8 Neutron Activation Analysis of Gold Impurity Levels in Silver Coins and Art Objects ADON A. GORDUS and JEANNE P. GORDUS The University of Michigan, Ann Arbor, Mich. 48104 The levels of gold impurity in most silver coins minted more than 100-200 years ago appear to be an indication of the levels occurring in the original silver sources. Thus, it is often possible to group coins according to their gold:silver content ratios and to suggest possible geographic origins of the silver. During the past six years over 8000 coins and metallic art objects were analyzed using samples in the form of metal streaks on etched quartz tubing. Summarized here are analytical data for Sasanian (Persia: 224-650 A.D.) coins and silver art objects as well as Umayyad (Persia: 661-750 A.D.) and Byzantine coins. Data are also presented on the identification of modern forgeries of ancient and medieval coins and Sasanian art objects, the use of Mexican and South American (Potosí) silver in Spanish and other European coinage and its relationship to the price revolution of sixteenth-century Europe, and the questionable honesty in the minting of Zapatista Revolutionary Mexican coinage of 1914-1915. ' t r a d i t i o n a l l y , a r c h a e o l o g i c a l c h e m i s t r y has b e e n r e s t r i c t e d almost e n t i r e l y to t h e s t u d y of p r o b l e m s for w h i c h the o n l y r e m a i n i n g e v i d e n c e w a s the m a t e r i a l to be a n a l y z e d .
F o r e x a m p l e , earlier studies i n our
l a b o r a t o r y w h i c h p r o v i d e d results t h a t a l l o w e d i d e n t i f i c a t i o n of the geo l o g i c a l sources of H o p e w e l l o b s i d i a n artifacts ( I ) called classical archaeological
chemistry.
are w h a t c o u l d
M u s t we,
however,
be
restrict
a r c h a e o l o g i c a l c h e m i s t r y to p e r i o d s a n d areas w h e r e artifacts are the chief sources of i n f o r m a t i o n a n d a v o i d p r o b l e m s of i m p o r t a n c e i n m o r e recent times s i m p l y b e c a u s e w r i t t e n records exist?
C l e a r l y , the significance of
t h e p r o b l e m to be s t u d i e d s h o u l d b e the c r i t e r i o n , a n d t w o of the three studies r e p o r t e d here i n d i c a t e a close a n d p r o d u c t i v e i n t e r a c t i o n b e t w e e n 124
8.
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GORDUS
in
125
Silver
a n a l y t i c a l results a n d c o n v e n t i o n a l w r i t t e n h i s t o r i c a l sources.
In the early
days of the a l l i a n c e b e t w e e n c h e m i s t r y a n d studies of t h e past it w a s n a t u r a l for a r c h a e o l o g i c a l
chemists
to select p r o b l e m s
p r o d u c t i o n of a n y n e w e v i d e n c e w a s i m p o r t a n t .
for w h i c h
the
H o w e v e r , n o w , as the
scientific t e c h n i q u e s b e i n g u s e d h a v e b e c o m e m o r e precise, refined, a n d s o p h i s t i c a t e d , the a t t e n t i o n of t h e investigator s h o u l d t u r n t o w a r d p r o b lems w h i c h are subtle, m u c h d e b a t e d ,
a n d sufficiently significant to
justify the i n t r i c a t e w o r k i n v o l v e d . L e t us consider coins as o u r sources i n t h e same w a y that historians a p p r o a c h t h e i r sources.
C o i n s are of l i t t l e use unless t h e y are c o r r e c t l y
c a t a l o g e d . A great c a t a l o g , s u c h as some of t h e B r i t i s h M u s e u m catalogs, is, i n t h e historian's v i e w , e q u i v a l e n t to a w e l l e d i t e d text. A p o o r c a t a l o g , l i k e a p o o r l y e d i t e d text, is often useless.
A r c h a e o l o g i c a l chemists c a n
p r o v i d e analyses w h i c h a i d i n c a t a l o g i n g coins, thus m a k i n g these m a t e rials v i a b l e sources of i n f o r m a t i o n . V e r y often, results of c o i n analysis correct false impressions e a s i l y obtained from historical a n d economic documents.
E v e n a l l o w i n g for
w e a r , coins s e l d o m meet the l e g a l s t a n d a r d of w e i g h t a n d silver content r e q u i r e d b y the l a w at the t i m e t h e y w e r e m i n t e d .
