperature of the burnout is correct and higher than the luke warm
temperatures used when patinating silver with potassium sulphide. And
yes, again, if it was due to sulphur, then at those temperatures,
they would more than likely produce sulphur dioxide.

Also, as you say, liver of sulphur patina won’t usually be removed
by normal pickles but as the poster said, the black was easily
removed by pickle and was thus probably the normal copper oxide
rather than silver sulphide.

I am often too quick to respond, sometimes before properly thinking
it through - you know the mouth in gear before the brain. Another
failing I’m afraid.

Helen