Lost wax casting is a jewelry-making process that is used very frequently by goldsmiths. To say this technique is tried and true would be an understatement. It has been used for thousands of years, with the oldest known example being a 6,000 year old amulet from the Indus Valley Civilization in South East Asia.
Technology has advanced, but the technique remains the same. Let's talk about what lost wax casting looks like now.
The Wax
There are several types of wax used in this process, which all have different properties and purposes. There are harder waxes that are used for carving and more malleable waxes that are used for molding. Casting waxes can be purchased in sheets, wire, blocks, ring tubes, and more. Since wax is very easy to work with, goldsmiths can get very creative and make unique pieces out of the wax before turning it into a precious metal.
Once the piece of jewelry has been created out of wax, the piece must be carefully attached to sprues, which hold the piece up during investing and create the channels for which the molten metal will come through to to the piece during casting. Once all the wax pieces going into a casting cycle are secured by sprues onto the rubber base of the flask, we move on to the actual casting.
Casting
First, the rubber base is secured to the circular metal flask, closing the sprued wax pieces inside. The next step is to mix investment, a powdered plaster-like substance, with water and vacuum it before pouring it into the flask. After pouring the mixture into the flask, it must be vacuumed again and allowed to air dry.
Once dried, the flask is placed in the kiln for burnout. The burnout process takes 8 to 12 hours and requires the temperature to hold at 1350 F for four hours to totally eliminate the wax (or other burnable items used).
After burnout, the flask must be cooled to the temperature needed for the precious metal being used. Each type of metal, including different karats and alloys, requires a specific temperature for the next step.
The flask must be held at this casting temperature for a specified amount of time to ensure the temperature is consistent internally and externally. During cooling, the metal can be melted using an electromelt or manually with a torch. The molten metal will be at an extremely high temperature, much higher than the specified temperature for the flask.
A casting assistant will then take the flask out of the kiln and the goldsmith must immediately pour the molten metal into the flask to fill the investment cavity, while both are at the proper temperatures.
The flask is then allowed to cool slightly before immersing it in water, which will cause the investment to boil and dissolve, leaving only the metal pieces behind.
Finishing
After casting, you'll be confronted with a hunk of metal shaped exactly as your wax and sprues, and it won't be shiny or beautiful.
First, the goldsmith will remove each piece from the sprues and carefully file and shape the space where sprues had been attached. Filing, fitting, soldering, and setting are some of the primary processes done in this stage.
The process of completely and professionally finishing a piece are extensive and require a significant amount of time and care. To learn more about finishing, keep an eye out for our next goldsmithing blog!
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