N e v e r t h e l e s s , i t is
s t i l l s u r p r i s i n g to find that coins f r o m t w o different cities ( L u c c a a n d P i s a i n the e l e v e n t h a n d t w e l f t h c e n t u r i e s ) differ c o n s i d e r a b l y i n w e i g h t a n d silver content i n a p e r i o d w h e n b a n k e r s h a d access to a s s a y i n g techniques. It is e v e n m o r e s u r p r i s i n g to note the difference b e t w e e n t h e i r r e a l v a l u e , as r e v e a l e d b y analysis, a n d t h e i r p e r c e i v e d v a l u e as r e v e a l e d b y
docu
ments of the p e r i o d . A l s o , as o u r report o n t h e P o t o s i silver w i l l s h o w , a great e c o n o m i c theorist of s i x t e e n t h - c e n t u r y F r a n c e , w h o s e hypotheses are i n f l u e n t i a l even t o d a y i n some e c o n o m i c circles, b a s e d his theories o n w h a t he p e r c e i v e d to b e the c a s e — a n d w h a t w e find to be, at best, a h i g h l y q u e s t i o n a b l e f o u n d a t i o n f r o m w h i c h to p r o c e e d . It m a y seem that the r e l a t i o n s h i p b e t w e e n a l i t h i u m - d r i f t e d g e r m a n i u m g a m m a - r a y detector
and
societal attitudes t o w a r d m o n e y a n d r e l i g i o n i n s i x t e e n t h - c e n t u r y S p a i n is h a r d to establish, b u t the strength of a r c h a e o l o g i c a l c h e m i s t r y has always been
the p r o d u c t i v e
c o u p l i n g of
incongruous
techniques
and
concepts. Experimental B e c a u s e of the n a t u r e of the samples i n v o l v e d i n this s t u d y , s p e c i a l , p r e f e r a b l y n o n d e s t r u c t i v e a n a l y t i c a l procedures m u s t be used. I n a d d i t i o n most m u s e u m s w i l l not p e r m i t , or at least find i t difficult to a r r a n g e for, the transfer of a c o i n o r art object to a n o u t s i d e l a b o r a t o r y . F u r t h e r , the types of d a t a to b e a c q u i r e d w i l l g e n e r a l l y b e significant o n l y i f a statistically large n u m b e r of samples is a n a l y z e d . T h e s e p r o b l e m s are a l l q u i t e different f r o m those n o r m a l l y e n c o u n t e r e d i n a n a l y t i c a l c h e m istry a n d thus restrict i n some cases the a c c u r a c y a n d p r e c i s i o n of the d a t a .
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T h e methods used must, ideally, permit sampling i n a museum, p r e f e r a b l y w i t h o u t m o v i n g the objects f r o m the d i s p l a y area. T h e a n a l y t i c a l m e t h o d s s h o u l d not b e p a r t i c u l a r l y t i m e - c o n s u m i n g or expensive because of the v o l u m e of d a t a g e n e r a l l y r e q u i r e d for v a l i d c o n c l u s i o n s . T h e s e f a i r l y stringent r e q u i r e m e n t s c a n n o t be m e t i n a l l cases, y e t w e h a v e a t t e m p t e d to use t h e m as a g u i d e i n o u r studies. Analysis Methods. U n t i l a f e w years ago p r o b a b l y n o m o r e t h a n 2000 analyses of a n c i e n t a n d m e d i e v a l coins h a d ever b e e n p e r f o r m e d — a n d t h e n o n l y o n m o r e c o m m o n coins. T h e reason was s i m p l y t h a t analysis of these artifacts r e q u i r e d c h e m i c a l m e t h o d s w h i c h d e s t r o y e d at least a p o r t i o n of the c o i n . E v e n the less h a r m f u l m e t h o d , s p a r k spec troscopy, is n o t p e r m i t t e d b y the m a j o r m u s e u m s since i t leaves a v i s i b l e b u r n m a r k o n the e d g e of the c o i n a n d r e q u i r e s t e m p o r a r y transfer of the c o i n f r o m the m u s e u m to a l a b o r a t o r y .
Figure 1. A small amount of non-corroded metal alloy from the edge of the coin is obtained as metal streaks on the roughened quartz tubing Specific g r a v i t y c a n be u s e d to a p p r o x i m a t e t h e content of g o l d coins b u t o n l y i f a b i n a r y m i x t u r e is a s s u m e d ( u s u a l l y A u + A g ) . D e n s i t y m e t h o d s for s i l v e r objects are not v e r y r e l i a b l e , n o t o n l y because of the necessity f o r a s s u m i n g a b i n a r y m i x t u r e , b u t b e c a u s e the d e n s i t y of c o p p e r (8.94 g / m l ) is r e l a t i v e l y close to that of s i l v e r (10.5 g / m l ) . P o s s i b l e a i r b u b b l e s i n the alloys ( b o t h g o l d a n d s i l v e r objects) f u r t h e r l i m i t the r e l i a b i l i t y . F i n a l l y , the d e n s i t y m e t h o d p r o v i d e s no i n f o r m a t i o n o n m i n o r or t r a c e - l e v e l constituents. I n those f e w cases w h e r e a c o i n or art object c a n b e b r o u g h t to a laboratory, certain nondestructive analytical methods can be used. Such methods i n c l u d e x - r a y fluorescence s p e c t r o m e t r y a n d w h o l e - o b j e c t n e u t r o n a c t i v a t i o n analysis. W h o l e - c o i n n e u t r o n a c t i v a t i o n i r r a d i a t i o n i n a r e a c t o r is not feasible for large-scale studies even t h o u g h a f e w E u r o p e a n w o r k e r s h a v e u s e d this m e t h o d to a n a l y z e a b o u t 1000 coins. T h e p r o b l e m arises f r o m the p r o d u c t i o n of detectable amounts of a 255-day h a l f - l i f e isotope of s i l v e r ( A g ) . A l t h o u g h a s i n g l e c o i n i r r a d i a t e d i n this w a y w o u l d possess o n l y a s m a l l a m o u n t of r e s i d u a l r a d i o a c t i v i t y , a m u s e u m h a v i n g m o r e t h a n 1000 i r r a d i a t e d s i l v e r coins w o u l d , l e g a l l y , b e r e q u i r e d to o b t a i n a n A t o m i c E n e r g y C o m m i s s i o n radioisotope-storage license. 1 1 0 m
8.
