CHA GES SUFFERED BY COINS I N THE COURSE OF TIME AND THE INFLUENCE OF THESE ON THE

RE ULTS OF DIFFERENT METHODS OF ANALYSIS

BY

J. CONDAMIN and M. PICON

REPRINTED FROM

METHODS OF CHEMICAL AND METALLURGI CAL

INVESTIGATION OF ANCIENT COINAGE Edited by E. T. HALL and D. M. METCALF

ROYAL NUMISMATIC SOCIETY SPECIAL PuBLICATION No. 8

1972

,.., .; ......

1

Changes Suffered by Coins in the Course of Time and the Influence of these on the Results of Different Methods of Analysis

}. CONDAMI N and M. PICON

(PLATES V-Xl)

THE composition of ancient coins has always interested numismatists and bas been determined by various techniques. Thus, except for the measurement of density which bas always been used, the analysis of ancient coins has gone through three successive stages, namely:

the first stage, using destructive chemical methods (wet-chemical analysis); the second, using non-destructive physical methods of analysis of the surface of the

coins (optical emission spectroscopy and X-ray fluorescence spectroscopy); the third, using non-destructive physical methods of analysis of the whole coin (and

more particularly sorne of the activation methods). We may assume that non-destructive methods of analysis of coins will gradually

replace all the others, and that we will thus avoid destroying ancient specimens. Unfortu­nately the use of activation methods does not solve all the problems, because ancient metallic objects have suffered important changes in the course of time. Consequently the chemical composition of these objects, at the present, is sometimes very different from their original composition; but the difference between the present and the original total composition is generally quite small, as the changes often occur only in the superficial part of the alloy.

We shall discuss the influence of these changes on the results, first, of the methods that give the total present composition and, secondly, on those of the methods that give only the present composition of the surface of the coin. We shall be concerned only with the titration of the main constituents.

In this study we have used the following material: (a) silver-copper coins: mainly Roman denarü from the second and third centuries

(about 100 specimens); (b) copper-tin coins and copper-tin-lead coins: mainly Greek and Hellenistic (about

zoo specimens); (c} copper-zinc coins and copper-tin-lead-zinc coins: mainly sestertii (about fifty

specimens). c 8764 E

":' --r. ..

so J. CONDAMIN AND M. PICON

PRESENT TOTAL COMPOSITION AND ORIGINAL TOTAL COMPOSITION

The difference between the present total composition and the original total composition results mainly from two groups of phenomena, nam ely (A) the oxidation of the alloy; (B) the formation of holes inside the alloy. We shall consider each of these in turn.

A. The oXl·dation of the alloy. As a rule the volume of an oxide is higher than the volume of the metal it cornes from. The following table gives indications for the oxides we are interested in:

Metal: Copper Tin Lead Zinc

Oxide Cu20 Sn o. PbO ZnO Volume of metal 1 1 1 1 Volume of oxide x·6s 1·32 J•27 1·56

As the volume of the coin does not vary, the increase of volume due to the formation of oxide is necessarily accompanied by the elimination of a part of the alloy towards the surface of the coin. This part, which has been eliminated, is to be found in the crusts of oxides and carbonates that are frequently seen on the surface of ancient coins before they are cleaned.

The elimination towards the surface of the coin of part of the alloy entails a change in the chernical composition in the following cases:

(a) when dealing with a monophase alloy one of the constituents of which is eliminated towards the outside more easily than the other;

(b) when dealing with a polyphase alloy lia ble to the above mechanism, or when dealing with a polyphase alloy of which one of the phases becomes oxidized more easily than another.

The above considerations also apply to various compounds other than oxides which, like the latter, have been formed in the course of time under the action of the surround­ings. Usually they are chlorides, and sometimes even carbonates and sulphates in the levels doser to the surface. A few examples, among those most frequently met with, are given in the following table, which also shows the volumes of those compounds that correspond with one unit volume of metal.

Metal: Silver Copper Lead

Chloride AgCl Cu Cl PbC11

Volume of metal 1 1

Volume of chloride z·so 3"36 z·s8

CHANGES SUFFERED BY COI NS IN THE COURSE OF TIME 51

B. The fomwtion of holes inside the alloy. Oxidation is generally accompanied by the formation of holes. These holes result in a variation of the chemical composition in the following cases:

(a) when dealing with a monophase alloy in which the formation of holes is accom­panied by the redeposit of one of the constituents as a metal or as compounds;

(b) when dealing with a polyphase alloy of which one of the phases is more liable to the formation of holes.

