Brad Hunter asked about casting a replacement knob.  I sure wouldn't go
to a lot of trouble to cast a pot metal knob.  If a foundry can't keep
out impurities that cause pot metal failure, you can bet that you won't
be able to either.  Plus, zinc die casting isn't a job for the home,
using hot, liquid metals that giving off dangerous fumes.  Better to do
the part of the job that you can do successfully and let an expert do
the fussy stuff.

Here's how.  Make a small box or container that will fit the part, with
a space around it for the molding material, a minimum 1/4" around in
all directions.  For a small knob, perhaps a plastic film canister
would do.  But you want to use something from which the final mold can
be removed easily.  The box serves two purposes: it holds the liquid
mold material until it hardens around the part, and it supports the
mold when you pour in the material to make the new part.

Buy some liquid mold making material.  Many companies make a room
temperature vulcanizing rubber.  Micro-Mark is one such company.  If
you are in a hurry, you can buy dental molding material.  Neither of 
these need a parting compound, and they will not stick to the original
part.  The dental material is a lot faster (and more expensive).

You need to make provision for removing the part from the mold.  For a
knob, you should put a dowel in the hole and also fill the screw hole
with something like wax.  You'll have to figure out the best way to
pour the mold so that the part stays in the middle.  For instance, you
might pour a quarter of the liquid rubber compound in first and let it
harden.  Then wet the bottom of the part with liquid rubber, pour a
little more rubber into the mold, and put in the part.  The rubber
sticks to itself but not the part.  Then fill up the rest of the mold.

You can either plan a parting line where the mold can be separated into
two or more pieces to get the part out, or you can carefully cut it out
with a razor knife.  Lastly, make sure that you have a place in the
mold (like the dowel sticking out the top) where you can later pour in
the metal or other material to make the part.

When the molding material has hardened, remove the original part
unharmed.  The casting rubber won't take too much heat (maybe 500-600
degrees Fahrenheit) so you can use low temperature metals only, cer-
tainly not brass or steel.  There are a number of eutectic alloys of
bismuth, lead, and tin, sometimes called cerrometals.  The problem is
that they might melt at too low a temperature.  So what to do?  The
easy way is to avoid the casting part of the process altogether.

Cast your part out of wax by heating the mold a bit and pouring in a
melted candle.  Then send the wax parts, as many as you like, to a
short-run foundry that specializes in one-off parts for custom jewelry
makers.  I've made several parts this way, including a replacement for
the silver-coin earring that my wife lost.  It worked just great.  Not
only can you read the date on the coin but you can tell the age of the
man who cast it 1800 years ago.

There is a minimum charge of about $25 (a few years ago), but small
parts are only a few dollars each, even in silver.  For an additional
charge, they will even make the mold for you.

Regards from upstate New York,
Craig Smith

P.S.  Depending on the application, perhaps even an epoxy copy would
do.  Years ago when they restored the old City Hall here, I made re-
placements for an ornate brass doorknob and a 6-inch decorative iron
gargoyle that were stolen.  The newspaper did a story about the job,
and they couldn't tell which was the original and which was the copy
when they published a picture of it.