Wednesday, 9 January 2013

Process of Casting in Jewelry Making

Use the thickest part of the bone for casting. Cut away the top and bottom ends so that it is almost square. Casting is a technique by which it is possible to create shapes that would otherwise involve an enormous amount of waste. It is also used to re-create gently flowing lines of the kind that are comparatively easy to achieve in a soft medium such as wax but that are difficult to reproduce in metal. Casting is also, of course, a means by which many identical pieces can be made, either for a mass-market or to make, for example, chains with identical links.

There are three main methods of casting. Two of these cuttlefish casting and the lost wax method are available to people working at home, while the third involves the production of a rubber mold, which, while possible to do in the home workshop, is expensive and requires a lot of space and special equipment, It is not, therefore, a viable method unless it is being used almost every day and is most often carried out by a commercial company working from a model you have made yourself.

Tools and materials

several kinds of wax are available for creating the models used in casting. Among these are sheets of flexible wax, supplied in thicknesses from Vs to 1 inch; blocks for carving; molding wax, which is worked by hand; tubes and bars, which can be used to make rings; and sprue wax, which is supplied as rods in different sizes for attaching to models. Choose the most appropriate wax for the model you want to create. 

In the top of the bone, cut out a rounded channel. This channel, which must be slightly deeper than the design, is the duct through which the molten metal will be poured. Make it fairly large. You will also need one or two little carving tools and a craft knife. Keep a small flame burning the pilot light of a soldering torch or a little alcohol lamp with a small wick, for example so that you can keep warming the carving tool when you need to join pieces of wax together and to smooth over the surface of the wax. An electric "pen" worker can be used to join different grades of wax and is useful for shaping and molding.

Cuttlefish casting 

This is a very quick and simple method and can be done in one of two ways. The first, as shown in steps that follow, is simply to carve the required shape and depth of pattern into one side of the halved cuttlefish bone.

The alternative method consists of using a cuttlefish bone to make a model in plexiglass or metal, for example or to use a found object of a suitable shape. The bone is prepared as described in steps 1 and 2, but the object is then pushed into half the bone until it is buried to about half its depth. The model should be positioned so that its heaviest part is toward the bottom of the bone.

Lost-wax casting 

Finish the edges of the casting and burnish to highlight the high edges.
This technique involves making a wax model that is supported on a conical stand mounted on a rubber or ceramic base. The model is surrounded by a metal sleeve or flask, which fits tightly onto the base; and a plaster/silica mix, known as investment, is poured into the flask and allowed to set. The flask is then placed in a heated kiln to burn away the wax, which leaves an empty mold within the investment. The wax melts and burned away, and the impression of the mold is left in the hardened investment. When all the wax has burned away and the investment is the correct temperature, the flask is removed from the kiln and placed into a "centrifugal" casting machine which is held in tension by a spring. The metal is then melted in the crucible which is secured in place against the open end of the flask, and when it is completely molten, the heat is removed, the spring latch of the casting machine released, and the molten metal flung into the mold.

 When the wax model has been completed and sprued, it is weighed, and weight of the wax is multiplied by 11 for silver, by 18 for 22-carat gold, by 14 for 14-carat gold, and by 12 for 9-carat gold to find the weight of the metal needed for casting.

Preparing the investment

Mark out the design on the modeling wax. The design can be accurately drawn with dividers, a scribe, or something similar, and then the details of the motifs can be cut away with modeling tools. Bear in mind that the pattern can be cut and modeled very finely, and that the finished piece in silver will weigh about 11 times more than the wax model.Mix the investment in water that is at room temperature in the proportions of 4 parts water to 10 parts investment. Once the investment has been added to the water, it must be mixed and poured as quickly as possible because it begins to harden after about 10 minutes. If the wax model is exceptionally detailed, apply a coat of investment before you begin pouring; this will help to prevent air bubbles from being trapped on the surface of the model, which is to be avoided at all costs because they would be cast as silver blobs.

When pouring the investment into the flask, it is important to bring as many air bubbles as possible to the surface. Hold the flask in both hands and gently bang it on your working surface a few times.

The rubber or ceramic base is then removed from the flask and the flask placed into the kiln. It should be placed with the sprue opening facing the floor of the kiln and it should be supported, either on ceramic legs or between two ceramic kiln bases, so that the wax can melt and drop out of the investment. The kiln is set to approx 150°C and this temperature should be held for approx 1 hour to allow all the wax to burn out and then increased to approx 370°C for another hour. The temperature is then increased further up to approx 700°C for the last two hours of the burn out.

Melt the sprue onto the top of the base of the flask and paint the model with a wetting agent. Use long-handled tongs to remove the flask from the kiln and place it in the centrifugal machine. Place the weighed and cut up metal in the crucible with some flux powder, and use a strong torch to melt it. Do not include any solder in the metal used for casting, and when you are cutting up scrap, put any soldered joints back into your scrap box. 

The machine is sprung into position and released, and the centrifugal force created shoots the molten metal out of the mouth of the crucible into the sprue channel of the flask. Pick up the flask with the long-handle tongs and place it in a bucket of water. The investment will break away to reveal the metal casting. Remove the sprue and file and finish the piece. 

Commercial casting 

Pierce away the sprue and file the edges to make the piece ready for finishing.

This is the process by which a rubber mold can be used many times to create large quantities of an article. In essence, a rubber mold is made around a silver or rhodium-plated model which has a sprue attached. The rubber mold is cut in half to release the model in such a way that it can be exactly relocated. Warm wax is injected into the realigned mold and removed so that more wax can be introduced to produce yet another model. All the wax models are mounted on a "tree," which is placed in an investment and cast in the same way as the lost wax method, but on a much larger scale.

To make a good casting, you must make a really well-finished model, which is sent away to be cast commercially. If you are in any doubt about the positioning of the sprite, ask for advice from the company or leave it to the company to attach the sprue.

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