Expert: Thomas - 2/22/2004

Question
Hello Thomas,
I appreciate your sharing of the art with me...

So I have my twice self refined pure gold shot and I want to make something with it. I have a fused silica bowl type crucible, some tongs that lock on to it, a bernzomatic propane torch, an oxy/ acetelyne torch and the ability to hook it up to a barbeque bottle of propane to have oxy/propane heat.

Please explain the process that I might use to make a "blob" of about 1/2 OZT that I can cold work (press) into a shape.

I am asking about the heat required, use of a flux compound (borax? something better?) Do I need a furnace or will the torch be OK? Propane, o2/propane or o2/actylene.... something better? What type of flame do I use?(neutral, carburizing[rich],oxidizing[lean]? Will pure gold temper? Is there a process of annealing? Will impurities emerge from the molten gold and tarnish the surface? If so can they be removed?

I have a one time ignorant expierence with this endeavor, I made a cup from stainless steel and played with the different heat, I melted it kinda OK with propane but just didnt get enough heat to get a smooth "silver solder type bead". When I used the borax it welded to the SS cup. Then...I have practical experience of overheating with the acetylene/O2 I think the nickle came from within the SS..... I contaminated about 2.3 grams.

I want to acheive a soft, gold, shiny bead of about 1/2 Ozt. that can be cold worked (pressed).

Thank you for an anticipated answer to my query,
Kurt

Answer
Dear Kurt,

My answers might not be the textbook answers but are methods which have worked well enough for me and others I know.

GASES.  For years I used acetylene/oxygen for all my work.  I used it for soldering, casting, melting and making ingots of gold or silver.  The hot flame makes easy work of most melting procedures.  The problem with acetylene is the soot and the high carbon content of the flame.  With acetylene, I always use a neutral flame and this requires only low oxygen pressure.  Then again, acetylene is not recommended for some metals such as certain white gold alloys or for platinum work. There is a chance of contamination from the flame gases.

My choice would be propane/oxygen.  With a large enough torch tip, a sufficiently hot flame is produced and the burn is cleaner than with acetylene.  Still, use a neutral flame to prevent problems from too much gas pressure and from too much oxygen.  Platinum is the only metal I work with an oxidizing flame.  All the others are worked with a neutral flame.

With pure gold, the problems some alloys have with oxidation of the mix of metals is pretty well eliminated.  Still, use a neutral flame with enough overall heat (large enough tip) to melt the metal fairly rapidly.

FLUX.  There are fluxes made specifically for casting use and these work well in simple melts.  If this is not available, the general mix of borax and boric acid works pretty well.
I make sure the borax is well powdered and use about 1/3 borax to 2/3 boric acid. If the borax is not handy, boric acid alone will suffice.  Still, the mix appears to do a slightly better job.    

CRUCIBLE.     The ceramic or fused silica crucible is fine.  A casting crucible will work fine also for the melt.  Be sure to heat the crucible to red heat and sprinkle on the flux, turning the crucible while under the flame to line the crucible with a fine layer of flux. When cooled, the look is like a fine “glass coating” on the surface.  This will help protect the crucible and prevent in some ways little pieces of crucible from getting into you metal when molten.   Once prepared, you are ready to melt the metal with the propane/oxygen.  Preheat the crucible to cut down on torch time for the metal melt. Put the metal in the crucible, sprinkle on a pinch or so of flux and do the melt.  It is a good idea just before you pour the metal to add another pinch.   When the metal melts and there is “swimming” look to the surface, keep the flame on the metal and do the pour.


Ok, I said “pour” but what to pour the metal into to get a workable form?  That is next.
With ½ troy ounce of gold, you might want to make a small “ingot mold” for the pour.  I have found pouring a small amount of metal is often more difficult than a larger pour. For this reason, you might consider making a “mold” and melting the metal directly in the mold.

INGOT MOLDS.  Typically, ready made ingot molds are designed for larger volumes of metal.  Since pure gold is quite dense, the metal takes up little space compared to the weight.  I figure ½ troy ounce of pure gold will occupy a cube about 8mm x 8mm x 8mm, too little metal for most ready made ingot molds.   I suggest you use a charcoal block of the kind jewelers use for soldering, annealing, etc.  These are specially treated and fairly dense charcoal.  This charcoal does not burn(catch on fire)  but will eventually be damaged by high heats, turning to a sort of ash or flake away.   To melt you pure gold, make a cut in the charcoal about the size mentioned or some form of the same total volume, shallower and wider, longer and shallower, etc.   Place your gold in the cut and see if the volume looks about right.  Remember, air space between the gold shot will not count when the melt is done.   With this, put some flux on the metal and apply the heat. Melting will be fairly quick since the charcoal helps retain heat. Flux just a tiny bit when the metal is molten and let it cool.  If you wish, you may place a flat piece of the charcoal on top of the molten metal to help flatten the natural curve caused by surface tension.

Another mold that will work is one made of casting plaster.  The plaster is mixed with water to form a slurry just thicker than a milkshake.  Do this in a form of some sort to hold the plaster.   After about 45 minutes, take a sharp tool and carve the mold shape into the plaster.  Now, a furnace or long stay in a lower temp oven is needed to completely dry the plaster.  A home oven will do to dry the plaster. We do not need the higher temperatures used to burnout waxes for casting.  A slow heat up to boiling temp and then maybe an hour at 100 degrees hotter will dry the plaster.   Put some flux in the plaster mold, flux the metal and melt.

If no form of mold is available, you may simply melt the gold on a flat not flammable surface (not metal), like a jewelers soldering pad.  The molten metal will melt and form a half ball then will cool in that shape.  Careful the metal doesn't run off the surface when molten!  : )


TARNISH AND DISCOLORATION.  Generally, pure gold may be heated to dull red and the surface shows little if any discoloration.  For a melt, I am not really certain how clean the metal will look when it cools.  This might depend somewhat on the mold of surface used for the melt.  Most discoloration may be removed with surface sanding or if a mild sulphuric acid is available a short dip in the acid will remove tarnish.  Still, some polishing is needed.  Surface impurities such as bits of stuck on charcoal or plaster may be abraded away fairly easily.  The smoother the surface of the mold the cleaner is the new formed gold piece.

WORKABILITY.  Pure gold is the most ductile and malleable metal.  It may be pounded into gold leaf so thin that a light behind will show a greenish color, since it is so thin some light will go though.   You should have no problem reforming the metal.  There is a long way to go in working before “hardening” has much effect.  Still, it might be wise to anneal (flux and heat to low red color in dim light) the metal after reducing it to about 40% of the original thickness.  I simply do not work much pure gold and throw that in as a precaution.  In reality, I have worked pure gold beyond that point before feeling a need to anneal.

YOUR STAINLESS STEEL EXPERIMENT.  I  solder to stainless using a suitable flux, like those containing fluorides.  The fumes are not safe.  As for welding or fusing the metal, my welder friends say this must be done with an inert gas and special electrical welding machines.  Certainly, don't try to use stainless as a crucible!  Kurt, I hesitate to tell of the MANY times I have done things that look ignorant today!  Well, I did learn something from each of those mess-ups.  Sometimes all I learned was to ask myself why in the name of good sense did I even try it!  Ha Ha.

If this information is useful, I am pleased.  If you need more or an explanation of some point, do not hesitate to get back.  You may use the “follow-up” option to this question or simply ask again for a clean page.

God Bless and Peace.                 Thomas.                     2/22/2004 4:51 PM