Some of the metal pieces offered in our listings were created by the Arc-Casting process. This process was used either to obtain the appearance of an arc-cast pellet or in some cases because of extremely high melting points (iridium, osmium and others).
In a nutshell the arc-casting process works like this:
- The raw metal material (powder, sponge, etc.) is measured on a gram scale and placed in a copper or graphite mold within the arc-melting chamber.
- The arc-melting chamber is then sealed and evacuated with a vacuum pump to create a vacuum inside the chamber. The chamber is then back-filled with Argon gas, which is a completely chemically inactive gas. This gas will ensure that the metal piece being melted will not have any air (oxygen and nitrogen) to react with during melting.
- Electrical current is then passed through a tungsten electrode inside the chamber which is positioned close to the metal being melted. An electric arc will jump the gap between electrode and metal and will flow through the metal, heating it very rapidly until it melts. The copper or graphite mold are cooled by a continuous flow of water or coolant to ensure that the mold itself does not melt during the arcing process.
- Once the metal is sufficiently melted the current is turned off and the arc-cast piece allowed to cool. After cooling the melting chamber is opened and the piece removed and inspected.
Note that for precious metal arc-cast pieces the listed prices are higher than the market price of the raw metal due to the cost of the labor and equipment of casting these pieces. This is analogous to silver bars and gold bullion coins which typically sell for a few cents or dollars above the daily quoted spot market price for these metals due the cost of fabrication.
The arc-melting process produces cast pieces of a purity that is equal to, or better than the purity of the original raw material. Our suppliers have assured us that they have validated their arc-melt process on a regular basis to ensure that this is the case.
The arc-cast pellets offered here are cast in a spherical mold to create the button shape. The mold can be fabricated to create other cast shapes, such as rectangular bars or flat disks. The precious metals (osmium, iridium and others) can be cast in pieces ranging from 1 gram up to about 100 grams.
Here is a 100-gram piece of iridium that was recently made special for a customer. It is over 1 inch in diameter (25mm) and feels incredibly heavy in the hand as well as being a beautiful piece to view:
Here is another custom-made piece, a 140-gram Ruthenium bar. It is over 2 inches long and over 1/2 inch in diameter:
A perfect Osmium 10-gram piece even made it into a ring that I proudly wear:
Rare-Earth Metal Chunks
All of the rare-earths are offered in our listings (except for the radioactive Promethium of course) and come in two general forms: Cut from cast ingots, or distilled (dendritic).
Lanthanum, Cerium, Praseodymium and Neodymium are cut from larger ingots that have been cast in a mold. These pieces are completely solid and uniform with no internal voids. Since the melting points of these metals is relatively low (about 1500 degrees F) casting is not difficult.
Some of the other rare earths are in dendritic form. These metals were purified by either distillation or sublimation (yes, from a vapor!) under high vacuum and condensed directly from the vapor. The solid metal is then removed from the condensing chamber and has what is called a dendritic structure. Samarium is probably the best demonstration of this structure. The metal is solid but has a definite "grain" to its structure, forming as long and thin "dendrites" which can be broken off from the main bulk. Sometimes this structure creates a very attractive and shiny crystalline appearance. The metal pieces will have internal voids and may be "stringy" in appearance.