GORDUS
AND
GORDUS
Gold Impurity
127
in Silver
Irradiation w i t h charged particles accelerated i n a cyclotron, w h i l e e l i m i n a t i n g t h e p r o b l e m of l o n g - l i v e d s i l v e r isotopes c a u s e d b y the different n u c l e a r reactions i n v o l v e d , is not f e a s i b l e for a n y large-scale s t u d y b e cause of the cost i n v o l v e d . U n l i k e a n u c l e a r reactor, w h e r e n u m e r o u s different samples c a n b e i r r a d i a t e d s i m u l t a n e o u s l y , o n l y a single s a m p l e can be irradiated using a cyclotron beam. Neutron Howitzer Irradiation. W h o l e - c o i n i r r a d i a t i o n , h o w e v e r , has the a d v a n t a g e that the i n t e r n a l p o r t i o n s of the c o i n are i r r a d i a t e d a n d a c t i v a t e d , thus p e r m i t t i n g n o n d e s t r u c t i v e analysis of the entire c o i n . O n e m e t h o d w e h a v e d e v i s e d is b a s e d o n s u c h i r r a d i a t i o n b u t i n a n e u t r o n b e a m that is a b o u t 100,000,000th as intense as that i n a reactor. O n l y the s i l v e r i n the c o i n is a c t i v a t e d sufficiently for d e t e c t i o n . N o n e of the 255-day h a l f - l i f e isotope is d e t e c t e d , a n d t h e analysis is b a s e d o n s i l v e r isotopes w i t h h a l f - l i v e s of 24 sec ( A g ) a n d 2.4 m i n ( A g ) . T h i s s i l v e r a c t i v i t y dissipates i n 10-15 m i n , a n d the c o i n is u n h a r m e d . W e h a v e a n a l y z e d over 4000 coins b y this m e t h o d w h i c h is d e s c r i b e d i n Refs. 2 , 3 , 4. Streak Method of Analysis. O t h e r m e t h o d s of analysis, besides d e s t r u c t i v e c h e m i c a l analysis, r e q u i r e s a m p l i n g b e n e a t h the c o r r o s i o n l e v e l to o b t a i n a s a m p l e representative of the o r i g i n a l c o i n . W e h a v e u s e d a streak m e t h o d of s a m p l i n g ( F i g u r e 1 ) b a s e d o n a p r o c e d u r e d e v i s e d b y Ε. V . S a y r e of B r o o k h a v e n N a t i o n a l L a b o r a t o r y . A t i n y area ( a b o u t 3-4 m m ) o n the e d g e of a c o i n or m e t a l l i c art object is first s t r o k e d a b o u t five or six times w i t h fine-grain e m e r y p a p e r to r e m o v e a b o u t 0.03 c m of surface l a y e r . T h i s b r i g h t e n e d area is r u b b e d w i t h a s m a l l p i e c e of r o u g h e n e d h i g h - p u r i t y q u a r t z t u b i n g to p r o d u c e a m e t a l streak o n the q u a r t z . T h e s e streaks w e i g h less t h a n 0.0001 m g . A s e c o n d a n d o c c a s i o n a l l y a t h i r d streak is t a k e n at the same p o s i t i o n . T h i s m e t h o d of s a m p l e t a k i n g o c c a s i o n a l l y causes c o n s t e r n a t i o n a m o n g m e t a l l u r g i s t s w h o , t h r o u g h experience, are f a m i l i a r w i t h the i n h o m o g e n i e t i e s w h i c h exist i n m e t a l alloys. W e d o not d e n y t h e existence of s u c h i n h o m o g e n i e t i e s b u t s i m p l y note that the areas over w h i c h w e r u b the q u a r t z t u b i n g a p p a r e n t l y are l a r g e e n o u g h to g i v e consistent sets of d a t a . T h e s e d a t a are also consistent w i t h s i m i l a r streaks t a k e n i n the s a m e w a y f r o m other regions o n the e d g e of the c o i n , i n d i c a t i n g that there are no gross i n h o m o g e n i e t i e s i n ancient a n d m e d i e v a l c o i n alloys. T h e streaks are b r o u g h t to A n n A r b o r a n d together w i t h streaks f r o m alloys of k n o w n c o m p o s i t i o n are i r r a d i a t e d for 2 h r i n t h e U n i v e r s i t y of M i c h i g a n n u c l e a r reactor, at 3 X 1 0 n e u t r o n s / c m / s e c , r e s u l t i n g i n d e t e c t a b l e a m o u n t s of r a d i o a c t i v e silver, c o p p e r , g o l d , arsenic, a n t i m o n y , a n d z i n c . T h e a c t i v i t y is m e a s u r e d u s i n g a p u l s e - h e i g h t m u l t i c h a n n e l g a m m a - r a y a n a l y z e r as d e s c r i b e d b e l o w . A m o r e serious c o n c e r n is that of c o r r o s i v e d e p l e t i o n of the m o r e r e a c t