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Bi-Metal Casting

#1

I have a problem I thought that some of you more experienced casters
might have the solution for, I have just cast a small sculpture in
bronze of a figure clutching a silver fish.

I cast the fish and cleaned it up and poured the wax for the figure
around it, I then burnt it out, quite a few hours, it weighed a
couple of kilo’s , when I opened the flask the fish was encased in a
hard grey amorphous blob of plaster which when pressure washed off
the silver had been eaten away as if etched. It seemed to have lost
about a millimetre of metal off the surface. Why?

I am trying a sample piece today with boric acid in the mix ,4
percent like the stone in place casters do , I know it is not
exactly a jewllery item but I thought it is more in this line than
normal sculpture casting. Any one got any ideas?

Many thanks,
Tim Blades.

#2

Hi Tim,

I use silver wire in hollow wax designs as reinforcing rods. Some
times I have the same problem you described. The silver wire is
coated with a hard coppery colored investment layer.

I break off the coating and use the wire as scrap silver in my next
pour. The wire is just like you described. It is rough. I have
noticed that the wire does not melt as rapidly as sterling silver.

It may be just a stupid guess but I believe copper is precipitating
out of the sterling and imbedding itself into the investment around
the silver. That is what causes the hard copper colored investment.
Is it possible the silver has been somewhat refined into fine silver
and melts at a higher temp?

I find the coating problem occurs in flasks that are in the back of
the burn out surrounded by other flasks. My guess is that these
flasks see a higher temp than the ones in the front of the burnout
oven.

You might try lowering the burnout temp.
Lee Epperson

#3

Hi Jim,

The sample piece I did with Boric Acid in the mix came out fine. I
also did various other flasks when I cast the first one and they
were all OK. It has been suggested to do the bronze first, but the
fish is 50 gm,and the sculpture 2Kg . I don’t think I can go that
way.

I will try again with a big test piece (not the customers!) and see,
the metal that was eroded was not in contact with the bronze .

many thanks for your thoughts,

Tim.