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Brittle argentium castings.... again


#1

Hi Orchid, First let me apologize for the possible lack of spaces
between some words. I’m not are why but this happens in my posts to
Orchid, maybe it’s yahoo mail? As I’ve written before I’ve
experienced problems intermittently with extreme brittleness in my
argentium castings. I thought I’d solved the problem but it has
returned with a vengeance. The last few times I’ve casted about half
of my castings have come out so brittle they break with even just
the slightest bit of bending. It seems likethey’re full of
micro-cracks that require only a little bit of stress to reveal
themselves. I thought before that it was an issue with incomplete
burnout due to lack of oxygen, so I added a vent hole in the door of
my neycraft kiln and have started to crack the door of the kiln open
for a few minutes at the point where the wax fumes begin to ignite
around 950-1000 degreesF. This results in some flames exiting the
top vent, something I assumed meant there was adequate oxygen for
combustion.

I use a high ratio of of new metal to old and make sure to clean the
old metal very meticulously before adding it to the melt. I melt
with an electro-melt furnace using a small amount of casting flux.
All the variables I can think of have been considered. This evening
I poured two flasks of filigree earrings, the flasks are 3x5, burned
out for about 9 hours. the melt temp was 1830 F and the flask temp
was 1025. The flasks appeared to be fully burned out, with white
investment and no discoloration around the sprue hole. The castings
from the first flask came out grey-black in color and extremely
brittle, with yellowish investment surrounding the castings. The
second one was farther from the door and came out slightly better,
though still a bit grey-black towards the button and more white at
the top. Castings that come out white in color are fine while those
that are more grey-black, sometimesmottled tend to be the problem
ones. The black color seems to seems to indicate incomplete burnout,
but when I remove the flasks from the kiln to cast they seem to be
fully burned out. Is it possible that I am overheating the
investment causing it to break down and release sulfur? I make sure
not to get it over 1350 F but perhaps my thermocouple is incorrect?
Also I’ve noticed the investmentaround walls of the flask
occasionally has a bit of rust color, presumablyfrom the flask,
though I do clean them before investing. I am just about at my wits
end here, If anyone has any ideas about what may be the cause of
this problem I would really appreciate hearing them. I’ve been
casting for more than ten years and its quite frustrating to be
unable to diagnose this issue. Thanks. D.


#2

I cast Argentium a few times a week without any of your experience.

I am not sure about why you have the problem. Let me point out some
things that I do different.

Over 40 years of casting I have never seen flames in my oven. I used
to dewax at 300f for an hour and let the wax melt into a tray that
was removed before increasing the temperature. I now use boiling
water to dewax. (I use a rice cooker) There is nearly nothing to
burn.

I use a short burnout time, my flasks are smaller than yours. They
are 2 1/2 diameter and 3 inch occasionally a 4 inch flask diameter
and never more than 4 inches tall. I most often have only 4 flasks in
the oven. My temperature control is very good.

My burn out time is
after dewaxing
start -15 min at 300f
then - 30 min at 500f
then - 30 min at 900f
then - 1hour 45 min at 1350
cool to temperature to cast the first flask 45 min.

cool to each successive temperature 30 - 45 min depending upon the
difference.

I use a torch to cast and cannot tell you the temperature of the
metal. I can tell you for filigree I will cast with a flask temp of
1100f. The metal is melted untill it balls up and will swirl when the
arm of my centrifugal cast is moved and it is then released.

I usually do not quench, I break out when fully air cooled. If I do
not wait until fully air cooled, the flasks are broken out in water,
but the flask is probably not more and 200f - 300f. (cool for 1/2
hour before breakout)

Things that you wrote that concern me…

the flames in your oven the low flask temp for filigree and perhaps a
high metal temp to compensate if the reused metal is compromised
because it has been cast and has come out as you described, I would
not reuse any of it in another cast. It sounds like it is ruined.

Below is a copy of a recent post that I made about casting
Argentium…

I am usually casting 2 1/2 inch flasks, sometimes 4 inch flasks (all
custom work, small items)

  • centrifugal casting - torch (propane & oxygen) to melt and I use a
    somewhat hotter flame than I use for regular silver (for regular
    silver the flame I use is somewhat reducing, I use more oxygen in the
    flame when casting Argentium) The usual amount I cast is from 10 to
    60 grams in a flask

i) I find that I need a mix of new grain to old buttons of 60% new
and 40% old (50/50 shows some signs of a poorer cast on occasions,
but not always) I make very sure the old material is very clean

ii) I coat the material to be cast with boric acid dissolved in
alcohol, then burn off the alcohol, I use no other flux The boric
acid solution is about 30 – 40 % saturated

iii) I am careful to not over heat the metal (molten Argentium looks
different from molten sterling) I cast shortly after the metal has
become fully melted. The Argentium 935 pro appears to have a large
slush area. It does not roll as easily like molten sterling silver. I
notice that I have a few bits still adhered to the flask after
casting

iv) the flask temperature I use is slightly higher than what I use
for regular sterling (1100f for very fine items down to 850f for
larger)

v) I always let the flask cool to about 250f - 300f or right to room
temperature and I break out the cast, I do not quench. (I have
quenched hot flasks in water, the casting will crack!)

vi) The just cast material is very white. I ultrasonic clean off
investment for a few minutes only, pickle in hot pickle for a few
minutes, then ultrasonic again about one minute.

My castings are always free of porosity, never have surface pits
from contamination and work and polish well. I very often fuse (weld)
pieces together and will use Argentium hard solder and medium only. I
have never had a need to use easy solder.

I always heat treat in a small kiln holding the temperature at about
500 – 550F for an hour.

The finished cast jewellery is very white in colour, reasonably hard
and has a very good polish. Never has fire scale been a problem and
when doing torch work, I do not coat with anything at all.

My customers tell me that their items remain bright and they are
pleased. I have a little bit of evidence that those that can turn
sterling silver black when worn have much less of an issue with the
Argentium sterling silver.

I hope that I have been helpful and certainly welcome any question
that you have.

Regards,
Franklin