In fact, the phenomena of oxidation and formation of holes rarely appear separately. In particular, holes are never to be found without there being also sorne formation of oxides or other various compounds. It is, then, often difficult to dissociate the two phenomena.

Let us also point out that there are, in ancient coins, numerous types of corrosion that play a minor part in the variation of the total composition. There are, for example, the case of corrosions under stress, corrosions of slip-bands, and so on.

We shall now concern ourselves with the variation of the total chemical composition in the course of time in the following alloys: (1) silver-copper alloys; (z) tin-copper alloys; (3) alloys containing lead; (4) copper-zinc alloys.

1. Silver-copper alloys. It is these alloys that show the most important variation between the present total composition and the original total composition. This is due to severa! factors, among which we may mention the fact that we are dealing with a poly­phase alloy; that there is a great difference of affinity for oxygen between the two phases; and that there is a high coefficient of increase of volume for the copper oxide. Oxidations which are very extensive can thus sometimes be noticed (Pls. V-VI, 1- 5). The oxidized phase is the richest in copper and the oxide is, as a rule, Cu20.

These oxidations are responsible for a more or less important variation between the present total composition and the original total composition, which can be known from the interior of the coin which is unoxidized. Sorne measurements carried out on four Roman denarii of which only a small unoxidized inoer part was left gave the following results:

Present total composition: % silver

8x 64 59 ss (Wet-cbemical analysis)

Composition of the unoxidized innee part: % silver

71 48 49 45 (Optical emission spectro­scopy, condensed spark)

Percentages have been calculated from the sum of the metallic elements, that is, withôut taking into account the weight of oxygen which is in the oxidized part of the coins. The specimen whose total composition has a silver percentage of 64 is illustrated as Pl. V, x.

52 ]. CONDAMIN AND M. PICON

If we assume that all the copper initially present in the alloy is oxidized, we may easily calculate the expécted increase in the silver percentage. For instance, for the above specimen of which the silver percentage was initially 48, the figure would become 6o after the coin was oxidized. The percentage measured is slightly higher than the per­centage calculated: although the alloy has in fact not been complete! y oxidized, it reaches 64. This is due to the fact that there are holes in the place of former copper grains. In many cases the proportion of holes is much higher than in the specimen quoted above, and the increase of the silver percentage is much more important.

I nflue nce of corrosion on t1Je Jlercen tage of sih·e r i n an ciout coins

Allo y : Ag- Cn

Ass ump t ion: di snpJ>Ottrauce of two- t1Jird '! of t h e coppor in corro<le<l parts

1

!==== r--- :;;tJ ~ .. :;,~ corrod l;'d volume ..]_ _]_ tota l Yo lum c 6 2

Sih·cr p crcentuge of 47 47 thE' non corrodcd coin

Ai h·or pcrcentnge of 1111' ('OIToClNl <'OÏ Il

50 5 6

FIG. 1.

As far as we can judge from our own experience, there is no connection between the presence of oxides and holes inside the coin, and a bad condition of the surface. It would appear rather to the contrary, as most of the coins whose inner part was very oxidized bad a very good-looking surface. This is apparently due to the redeposit of silver, which we shall be dealing with below.

T? give an idea of the.~mportance of these phenomena we may say that out of a group of nmety Roman denaru, about 10 per cent showed an oxidized volume in excess of 8o per cent of the total volume of the coin, and had lost from one-third to one-half of the whole copper initially present. In numerous specimens the oxidation was Jess important.

The consequences of these phenomena of oxidation and corrosion are increased by the fact that in ancient coins the edge is the part that suffers the greatest changes. These altera~ions that are more important on the edge result first of aU from the moving of matenal that occurs more especially in that area at the time of minting. They play a prominent role in the difference between the present total composition and the initial total composition, as the relative volume of this area is very important. The foregoing observations will serve to show that in the case of wet-chemical analysis, when only a fraction of the coin is eut off, it is better not to take only the edge (see Fig. 1).

CHANGES SUFFERED BY COINS I N THE COURSE OF TIME 53

2. Copper-tin alloys. These alloys have suffered various changes according as they are monophase (phase a) or two-phase (phases a and S) alloys, and, in the fust case, according as they are homogeneous or as they have a dendritic structure. The coins made of a monophase alloy with an homogeneous structure are less affected while the dendritic structure is more lia ble to these changes; in this case the axis of these dendrites is either preserved or attacked (Pl. Vll, 6 and 7 ). The two-phase structure of the alloy makes the alteration easier than when dealing with a monophase alloy, and it is sometimes the eutectoid (a-S) that is altered, and sometimes the rest of the alloy (Pl. vm, 8 and 9). In copper-tin alloys the phenomena of oxidation are thus quite complicated. In the specimens we have studied these phenomena result in an increase of the tin percentage. It is very probably a general feature that seems to be connected with the low mobility of the compounds of tin; these compounds do not migra te, wh ile the compounds of copper migrate more easily toward the outside. However, more numerous studies of these cases would be required.

