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Casting Coral

Greetings all.

A student (retired dentist) in my casting class is telling about
knowing someone years ago who (sounds like a fairytale already) who
was taking coral fronds, investing them then using an acid to
dissolve out the coral and casting from there. His question was
about the acid used that would not harm the investment. I plead
absolute ignorance about if this is even possible much less what
would be used.

Comments and instruction please. Insane laughter is all right too.
Either way I get edgeecated and he gets an answer.

Bill Churlik

coral fronds, investing them then using an acid to dissolve out
the coral 

Since coral is calcium carbonate, 10% hydrochloric should do it.
Most common acids would (except HF, which you don’t want to mess
wiith anyway), but HCl is the easiest to come by.


Thanks, this may be a part of the possible answer.

That most important concern is will the acid also attack the gypsum
binder (calcium sulfate ? ) in the investment ( phosphate binder for
platinum investment)? Of course there is the issue of fumes and
burnout. Thank you Teri.

Just think, I could have told my student,“You are nuts, just do as I
say, go away don’t bother me.” The correct answer is important, so is
the thrill of the hunt!

Bill Churlik

Any acid that would etch the coral away would dissolve the gypsum
binder in the investment as well. Possibly this person could have
made a rubber mold of the coral and then used the acid to dissolve
the coral but I have a hard time seeing how one would do it with

Jim Binnion

James Binnion Metal Arts
Phone (360) 756-6550
Toll Free (877) 408 7287
Fax (360) 756-2160

Member of the Better Business Bureau

Some time ago I came across a web site for casting bird skulls, I
can’t remember the site. The important thing was that it was done
with shell casting , this is a kind of mould made with a clay slurry
and ceramic powder put on (dipped) in alternate layers, this is then
baked like a pot and the bone could then be etched away with acid .
The mould would be unaffected. Presumably coral could be done in the
same way.

Tim Blades.


Thanks for the input. I had not thought of a ceramic shell. Got to
try that technique soon. I was looking at a piece of coral that I
used as a display piece. I would think trying to clean out the
ceramic casing would be difficult if not impossible… The piece would
also be impossible to rubber mold and then cut loose. This all may
depend on how convoluted or “branchy” the branch is.

At this point if someone were wanting to get the coral look in
metal, I would recommend electroforming a copper layer on the coral
then plating as an easy out.

Bill Churlik


I wouldn’t want to do it, I have seen someone take a week getting
the shell out of a heavily textured sculpture !

If it were me I would do a soft thin RTV mould and pressure cast the
wax in ( pour in very hot wax and put in a pressure tank and squash
out the bubbles , The thin mould gives you more chance to peel it
off. A seroiusly 3D piece would be a real problem .

This thought will no doubt inspire some artist to insist that
someone like me has to work out a way !

Tim Blades

How about using the technique often used by people who cast plaster
figures etc.? Make a mould of the coral by applying a thin (1/8")
skin of rubber latex all over and, before removing the coral, cast
this whole lot into a block of plaster of Paris (may need to be a
sectional block). Now remove the coral and rubber from the plaster
and peel the rubber off the coral. If you now put the rubber mould
back into the plaster block, this will support it while you cast
waxes from the rubber etc.

Best Wishes,

Ian W. Wright
Sheffield UK