A few days ago, I had a lesson from Mr. Wade, a silversmith who works at Gaumers. At our previous lesson, we had carved a wax ring, and at this lesson we cast and finished it. So, lucky ol’ you gets to learn how the Lost Wax Casting process works.
To begin, the smith takes some wax, usually made in part from beeswax. The wax’s melting point must be very high; as it is crucial that it does not become soft and mushy while being worked. Using a variety of tools and techniques (It would take a pretty long blog post to describe the entire carving process) the smith carves out a ring
Then, the smith attaches, or melts, a sprue (a thin stick of wax) to the wax object. This will be for channeling the molten silver later.
The object is then immersed in wet plaster, making sure the sprue is sticking out of the plaster, and is left to harden.
Once the plaster block has hardened, it is placed in a kiln, and heated up. Many smiths just like to melt the wax out; placing the mold upside down and letting the melted wax slowly run out. My teacher places it in the kiln, face up, and turns up the heat! He doesn’t melt the wax, he disintegrates it! Anyway, all that matters is that the wax is somehow removed from the mold. Once this is done, there is a cavity inside the plaster that has retained the shape and design of the wax. I have even heard of many people’s casting come out with their fingerprints on them, so precise does wax imprint upon the plaster.
So, now the smith has an empty mold. The mold is left still very hot; if not, the incoming heat would possibly shatter the plaster. So, the smith takes the mold out of the kiln and places it with its sprue hole upwards. Now, the smith takes some of the metal to be cast (I’ll just use silver as an example) and, after placing it in a crucible, melts it, either with a torch or in a kiln.
Once the silver has been melted, it is poured into the mold through the place where the sprue had been burned out of the plaster. The molten silver travels through the tunnel created by the sprue and into the main cavity, where it then flows to the back of the ring; the rest of the silver filling up the whole cavity and any little inlets, designs, even fingerprints and small scratches that had originally been made in the wax. Once the cavity has been completely filled up, the smith stops pouring. The silver in the mold cools very quickly, and hardens almost immediately. Once he is sure the silver is hard, he (using tongs) picks up the mold and swishes it around in the water. The plaster, which was able to stand up to so much heat, dissolves in the cool water, and the smith only has to fish the silver object out.
The silver, having been poured into the mold, became an exact replica, in silver, of the wax model made previously; sprue and everything.
All that remains to do is cut off the sprue and any bumps that may have been caused by bubbles in the plaster, and finish and polish the piece. Depending on what it was, more or less finishing would, of course, be necessary. If a stone was to be set, it would then be set. The casting is done.