Let us take a theoretical example, qui te dose to th ose we have come a cross: an alloy with 88 per cent copper and 12 per cent tin, after half the copper and half the tin have been oxidized, would have a tin percentage of rs, if we assume that the whole of the oxidized tin does not migrate. Phenomena of this type affect only 4- 5 per cent of the specimens we have studied.

3· Alloys containing lead. Oxidation and corrosion are made easy by the fact that we are dealing with polyphase alloys in which lead is found in isolated grains. The variation of the total composition in the course of time generally cornes from the fact that the grains of lead are more easily attacked. These grains very often disappear, and their disappearance quite frequently results in important variations in the lead percentage. The other constituents of the alloy are far less affected (Figs. 2 and 3). Phenomena of this type affect 4- 5 per cent of the specimens we have studied. The percentage of tin and lead at various depths in the coins in the following illustrations are represented in the graph, Fig. 4· It should be noted that these altered specimens (and also the coins at Pl. VII, 6-7) come from Egypt, and that in a group of coins of this origin one may find a much higher percentage of altered coins.

4· Copper-zitzc alloys. These alloys are among the Jess altered in the interior of the coin. This is mainly due to the fact that the alloy is formed of a single phase that has been made homogeneous. This homogeneity cornes from the treatment undergone by the alloy in the course of manufacture of ancieht brass coins. We may theo consider that the alterations of these alloys are essentially superficial if we leave out, as we have already said, the corrosions under stress, which are particularly numerous in brass coins. These corrosions hardly affect the total composition.

There are, however, sorne cases where corrosion attacks the interior of the coin, but only a small volume of it. Such corrosion is often connected with a redeposit of copper (Pl. IX, 10 and n ). As they are not very extensive, these phenomena result in only a small alteration of the total composition. For example, let us assume that one-fifth of a copper-zinc alloy with 20 percent zinc has been attacked (which is already exceptional)

., ....

FIC. 2. Cross-section of a large Ptolemaic bronze made of a copper-tin-lead alloy, with 4 percent tin and 19 pcr cent lead. We may notice that the lcad grains have bccn particularly attacked and been more or less completely replaced by oxidation compounds, mainly PbCI1• Ont y the inner part of the coin has not been altered. The oxidized layer shows a zoned structure. In particular, one may notice, in the deepest part, an area where very numerous holes have been fonned at the expense of the lead grains and also of part of the alloy surrounding these grains. The presence of holes and the transformation of lead into chloride is liable for a great difference between the original total composition and the present total com­position. The percentages obtained in spectrography at various levels are those shown on the curves b of Fig. 4·

x 6·s, as polished.

FIG. 3· Cross-section of a bronze drachm (Antoninus Pius) made of a copper-tin-lcad alloy with 6 per cent tin and 21 percent lead. One can notice that the lead grains have been attacked very similarly to those of Pl. VII, 7, with an arca that has not been altered in the centre of the coin. T he variations of concentration obtained in spectro­graph)• at various levels are those shown on the curves a

of Fig. 4· x 6·s, as polished.

CHANGES SUFFERED BY COINS IN THE COURSE OF TIME 55

and that the zinc has been almost entirely eliminated whereas the copper has been redeposited. The percentage of zinc in the final state of the alloy will then be about 17 (percentage calculated from the sum of the metallic elements). This redeposit of copper also appears in corroded bronzes.

Percentage of tiu and lead in successiYç layers of an ci en t coin s Alloys : Cu·Sn·Pb

--- ---· a Pb

.-·-b ~

/'fb

"Pb\_. j\ ----·~ \ _,_/-·

\/'·~ ...._Sn ~-~ Sn ·-·-·-·__.....__ / ---, ----~

· "- ---- Sn b ---· .___. ---·------Sn

• a

0 200 ~00 600 800 1000 1200 HOO 1600 1800

D epth below the surface of the coin (microns)

FIG.+

Summary. In short, silver-copper alloys are those which are most Hable to alteration of their total composition in the course of time. Next come alloys that are rich in lead, and bronzes; and fin ally brasses.

Furthermore, when dealing with identical compositions, the coins for which the blanks were cast are more easily oxidized in their interior than those of which the blanks have been hammered and tempered, because the dendritic structure resulting from the casting makes the processes of oxidation easier.

