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Whitish coating on silver castings


#1

Hi - I’m new to casting but have had mixed success. If I put the
flask on the vacuum table with the vacuum on but not sealed properly
the item comes out bright silver but with very little detail. If I
put it on full vacuum and sealed the item comes out in fine detail
but with a coating.

This coating is whitish and scrapes off but requires lots of buffing
and polishing. What am I doing wrong?

It seems to me that maybe the vacuum is too strong and is sucking in
some of the investment onto the silver. I have tried turning the kiln
temp down. I melt the silver with a handheld gas torch and pour it as
soon as it is melted.

I use fine silver granules.

Thanks for any advice you can give!

Bronwen
Bronwen Channon


#2

Hi- You might want to double check your investment slurry. Make sure
you mix it exactly as the investment manufacturer recommends. It may
not be dense enough allowing fine pore to form.

I was first taught to mix investment by feel. That did not give very
good results. Since then I have got it down to a science using the
investment manufacturer’s directions. I use 2.5 grams of investment
for each milliliter of water used. I determine how many milliliters I
need by filling my flask with water to about 60% (for 3" or larger
flasks) to 65% for the smaller flasks.

I have attached my instruction sheet for all who care to look at it.
I hope that is okay.

Good luck,
Ken Moore
www.kenworx.com


#3
flask on the vacuum table with the vacuum on but not sealed
properly the item comes out bright silver but with very little
detail. 

The metal is entering the flask more slowly, and being resisted by a
bit of remaining air pressure. So in essence, it doesn’t actually
fill all the way. That allows the metal surface to pull away,
slightly, from the mold wall, which allows it to remain bright, as
it’s not picking up the full matte texture of the investment. Casting
with the mold and/or metal hotter would improve the castings if this
were a consistent situation. But you then say…

If I put it on full vacuum and sealed the item comes out in fine
detail but with a coating. 

That’s about how it SHOULD come out. The metal has fully “wetted” the
mold surface, and picked up all the detail, including the grain of
the investment. In doing so, it also slightly bonds to the thin
surface layer of the investment, which is your “coating”. An
ultrasonic cleaner after pickling will remove it. So would brushing,
or just about any other mechanical means of scrubbing. Part of the
reason you get as stubborn a film as you do, is the higher
temperature of your metal (fine silver) instead of sterling.
Sterling, both because it melts at a lower temperature to begin
with, but also because it solidifies more slowly and over a
temperature range, can be poured at a lower metal temperature. That
reduces the degree to which the investment adheres to the surface.
Also, even though you’re using fine silver, which may appear bright,
pickling after removing the casting from the investment will greatly
help in removing the rest of the investment. The acid weakens the
investment and it’s bond to the metal.

Peter Rowe


#4

Bronwen

The vac sucks from the mold cavity into the investment, no way for
the investment to get into the metal It sure sounds like your burn
out and/or flask casting temps are way too high. Castings do not come
out bright and shiny, usually a matte white for silver but no coating
you can scrape off. I have seen a hard white investment coating a
couple of times but only when the kiln controller took the evening
off and maxed out the kiln. :frowning: Pure (fine) metals are harder to cast
than alloys, something about the liquidus and solidus temps being the
same. With alloys you get two different numbers.

jeffD
Demand Designs
Analog/Digital Modelling & Goldsmithing
http://www.gmavt.net/~jdemand


#5

Are you sure the “white coating” isn’t just the unpolished surface of
the silver? Many people confuse the two. If it is just the silver
surface, all you need to do is brush or burnish it. If it is
investment residue, and you brush it in a bucket of water you will
see residue in the bottom of your bucket…

Mary Ellin D’Agostino, PhD
www.medacreations.com