Archive for March 2011 | Monthly archive page
There are two new designs soon to be available at whirling sun. One is the Kolovrat, a Slavic sunwheel symbol. The Other is the Germanic Schwartze Sonne, or “black Sun”. The Kolovrat (lit.: “turning wheel”) was present in pre-Christian Slavic mythology. It was dedicated to the sun god Svarog and often called “The wheel of Svarog.”
I had attempted to carve the black sun a number of times in the past with unsatisfactory results. There is a lot to it and I could never get the geometry just-right. This time I used a combination of the lathe, the rotary table, and several hours of hand carving; I am happy with the result. Molds are being prepared already.
I know several of my good customers have been asking about these two pendants for quite some time. I am very pleased to finally be able to offer them.
Once the flask has matured in the burnout oven, it is time for the fun to begin: injecting it with glowing hot, molten metal!
The centrifugal casting machine has a spring driven arm that holds the hot flask and a crucible that contains the molten metal to be injected into the flask. The arm of the machine “breaks” near the crucible end to help keep the metal in the crucible as the machine accelerates up to speed. The other end has adjustable weights to help balance the machine. The heavy spring inside the base provides the centrifugal force that forces the molten metal into the flask.
It is important that the flask be just the right temperature for casting. If it is too hot, the metal will not cool and harden and will just flow back out. If it is too cool, the metal will solidify too quickly and form a clog inside the mold.
With the burned out flask simmering in the oven at the correct casting temperature for whatever metal is to be used, the centrifugal casting machine is prepared for launch. It is important to bring the flask to just the right temperature for casting. If it is too hot, the metal will not cool and harden and will just flow back out. If it is too cool, the metal will solidify too quickly and form a clog inside the mold.A typical centrifugal casting machine for casting jewelry A centrifugal casting machine with crucible and flask in place ready to cast sterling silver Thor’s Hammer pendants.
The metal to be cast is weighed out and placed into the centrifuge’s crucible. The right amount of metal is determined by weighing the wax model multiplied by 10 for silver, or 13 for gold. Then add 20% for the “button” – a reservoir of metal for the casting to draw from as it shrinks.
A vial of .925 sterling silver casting grain
The grain is placed into the crucible and then melted using an oxy/acetylene torch. A small amount of flux is sprinkled onto the metal to help clean it. Its heated until it turns glassy and rolls like mercury and then the arm of the centrifuge is released, spinning the metal into the flask.
After the machine comes to a rest, the hot flask is removed from the cradle of the casting machine and plunged into a bucket of water. The thermal shock causes the “investment” to explode, releasing the metal casting inside.
The casting is done, and all that’s left is finishing and polishing.
Once wax duplicates are made from the RTV mold, they are ready to be sprued onto the rubber flask base. The rubber base has a raised cone in the center. This will form a funnel in the prepared mold and also creates a reservoir of extra metal (called the button) to fill the mold.
The wax models can be attached directly to the base, or attached to a sprue, which is melted onto the rubber base, forming a “tree”.
Next a stainless steel tube is placed around the rubber base and the mold is ready to be “invested”.
Sprued wax models prepared for investment
The investment is a plaster-like powder that is carefully mixed using precise ratios of water to powder. It is vacuumed until it boils,and one minute thereafter, then carefully poured into the prepared flask. Then it is vacuumed again to make sure no air bubbles remain clinging to the wax model. It is left to dry for at least 2 hours.
Once the investment has set, it is time to begin the “Burnout”. The flask is placed into a special oven, nozzle down, and the temperature is gradually increased over the course of hours to cure and temper the investment in preparation for being injected with molten metal.
The oven is preheated to 300 degrees and the flasks are placed in for a few hours, then the temperature is raised to 700, then 1350 degrees, finally the temperature is brought down to the proper casting temperature for the metal to be cast.
Three invested flasks inside the burnout oven
The Burnout oven pyrometer at burnout temperature
Once master wax model has been created, it can be “invested” in plaster and burned out to create the void into which metal can be injected. The wax master will be gone forever, replaced by silver or gold or bronze. But what if you wanted to make a few of the same piece?
A new master doesn’t have to be carved for every identical piece you want to create. Instead, a rubber mold is made from the master. This rubber mold can be filled with wax over and over to create hundreds of duplicates of the same piece.
What follows is the method preferred at Whirling Sun: RTV silicone.
