Firescale has been the scourge of sterling silver casters since the first silver was poured into a mold. It has been a thorn in the side of silver fabricators. Fortunately there are ways to prevent fire scale while silver soldering.
This paper describes a simple, inexpensive method of preventing fire scale during vacuum casting.
Firescale is cuprous oxide, the combination of copper in the silver or gold with oxygen. Firescale forms whenever silver or gold containing copper is heated above 1000 degrees F. Unlike cupric oxide, the black coating on silver, firescale cannot be removed in the pickle pot. Normally it is removed by abrasion, electro striping or left as a surface finish.
Firescale usually shows up, as a purple or copper stain in the metal, during the polishing process. It prevents sterling silver or gold from being polished to its fine brilliant luster.
The method of preventing firescale during the casting process consists of preventing oxygen from reaching the metal while it is cooled in the investment mold. The use of a reducing flame and graphite crucible during the melting process will help in keeping oxygen from the metal as it is melted. Oxygen must be removed during the cooling process in order to prevent firescale. The method described in this paper to prevent the forming of firescale during the cooling process is very simple. It allows the metal to cool in a reducing atmosphere. Castings, when removed from the mold, are bright silver. They do not have to be pickled to remove the black coat. In some instances they can be polish with jewelers rouge after removal from the quench water. I vacuum cast my projects. I use an electric Handy Melt Furnace. The process requires two smooth soldering pads, scrap wax shavings and an empty flask the next size larger than the mold flask.
I place some wax shavings on a solder pad before I start the pouring. Once the metal is poured I place the mold flask on the wax covered solder pad. I then place the larger flash around the mold flask. Wax is then thrown on the sprue button and the second solder pad is immediately placed on the cover flask. The wax inside the cover flask absorbs the oxygen as it attempts to burn. The flask should be cooled for a longer period of time then normal as the heat does not dissipate as rapidly because of the cover flask.
When the flask is first quenched the wax forms seals with the investment and prevents it from breaking up. The flask has to be removed from the water and the surface of the investment scrape to remove the wax/investment skin. The flask is then quenched as normal.
This is a sample of a sterling silver casting that was cooled in the reducing atmosphere created by the process described in this paper. The casting was quenched seven minutes after the pour. It was brushed with a brass brush to remove the investment. Investment can still be seen under the bear. The casting was not pickled and does not need to be pickled before polishing it. It is free of both cupric oxide, the black oxide, and cuprous oxide, firescale.
Place about a tablespoon of wax shavings on a solder pad prior to pouring the metal.
Place the hot mold flask on the wax shavings. The wax will immediately melt and seal the bottom of the flask to the solder pad, which prevents air from entering the investment mold and metal from the bottom side.
Place the cover flask around the mold flask. The wax will seal the cover flask to the solder pad.
Throw wax shavings on the sprue button.
Place a soldering pad on the cover flask. The wax shavings will attempt to burn and will absorb all the oxygen around the mold flask and the metal it contains. A reducing atmosphere will be created around the mold flask. There will be no oxygen remaining in the cover flask to combine with the copper in the sterling silver to form firescale.
The cooling of the casting flask will take longer due to the fact it is inclosed in the cover flask. The cooling time before quench should be increased by a minute for small castings to several minutes for larger castings.