The lost wax process is documented in detail in the molding of the Broderbrunnen. When you click on the following headings, you come directly to the respective step in the work on the Broderbrunnen.

A model created by the artist is cast in metal. It can be made of various materials: depending on the surface structure desired, sculptures are formed from wax, plaster, clay or similar materials.

The Negative
The model is coated with multiple layers of an elastic material (here: silicon). The layers have different colors (here: black). The viscous mass hardens on the surface of the model and precisely reproduces the model in the negative.

Plaster Shell Mold         
A plaster shell mold is created to support the negative. This supporting mold can consist of one piece in the case of a relief, but usually consists of two or more segments.

Removing the Model     
The segments of the plaster mold are separated and the negative (silicon) is removed. The plaster mold consists of segments so that when they are separated, the model is released. In the picture, only the front part of the mold can be seen.

The silicon negative is placed in the supporting mold so that it can, in the next step, be used to form the wax positive.

Wax Positive         
The silicon negative form is coated with liquid wax using a brush. The first layer of wax has to be applied very carefully so that air bubbles do not form when further wax is poured in.

After the first layer of the wax positive has cooled, the two parts of the mold are put together again and further wax is poured into the hollow cavity. When the wax layer is approximately four to five millimeters thick, the rest of the liquid wax is poured out. The inside of the wax model, therefore, remains hollow.

Once the wax has hardened, the plaster-silicon mold is opened and the wax reproduction of the model is removed.

Casting System
After the wax model has been retouched, a system of sprues and risers is attached. The sprues and risers provide an entrance for the molten metal and serve as exits for the escaping air. Nails are inserted in the wax model in order to fix the core to the outer form once the wax has been melted out.

Chamotte Mold
A sturdy cylinder is build around the model and “packed” with chamotte. The wax model with its casting system is carefully immersed in the chamotte until the wax model has completely disappeared.

The completely dry chamotte mold is heated in the kiln until the wax inside it melts and runs out. There is now a hollow cavity in the chamotte cylinder that was previously occupied by the wax. The temperature in the kiln increases to ca. 700 degrees Celsius and remains at this high temperature until all bound and free water is evaporated from the form. (Picture: cross-section of the chamotte mold)

The molten metal is now poured into the hollow within the chamotte shell mold. The air escapes through the vents and the cast piece then has to cool down gradually. Once the bronze has hardened, the chamotte shell mold is shattered, the sprues are sawed off and the raw casting emerges.

The raw casting is now chased in order to remove the remains of the sprues, as well as errors, air bubbles, and other imperfections. If the figure has been cast in multiple sections, these have to be put together and holes welded shut.

During patination, various chemicals affect the surface of the metal. Methods and techniques of patination are often well-guarded secrets of the foundry or the artist. Finally, the bronze can be waxed or sealed with a protective layer.