Since the early 1970s, the U.S. Mint has intentionally frosted the recesses of proof dies, ensuring that the corresponding raised elements of every coin would have a “cameo” appearance.
This frosting actually constitutes intentional damage to what were originally sharply defined features. Under a microscope, the frosted elements look indistinct compared to an early die state circulation strike or a Proof coin produced before 1971.
Methods of ‘frosting’
Intentional frosting was originally applied to a Proof die by sandblasting the entire surface and then polishing the field portion of the die face to a mirror finish (Coin World, Sept. 14, 2009). The frosted recesses of the die face lie below the plane of the field and are thus protected from polishing. Chemical etching of the die face and striking the die against a gritty emery disc are other techniques that were occasionally used to create a frosted surface.
Traditional methods of frosting and polishing dies occasionally led to mistakes. Sometimes a die was polished too aggressively, leading to thinned, tapered or undulating letters and numbers (Collectors’ Clearinghouse, Feb. 21, 2011). Sometimes the polishing was uneven, leading to wavy fields (Collectors’ Clearinghouse, June 30, 2008). In one case a distinct step was produced (Feb. 14, 2011).
In the last few years, pinpoint, computer-controlled laser etching of the recessed elements has replaced the traditional methods. In this process, frosting becomes the last step of die preparation (Coin World, Sept. 14, 2009). This technique also yields the occasional mistake. Offset frosting and misplaced islands of frosting have been observed (Feb. 21, 2011).
New Hampshire quarter oddity
Dale Stokes recently sent me a Proof 2000-S New Hampshire quarter dollar with a frosting mishap that I’ve never previously encountered. It involves the label OLD MAN OF THE MOUNTAIN, which appears in incuse letters within the eponymous (and now collapsed) rock formation. OLD MAN, OF THE and MOUNTAIN are arranged in three lines.
In Stokes’ coin, each line is surrounded by a precisely ruled, tight-fitting block of very dense frosting (see photos). The density of microscopic bumps that compose the frosting is greater than elsewhere on the rock face. Stokes sent me two comparison Proof sets, and their New Hampshire quarter dollars showed no difference in frosting density across the rock face (see photos above).
In 2000, the traditional methods of frosting were still being used, so I presume that polishing of the Proof field was the last step in preparing the die that struck Stokes’ coin. The incuse letters of OLD MAN OF THE MOUNTAIN are polished just like the field.
The edges of the blocks of dense frosting conform precisely to the outer edges of the incuse letters. On the left side of the first two lines, the dense frosting ends at the outer margin of the two O’s that begin each line. Such precision suggests the use of a stencil to guide the frosting process.
Matter of speculation
Why a stencil might have been used on this die, and why the frosting density should be different in this area, is a matter of speculation.
The design of the New Hampshire quarter dollar does present unique challenges when it comes to frosting and polishing. The mountainside is riddled with cracks, crevices, and cavities, some of which are quite deep. The cavities on the coin are, of course, generated by features that rise up from a larger basin in the die face. While I do not have the equipment to take precise depth measurements, it seems to me that the deepest cavities approach or match the depth of the incuse letters they surround. This could pose a problem if the polishing tool removed the tops of these raised features and left the floors of the deepest cavities mirror smooth.
To prevent such an accident from occurring, a multistep process may have been used to frost the die. Or perhaps an accident such as I described did occur, and this represents a repair job. Beyond these broad conjectures, I cannot speculate.
One interesting and perhaps relevant observation concerns the floors of the incuse letters in the anomalous quarter dollar. In most of the letters, the mirror-like finish is marred by sparsely distributed specks of frosting. Neither of the two normal coins shows this. It is perhaps another indication that something went wrong in the production of this Proof die.
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