NNP

Undercut, Undercutting.  Modeling of relief between the design and its background; the carving of overhang of design relief; a negative slope of relief also called back draft or under bevel. Undercutting is indicative of full-round sculpture even though it is attached to its background; it intensifies a contour line or relief form by casting a shadow behind the relief. In the medallic field undercut designs can only be reproduced on bas-relief plaques (and then made only by flexible molds). Undercutting must be avoided in modeling for a numismatic or medallic design which is to be diestruck or electroformed.

The slope in which the relief rises from the background is most important in modeling. If the model is a pattern for any kind of diework (and most casting) it must have an ideal bevel, a draft or taper of at least 15°. This will allow a die made from that model to strike a piece and to withdraw (or a cast piece to be released from a mold made from that model). Anything less than 5° draft will cause a formed piece to "hang up" or freeze in the die or mold. And, obviously, since a die cannot strike under relief, undercuts in models must be avoided ("at all costs" by every mint and medal official). Should such a model be prepared with undercuts, they must be filled in first before any die preparation is begun.

Low relief – a design without any bevel at the sides – is easiest to strike. Stiacciato, the lowest of relief, is ideal for coin designs which must be coined on high speed presses. But once the designer heightens the relief he must create an angle on the side of the relief. A steep vertical relief without any bevel is most difficult to strike, creates the greatest stress for dies and causes difficulties in striking. The displacement of surface metal of the blank is greater and the wear to the dies is at its maximum (which leads to diecracks and diebreaks). As the artist shapes the sides of the relief in his design during modeling he must be aware of this angle or bevel at all time.

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