i v e metals near the surface. I d e a l l y , a n i n t e r n a l d r i l l i n g s a m p l e s h o u l d be t a k e n for analysis. W h i l e this is p r e f e r a b l e to a streak s a m p l e , p r a c t i c a l l i m i t a t i o n s are i m p o s e d b y the types of objects w i t h w h i c h w e are w o r k i n g . W e d o u b t i f a n y o w n e r or m u s e u m c u r a t o r w o u l d a l l o w us to d r i l l i n t o a c o i n . H o w e v e r , s o m e m u s e u m s w i l l o c c a s i o n a l l y p e r m i t d r i l l i n g s to be t a k e n f r o m m e t a l l i c art objects, a n d d u r i n g the past y e a r a group from Brookhaven National Laboratory ( including P. Meyers and Ε. V . S a y r e ) has o b t a i n e d d r i l l i n g samples f r o m silver art objects w h i c h w e 1 1 0
1 0 8
2
13
2
128
ARCHAEOLOGICAL
CHEMISTRY
h a d p r e v i o u s l y a n a l y z e d b y u s i n g streak samples. T h e B r o o k h a v e n g r o u p f o u n d a n average of 2 0 % m o r e c o p p e r i n t h e d r i l l i n g s . T h i s is u n d e r s t a n d a b l e since the r e a c t i v i t y of c o p p e r c a n result i n p r o n o u n c e d surface d e p l e t i o n of that element. H o w e v e r , for most of o u r studies, this differ ence i n c o p p e r l e v e l b e t w e e n streak a n d d r i l l i n g samples is of m i n o r c o n c e r n , e s p e c i a l l y for objects w i t h m o r e t h a n about 9 0 % silver. I n these cases, a streak silver content m i g h t i n d i c a t e 9 4 . 0 % , for e x a m p l e , whereas a d r i l l i n g s a m p l e w o u l d s h o w 9 2 . 9 % . Since the e x p e r i m e n t a l u n c e r t a i n ties i n the c a l c u l a t e d silver contents are t y p i c a l l y a b o u t ± 1 - 2 % , this difference b e t w e e n streak a n d d r i l l i n g d a t a w o u l d a p p e a r m i n o r . H o w ever, i n cases w h e r e it m a y b e i m p o r t a n t to a p p r o x i m a t e the silver content m o r e a c c u r a t e l y , this general difference c a n a l w a y s be t a k e n into account i n the i n t e r p r e t a t i o n of the d a t a . O n the other h a n d , the B r o o k h a v e n g r o u p f o u n d about 1 0 % less g o l d i n the d r i l l i n g samples vs. o u r streak samples. A s n o t e d b e l o w , it is the g o l d : s i l v e r ratios that are especially i m p o r t a n t , a n d the B r o o k h a v e n Table I.
Sample l
b
2
6
3
&
4
6
5
6
6
7 8 9 10 11 12
Analysis Data for Edge vs. Inner Sample Streaks"
Identification R o m a n , Vespasian, 75 A . D . Roman, Early Republic 205-195 B . C . Roman, Republic, C l a u d i a ca. 41 B . C . R o m a n , T i t u s , 80 A . D .
edge inner edge inner edge inner edge inner R o m a n , Severius edge A l e x a n d e r 222-235 A . D , inner S a s a n i a n , S h a p u r I , ca. edge 250 A . D . , highly corinner roded Sasanian, Shapur I edge ca. 250 A . D . inner Sasanian, Yezdegird I edge ca. 400 A . D . inner Sasanian, K h u s r a u I edge ca. 575 A . D . inner Sasanian, K h u s r a u I I edge ca. 598 A . D . inner Sasanian, K h u s r a u I I edge 620 A . D . inner F o o t of a s m a l l I s l a m i c edge silver b o w l inner
Ag, % Cu, % Au,
%Cu/ % % Ag
%Au/ %Ag ( X 100)
75.8 76.6 85.7 83.4 97.4 97.4 94.6 92.8 44.0 43.9 15.2 12.6
23.9 23.1 13.8 16.1 2.1 2.1 4.6 6.5 55.4 55.6 84.2 86.9
0.34 0.34 0.45 0.43 0.47 0.46 0.73 0.68 0.20 0.20 0.065 0.057
0.315 0.302 0.161 0.193 0.022 0.022 0.049 0.070 1.26 1.27 5.54 6.90
0.44 0.44 0.52 0.52 0.48 0.47 0.77 0.73 0.45 0.45 0.43 0.48
91.8 86.4 96.2 95.9 94.6 96.9 97.6 97.5 96.5 96.3 74.9 75.8
7.7 13.1 3.2 3.5 5.4 3.1 1.8 1.9 3.4 3.6 22.7 22.1
0.50 0.46 0.63 0.63 0.016 0.015 0.57 0.57 0.018 0.019 2.4 2.4
0.084 0.152 0.033 0.036 0.057 0.032 0.018 0.019 0.035 0.037 0.303 0.292
0.55 0.53 0.66 0.66 0.017 0.015 0.58 0.58 0.019 0.020 3.2 3.1
Negligible zinc, arsenic, and antimony were detected. Roman coins were borrowed from G . Carter who had filed down one side of the coin for x-ray fluorescence analysis. The inner samples were taken from this filed-down side of the coin. The x-ray analysis data of Carter for each of these five coins were within dbl.5% silver of that found by the streak method. a
6
8.
