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Understanding Lustre

18-Jul-2009

When we talk about lustre on a coin, we numismatists refer to the original mint lustre, the way light reflects off the coin's original surfaces. This is caused by the light reflecting off the naturally grooved surface of most coins and gives off a unique pattern of light known as cartwheel lustre.


The above is an example of a coin with original, brilliant surfaces, notice how the coin is brighter from 10 o'clock to the centre of the coin, then to 2 o'clock? These angles change per change in the angle of incoming light to rotate fully around a coin. This is what we call cartwheel lustre.


This is how the surface of a typical, brilliant finish circulation coin looking in the direction of a tangent through a cross section of the coin should look. As we change the angle of incoming light, the amount of light entering a groove and the angle of reflection changes and as a result the brightness of that point when viewed from specific angles changes too. If the light comes from directly above and we look from above, the entire surface is iluminated and we don't see the cartwheel effect. This makes for an effective photographic technique as seen here (enlarged 4x):


Note how crisp the detail is, however this technique isn't always favoured as lustre isn't shown as all of the light reflects directly back up from all points making it equally bright.

These grooves are not always consistent between coins, the wider the grooves, the more reflectivity you have on the surface of the coin and with a completely flat surface, as seen in the fields of many modern proofs we see a completely mirror finish. This is because the wider the grooves are, the closer the angle the light reflects off the surface is to the angle it comes in at, which is the way light reflects off a perfect mirror, thus producing a mirror finish.

Modern day circulation issues have much flatter surfaces hence why appear more reflective than pre-decimal issues, or even early decimal coins. This is how we can expect a typical current circulation issue to look.


Modern proofs have ideally flat surfaces in the fields producing a mirror effect as seen with the coin below:


The more relative depth the grooves have (relative to the width), the more intense the lustre is, this is because the ratio of the internal surface area of the groove per surface area of the coin is greater and thus more light is reflected per any particular area of the coin.




The above two threepence (enlarged 2x) show the effects of different groove depth, the 1962 has much deeper grooves providing a much more intense cartwheel effect while the 1963 is much lighter but still complete. Deeper grooves seem common among the even years from 1960 to 1964 from the Melbourne mint while the odd years tend to have more shallow grooves. In this case this is most likely caused by strike pressure.




The 1962 will likely look like the first image above, and the 1963 like the second. Note how the grooves are much deeper relative to the width of the groove in the first allowing for more light to reflect. Width can vary too, though this is unlikely between denominations from the same mint in such a short span of years. An example of width variance can be seen between the various mint WW2 silver. The American mint threepence had a more smoother finish causing a more mirrored appearance and deeper grooves enhancing lustre intensity. For example the following pair of sixpence (enlarged 2x), both dated 1942, the former from the Melbourne mint and the latter from the Denver mint.




Note how the latter seems more reflective and portrays much stronger lustre, in the coin manufacturing industry, this is considered a positive quality but among numismatists this is highly subjective and I personally prefer a less reflective lustre, at least on my circulation coins.

When a coin is dipped, residue fills the grooves and the the depth decreases, thus decreasing the strength of the lustre. Some dipping materials may also corrode the tips of the grooves again decreasing relative depth and thus the strength of the lustre.

This effect of filling the grooves also begins to leave a flat top which light reflects off similar to a naturally flat surface on many modern proofs, this causes a slightly mirrored effect, though the make up of the dipping substance tends to be non-reflective and thus is more likely to create a matte finish, though some types of silver polish used to clean coins in the past do create the mirror finish instead.


This is the effect of dipping a coin with a non-corrosive substance, even dipping with this substance which leaves residue in the devices, will hide lustre, and excessive use will result in a dull, matte coin.


This sixpence (enlarged 2x) is how the matte effect of a heavily dipped coin looks, notice the absense of lustre. Oddly enough, this coin was offered on eBay as Gem Brilliant Uncirculated!

Many dipping substances are corrosive and wear away the top layer of the coin with the intention of exposing a brighter metal below. This results in the damage to tips of the grooves and residue left in the grooves:


When this occurs, the finish of the coin becomes even more reflective producing a strong mirror finish however less cartwheel lustre is present.

When a coin is worn down, the first to be hit are the tips of the grooves on the highest points of the coin, then the grooves in the exposed surfaces, the surfaces within the devices and legends tend to be untouched even down to the VG level, though in silver and copper they'll be hit by environmental damage by then evening out the surfaces, losing its natural lustre anyway, though it's not unusual to see a gold coin with lustre in the legends even in VG condition.


This is how the surfaces of a typical EF coin looks, there's considerable wear throughout, at least at the microscopic level, to the naked eye this will only be detectable by a slight dimming of the lustre in the exposed areas, best identified by comparing the surfaces within the legends with the exposed surfaces. There will be a noticeable difference by this stage.


Here is a sovereign (enlarged 2x) that grades extremely fine, notice how lustre is much stronger within the wordings of the legend, this is because when the coin is touched, such fields are protected by the high legends. Yet lustre is still present within the exposed surfaces, this is a key to a coin grading extremely fine and a great determinier for the old strike vs wear debate.


You probably can't tell from the photo, but this coin has traces of lustre within the fields indicating a grade of nearly extremely fine, yet while the coin has 8 pearls, the 2nd set appear too worn to grade what we would normally consider nearly extremely fine and position it closer to a good very fine grade, this indicates that the obverse of this coin is struck from a later die state, despite a strong strike and suffers from some die filling, particularly in the band. Seasoned collectors would be able to determine that this is a London mint obverse as such missing detail despite a strong strike is a characteristic of early London mint silver.


Then on the other hand you can have a half sovereign (enlarged 2x) like this which appears to have full, unworn details indicating an extremely fine or better grading, yet the coin only has lustre within the devices indicative of a good very fine grading. This indicates that the coin is a stronger than average strike and in fact has the surfaces of a good very fine coin and just appears to have more detail because it was struck with more detail in the first place. Most coins, however will have their surfaces agreeing with detail.

While toning is generally accepted by numismatists, it too damages the surface of the coin. Essentially toning is a chemical reaction which alters the surface of the coin.


However, toning acquired over a long period of time will alter the surfaces in a way that grooves are still present with sometimes only minor changes in depth thus we can still have strong lustre beneath a tone, this is highly desirable and produces some of the most beautiful coins.


In the end, if what we aim for is a coin closest to its original state, then we cannot dismiss original lustre as an important grading criteria.

Application in circulated coin Grading

AU58: Full pattern of lustre lightly subdued in the fields and absent on minor high points.
AU55: Full pattern of lustre heavily subdued in the fields and absent on the high points.
AU53: Considerable lustre with traces in the exposed fields.
AU50: Some lustre confined to the legends.



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