We must keep in mind the foregoing remarks on oxidation and corrosion when studying results from ali methods that provide the present total composition of ancient coins-­destructive chemical analysis (wet analysis) and analysis by neutron activation. From the latter methods we must leave on one side the streak method, which gives the composition only of the surface and raises further problems, that are discussed below.

--., .....

s6 J. CONDAMIN AND M. PICON

PRESENT TOTAL COMPOSITION AND SUPERFICIAL COMPOSITION

Sorne non-destructive methods such as optical emission spectroscopy, X-ray fluores­cence, and the streak method of neutron activation do not indicate the total composition but the superficial composition, which is almost always different from the present total composition, even in the case when this latter is not perceptibly different from the initial

Pe1·ccnta~e of ti n , lead , zinc in successive layers of coins

Zn .--­. --.~

-··

/--._Pb

1 20

:l Alloy: Cu-Zn ; · ~ 15 15 15 ......

;.o Alloy:Cn-Sn-Pb ~ ~~ ~ .\ AUoy:Cu:Sn IWAl toy:Cu-Sn-I> ~1 ~ ~~l 10

~ ._ Sn\

.-'"'"-.._Sn 5~~~~---r--~5~--~--r-~----~5~--r-~--~--J

0 100 200 300 0 100 200 300 0 100 200 300

Depth b elow the surface of t h e coin (microns)

FIG. 5·

composition. Using optical emission spectroscopy wc can reach only a thin layer-3 to 4 microns under our experimental conditions (condensed spark, capacity zx xo-9 F .; secondary voltage, Is,ooo V). Using X-ray fluorescence under 45° (the conditions of our measurements), the zone that provides so percent of the fluorescence radiation is from 6 JJ. thick for PbLy to 40 JJ. for SnKœ. (Intermediate values occur in the other radiations we have used: CuK/3, ZnKœ, AgKœ.) For numerous ancient coins the depths reached by these two methods seem less than the depth of the altered arca in the vicinity of the surface. Fig. 5 shows a few examples which are typical of the percentages of tin, lead, and zinc at severa! levels starting from the surface. If the depth of the altered zone is small in comparison with the thickness of the coin-which is generally the case-the . variations do not entai! important differences between the present composition and the composition of the central part of the coin. But the results obtained by surface methods of analysis greatly depend upon the more or Jess important alteration of the surface.

CHANGES SUFFERED BY COINS IN THE COURSE OF TIME 57

One must also take into account the fact that the emission of metallic atoms that are part of compounds is often different fr.om that of the same atoms whert 'they are part of the metal. It also depends on the characteristics of the method of analysis used (X-ray fluorescence, optical emission, and so on). As a result, the percentages at the various levels of the altered parts (Figs. 4 and 5) and also almost al1 the superficial concentrations must be regarded as apparent concentrations depending on the conditions of analysis.

An experimental study has been made of the changes shown by the surfaces of alloys when they are analysed, on a selection of coins of various origins (including silver-copper alloys, bronzes, and brasses). For each group of coins, and for the two methods of analysis (optical emission spectroscopy and X-ray fluorescence) we have prepared graphs repre­senting the superficial percentage of the various elements of the alloys, plotted against the percentages of the same elements in the central part of the coin. Thus there is a pair of graphs for each of the types of alloy, or constituent element, which are discussed in turn below .

Silver-copper coins. T hese coins have been cleaned so as to obtain a metallic surface, as is the practice among numismatists. Figs. 6 and 7 show a systematic enrichment of silver in the superficial layer (let us point out that wet-chemical analysis has provided the total composition and not the inner composition, but we may consider these com­positions as identical for practical purposes because very oxidized coins have been excluded from the diagrams). Metallographical examination of these coins shows that superficial enrichment in silver does not depend on an ancient blanching process. As a rule, it results from two main causes: the oxidation of the grains of copper, and the relief of the silver grains on the surface. This relief, due to the modern cleaning of the coin, results, when using X-ray fluorescence, in a superficial enrichment measured in silver. This enrichment is ali the greater as the incidence of the radiation is more grazing . Another cause of the superficial enrichment in silver is the redeposit of silver which replaces more or less completely the grains of copper oxide in the vicinity of the surface. This phenomenon, still incompletely understood, affects the composition only of speci­mens that are strongly oxidized (Pls. X- XI, 12-15).

Bronze and brass coins. The condition of the surface of these coins showed ali the possible cases, ranging from carefully cleaned coins with a shiny surface, to coins whose patina had been fully preserved. In spite of these various aspects, sorne permanent features were noticed. Tin, lead, and zinc behave differently. This seems to be connected less with the affinity of each of these rn etals for oxygen, than with the possibility for these same metals to give, through oxidation and corrosion, sorne compounds that are easily eliminated towards the surroundings of the alloy.

