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Preparing a new crucible

Can anyone help me with the proper procedure for preparing a new
crucible for use with sterling silver and/ or gold? Can one be used
for both metals? Or is it better to keep them separate. I am new to
the forum and may have missed it if this has already been covered,
but I came up blank when I searched the database. Thanks in advance
Jim Malone

Keep them separate! If you use one for gold, then use the same
one for silver, your silver will turn out yellow.

Make a paste using boric acid & water. Coat every part of the
inside. Place in a kiln -400 degrees or so, for 10-15 minutes.
Remove the crucible and let it cool.

I’d suggest separate crucibles each for silver, white gold, and
yellow gold. Heat them gradually , pointing a torch into the
bowl, till the inside surface is a nice cherry red. Sprinkle
borax (no 20 mule team, it has perfumes added) from a jewelry
tool supplier or chemical supply company. Keep sprinkling and
heating until it has a nice glassy coating inside. That’s it.

Hi, I would say that it is an absolute neccesity to have
different crucibles for each type of metal you cast as well as
each karat of gold and each type of sterling (standard Sterling
Alloy As well as Deox sterling alloys) Cross contamination will
cause problems and potentially lower the Karat content of your
gold .

  Check out the casting pictures on my website under "workshop"

Hi Jim, You didn’t specify if the crucible is for a centrifugal
casting machine or the hand held type. A method that works well
for me is to melt borax powder in the bottom of the crucible and
swirl or rotate the crucible around until the whole inside is
coated. Pour off any excess borax while keeping the torch on the
crucible. I use an oxidizing flame to actually “blow” the borax
out of the crucible. You don’t want any excess to get into the
casting. If using in a cent caster, load an empty flask in the
machine and spin the crucible while hot. Any excess borax will
be slung out of the crucible into the flask. I wouldn’t
recommend using the same crucible for both gold and silver, or
for different colors of gold. Ken

Jim: I would recommend boric acid powder, it goes liquid a
little quicker than borax (maybe its my imagination) but I think
really either one will work fine. All you do is use a hot torch
flame, preheat the inside of the crucible then pour in a good
handful of boric acid or borax, available at any pharmacy. Heat
the stuff until it goes liquid then using tongs rotate the
crucible so the liquid glass coats the inside really well,
anywhere the metal is going. Use plenty of whichever and when
its all good and coated pour off the excess keeping the flame on
it all the time. Make sure if this is a centrifugal casting
crucible that you left the hole open and clear for the metal to
shoot through. If it gets plugged heat it from the outside and
use a coat hanger wire or something to unclog it and pour out
the excess. Its really very simple. After casting several times
the crucible will get a bunch of grunge and old metal bits left
over. You’ll need to clean the crucible by filling it with lots
of borax, melt it and scrape and pour out the garbage…Dave

Dear Jim, when I was an apprentice it was called “dressing” the
crucible and it was the apprentice’s job - along with sweeping
the floors, picking the cigarette butts and broken sawblades out
of the dust - no, I won’t go on. Anyway, back to the crucible. We
always used ceramic crucibles in that workshop. Some of the
crucibles didn’t have a very good pouring spout, so one would
have to grind a slightly deeper groove in one side, usually
with an old coarse handfile, making sure there were no sharp
corners or edges around the spout. Once the shape and depth of
the spout was correct, one warmed up the new crucible, applying
the heat evenly all over its top, sides and underneath until it
was hot enough to make sprinkled borax melt and stick to its

  The next step was to liberally sprinkle powdered borax all over
  the hollow and continue heating until the borax went clear and
  syrupy-runny and covered the whole of the hollow with an even
  thin layer. The last step was to pour out the excess borax so
  that the spout was also glazed with the molten borax. Then it
  was ready. It was quite simple and straightforward. I discovered
  that it was best to keep separate crucibles for specific
  metals. My best melts have invariably been out of clean
  crucibles. Hope this helps. Kind regards, Rex from Oz

Most of the folks who’ve posted are using borax to prep crucibles and as a flux.
But down here on the gulf coast many of the jewelers I know use boric acid instead.
I tried both but those big plastic bottles of boric acid powder seem to work very
well while borax particularly in its original box absorbs atmospheric water and
forms big clumps. Other than conveniance is there any great reason to use one over
the other? Geo.

Does it have to be a kiln? What about a oven? I guess that would not be very smart though. How about a hot flame?