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esnible Jan 16, 2004 07:12 PM

Coin surfaces
 
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I've been trying to understand what 'crystallization' looks like on a coin.

Linked is a genuine but crystallized coin from Doug Smith's web site. Doug describes the surface as 'reticulated'. I believe coins like this are described in auction catalogs as 'heavy crystallization.'
http://www.ancientcoinmarket.com/ds/...ature20/1.html

(Doug, if you are reading this: I have several coins that are that bad too!)

Below is a diobol from my collection. I believe it is genuine. (The dealer who sold it to me did not. He would not sell it except 'for study', and put a note on my receipt saying it was probably counterfeit.)

Under the loupe there are patterns on the surface that look like little silver rows of corn on a cob. I believe those corn-row patterns what I sometimes see called 'light crystallization' in auction catalogs. Is that correct?

(I'm putting this topic in the forgery area because crystallization is an important clue to authenticity. The topic seemed to fit best here.)

wgsant Jan 18, 2004 09:07 PM

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Ed;

Crystallization is just what it sounds like. The metal of a coin takes on a molecular structure that is geometric like a crystal. It sometimes looks reticulated, like Doug's coin, and sometimes just porous like your Apollonia diobol. Often, the surface of a coin may look normal but the interior is crystalline. These coins break easy and you can immediately see the crystallization once the interior is exposed. It has been argued in the past that crystallinity is a sign of authenticity because it is not possible, nor desirable, to duplicate it mechanically or chemically. This is unfortunately not the case. I have seen several modern forgeries that were crystalline. Below are images of a die pressed Rosa test piece that has a crystalline center. The enlargement shows the edge looking into one of the flan cracks. It is hard to see the detail, but the metal has the appearance of tiny pebbles and actually sparkles when you move it in the light.

Wayne

Wayne G. Sayles
wayne@ancientcoins.ac

esnible Jan 19, 2004 08:57 AM

Wayne-

I see terms like 'crystallized' or 'porous' in auction catalogs. A person with a good set of auction catalogs can infer what these terms mean. What I haven't seen anywhere is a visual dictionary of what these surfaces look like under the binocular microscope.

When I got my microscope I tried to describe the various corrosions and surface problems I was seeing. I found my vocabulary to be lacking. I'd like to learn the vocabulary and then produce a visual glossary for the Articles section of ancients.info.

You say the manifestation of crystalization on my coin's surface is 'just porous'. My reference for 'porous' is from Doug Smith's "Ancient Coin Vocabulary". http://dougsmith.ancients.info/voc2.html . Doug illustrates 'heavily porous' and explains it as 'roughened by corrosion.' Unfortunately I can't tell from Doug's small picture if his coin's roughness is regular or irregular.

I guessed that 'porous' meant 'irregular roughness' and 'slight crystallization' meant 'regular roughness'.

-Ed

bruce61813 Jan 19, 2004 09:17 AM

Think of the newly minted coin as a smooth uniform solid. Remember that it is a mixture of two types of metal; brass = copper + zinc, or bronze = copper + tin.
Over time various chemical will react with these components, for good or bad. Chlorides will attack the tin in the bronze matrix, and enough over a long period of time will dissolve the tin, leaving something that resembles a sponge. From the outside, it may look slightly rough, with small pits, but inside it is nothing more than a loosely bound shell.
The term crystalline is in a sense a mis-nomer, the coin is brittle because of elements removed, leaving the weak lattice structure. True metallic crystalization requires heating to a semi-liguid point and slow cooling, where the two elements seperate and and form large areas of the native metals, that do not bond together as well as the smaller crystal structure formed by rapid cooling.
Mechanical wear, like bending, can produce this effect in a local area also [rarely seen in coins, but think of a can lid, or piece of wire bent repeatedly]. Mechanical impact can produce a hardening, but this is not something that generally affects coins, but rings are subject to this.

Bruce

wgsant Jan 19, 2004 12:21 PM

Ed;

I think there is definitely a need for a visual glossary. As you have already discovered, there are a lot of "nebulous" terms used by numismatists that do not really have a solid foundation. When we say that a coin is "porous" or "crystalline" most people who have seen a lot of coins will understand what that means. But, what is the difference between porous and pitted? or corroded? As Bruce points out, what passes as "crystalline" may simply be the leaching of certain elements from the composition. I have taken to describing surface conditons simply as "smooth" or "rough". This is a very loose terminology, but most people at least understand the terms.

