QuestionQUESTION: Greetings!
Time to ask an expert since I am approaching the frustration level. A little background.
Gathered up a bunch of odd and ends 10k, 12k and 14k scrap jewelry we had. Placed it in a graphite crucible, used an oxyacetylene torch, fluxes with borax. Metal melted, formed a nice swirl and poured it in to a lost wax cast vacuumed investment. Broke the investment apart and I had metal on the bottom of the ring and almost a lava like black substance on the top half of the ring. The black stuff was brittle and broke off easily. I returned the metal to the crucible and melted it down dribbling it into a water bath making splatter pellets. Lots of carbon like substance so placed all the metal and carbon residue in HCL H2O2 solution to recover the gold. Gold recovery is currently under way.
My assumption is that this is the first time I have used oxyacetylene to melt gold and believe I introduced massive carbon into the melted amalgam via the acetylene gas and overheating. In the past I have used a small Carlisle Oxy propane torch. I did not use the OP torch this time since my total weight of melted mass was 55 grams and my tip on that torch is tiny for intricate neon glass work.
So before I go ahead and do this all over again, I want to ensure I eliminate the black carbon. I will purchase a Hoke hand torch Oxy propane but I do not know what size tip I should use.
Once my gold is clean via the AP solution I plan to mix with silver and copper pellets. Should I heat all of this to liquid form, let cool, and reheat before I pour or would it be better to pour after the first heat?
The bottom line what should have been a simple process has become a chore during this casting. I do not want to make up another casting and find out I will have black residue problems. So trying to do it right the second time. Thank you so much in advance for your knowledge transfer and experience.
With regards
ANSWER: Freddy, my recommendation for torch is the Meco Miget, available at most any welding supply house and gas supplier. These can almost always be purchased locally. This torch is far superior and more versatile than "jewelry torches". Which tip? Humm,,the largest coming with most of these torches sold from jewelry suppliers is the #3. With propane that might do but honestly a graphite crucible soaks up so much heat you might need the larger tip, such as seen in this web page:
http://www.tinmantech.com/html/all_meco_related.php#pro
With propane, a good way to introduce youself to this torch is with NO tip. Play with the fuel gas and oxygen to see the flames available with the open nozzle then go for the tip installed.
Recommended: Get another crucible. One of the white clay silica crucibles like those sold by Rio Grande are ideal for this melting. Choose almost any shape just be certain you have a means to hold and turn the crucible for pouring neatly.
http://www.riogrande.com/Search/melting-crucible
Prepare the crucible with boric acid, not straight borax. Heat the crucible and sprinkle on boric acid, turning under heat to coat the surface. This helps the crucible last longer and helps prevent any surface flakes from getting in the melt. (Most non metal stuff will be off to the side or floating on the melted metal near pouring time.)
Get ready to melt your metals by heating the crucible to red, add the metal and put on a melting flux. Boric acid will work ok and does better than borax. If you are going to buy a new crucible, go ahead and get some melting flux powder while you are at it.
THE BLACK GLOBS ON YOUR MELT...I have done that:
Carbon likely caused the color but would not form a residue like you describe. I suspect there is too much carbon in the oxygen/acetylene mix and the flame may be too rich. Then, it sounds like the flux was possibly in excess. Flux is the primary reason for residue like you describe with color supplied by the carbon and impurities from the materials melted. I used acetylene for years and it did well for me, except for some soot in the air from firing up the torch.
You need a NEUTRAL flame, certainly so with acetylene or carbon can infuse into the melt, sometimes messing up white gold work. Propane will work very well and there is simply no carbon problem.
Freddy, certainly do it again and if possible use a clay/silica melting crucible. Use propane with a hot but not overly oxygenated flame. Sprinkle on some flux during melting, stir the melt if you have a suitable stirring stick, then when the metal is balling up sprinkle on a little more flux and as soon as the metal gives a spinning look to the ball hit the vacuum switch, pour when vacuum is right but try to time well so metal is not over heated or vacuum cools the flask.
What you did sounds pretty much on target but the flux has me concerned and a too rich flame trying to heat a lot(?) of metal in a graphite crucible.
I hope my advice helps. Others will offer different advice but this is mine and has worked for me for many years, melting for vacuum casting or for pouring ingots and wire molds full.
God Bless and Peace. Thomas
---------- FOLLOW-UP ----------
QUESTION: Thomas,
Thank you! New torch ordered, new ceramic crucible and hider ordered, boric acid and flux in hand, so I will try it again based on your advice. I was able to get rid of all the black residue via soaking in PA and I have certainly concentrated the karat since I probably, as you mentioned, overfluxed, over heated, and had too rich of a flame. Live and learn but I am glad to be on the right road again. Thank you!
AnswerFreddy, I am very happy to be of help to you. I am willing to say you will do fine the next time and I do believe you will like using propane in this case. Years ago we would visit your state on "gem hunts" and dearly love the mountains there. We also love the coast and the Outer Banks but have not been able to get back for way too many years. Your enthusiasm is wonderful.
God Bless and Peace. Thomas.