GORDUS
AND
Gold Impurity
GORDUS
129
in Silver
d r i l l i n g samples f r o m some of our coins l i s t e d i n T a b l e I s h o w o n l y a 6 . 3 % l o w e r v a l u e vs. our e d g e A u : A g values. H o w e v e r , these same B r o o k h a v e n d a t a are also 4 . 2 % l o w e r w h e n c o m p a r e d w i t h our i n n e r A u : A g ratios, whereas w e s h o u l d b e i n agreement for this set of d a t a since s a m p l i n g is f r o m the same r e g i o n . T h i s d i s c r e p a n c y is a p p a r e n t l y c a u s e d b y differences i n standards w h i c h w e r e c e n t l y c o m p a r e d a n d f o u n d to differ i n A u : A g ratios b y 4.0 ± 1.4%. T h i s difference error m u s t be associated p r i m a r i l y w i t h o u r o w n g o l d - s i l v e r standards w h i c h are s i m p l e jewelers' a l l o y s ; the B r o o k h a v e n standards w e r e p r e p a r e d u n d e r m o r e e x a c t i n g r e q u i r e m e n t s . H e n c e , a l l of our p u b l i s h e d A u : A g d a t a , i n c l u d i n g those here, s h o u l d be m u l t i p l i e d b y 0.960. T h e a c t u a l difference b e t w e e n edge A u : A g ratios a n d d r i l l i n g A u : A g ratios, there fore, is p r o b a b l y o n l y o n the order of 6.3 — 4.2 ~ 2 % . ( T h e d a t a of T a b l e I, for e x a m p l e , s h o w that the A u : A g ratios f r o m the i n n e r samples are, o n the average, o n l y 0 . 7 % less t h a n t h e ' e d g e A u : A g ratios.) Streak Data Calculation. I r r a d i a t e d streak samples are m o u n t e d o n 2 " X 2 " cards a n d p l a c e d next to the face of a h i g h - r e s o l u t i o n ( F W H M = 2.2 k e V at 1332 k e V ) G e ( L i ) detector. A l u m i n u m shields, 1 / 1 6 " t h i c k , are p l a c e d o n e a c h side of the s a m p l e to p r o v i d e sufficient mass to a l l o w for r e p r o d u c i b l e C u p o s i t r o n a n n i h i l a t i o n . T h e g a m m a - r a y s p e c t r a l d a t a ( r a d i o a c t i v i t y counts ) are d e t e r m i n e d u s i n g the f o l l o w i n g s p e c t r a l peaks : A u (2.70 d a y , 411.8 k e V ) , Z n (13.8 h r , 438.7 k e V ) , C u (12.8 h r , 511.0 k e V ) , A s (26.3 h r , 559.2 k e V ) , S b (2.75 d a y , 564.0 k e V ) , Ag (255 d a y , 657.8 k e V ) , a n d N a (15.0 h r , 1368.4 k e V ) . T h e N a a c t i v i t y arises p r i m a r i l y f r o m s m a l l i m p u r i t i e s i n the h i g h - p u r i t y ( S p e c t r o s i l ) s i l i c a . H o w e v e r , since N a also results i n a d e t e c t a b l e 511.0 k e V p e a k w h e n G e ( L i ) detectors are u s e d , the C u p e a k is c o r r e c t e d for the N a , 511 k e V c o n t r i b u t i o n . T h i s c o r r e c t i o n i n o u r d e t e c t i o n system is about 5 % of the 1368 k e V N a p e a k a n d u s u a l l y affects the c o p p e r d a t a b y less t h a n 1 % . I n a d d i t i o n , A s emits another g a m m a - r a y of lesser i n t e n sity at 657.0 k e V , thus i n t e r f e r i n g w i t h the A g peak. T h i s A s correc t i o n of the silver p e a k i n our d e t e c t i o n system is about 7 % of the 559 k e V A s peak b u t almost a l w a y s has n e g l i g i b l e effect o n the c a l c u l a t e d silver d a t a because of the l o w arsenic contents. O n l y i n h i g h l y d e b a s e d coins, w h i c h possess h i g h e r arsenic ( a n d l o w e r s i l v e r ) contents, does this arsenic c o r r e c t i o n of the silver d a t a represent m o r e t h a n 1 % of the silver a c t i v i t y . S i n c e w e do not w e i g h these streaks, w e assumed that the A g + C u + Z n + A s + S b + A u = 1 0 0 % . B y c o m p a r i n g the r a d i o a c t i v i t y levels i n the streaks w i t h those i n the s t a n d a r d a l l o y streaks ( e a c h c o r r e c t e d for d e c a y to a c o m m o n reference t i m e ) w e c a n t h e n c a l c u l a t e the percent r a t i o s : C u / A g , A u / A g , Z n / A g , S b / A g , a n d A s / A g . T h e c o m b i n a t i o n of these r a t i o d a t a a n d the a b o v e e q u a t i o n a l l o w s us to c a l c u l a t e the i n d i v i d u a l p e r c e n t values. V a r i o u s c h e m i c a l analyses r e p o r t e d i n the literature for silver coins a n d art objects i n d i c a t e that other elements s u c h as l e a d a n d t i n ( w h i c h w e d o not d e t e c t ) are u s u a l l y present at less t h a n 2 % . T h e r e are n o t a b l e exceptions h o w e v e r . S o m e types of coins f r o m c e r t a i n p e r i o d s c o n t a i n u p to 1 0 - 1 5 % P b + B i . A s a result, w e h a v e a l w a y s p e r f o r m e d d i r e c t n e u t r o n H o w i t z e r silver analysis o n at least a f e w coins of e a c h g e n e r a l t y p e that are a n a l y z e d b y streak analysis. T h e s i l v e r d a t a for the H o w i t z e r analysis are i n v a r i a b l y l o w e r t h a n those for the streak analysis, b u t this is to be e x p e c t e d for t w o reasons: the 6 4
1 9 8
6 9 m
7 6
6 4
1 2 2
1 1 0 m
2 4
2 4
2 4
6 4
2 4
2 4
7 6
1 1 0 m
7 6
7 6
130
ARCHAEOLOGICAL CHEMISTRY