For tin (sec Figs. 8 and 9) this elimination is difficult. The oxidation products that are on the surface stay there, whatever the mechanism of their formation may be. T his latter may either be a superficial attack on the alloy by the surroundings, with elimination of the copper compounds, or an oxidation of tin supplied by the diffusion of this element from the interior of the coin. In both cases the tin compounds gather in the vicinity of the surface and result in a local increase of the tin percentage. For a given coin, this increase is not al ways the same by optical spectrography and X-ray fluorescence, although the general distribution of the superficial composition with regard to the total composition

,..,

sB ]. CONDAMIN AND M. PICON

Surface percentage of sHver compared to interior · percentage in aneient coips

Alloys ; Ag· C.u 100r-------------------~--~----~--------------~------~

• . . . .

.. •

.. . . 90 . .

~ 0

<V 'QO

~7 :::= <V Q J-4 <V ~

2$6 ~ ~ r:s

00

•

• •

•

• • • • •

• • • -.. • ..

•

• • •

50 60 70 8C 90 100 Interi01· 1>ercentage of silvex

1\Iethods of analysis : .sudace : optical emission spectro­·Scopy (condensed sparl{)

.interior: wet chemical analysi s Fic. 6.

is nearly the same by the two techniques of analysis, as shown by the general corre­spondence between the diagrams. These particular differences are often due to the fact that the portion that was analysed was not exactly the same by the two techniques. It is

CHANGES SUFFERED BY COINS IN T H E COURSE OF TIME 59

Surface percentage of sHver compared to interior percentage in .ancieut ·c.oins

Allo ys: Ag_-- Cu 100r---~--~~------------------------------------------~

9

~ Q,)

> -·~ r:JJ 6 ~ 0

Q,) 'Q.() ce .... 1:17 Q) Q J-4 Q)

~

<V

~6 ~ ~ r:s

00

•

•

• •

• •• ••

• • • • • 0

• • •

• 0

•

•

50

. . .. • . .· . •

0

0

60 70 ~0 90

Interior p ercentage of silver 100

Methods of anaJysis: surface : x- ray f i uorescence interim:: wet cltemical analysis

FIG. 7·

also due to the fact that when analysed the oxidation compounds behave differently from the metal, and their reactions vary according to the nature of the compound and according to the technique of analysis.

:'!-; .. ~ ....

6o J. CO N DAM I N AND M. PI CON

Surf~ce pe.rcentage of tin compar~d to interior percentage in an cien t c.oins

All o.ys : "' Cu- Sn • Cu-Sn-Pb o Cu-Sn-Zn

21'h------o- Cu-Sn -P b -Zn--------:t

0 0

•

0

"' •

"' "'

"'

"'

10 15 Interior percentage of tin

20

J\Iethod of analys is: optical emission spectroscopy (conden sed spark)

FIC. 8.

For zinc (see Figs. xo and n ) the essential difference in comparison with the preceding diagrams is due to the fact that the oxidation products that are formed on the surface are

CHANGES SUFFERED BY CO INS I N THE COURSE OF TIME

Surface ]Jercen~age of tin compar~d to interior p e r centage in ancient coin s

Alloys : "' Cu-Sn • Cu- Sn-Pb o Cu-Sn-Zn

20------o Cu-Sn-Pb-Zn------~

0

0 • A :A

• ~ ·~

~œ~. ~ ~v ~ .

A

"' "' A

5 10 15

Interio:t percentage of tin 20

Method of analysi s : x-ray fluorescence FIC. 9·

6x

easily scattered into the surroundings. As a consequence, the superficial zinc percentage tends to be lower than the interior percentage.

j • ....

62 J. CONDAMIN AND M. PICO N

S u rfa.ce pe.rcen tage of :r.i ne compar e <l to i n terior

J)e r centage jn a ncient coi n s 30r---------------------------------------------------------~

5

Alloys: • {;n- Zn

a Cn -Sn-Zn

o Cn- Su-Pb -Z 11

• • •

•

• D • a D

0 D • • • • 0 • • •

D D

0 . D

0 0

0

0

5 1 15 20 25

lnter ior _pe rcentage of· zi ne

Meth o(l of a nuJys.is : optical e mission ::;pectroscop y ( condcused sparl\)

FJO. J O.