Wayne

Wayne G. Sayles
wayne@ancientcoins.ac

esnible Jan 19, 2004 07:01 PM

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Here is a holed Neapolis hemidrachm. It has a reticulated surface. The grains are the same size as Doug Smith's Corinth trihemiobol.

The reticulated pattern follows the ancient hole deep into the coin.

This coin is genuine and of the same dies as SNG ANS 450.

esnible Jan 19, 2004 07:10 PM

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This is a rare litra from Segesta in Sicily. It is jam-packed with interesting devices: A facing nymph, a dog, a murex shell, a gorgon, and a retrograde inscription. The coin would be quite valuable if the surface hadn't nearly flaked off.

A rough metallic core has been revealed where the surface fell off. In those places where the surface of the coin remains the metal looks good and well preserved: the muscles of the dog are still visible.

Where the surface has flaked off only the barest details remain. The gorgon is barely a face.

I'm not sure how to describe this coin's surface damage. It's chipped. If it was a fourree I would say 'very little silver remaining'. It's not a fourree though, I think it is a genuine ancient silver coin.

bruce61813 Jan 19, 2004 07:27 PM


This image may help. [Sorry about the quality, it was done on a scanner]. The top portionn is part of a brittle coin. It litterally shattered while I has hand cleaning it. The dark areas are where the metalic matrix has been completely replaces by oxides, no metal fabric remaining. The lower with the bright yellow [soild metal !], the red is copper, you can see the tail-off from yellow to red to black, or solid to porus to what is mostly dirt held together by lime. When I first saw this coin, it was looking to be a great one, good detail, I went to scrape away one spot and crunch! It broke into very small fragments.
I have coins with very rough [pitted] surfaces that are still solid, and some with smooth exteriors that i suspect are very fragile.
The worst are the silver coated bronzes, follees. The corrosion runs under the silver, and the silver layer is peels off. Hope he picture helps.

Bruce

esnible Jan 19, 2004 08:07 PM

Wow, I think this is the first time I've seen gold in an uncleaned coin!

Bruce, I don't understand how the inside of the coin can turn to oxides (= rust?) while the outside continues to show good detail. I know that is what happens but what keeps the outside of the coin looking metallic when the inside is like a geode?

bruce61813 Jan 19, 2004 08:26 PM

Yes and I have many more gold coins, just like this- just wait until the paint dries. I am not sure why it came oout gold in the scan, it was bright shiny copper!!

I am not sure of the entire mechanism, but your comparion to a geode is very apt. One possibility goes like this: the coin is coated with a layer of calcum and minerals, they form a carbonate layer with the copper, this is tha nice hard dark green that is well liked [copper carbonate = malachite]. Over time more water works into the pores of the coin, the water leaches away the softer metal, or chlorides are present and they attack the tin, forming tinchloride, whick may be more brittle, the freed copper [dull red] is left stuck to itseld with a mix if tinchloride, lime and other things filling the space. None is as strong as the original, but the outer layer acts as a mold. As we start cleaning we see the top layer, mold perfect, but the inner is now replaced with the brittle mix [like picking up a cast of dried mud, accurate, but very
brittle]. So any pressure on the "coin" and the whole shatters.
This is chemical displacement, one element replaces another, and the way fossils are formed.

Hope this makes sense. Try this, prss a coin into clay, fill it with mud and let it air dry. You should have a great look half coin, but
if you presson it - it shatters.
Oh, yeah, I do know how geodes are formed.

bruce

bruce61813 Jan 19, 2004 08:39 PM

A followup:
esible first coin, the Neaoplis hemi. has what should be termed "surface etching". The softer areas have been disolved away leaving the hardeer layers exposed. his could be due to a very high quality alloy, well mixed, and not exposed to a long term harsh environment. Oh - neat gorgon!!
The second coin [Litra] is unfortunately to common. I would expect two things. First a poor alloy, not well mixed, with some "streaks " of the pure metal [like a swirl icecream cone]. When the flan was struck, a minute fracture was formed, as the two metals may not have had the same cooling rate. Water and other chemicals used this as an entry, eating the softer layer from underneath. As with the silver, the surface was underminded, and eaten. So when it was cleaned - it broke away, leaving the rought harder layer.
I will post another example when I get a chance of the "copper boils". It is similar, I just don't have a scn handy.

