Lost Wax Casting Illustrated
In the process of shopping for or commissioning a major jewelry purchase, the terms of lost wax casting are often heard. The wax casting process is often discussed over the countertop but little understood by jewelry customers. Jewelers often take it for granted that everyone understands this ancient technique; perhaps assuming that knowledge passes by osmosis. Nothing, of course, could be further from the truth.
This grand and ancient technique of making something quite literally from nothing was practiced by many different and widely separated civilizations all over the ancient world. Beautiful examples of cast gold can be found from Aztec dominated Mexico to the gold bearing regions of Africa to Imperial China. It appears that if you, as a culture, are smart enough to know how to melt precious metals, you are, ipso facto, intelligent enough to have figured out the casting process.
For the scope of this discussion, I will be eliminating the front and back parts of the Jewelry-making process, namely, the design and wax carving process in the front and the finishing and stone setting at the rear. We are only concerned here with the magic that happens in the middle, the making of something from nothing.
In the photo on the left, we see six round Diamonds and a large oval Ruby which are to be set into the finished Gold ring. At this stage however, they are merely used for the layout of what will be settings for these stones.
Notice the y-shaped pink wax structure attached to the shank of the wax ring. This is what makes the whole process possible. Called a gate by sculptors or a sprue by Goldsmiths, this form will melt out leaving an open channel through which molten metal can flow into the now vacated form previously filled by the wax ring
The process of Lost-Wax casting in essence is simplicity itself. That said, it must have taken a great leap of mental prowess to make it happen for the first time. In the simplest terms possible, it happens like this; a "wax" is connected to wax wires called sprues. This structure is then connected to a rubber base whose sole purpose is to temporarily hold a steel cylinder called a flask. The flask, now an open-topped container, is then filled with a special type of plaster called investment. Similar to Plaster of Paris, the investment is able to endure the high temperatures required to melt the wax model thoroughly. After setting into a brick hard solid, the flask, now minus the base, is set into an oven and slowly heated to 1350 degrees Fahrenheit. This great heat melts and then vaporizes the wax, which, although now encased within the investment, is open to the outside because of the sprues. Having now melted and dripped out of the plaster, the sprues are now tubular channels through which molten gold or silver is allowed access to the now void and negative form of the wax. The investment is an amazing fluid which conforms to every detail of the wax model right down to file marks and other imperfections. If adequate force is given to the liquid metal, every detail of the wax model, which became the inside surface of the investment, is picked up in the now metal casting.
I use a centrifugal casting system. Essentially a large open wheel, the centrifuge is powered by a large spring, which is cocked and locked for the process of melting the metal of choice. When released, it gives enormous pressure to the molten metal now flowing outward and into the vacated flask. Notice the circular cradle in the middle of the wheel. This cradle serves to hold the flask, which is at the requisite 1350 degrees Fahrenheit and the wax model has melted out 
leaving a perfect negative copy of itself in the plaster and is ready for casting. The cradle also is free to swivel upon release of the spring giving that much more impetus to the free-flowing metal.
A short detour into basic metallurgy is required here and forgive me in advance for using the word alloy too much but there is no suitable synonym. In a discussion of alloys,,,, An alloy is any metal comprised of a mixture of two or more metals and the world around is largely built from them.
In order to produce my own gold alloys I start with 24 karat or fine gold. Precious metals are usually purchased in the form of small round shot better facilitating accurate measurement. Determination of just how much gold I'll need is the first order of business. Wax has the specific gravity of water, determined to be the value of 1gram/cubic centimeter. 18k gold has a S.G. of 15.5; therefore by multiplying the weight of the wax model with its sprues attached by 15.5 grams gives me an accurate knowledge of the finished weight of the casting. In this case fifteen grams are required for the ring itself but thirty grams are needed for the whole casting. In addition to the sprues ,which can be weight intensive, is what's called a button. This is a bit of extra metal required at the base of the casting to give additional inertia to the casting. This helps the metal to completely fill the space at hand and to be free of what's referred to as porosity. Suffice it to say that porosity is a goldsmith's nightmare.
If thirty grams of gold is needed for the casting, I multiply this by .75 which is the percentage of gold to alloy metal in 18k gold. The alloy metal is typically an alloy itself of silver and copper. Different percentages of these two metals can give a finished gold with vastly different colors and working characteristics. In this photo, I have measured out 22.5 grams of fine gold and 7.5 grams of alloy. Appearances here are quite deceiving as it seems that there are roughly equal amounts of both metals. The gold has twice the weight per volume or density as the alloy and the alloy comes in larger pellets so it appears to occupy an equal volume.
The metal is carefully melted with an enormous flame produced by an oxygen/propane torch. You'll notice a long thin black rod leaning against the crucible in the first image and being inserted into the molten gold in the second. It is made of rather pure carbon and is needed to stir and thoroughly mix the gold and alloy as well as to clean impurities from the liquid and really hot metal. The hands belong to me.
The fine gold is melted first as it has a dramatically higher melting point than the alloy addition. Once molten, the gold is ready to accept it's alloy. This part can be rather touchy because with every grain of foreign metal added, the gold is now an alloy has a lower melting point and we are therefore a little closer to its boiling point. You don't want to come anywhere close to gold's boiling point because there lies more trouble in the dreaded form of porosity.
With a good flame playing across the now molten and shimmering gold, I give it a stir with the carbon rod and then sprinkle the surface of the metal with borax. This acts as a flux, from the Latin for "to flow". This chemical and several others as well help to prevent the oxidation of molten metals thus helping them to join together as an alloy and to flow into the cavity awaiting. After one final stir 
of the rod I release the spring and the centrifuge spins to life. I have no idea as to acceleration rates or speeds but it becomes a blur of motion in the blink of an eye. This always used to be the hardest part, just waiting for the centrifuge to stop its incessant spinning so I could see the new wonder lying within the hot plaster. After a few minutes the flask has cooled sufficiently to dunk it into a 
bath of water which boils as it comes into contact with the hot plaster helping to free the solid casting. Although there is some waiting for things to cool, the gold itself froze into its new shape in seconds. However, if really hot gold is doused with cold water immediately upon casting thermal shocks happen often tearing a piece apart as different parts cool too quickly and shrink tearing away from other hotter portions. This also gives rise to the dreaded porosity.
Having cooled and been released from its bondage within the investment, the gold ring is now scrubbed, cleaned and dipped into a weak bath of hot muriatic acid. I use the exact same chemical that one might use to keep their pool fresh and clean only in much stronger concentrations. This removes any oxidations that formed on the hot metal in the casting process. 
Although gold itself is impervious to oxidation the copper in the alloy is not. Notice the black surface of the ring being scrubbed with a toothbrush as opposed to the buttery gold color of the ring after an acid bath.
The casting is complete and now on to the finishing and setting of stones but that is a different article. Thank you for your attention, I hope this has been of interest and assistance.