30

Asto lead (see Figs. 12 and 13), if we consider the scattering of its oxidation products in the surroundings, it should apparently rank between tin and zinc. But in its case, the phenomena are rather complicated, because in optical spectrography the emission from the oxidation products of lead and in particular from lead carbonate is weak compared to that from copper, and also because in X-ray fluorescence the absorption of the radiation from lead by the copper matrix is very important. These phenomena, which play a

CHANGES SUFFERED BY COINS I N THE COURSE OF TIME

Surface Jl ercentage of zinc compal'ed to int~rior

p er ce11tage ·jn an ci ent. coin~

30r-----------------------------------------------------~

0 ~ .,... N

'-' 0

Q.) 'CD d +"

5

0

A lloy.

0 0 0

5

• Cu - Zn a Cu- Sn -Zn

o Cu- Rn-Pb -Zn

• • •

• •

• 0

10 15 20 25 I nter ior p erceu tage of zin c

Metho.d of anal~· s i s: x -1·n.y fluoreAceuce FIC. Il.

30

secondary part in the analysis of tin and zinc, cannot be neglected here, where they are very important. ln X-ray fluorescence, for example, the absorption due to copper is much lower in the patina, which is generally less rich in copper than the alloy. As a result, the apparent superficial percentage is higher than the inner percentage, even if most of the lead compounds have been eliminated and even though lead is generally very corroded, as the alloy is a polyphase alloy. By contrast, in optical spectroscopy these residual

...

J. CONDAMIN AND M. PICON

urfnce p er ceu tago of l end compared to i11 tedor perceJJ t aae in anc ie n t· coins

JS·r-------------------------------------------------------------7 Alloys:

3

..... 0

~

'OIJ 20 « .... Q

~ <.)

"'" c. Q, 15

0 <:..

r.r. 0

0 oo

s

• Ctt· Sn-Pb

o Cu·• n-Pb- Zn

• •

• •

• , • •

0 • •

0 • .. • o• 0 0

•

• 10 15 20 25 Interio1· pc rcon tage or lcad

30

Mothod of ana lys is: optical emiss ion spoetroscopy (con d c.uscd spM·k)

FIC. !2.

35

compounds will appear very little during the analysis, as they are not numerous and as their emission is weak compared with that of copper. We shall therefore generally observe an apparent superficial percentage lower than the inner percentage.

Let us finally point out that the different reactions of tin, zinc, and lead towards the surroundings are more or less the same as the reactions of these three constituents towards the cleaning undergone by coins after their discovery. These actions seem to account for most of the aberrant cases that are to be noticed in the previous diagrams.

""' 0

CHA N GES SUFFERED BY COINS IN THE COURSE OF TIME

S ur.face p e r cent age of l ead .comJ)ar ed to in t.edor p ercen ·

-tage in ~tnci ent coins

3Sr-----------------------------------------------------------~ .\llo ys : • û n· f:i n -Pb

o Cu-Sn- Pb· Zn

• 30 •

•

•

~ 20 'al ~

:::1 ~ C) .... ~ ~ 15

0

0

•

0 0

•

•

10 15 20 25 luterior p ercentage. of l ead

30 35

Mothod of ann,lysis x-ray fluorescence F IG. 13.

6s

Summary. It seems obvious that, when using methods of superficial analysis, only specimens that have been very carefully scoured can give significant results. But we must point out that the scouring must be made to a depth of one- or two-tenths of a millimetre if we are to obtain (for homogeneous alloys) a percentage close to the interior percentage. Concerning all the alloys which do not form an homogeneous solid solution, it will always be difficult to be as positive, when there is no metallographie investigation of the area to be analysed; the metallic aspect of this area is not a sufficient criterion. If it should be

C 87M F

.., .....

66 J. CONDAMIN AND M. PICON

absolutely necessary to undertake the analysis without the coin having been deeply scoured, the risks of error could always be estimated from the previous results. There may, however, be cases where the exceptional preservation of the specimens makes it possible to use surface methods of analysis without strong scouring; but it is obvious that a previous study is necessary to make sure that the inner composition corresponds with the superficial composition. For instance, we are now analysing about I,ooo sestertii whose excellent condition makes it possible to use X-ray fluorescence. These sestertü come from a treasure found in the Garonne, near Bordeaux.

In the case of superficial analysis we have, deliberately, limited our study to the incidence on the analytical results of the changes suffered by coins in the course of time. Other factors, however, must be taken into account, in particular the shape of the objects (relief of the coins) and the surface variations of composition which are due to the processes of manufacture of the specimens. As an example we may quote the blanching of coins bef ore striking, the oxidation due to hot-working, the segregation of lead towards the inner part of the cast metal (which is very frequent), the opposite segregation of tin (which is less usual), the surface dezincification of hot-worked brasses, and so on. AU these factors also contribute to limit the field of application of the methods of superficial analysis.