First, a wax sprue is attached to the master. Sprues are attached by heating a tool over a flame and “welding” the sprue to the wax master model with it. The sprue should be thick enough to form an adequate channel through which the hot wax can be injected. It should also be placed strategically so that no detail is obliterated by it. This sprue will probably also be used when it is time to inject molten metal into the casting flask, so it is good to make sure the metal will have a path that is clear of turbulence or right angles. It is bad for metal to be injected through a small opening into a big one, like an hour glass. This causes the metal to “spray” into the cavity like putting your thumb over a garden hose, and can cause a “blowout”. More sprues should be added to avoid this.
The sprued master is then attached to a cone in the bottom of a mold frame.
Once the mold frame is ready, a batch of RTV silicone is prepared for it. RTV stands for “Room Temperature Vulcanizing”. As the name implies, it does not require heat to vulcanize, such as natural rubber would. Such heat would melt and destroy a wax model. RTV comes in 2 parts, the rubber and the hardener.
RTV is usually mixed in a 10/1 ratio by weight -ten parts rubber to one part hardener. It is thick like warm taffy and is a bit tiring to mix, but you should mix it with a spatula or putty knife until it is all a uniform color.
Once mixed, it is placed under a bell jar and vacuumed for 5 minutes to release all the entrained air bubbles. After this it is slowly poured into the mold frames, being careful not to knock the wax model off its sprue. Next, the RTV filled mold frame is placed under the bell jar and vacuumed for 5 more minutes. It will most likely boil over during this operation, so it is advised to wrap masking tape around the top of the mold frame to contain as much RTV as possible. a plastic container lid under placed under the mold frames during vacuuming can catch the spilled rubber so you can pour it back in.
Mold frames containing wax models and RTV rubber being vacuumed in a bell jar
After the vacuuming is complete, the pressure is released and the overflowed rubber scooped back into the frame. The Rubber takes around 24 hours to vulcanize. The process can be accelerated with heat, but remember, there is a wax model in there!
After the rubber has cured, it is time to remove the mold from the frame and cut the model out of it. Working slowly and deliberately, an incision is made around the entire perimeter of the mold using a hobby knife or a scalpel. Next, zig-zag cuts are made so that the 2 mold halves will line up again later. When the wax model is reached, special care is taken to ensure it is not cut. The rubber is pulled apart to gain access while cutting out the model.
Finished RTV mold of the “Skane Mjolnir” Thor’s Hammer pendant
Finally, the mold is held between two metal plates and the opening is pressed onto the nozzle of a wax injector. The wax injector is pressurized and filled with molten wax. The best results are achieved when the temperature and pressure are at the lowest possible settings where the wax still flows and fills the mold. Too hot and the wax will shrink as it cools. Repeat this over and over to create duplicates of your wax masterpiece.
The wax copies are perfect reproductions of the original master.
In this article we will walk you through the process as a piece of jewelry transforms from an idea on paper to a piece of finished jewelry.
It all starts out with an idea and a sheet of paper. In this example, we are going to make a wax master of the Irminsul to be used for pendant and pin castings. The image can either be hand drawn or printed from a computer illustration. It is then glued onto a piece of “carving wax”, a specially formulated wax that carves easily and can be burned out completely and cleanly from a mold cavity. Ordinary white glue works well to glue the paper to the wax.
Next, the outline of the image is transferred onto the wax by poking holes along the lines with a pin, scribe or a hobby knife to form a sort of pointillism image on the wax (Think “connect-the-dots”). This will be the guide by which the piece can be carved.
The paper is then removed and the rough shape carefully sawn out of the wax block. It is sawn out using a jeweler’s saw, close to the line, but not on the line. The final millimeter or so will be carved down with files.
Next, tools like hobby knives, files and wax carving tools (these are very similar to dentist’s tools) are used to carve the details in. The shape of the wax master can be formed either by carving away the rough shape until the finished piece is achieved, or globs of wax can be melted and applied to the master to build up the form you want.
Alternately, a wax pen, , a device a lot like a soldering iron but with sensitive heat control can, be used to perform many of the carving operations.
When the details are carved in or applied onto the piece, it can be polished by gently rubbing it with a felt polishing wheel. This step is optional as the final polishing can be done on the finished metal jewelry piece made from the master without risking damage to the delicate wax sculpture.
Another option, only for the brave, is called flame polishing; this is where the finished wax master is passed near an open flame to melt the surface for a fraction of a second, smoothing it. This method usually results in a loss of the sharp details that have been painstakingly carved and risks ruining the whole piece altogether.
The wax master featured in this article was used to make several popular items on the WhirlingSun store.