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 helps to prevent the oxidation of molten metals thus helping them to blend 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. For many people, this is the hardest part because of the sudden violent action. I know one jeweler who casts everything with the vacuum method simply because of this. This always used to be the hardest part, for me, 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.