c o p p e r surface c o r r o s i o n effect a n d the a s s u m p t i o n i n the streak analysis that P b , B i , F e , a n d other m i n o r elements are absent. I n some cases w e h a v e s i m p l y c o n s t r u c t e d e m p i r i c a l c o r r e c t i o n curves ( b a s e d o n d a t a for coins s u b j e c t e d to b o t h H o w i t z e r a n d streak analysis ) to adjust the streak d a t a for other coins of t h e same type. ( A flameless a t o m i c a b s o r p t i o n s p e c t r o m e t r i c m e t h o d w h i c h w e p l a n to use w i l l a l l o w us to a n a l y z e the streak samples for P b , B i , F e , a n d other, p r e s e n t l y u n d e t e c t e d ele ments. ) T h i s streak m e t h o d has b e e n d e s c r i b e d i n Refs. 3 a n d 4. Gold:Silver
Ratios
E x a m i n a t i o n of the g o l d : s i l v e r ratios f r o m over 9000 s u c h m e t a l streaks shows clusterings i n values that u s u a l l y correlate w i t h t h e m i n t s a n d dates of t h e coins. T h i s i m p l i e s that e a c h s i l v e r source u s e d d u r i n g e a c h p e r i o d tends to h a v e a c h a r a c t e r i s t i c g o l d / s i l v e r l e v e l . Some geolo gists m i g h t object to this statement since some silver m i n e analysis records f r o m the 1800's s h o w a c e r t a i n v a r i a b i l i t y i n the g o l d l e v e l of a s i n g l e silver m i n e . H o w e v e r , the m i n e analyses w e r e p e r f o r m e d o n the t o t a l silver - f g o l d e x t r a c t e d f r o m the ore rocks. T h e ancient a n d m e d i e v a l m e t a l l u r g i s t s , o n the other h a n d , w e r e able to extract o n l y c e r t a i n fractions of the silver i n the ore. S i n c e these g o l d i m p u r i t y levels w e r e almost a l w a y s less t h a n
1%,
t h e i r presence d i d not color the silver v i s i b l y a n d therefore w e n t u n d e tected. F u r t h e r , the c h e m i c a l s i m i l a r i t y of g o l d a n d silver r e s u l t e d i n the g o l d b e i n g c a r r i e d a l o n g i n the r e f i n i n g process, a postulate of ours that has r e c e n t l y b e e n c o n f i r m e d b y M c K e r r e l l a n d Stevenson ( 5 )
w h o sub
jected silver ores to a l e a d e x t r a c t i o n - p u r i f i c a t i o n process s i m i l a r to that u s e d i n a n c i e n t times. T h e g o l d i m p u r i t y l e v e l i n a c o i n , therefore, c a n be a d i r e c t reflection of the l e v e l i n the silver i n the o r i g i n a l ore, a n d coins can b e g r o u p e d a c c o r d i n g to these g o l d levels. T h e i n t e r p r e t a t i o n of s u c h g o l d l e v e l d a t a is a little m o r e c o m p l i c a t e d since coins a n d art objects w e r e m a d e not o n l y of n e w s i l v e r b u t sometimes of v a r y i n g amounts of silver o b t a i n e d f r o m m e l t i n g o l d e r coins or art objects. It has b e e n i m p l i e d ( 6 )
that the o n l y silver a v a i l a b l e w i t h i n the
P e r s i a n E m p i r e w a s that present as a n i m p u r i t y i n l e a d ores. T h e extrac t i o n of silver f r o m l e a d ores was c e r t a i n l y p r a c t i c e d b y the R o m a n s , e s p e c i a l l y w h e n h i g h - g r a d e silver sources b e c a m e d e p l e t e d .
However,
the fact t h a t s u c h e x t r a c t i o n t e c h n o l o g y w a s k n o w n does not m e a n that the P e r s i a n s , for e x a m p l e , necessarily u s e d this m e t h o d , especially if h i g h g r a d e silver ores or m e t a l l i c s i l v e r sources w e r e s t i l l a v a i l a b l e . I n fact, A r a b c h r o n i c l e s , e v e n those of the t e n t h t h r o u g h t h i r t e e n t h c e n t u r i e s , c l e a r l y d i s t i n g u i s h e d l e a d sources, s i l v e r sources, a n d sources f r o m w h i c h b o t h l e a d a n d silver w e r e d e r i v e d (7, 8).
T h e phrase " s i l v e r s o u r c e " as
u s e d i n this a r t i c l e is m e a n t to i m p l y a g e o l o g i c a l f o r m a t i o n that was
8.
GORDUS
AND
Gold Impurity
GORDUS
in
131
Silver
v i e w e d b y the p e o p l e of the p e r i o d p r i m a r i l y as a source of s i l v e r , as c o m p a r e d w i t h a f o r m a t i o n u s e d , for e x a m p l e , p r i m a r i l y as a source of l e a d . This
d e f i n i t i o n , therefore,
(Ag S), 2
cerargyrite
includes
metallic
silver
sources,
( A g C l ) , a n d other m i n e r a l formations mg
ο
GOLD PER
100
^
mg
SILVER
§>
Κ)
Λ i t ft) * CP
ft
υ
ο ο * ο ο ο
ο
0
ο
σ0
σ
©o
fl
ο
argentite containing
ο
ο
^
° Ο DIMISHK ARMENIA ο AL RAYY HAMAOHAN
y
0
°
3$ο
*
ΜΑΗ I AL TAJ MARA
°
ο ° °° ο
0
~ 1 . 0 0 % g o l d ) w e r e e x p l o i t e d b y the U m a y y a d s . O n e m u s t h a v e b e e n v e r y close to the H a m a d h a n , M a h i , A l - T a i m a r a region.
S i m i l a r l y , the l o c a t i o n of
some of the r e m a i n i n g sources c a n be a p p r o x i m a t e d . i n - s i l v e r source
must have b e e n
The 0.75%
gold-
v e r y near the four cities of S u r r a q ,
Suk A l - A h w a z , M a n a d h i r , and N a h r Tira.