To bring this study to an end, we may then conclude that the changes suffered by coins in the course of time have repercussions both on the results provided by methods of total analysis and on the results provided by methods of superficial analysis. The former give a present total composition which is sometimes different from the original composition; the latter give the composition of the superficiallayer, which is generally very different both from the present total composition and from the original total composition. 1

1 Acknowledgemmts. This work bas been com­pleted th.anks to the help given us by a number of nunûsmatists, in particular Mm. J.-B. Colbert de

Beaulieu , R. Etienne, J. Guey, ]. L afaurie, G. le Rider, D. M. Metcalf, and H . Seyrig.

1. Cross-section of a Roman denarius (Septimius Severus) made of a silver-copper alloy whose silver ~n~ent will ?rigi~aUy have been 48% . This is the percentage of the central part that 'has not been ~xJdtzed and IS w~1te on the plate. ln the rest of the coin the copper grains have becn replaced by grams of cuprous OXJde, Cu~ O. after a fraction of the copper initial! y present has been eliminated the ,·olume of oxide being bigher than that of the metal it cornes from. A number of ho les have al~ ~ppeared at the expense. of the ~pper grains of the oxidized area. These various phenomena result m a de~rease of copper .m the com, and consequently in an increase of silver in relation to copper. Th us, m the course of tune, the total percentage of silver has changed from 48 to 64. Magnification

x 70, polished, but unetched.

2. Cross-section of a Gallic coin (Coriosolites) made of a silver-copper alloy with a low percentage of silver. The copper has been almost entirely oxidized; only the inner portion, which is white on the plate, has not been attacked yet. The cuprous oxide, Cu!O, which is light grey on the plate is the main oxidarion constituent. It is accompanied by numerous holes (black on the plate) and, in the central part, near the non-oxidized part, by a little cuprous chloride, Cu Cl, and silver chloride, AgCI. Inside the oxidized area. silver is to be found in white filaments. Magnification x 27, polished,

but unetched. "'.: - ., .. •

PLATE V

3 (left). Cross-section of another Gallic coin (Coriosilites) made of a silver-copper nlloy with a low percentage of silver. The copper has bcen entirely changed into oxide and chloridc, or has dis­appeared and been replaced by a large number of botes in the central part of the coin and in the

vicinity of the surface. Magnification x 27, polished, but unetched.

4 (right). Cross-section of a Tetradrachm (Claudius) made of a silver-copper alloy, in which only the central part has not been oxidized. Magnification x 6·s, polished, but unetched.

5· DctajJ of the above cross-section showing, next to the inner non-oxidized part, a thin layer of cuprous chloride, CuCI {clark grey on the plate), topped by a thjck layer of cuprous oxide, Cu,O (light grey). lnside these layers the silver grains bave generally kept the shape they had before the copper was oxidized. Above there i!. a layer of cuprous oxide without any silver grains. lt bas been formed out of the fraction of copper that has been eliminated outside the coin, as a result of the increase of volume coming from the transformation of copper into oxide. The more or Jess complete disappearance of this layer results in an increase of silver in relati~n to copper. Magnification X s8,

polished, but unetched. ,..,

PLAT E VI

6. Cross-section of a large Ptolemaic bronze made of a copper-tin alloy with 12% tin. One can observe that in the dendritic structure the areas rich in tin have been particularly attacked. The axis of the dendrites, containing little tin, has th us been preserved. ln that case there is an increase in tin if the copper is eliminated more easily than the tin toward the outside, this elimination being, as always, a consequence of the difference of volume that exists between the oxides and the metal they come

from. Magnification x 6·3, polisbed, but unetched.

. ,

~ •• 1 t .t

7· Cross-section of a large Ptolemaic bronze made of a copper-tin aiJoy with 9 % tin. One may notice that in the dendritic structure the areas that are poorer in tin have been particularly attacked. The axis of the dendrites are locally replaced by holes (black on the plate). As they are poor in tin, their disappearance results of course in an increase of the percentage of tin proportion ally to copper, but these phenomena are rarely important. Metallographie etching sets off axis of the dendrites on

the lower part of the plate. Magni6cation x 70.

PLATE VII

PLATE VIII

.... • • " • . • ·.' •• . . ,. • . . . • • • - • , ~ ... ' . .,. ., , .. . •

~ • • • ; • • • • . ~-: \ • • . t •

' .

1 • • • ' ,.

' •• •• • • 1 • ' .. •• • ' . • • . • • • • • : •. 1 • • •• • . . . ' • .-.

