In Part I of this series, I showed how a wax model of a ring is made while Part III covers finishing a cast ring. In this installment, I demonstrate how a wax model becomes a metal ring by discussing the investment and casting processes. Once a wax model is completed, there is a two day process before the ring is metal.
To create a path for molten metal to flow into a ring, jewelers add sprues to wax models. A sprue is simply a flexible stick of wax with a lower melting point than the carving material. I attach red sprue wax to each model by melting a bit of the wax- basically welding the sprue to the model. Once my models have their sprues attached, I begin to build a sprue tree by attaching my smaller sprues to a larger, central one. During this process it is important to keep everything neat. I also need to think about how the metal will flow into the model when I cast my sprue tree. If this process is done incorrectly, my models may not cast or be filled with porosity. Porosity is basically little air pockets and cracks in a cast piece. It looks terrible and can trash my hard work. When all of my models are attached to the tree, it is VERY important to weigh it! The weight of the wax is how I will determine how much metal to use when I cast. Finally, I attach my tree to a rubber sprue base, which is the round black object in the lower right of the photo.
When my sprue tree is ready, I will place my models in a metal tubular flask and cover them with investment, a product that is similar to plaster of Paris. I carefully measure water and weigh investment powder then thoroughly combine them in a rubber mixing bowl. To keep air bubbles from sticking to my models, I vacuum the bowl under a bell jar. This machine has an awesome pump and can pull a vacuum in about thirty seconds. Next, I pour my investment into the flask, careful to avoid pouring directly on the model since I don’t want to break my models or detach any sprues. I use the vacuum again and then allow my flasks to sit for about a half hour while the investment sets up. Lastly, I pop off the rubber base and load my flasks into the kiln to burn out the wax. The kiln will run overnight, reaching a maximum temperature of 1350 °F. The following morning, there will be no traces of wax left in my flasks, just an empty cavity ready to fill with metal.
Based on the weight of the wax, I calculate how much metal I need. In this case, I am using bronze, which weighs nine times more than wax. I also need to add some extra. Usually I only cast a couple rings at a time, so I like to use 15% more metal. However, larger sprue trees require more material. You may remember in Part I, I mentioned that there is plenty of room for error. This is where I went wrong, as you will see further down.The fun part
To cast my rings, my flask needs to be hot. So immediately before casting, I pull my flask out of a 1000 °F kiln. The kiln is incredibly hot, like “lake of fire” hot. I wear a leather apron and gloves during this part of the process. The gloves have to be loose enough to quickly shake them off, just in case they catch fire. I personally have never done this, but some of our gloves have suspicious holes in them…
Anyway, I load my flask into a centrifugal casting machine and light a large oxygen-acetylene torch. Next I pour my metal chunks into a preheated ceramic crucible, using a device we call a “taco.” I keep the flame directly over the metal as much as possible. Once the bronze chunks have melted into a pool, I give it a quick stir with a graphite rod to make sure everything has melted. Then I close the lid and the machine spins the extremely fast, pushing the metal into the flask.
I wait a few minutes for the centrifugal machine to stop spinning and give my flask a few minutes to cool off slightly. If I quench it too soon, the coefficient of expansion will punish me and my rings will crack from being cooled too quickly. That has happened before and it is not fun! To quench, I completely submerge my flask into a bucket of water. All of the hot investment blows out into the bucket, leaving just a little bit left on my now-metal sprue tree. Since I am holding the flask underwater with tongs, I wait until I can feel that the invest has emptied out.The moment of truth
When I lift the flask out of the water, I finally get to see if my models cast properly. This time they did not completely cast, as you can see in the photo. Epic fail. I mentioned earlier that I don’t usually cast more than a few rings at a time. I should have added more metal to my calculation. The extra weight probably would have pushed the bronze completely into the cavity.
Even though I have to carve my models all over again, I can at least reuse my metal. To do so, I need to get all the white investment powder off, so I put the casting into an ultrasonic cleaner with a bit of Simple Green. Now I am ready to repeat the process.
Be sure to check back for Part III, when I take a freshly cast ring to a finished piece of jewelry!