Having apprenticed under a half-crazed master jeweler, I have grown to be closed minded towards new ideas. Wait. Let me rephrase that. I have learned to be closed minded towards any good idea that anyone but myself has come up with. So having said that, let me begin.
Centrifugal casting was THE way and the ONLY way to cast precious metal. It took me some time to learn, but after a year, I believe I had acquired the skill to complete a successful casting. All the while, there was still a small spark in the back of my mind telling me that there were other- possibly more efficient- ways of doing things.
“JASON! Go heat up the car! i want to look at 800 acres not too far from here, where we can build our own Renaissance Festival!” (-spark gone-) “Yes boss.”
Those times are long gone now, but I have had the opportunity to keep casting, this time with vacuum assistance.
A whole new world had opened up, with new challenges, of which I had no one to help me through. I started with something simple, using the same burnout process and temperatures as a centrifugal casting, only to result in failure. I followed the instructions exactly as they were written, to no avail. This system had its own set of rules. And like many things in life, what works for one person, will not work for the other. So I had to figure this out on my own. Oh what joy. Besides working 40 hours at another job, providing love and support for 3 children, trying to keep my wife happy, and taking care of my house, I had the opportunity to fiddle around trying to perfect a method i knew nothing about, and figure it out soon, because we had waxes that were carved for paying customers, ready to be cast!
I am a patient person, and I thank God that He has provided me with a patient wife! With a little help from family and other outside sources, we were able to do our first successful cast. It came out with a few little imperfections here and there, but it was complete! Since that day, I am proud to say that in the 8 months of learning this method, we have only had 1 mis-cast. (incomplete fill of a pattern).
However, there was still a problem. Even though we were getting complete castings, they were coming out with something we call ‘pitting’. Pitting is the result of either foreign material falling down into the cavity left from the wax being burned out in the investment, impurities in the metal, or small incomplete fills. (not to be mistaken with ‘porosity’, which is the result of too-hot metal being poured).
Since I thought we had the incomplete fill problem solved, I disregarded that as the complication and worked on the other two options. I changed my method of transferring the flask from the de-wax to the burnout kiln; didn’t work. I stopped using a metal melter, thinking that the graphite powder from the flask was somehow falling into the cavity before the metal left it. I started torch melting to better control my metal temperature and so that I could use flux to purge any impurities that may be in the metal. Nothing worked. Castings kept appearing with pits. Although easily fixed, and not causing any lasting weakness in the piece, they were annoying and time-consuming to repair.
Frustrated, and ready to go back to the proven centrifugal method that I had known, I tried once more to get outside help. I inquired of a man who has owned a small business, and has been casting in his own shop for over 30 years. I sent him a picture of a recent cast which had this too familiar pitting. It was a ladies’ 5 stone white gold ring. The 5 stones were bar-set, cast in place and the pit was right on one of the bars.
Five hours later I received a reply. He immediately- and I’m sure instinctively- told me my problem. It was my skimpy sprueing method! “HUH?” I thought, “This is how i was taught to sprue, and this has worked for me before!” The sprue, for those of you who are new to this terminology, is essentially the feed line for the molten metal. An average sized ring, spin cast, took a sprue about 1/8” thick. But we were no longer using spin casting. “This is a whole new method, you idiot…” said this little voice in the back of my skull.
Well, the man was right. When one does a casting, there are 2 temperatures they are dealing with. The flask temperature, which should be around 950º - 1150ºF, and the molten metal temperature, which for gold and silver should be anywhere from 1650º - 1900ºF. Now, think of it this way: When something is 1700º, and it touches something that is 1050º, there will be some sort of instantaneous cooling. My problem was that round stock I was using to sprue the ring was too small, and when the molten gold flowed in the hole, it froze off too fast, not allowing a full flow to the ring. Imagine your 3” main septic drain being coated with 2” of hard gunk. Ahh.. that makes sense to me now.
I am happy to say that since I have started using a larger sprue, the past 3 castings have been a beautiful success. Smooth surfaces, free of pits and chinks.
I still wish to cast using a centrifugal system. It does have its benefits, especially with small, highly-detailed pieces. But until I get a big fireproof shop, vacuum assistance is fine for me.
