NNP

Background Plate.  A blank plaster disk often with a molded border upon which a design can be built by adding relief of clay, plasteline, wax or plaster. This plate forms the base upon which the model is created and is necessary for both cast medals of exact size, or of an oversize model which is later reduced by pantograph to a die for striking. A separate background plate is required for each side, obverse and reverse. The artist must also be aware that the surface of the background plate may, indeed, become that of the field of the struck piece if it is left intact with no added design or texture. Thus the surface of the background plate is the lowest in the model, it becomes the highest in the die made from that model, and also the lowest in the struck piece (for proof finish, the surface that is proof polish.

First background plates.  Renaissance medallists used a disk of smooth natural material, as black slate; Benvenuto Cellini describes using black glass, bone (or wood only as a last resort). Designs were built of wax on these surfaces, and when completely modeled they served as patterns for making the molds which would produce the cast medals.

Modern uses.  In modern times plaster of Paris is used for the ease of forming a desired background plate. The size is determined by the reduction ratio of model to intended die size (three- to five-to-one is ideal, but up to nine-to-one can be reduced on modern diecutting pantographs). Uniform plates are useful for obverse and reverse models of the same coin or medal; they are mandatory for producing a series of coins or medals where absolute uniformity is required.

When a number of uniform background plates are required (as for a series) a master pattern background plate is prepared and a mold is taken of it. Then any number of working background plates can be cast from such a master. This assures the uniformity desired for the series.

The plaster surface is made smooth by fine sanding or by emery. The border is

then prepared by modeling, or more often, by using a template (or even by using a milgrain tool) as the first step to preparing the design. The background plate can also be given a slight depression sloping towards the center from the border, and as such is called a basin. This will produce a camber in a die (or if the plate is absolutely flat the die camber can be created on modern diecutting machines when the model is reduced and the die is cut).

Like Cellini, modern artists use wood as a background plate only as a last resort. Wood being porous, it must be coated with shellac first, before any modeling is done. Otherwise moisture in the clay will be drawn into the wood, possibly deforming the clay model. Sturdy plastic, or other materials have been used as the composition for a background plate by modern artists, by only infrequently.

Once the modeling is completed on the background plate, irrespective of the media used to model the design, it is coated with a release agent and a casting is made. The artist generally works in positive for the original, the cast then is negative. He customarily touches up the detail in this cast and it becomes his matrix. He will make a second casting in the positive to submit for the pattern for the die to be cut. At this point there is no further need for the original model on the background plate, and it is usually cleaned off and used again if desired. (Or it may be stored intact in the artist's archives, but the original media is usually of such impermanent material that it does not keep well; the survival ratio of these models on background plates is quite small.)

excerpted with permission from

An Encyclopedia of Coin and Medal Technology

For Artists, Makers, Collectors and Curators

COMPILED AND WRITTEN BY D. WAYNE JOHNSON

Roger W. Burdette, Editor