T h e S o u t h e r n m i n t s of D a
rabjird, Sabur, and A r d a s h i r - K h u r r a must have been
near the
0.70%
g o l d i m p u r i t y source whereas Sijistan a n d K i r m a n shared a source h a v i n g silver w i t h a b o u t 0 . 8 0 %
gold.
T h e d a t a for each of the four m i n t s i n
the N o r t h e a s t e x h i b i t too large a range of values to p e r m i t suggesting the g e n e r a l l o c a t i o n of their silver sources, a n d the s p r e a d of these d a t a suggests the usage of m o r e t h a n one silver source.
B e c a u s e of the large
d i s t a n c e b e t w e e n D a m a s c u s a n d A r m e n i a a n d the other U m a y y a d m i n t cities, i t is difficult to d r a w conclusions f r o m the d a t a for the
coinage
of these t w o W e s t e r n m i n t s . D a t a s u c h as these c a n h e l p answer c e r t a i n h i s t o r i c a l questions.
For
instance, the m i n t c i t y w r i t t e n o n U m a y y a d coins c a l l e d M a h i m u s t be the one n o t e d here a n d not a s i m i l a r l y n a m e d c i t y near M e r v , w h i c h h a d b e e n suggested as a n a l t e r n a t i v e c h o i c e ( 9 ) , since the g o l d i m p u r i t y d a t a o b t a i n e d f r o m coins m i n t e d at M a h i agree w i t h the d a t a for H a m a d h a m a n d A l - T a i m a r a b u t do not agree w i t h the d a t a for M e r v . B y de t e r m i n i n g the types a n d amounts of silver u s e d i n each c i t y , i t m a y be possible to assess the m o v e m e n t of silver t h r o u g h o u t the E m p i r e .
Fur
thermore, possible locations c a n be suggested for ancient silver sources. T h i s c o u l d have i m p o r t a n t g e o l o g i c a l
implications.
F o r example,
the
G e o l o g i c a l S u r v e y of I r a n does not list a single silver source i n a l l of I r a n , a n d it is u s u a l l y stated that there never w e r e a n y silver sources in Iran.
( T h e o n l y silver f o u n d t o d a y
i n I r a n originates as a trace-
l e v e l i m p u r i t y i n l e a d ores; w e h a v e d e t e r m i n e d that the
gold-silver
ratio d a t a for these ores is m u c h less t h a n that f o u n d for any of coins.)
these
T h e d a t a of F i g u r e s 2 a n d 3, h o w e v e r , suggest that d u r i n g the
U m a y y a d p e r i o d there p r o b a b l y was a silver source i n the H a m a d h a n , M a h i , A l - T a i m a r a region.
T h i s r e g i o n , therefore, c o u l d be
appropriate
for p r e s e n t - d a y g e o l o g i c a l e x p l o r a t i o n . P o s s i b l y even the silver m i n e used d u r i n g . t h e U m a y y a d p e r i o d c o u l d b e w o r k e d t o d a y u s i n g m o d e r n extrac tion
methods.
Sasanian Coinage Silver Fineness.
F i g u r e 4 contains d a t a for the silver
fineness
of
a b o u t 400 Sasanian coins as d e t e r m i n e d b y t h e d i r e c t ( n e u t r o n H o w i t -
134
ARCHAEOLOGICAL CHEMISTRY
Reigns of
Sasanian Kings
A. o I
200
1
1
1
I
I
I
I
I
1
250
300
350
400
450
500
550
600
650
700
Date - A.D.
Figure 4. Silver fineness as determined by neutron Howitzer analysis for about 400 Sasanian drachms. Alternating open and closed circles are used to assist visually in associating data points with ruling monarchs listed at top of graph. zer) coin irradiation method.
A l t e r n a t i n g o p e n a n d closed circles w e r e
u s e d to h e l p v i s u a l l y i d e n t i f y d a t a points a c c o r d i n g to the r u l i n g m o n a r c h . T h i s g r a p h is a n u p d a t e d , e x p a n d e d v e r s i o n of the one r e c e n t l y p u b l i s h e d together w i t h a p r e l i m i n a r y d i s c u s s i o n o f the d a t a ( 1 0 ) .
R e c e n t emphasis
has b e e n p l a c e d o n the analysis of the coins of l o w - s i l v e r fineness, p a r t i c u l a r l y d u r i n g the r e i g n o f S h a p u r I . Since these d e b a s e d
coins a r e
s e l d o m f o u n d i n extremely fine c o n d i t i o n , they are u n d e r r e p r e s e n t e d i n m u s e u m collections. A s a result, a n u m b e r of S a s a n i a n coins that l o o k e d d e b a s e d w e r e b o u g h t f r o m dealers i n F r a n c e , E n g l a n d , a n d the N e a r E a s t a n d subjected to b o t h H o w i t z e r a n d streak analysis. T h e s e various d a t a , a n d e s p e c i a l l y those for the e a r l y coinage ( A r d a s h i r I - S h a p u r I I : 2 2 4 - 3 7 9 A . D . ) are discussed i n d e t a i l i n a separate article ( I I ) . Gold Impurity Levels.
S h o w n i n F i g u r e 5 are g o l d i m p u r i t y d a t a
for a p p r o x i m a t e l y 1300 S a s a n i a n coins w i t h almost h a l f o f these points r e p r e s e n t i n g analyses o f K h u s r a u I I coinage.
data
U n l i k e F i g u r e 2,
these g o l d l e v e l d a t a are p l o t t e d o n a l o g a r i t h m i c scale to s h o w subtle differences i n the l o w - g o l d d a t a points.
