diecutter said:
I should have figured it could not be that easy. I was basing the possible success on the fact that the Kerr Electro-melt furnaces and the large vacuum furnaces sold by Rio Grande both use graphite crucibles which are quite durable for that use.
They may present the appearance of being durable, but they aren't. Because the temperatures involved are well above the combustion point of carbon, they burn away rapidly, primarily from the shoulder upwards. That's one of the reasons they provide a cover, which limits the exposure to oxygen, but not enough to be totally effective.
When the furnace is up to heat, even for melting silver, there is a visible flame coming off the graphite around the rim. Failure of the crucible body isn't the problem, the problem is the top burns away such that the crucible is no longer supported by the rim, and there is no pouring spout to allow the crucible to be emptied in a safe fashion, where the discharge is desired.
I went so far as to coat the top of a crucible with a refractory wash in an attempt to extend the useful life of the crucible, all to no avail. It lasted longer, but not significantly. In the end, the expensive (at the time) furnace was relegated to a shelf in my storage room, never to be used again. They clearly are NOT a good idea.
The fact that the metal does not solidify in the crucible (at least not intensionally) probably is the reason for their durability.
As stated, above, they are not durable. The typical foundry crucible is far more durable, being made of graphite AND clay, which protects the crucible to some degree. They are also made of silicon carbide, which is far more stable.
Maybe if the sides of a graphite ingot mold were highly tapered it would make the mold more durable as the ingots cooled?
That's not an issue. Gold contracts upon cooling, it doesn't expand, so an ingot is inclined to end up loose, assuming it isn't keyed to the mold by lack of draft, or from undercutting by oxidation (graphite mold).
Melting in a metallic mold would be a horrible idea. Assuming one could use cast iron, remember, it has two properties that are less than desirable. One of them is that it is very poor in tensile----and subject to thermal shock. That's part of the reason it's so damned hard to weld. The other problem is that iron is readily dissolved in molten gold. You not only would struggle with your gold soldering to the iron, but you would suffer contamination of the pure gold from dissolution.
While I am not the least bit familiar with tantalum, I am of the opinion it, too, would be a huge mistake. It may not fracture from thermal stress, but it surely would be problematic in the way of contamination, and most likely soldering.
I guess the only sure way to be exact is to strike a planchet or ingot blank that has been already adjusted.
That is my opinion. Certainly, I'd avoid any ingots that had signs of having been altered---by any method. Shaving coins used to be a way of stealing----when the value of the coin was in the metal contained within. That problem was addressed by reeding the edges of coins, or otherwise making them such that any removal of material was obvious. It is for that reason that I never altered any of my ingots. They were always left as cast, although I did make attempts to create a smooth surface by controlled cooling. That appears to be common practice and leaves no damage to the surface of ingots.
For the record, casting ingots is not easy, not if you desire attractive ingots. Gold is far easier to cast, if for no other reason, oxygen isn't a problem as it is with silver.
I own several 100 ounce silver ingots made by one of the major refiners. They are die struck, which boggles the mind. I can only imagine the tonnage required! Very nice, they are :!: :!:
Steve commented that his graphite molds slowly formed a gray surface, which must be removed.
Yep, that they do. That's the result of the adsorbed oxygen being consumed as the silver solidifies. It can be limited, but unless one is able to melt and pour in an inert environment, it can't be eliminated. An inert atmosphere induction furnace would be the answer. Doesn't everyone have one of those in their garage? :lol:
I wouldn't use graphite for a mold, not under almost any circumstance, but then I also had the ability to produce my own cast iron molds. Nothing rivals a nicely blackened iron mold, especially if the iron is ductile instead of cast gray iron.
Harold
. Finally I added external heat and a graphite cover to reduce the exposure time of the molten melt to the air. It's very tricky for a single person to switch from pouring a white hot crucible to covering the top of the mold quickly, especially with bulky aluminumized gloves on.
