Reviewing GFRC Luster Descriptions

Originally published on August 25 2018

Freshly minted coins will emit "luster" or light reflections from the coin's surface. This is a simple concept but describing luster can be most subjective. To kick off this topic, lets visit with Wikipedia and their definition for "luster". It was immediately learned that the term luster is closely tied to mineralogy.

Lustre or luster is the way light interacts with the surface of a crystal, rock, or mineral. The word traces its origins back to the Latin lux, meaning "light", and generally implies radiance, gloss, or brilliance.

A range of terms are used to describe lustre, such as earthy, metallic, greasy, and silky. Similarly, the term vitreous (derived from the Latin for glass, vitrum) refers to a glassy lustre.

Lustre varies over a wide continuum, and so there are no rigid boundaries between the different types of lustre. (For this reason, different sources can often describe the same mineral differently. This ambiguity is further complicated by lustre's ability to vary widely within a particular mineral species.) The terms are frequently combined to describe intermediate types of lustre (for example, a "vitreous greasy" lustre).

When writing GFRC descriptions for silver and gold coins, I've settled on the following five terms and will attempt to define and clarify the requirements for each descriptor. Let's remember that evaluating coins is a subjective process with multiple variables that include lighting conditions, light source (natural sunlight, incandescent, halogen, fluorescent and now Light Emitting Diode (LED) generated light) and finally, the construction and age of the human eye ball.

When evaluating coins, I use three different types of light that are available at the office desk. There is natural light (from the office window), halogen light and incandescent light. Natural light is the most accurate and the least forgiving in terms of surface blemishes. This is one of the reasons for photographing coins under natural sunlight. Incandescent is probably the most forgiving and the least used when reviewing a coin.

Following are the luster terms and attempted definitions to be found within GFRC descriptions.

Frosty

Frosty luster results when a coin has considerable mint frost on its surfaces. Mint frost results when the dies have a textured or irregular surface that leaves micro bumps on a struck coin. The micro bumps will diffuse the intensity of reflected light since the light is reflective at multiple angles from the coin's surface. In some cases, the main devices of a coin can be frosty while the surrounding fields will have a different condition due to die polishing. Modern day "cameo proofs" are a good example of coins with frosty devices and heavily mirrored fields.

Satiny

Satiny luster results from coins struck with aged or eroded dies. After dies strike thousands of coins, the die steel characteristic will change due to ongoing strike force and metal stress. When a coin is struck, stress is applied that drives the planchet metal into die cavities. The planchet metal also moves from center to edge due to force. This stress eventually alters the steel die surfaces and forms metal stress channels that align from center to edge in a radial pattern. The phenomenon is typically referred to as metal flow lines and is best seen on older coinage like Capped Bust and Liberty Seated where dies were typical employed until cracked and no longer usable.

Reflective

Reflective luster results from proof or proof like surfaces. Freshly polished dies will have smooth mirrored surfaces. Those characteristics are transferred to struck coins. Proof coins are made with polished dies and polished planchets while circulation strikes may be made with polished dies and unpolished planchets. When either polished dies or polished planchets are introduced into the minting process, then struck coins while have mirrored or proof like fields. These surfaces will reflect concentrated light at a fixed angle. The result is a bold and intense luster that is labeled as reflective. The term mirrored surfaces are also employed to describe proof or proof like surfaces.

Steely

Steely luster is an "in between" descriptor. Surfaces are not completely frosty nor are they proof like. Instead the reflected light has characteristic similar to freshly rolled steel. Coin surfaces are smooth and probably result from a fresh pair of die steel that has not seen extensive polishing. Steely luster is the least often used term at GFRC for describing surface conditions.

Cartwheel

Finally, I will use the term "cartwheel" frequently to describe the behavior of the luster when a coin is viewing under a light source. All types of luster will produce cartwheel effects. Cartwheel luster is well described at the SpruceCrafts.com website as follows:

The cartwheel effect is a term that describes the rotating, windmill-like effect of light that mint state coins exhibit. The cartwheel effect is best known for appearing on Morgan and Peace silver dollars, due to the basically flat design of the coins, large planchet size, and die preparation methods. However, you can observe the effect on any mint state coin. Flow lines that occur during the coin striking process cause the cartwheel effect by reflecting the light off the surface of the coin. The effect is somewhat fragile and will disappear as a result of circulation or if the coin is cleaned. Coin collectors also refer to this as "mint luster."

In summary, every coin found on the GFRC price lists are thoroughly evaluated for surfaces conditions. Circulated coins are reviewed for originality while mint state coins are examined to ensure they offer natural mint state luster. Each coin description is carefully written to ensure that customers will not be surprised when a GFRC sourced coin arrives at their mail box. Since GFRC images are primarily true to color and do not capture luster, then it is imperative that customers read the descriptions to gain an understanding of luster type and characteristics.