The Lost Wax Investment Casting Process

Lost wax because a wax form in the precise shape of the jewelry is made and subsequently lost or destroyed in the production process. 'Investment' because that is the name of the material which preserves the form of the "lost wax" so that a form can be made from this 'loss'.

There are five steps to the creation of jewelry from wax. Each one is simple. Together they make up a complex system for the production of one of a kind pieces and the mass production of jewelry.

1. The wax: Creating the jewelry from wax
2 .The investment: Capturing the wax shape to preserve it
3. The burnout: This is where the wax is lost!
4. The cast: Filling the captured wax shape with molten precious metal
5. The cleanup and polish: Refining the surface of the casting to a perfect luster

The Wax
It all starts with an idea. Hm. Let's make a cat. Every body likes cats. Hey! Wow! Let's make a cat that's a local celebrity. Do you know of any cats that've saved their masters lately? No,  but how about a cat that was so revered by the Caloosa Indians that they made a statue of it with a human like body? Wow! Good Idea!

    The Marco Cat. While the artifact excavated on Marco Island in 1896 was on loan to the Collier County Museum, I took pictures. I resized a copy of the best pictureand traced the drawing onto a wax block from which I intended to sculpt the final cat.  The tracing is now etched into the wax block. Next, cut away the parts of the wax that is not the design in stages. First a very rough shaping,  then progressively more refined shapings until the final detail has been added.

Investment

Once the wax is finished it needs to be prepared for the investment step. A system of wax rods called ' gates' get attached. The gate is attached to a sprue base. A flask is fitted around the base. Grids get fitted into the sides of the flask to distribute the air in the form during casting. (picture of grids). The flask is filled with investment. Investment is essentially a plaster of paris with a quartz filler.The quartz helps the investment to tolerate temperatures up to 1384 degrees fahrenheit. It is also a health hazard as prolonged exposure to the fine quartz dust can result in pneumosilicosis, Black lung. We mitigate exposure to the dust with excellent ventilation and asbestos respirators. Once the investment has set up for two hours, it's reached its maximum green strength and it's time to dewax the flask. The sprue base gets pulled off the bottom of the flask.

The Burnout
    After 2 hours of steaming the wax out of the flasks  we put the flasks into the kiln. The burnout or flask baking process takes 3 to 15 hours depending on how many flasks are baking at the same time.Usually we cast more that one flask at a time.  The investment has to be heated slowly since rapid heating will cause steam explosions in the investment and damage the investment form. Once all the water has been driven out of  the investment, the flasks are heated up to 1350 F and allowed to bake at this temperature for several hours. This burns or oxidizes any carbon left on the walls of the investment. Carbon present in the cavity left by the wax when molten metal is introduced will cause a rough surface on the casting. Once the investment is baked cleanthe flask is cast.

The Cast
    Casting can be accomplished in two basic ways; By centrifical force or by vacuum assist.  My favorite is vacuum assist. It is less dangerous because there is not the chance for molten metal to be flung out into the room and I can cast larger quantities of items at one time using this process.
    Once the metal has been poured and cooled inside the investment, we need to remove the investment from around the casting. We take the flask, which is still much hotter than 500 F and plunge it into a bucket of water and allow the hot investment to boil out.  The castingfalls out with some investment still sticking to it.

Cleanup and Polish
    We scrub the investment off with a brush and proceed to cut off the gates. Clippers cut the gates most quickly. Sometimes a jeweler's coping saw is used to cut several gates attached to the same piece to avoid bending the casting. The surface of the casting is worked with finer and finer abrasive tools until it reaches the desired shape and sheen. Final polish is usually a burnishing step that brings up a truly mirror finish on this particular gold casting.
     This particular casting is a model, a piece of jewelry from which a series of production molds will be made. Production molds are made from raw rubber, packed around the piece, vulcanized, and cut away from the piece.
  Once the mold is made, we inject it with wax. Remember the process: wax form , form incased in investment, wax burnout, cast the molten metal , cleanup and polish. When we have injected enough waxes to fill our order, we start the lost wax casting process over again and make more metal jewelry from wax.

    We'd like for you to Contact Us us about this article. We'll answer any questions you may have to the best of our ability. We'll use your comments to improve future articles about jewelry manufacturing.