Non-Chemical Graphite Ingot Molds

[skyline27]

Do I need to use flux in a graphite ingot mold or can I just fill it with powder and hit it with the torch?

 

[Harold_V]

It is a MOLD, not a crucible or melting dish.

If you attempt to melt in the mold, you'll destroy it quickly. Carbon combusts above 1,100°F, and slowly burns away. Besides, it's one hell of a heat sink, so you'd have your work cut out for you melting the gold. BAD IDEA!

Melt your gold in a melting dish that has been prepared by preheating, then coated with a thin layer of borax. Not enough for any to pour out when you tip the dish to cast the gold. Heat the gold until it's fluid, and preheat the mold (at least above the point of boiling water, which will prevent any steam explosions).. The mold can be blackened with a sooty torch to improve the surface finish of the ingot. When the ingot is cast, play the torch (with a reducing flame) over the surface of the ingot so it cools from the bottom up, that way you can prevent the deep pipe characteristic of pure gold when it solidifies.

If your ingot has discoloration on the surface, what you should be doing is re-refining, not casting ingots. Pure gold will not discolor.

Harold

 

[skyline27]

Once it is cast would a propane torch be sufficient to heat it from the top?
(propane to save on oxygen and Mapp)

At what point do I know a pipe will not form?

 

[Harold_V]

Yes, propane would likely be adequate, especially if you're talking about a small ingot (ounce or less).

The pipe will form as long as the metal cools from the outside in (assuming your gold is pure. Contaminated gold often freezes without forming the pipe. A great deal depends on the nature of the contamination). Pouring an ingot with a flat and uniform top surface is VERY difficult to accomplish, which you will come to see when you try.

There's considerable shrinkage as the metal cools. If it cools uniformly from the bottom up, you end up with a thinner ingot, no pipe. If it freezes from the outside in----height (thickness) is established by the molten metal. The pipe formed is the result of shrinkage, leaving the ingot no alternative but to form the pipe-----the gold flows towards the cooler portion of the ingot until it has all frozen. That's how and when the pipe forms. It is the dying gasps of the cooling ingot. It happens very quickly when allowed to cool on its own.

Harold

 

[skyline27]

We tried melting the gold in the melting dish and pouring into the ingot mold. It was much more difficult than expected. The gold solidified so quickly that we couldn't get it into the mold fast enough. After 3 tries, we got one 1/4 toz ingot. I slowly cooled it with a propane torch. It looks nice with a slight terracing affect on the top. Rather than waste gas we made the remaining powder into a nice button (with deep pipe and crystalization).

It would take me a long while before I got bored with melting gold. It's the cherry on top of the whole process. Many thanks to all the forum members whose generosity and wisdom made it possible.

 

[Harold_V]

I always used either acetylene or natural gas when melting, along with oxygen. I may have lead you somewhat astray in suggesting that propane would be adequate. For that, I apologize. It's hot enough, although you may not be able to superheat the gold adequately to achieve the success you desire. If you can see your way clear to add oxygen to the process, you'll be better off by a long shot.

One thing that might help is heating the mold hotter. Get it just under the point where it starts to burn off the carbon coating you apply. That will limit how fast the ingot cools. I used to have my mold right along side the melting dish and heat them at the same time, insuring each was hot.
Also, heat the melting dish until it's red hot everywhere, particularly the pouring lip. That will keep the gold from freezing in the dish.

Small ingots are more difficult to pour than large ones. They tend to cool almost instantly.

Harold