• • • .. • ... 8. Cross-section of a Gallic coin made of a copper-tin-lead alloy, very rich in tin. The oxidation of the alloy is limited to phase li, so that not only phase a but a Iso the grains of lead, which arc generally very sensitive to the phenomena of oxidation, are left almost unattacked. On the plate the oxidation compounds of phase 8 are forming stripes, that are situated between the nearly parallel axis of the

dendrites. Magnification x 140, polished, but unetched.

9. Cross-section of a Gallic coin made of a copper-tin alloy very rich in tin. The inner part of the coin (white on the plate) has not been altered. In the rest of the coin it is phase a of the alloy that has been particularly attacked, wh ile the oxidation has not reached phase 8 which is to be noticed on the plate, standing out in white on the oxidation compounds of phase a. 1\lagnification X 115,

polished, but unetched.

---., . ....

to. Cross-section of a Roman sestertius (Caligula) made of a copper-zinc alloy with 15% zinc. The corrosion of the alloy goes with a redeposit of copper which is to be noticed at the top of the plate. These areas have replaced holes, sorne parts of which are still left and can be seen in black on the plate. The elimination of zinc and the redeposit of copper result of course in a decrease of the per­centage of zinc in relation to copper, but these phenomena are rarely important. Magnification x s8,

polished, but unetched.

11. Same cross-section as previously, but after the coin has been etched by a metallographie reagent. This etching shows the utter difference of structure between the grains of copper that have been redeposited and the rest of the alloy. lt is inconsistent, therefore, with the interpretation according to which one should see, in the grains of copper, residues coming from the manufacture of the

orichalcum. ·

- ... r. ..

PLATE IX

12. Cross-section of a Roman denarius (Geta) made of a copper-silver alloy with 53% silver. One can observe a redeposit of silver, together with a deep oxidation of the copper grains in the alloy. Here the redeposit has developed in the vicinity of the surface, as usual. Sometimes these phenomena result in appearances that, at first sight, migbt be mistaken for those of plated coins that have been

deeply altered. Magnificat ion X 350, polished, but unetched .

. ..

IJ. Detail of the cross-section of Pl. VI , 4 showing the same layers as Pl. VI, s. Jnside these layers the silver grains have generally kept the shape they had before the copper was oxidized. Yet one can see a slight change of the silver that results in a redeposit of this constituent in the vicinity of the original surface of the coin. Redeposit is to be found in the white area just below the layer of cuprous

oxide without any silver grains. Magnification x s8, polished, but unetched.

~ .....

PLATE X

14. Another detail of the cross-section of Pl. VI , 4 showing, in the vicinity of the edge, an important change of silver in the oxidation layer. Sorne redeposits bave formed and they have a very porticular zoned structure. There is an altemation of silver layers and levels of oxides where ho les, more or less lined up, ean be noticed. Redeposits of si! ver of this type are much ra rer th an those thnt occur very

close to the surface of oxidized coins. Magnification x sS, polished, but unetched.

15. Same cross-section as previously. General view. Magnification x 29, polished, but unetched.

,., -·

PLAT E X I

AJA A .VSM.V ANSNS BMC BNJ BSFN DOC IIBN IMC JESHO JMP JNG JNSI JRS MBNG NC NCirc NNA NNM NNUM NRom NS NZ RBN RIC RIN RN SM SNR WN ZfN

AB BREVIATlONS

American Journal of Archaeology American Numismatic Society l\1useum Notes American Numismatic Society Numismatic Studies British Museum Catalogue British Numismatic Journal Bulletin de la société française de numismatique Dumbarton Oaks Catalogue Hamburger Beitroge ::ur Numismatik Indian Museum Catalogue Journal of Economie and Social History of the Orient Jaarboek voor Muni- tm Pemzingkunde Jahrbuch for Numismatik und Geldgeschicltte Journal of the Numismatic Society of bzdia Journal of Roman Studies l\1itteilungen der Bayerischen mtmismatischw Gesellschaft Numismatic Chronicle Numismatic Circular Nordisk Numismatic Arsskrift Numismatic Notes aud Monographs Nordisk Numismatisk Unions Medlemsblad Numismatique Romaine. Essais, recherches el documents Numismaticky Sbornlk N umismatische Zeitscltrift Revue belge de numismatique Mattingly, Sydenham et al., Roman Imperial Coinage Riflista 1 taliana di Numismatica Revue numismatique Schrceizer Miinzbliitter Schtceizer tzumismatische Rundschau Wiadomofci Numizmatyczne Zeitschrift für Numimzatik

UNIVERSITY PRESS, OXFORD, RNO LA ND