Bruce

dougsmith Jan 20, 2004 05:15 AM

Surfaces
 
Years ago I was told by a collector whom I believe correct that what I called reticulation on that old page:
http://dougsmith.ancients.info/feac20.html
was a pretty certain sign that a coin is genuine (at least ancient). There is an important distinction between this condition and what I'd call porosity which is caused by corrosion and can be induced by chemicals. On my page I commented, " Note that the surface is reticulated with continuous high ridges separated by short (dark) recesses rather than scratched which would show continuous (dark) recesses. " I stand by this definition. Crystallization of the type I showed should take a few hundred years to develop (I do not claim to have a handle on this time line) and would be hard to simulate through artificial means. It only seems to occur on coins of a relatively high grade silver. Billon coins which break with similar ease show a completely different 'grain' pattern. There does seem to be a middle ground of alloy which does not develop either fault. This includes coins of both the US and Sterling standards so I do not expect to find what I called reticulation on a US or British coin from the 1700's. I would be interested in seeing photos showing that I am wrong here. The holed coin shown above is a great example and well photographed. I wonder how much a drop it would take to break a reticulated coin but I am not sufficiently interested to put that question to the test.

I might add that there are some relatively high silver coins that show no surface pattern but have broken from a little drop. I am not sure what is happening in those cases and hope someone can explain it. Was not there a dekadrachm of Akragas broken at a sale from a drop? I do not recall if it showed signs before or not.

dougsmith Jan 20, 2004 06:13 AM

[quote]Originally posted by wgsant
[b]

I think there is definitely a need for a visual glossary.
Wayne

[quote]

This forum could be an excellent place to begin a collection of photos and definitions for such a glossary. I, for one, would be happy to assist in shooting examples of significant terms. I have tried to do this on my pages in several places (grading and heads come to mind). What terms do others see as needing illustration? I can start shooting micro photos of surface situations that I find interesting but will not in every case know the 'correct' term for what I see.

Is anyone else of the opinion that there is worth in this idea?

esnible Jan 20, 2004 07:57 AM

Quote:

Is anyone else of the opinion that there is worth in this idea?
Many of the terms used to describe the surfaces of the coins are picked up by catalogers during their apprenticeship. When the terms are defined in books they are rarely pictured. I got most of my information from looking at Doug Smith's pages and auction catalogs. Doug chooses the worst coin he can find to best illustrate the term with a single photo. Auction catalogs are not printed with enough resolution to easily see the condition being described. It would be nice to have several pictures of each term, showing serious typical and slight manifestations, in both silver and bronze. Here are some terms which I only understand vaguely:

'pitting' http://www.cngcoins.com/coin.asp?ITEM_ID=15305
'porous' http://www.cngcoins.com/coin.asp?ITEM_ID=19520
'light porosity' http://www.cngcoins.com/coin.asp?ITEM_ID=19516
'evenly grainy' http://www.cngcoins.com/coin.asp?ITEM_ID=19711
'slightly grainy' http://www.cngcoins.com/coin.asp?ITEM_ID=20238
'granular surfaces' http://www.cngcoins.com/coin.asp?ITEM_ID=19713
'die rust' http://www.cngcoins.com/coin.asp?ITEM_ID=19677
'corrosion spot' http://www.cngcoins.com/coin.asp?ITEM_ID=20084
'casting bubbles' http://www.cngcoins.com/coin.asp?ITEM_ID=15728

(All these coins are genuine, although this is the counterfeits section.)

dougsmith Jan 20, 2004 09:19 AM

Should this be moved to another forum?
 
Please Ed, those are my best coins you are calling the worst available. :(
Many auction descriptors are a balance between the desire to be fair to the buyer and to the seller. There is no real hard standard where we pass from 'slightly grainy' to 'granular surfaces' any more than there is a firm agreement on the line between 'good fine' and 'about very fine'. Your vague understanding is more a matter of the subject being vague than it is of something you can correct.


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