O n l y d u r i n g the later Sasanian
p e r i o d do the coins bear m i n t m o n o g r a m s a n d dates g i v e n i n terms of the r e g n a l year. Identifications, w h e r e necessary, w e r e b a s e d o n G o b i
(12).
T h e s e d a t a suggest that v a r i o u s silver sources w e r e e x p l o i t e d , a n d most
8.
GORDUS
AND
GORDUS
Gold Impurity
135
in Silver
of these sources h a d m o r e t h a n a b o u t 0 . 4 - 0 . 5 %
g o l d i m p u r i t y except
d u r i n g the later reigns w h e n s i l v e r of l o w g o l d levels w a s o c c a s i o n a l l y used. P r e l i m i n a r y examinations of the g o l d levels i n K h u s r a u I I
coinage
g r o u p e d a c c o r d i n g to m i n t m o n o g r a m s i n d i c a t e d that the P a h l e v i m i n t s i g natures u s u a l l y i d e n t i f i e d as Z D a n d R D , f r e q u e n t l y c o n s i d e r e d v a r i a n t s b o t h r e p r e s e n t i n g R a y y , w e r e a c t u a l l y associated w i t h t w o separate m i n t cities (4).
It was also f o u n d that M R a n d M B m o n o g r a m s
represented
t w o separate m i n t cities a n d w e r e not s i m p l y v a r i a n t s of a M e r v signature ( 4 ). W o r k of this t y p e is c o n t i n u i n g a n d is b e i n g e x t e n d e d to the reigns of H o r m i z d I V a n d K h u s r a u I. T h e d a t a r e p r e s e n t e d b y the o p e n squares i n F i g u r e 5 w e r e o b t a i n e d f r o m streaks of m o d e r n fake S a s a n i a n coins (4).
N o t e that the g o l d levels
for these fake coins are u s u a l l y a p p r e c i a b l y less t h a n those for a u t h e n t i c coins.
T h i s is because silver u s e d i n the last 100 years almost a l w a y s 20
•
ι
ι
o\ooo.2-° 0011 200
•
•
ι
ι
ι
.·-.··
o
1
1
300
,
, 400
D A T E
ι
, 500
I
• 600
,
A.D
Figure 5. Gold impurity levels in over 1300 Sasanian drachms. Open squares are gold levels in modern fake Sassanian coins. h a d less t h a n a b o u t 0 . 1 % g o l d ; p r e s e n t - d a y s i l v e r u s u a l l y has less t h a n 0 . 0 1 % g o l d since m o d e r n r e f i n i n g m e t h o d s a l l o w e c o n o m i c a l r e m o v a l of most t r a c e - l e v e l g o l d . Sasanian Metallic
Art
A n u m b e r of silver plates, b o w l s , a n d p i t c h e r s h a v e b e e n i d e n t i f i e d as b e i n g m a d e i n the P e r s i a n E m p i r e d u r i n g the S a s a n i a n p e r i o d ( 13, 14,
136 15).
ARCHAEOLOGICAL CHEMISTRY
R e c e n t l y , some art historians h a v e reassigned the m a n u f a c t u r e of a
f e w of these objects to s l i g h t l y later periods or adjacent regions.
geographical
S o m e of the objects, p a r t i c u l a r l y those i n t h e shape of plates,
show a king (frequently on horseback)
h u n t i n g w i l d game.
Almost all
art historians still a t t r i b u t e these p a r t i c u l a r motifs to the Sasanian p e r i o d a n d region. T h e s e objets d'art are rare a n d c o m m a n d a h i g h p r i c e . T h e i n c r e a s e d d e m a n d b y m u s e u m s a n d p r i v a t e collectors for m e t a l l i c art of this p e r i o d has b e e n a c c o m p a n i e d b y a n increase i n the s u p p l y of these
objects—and
b y a n i n c r e a s i n g s u s p i c i o n o n the p a r t of art historians, m u s e u m curators, p r i v a t e collectors, a n d r e p u t a b l e dealers that some ( a n d perhaps m a n y ) of these s i l v e r S a s a n i a n art objects are m o d e r n forgeries.
V i r t u a l l y a l l art
historians a*gree that the S a s a n i a n objects i n the State H e r m i t a g e M u s e u m ( L e n i n g r a d ) are a u t h e n t i c , w i t h one e x c e p t i o n — a late n i n e t e e n t h c e n t u r y c o p y of a p l a t e i n the B r i t i s h M u s e u m . T h i s agreement is b a s e d to some extent o n the fact that the L e n i n g r a d c o l l e c t i o n has a s u i t a b l e "pedigree."
It i n c l u d e s objects that either w e r e
found
i n controlled
a r c h a e o l o g i c a l excavations or w e r e i n R u s s i a n collections as early as the mid-1800's. T h e present-day art m a r k e t is s u c h t h a t a p e d i g r e e is rare. If a p u r p o r t e d S a s a n i a n s i l v e r art object is p u t u p for sale, the s u r r o u n d i n g its o r i g i n are u s u a l l y vague.
circumstances
If it is a fake, the vagueness is
u n d e r s t a n d a b l e , b u t a vague o r i g i n w o u l d also be a s c r i b e d to a n a u t h e n t i c p i e c e since it is i l l e g a l to export s u c h objects f r o m I r a n , w h e r e t h e y are g e n e r a l l y c l a i m e d to be f o u n d near the C a s p i a n Sea b y "peasants d i g g i n g i n the h i l l s . " T h r o u g h the k i n d a n d w i l l i n g c o o p e r a t i o n of m u s e u m directors, p r i vate collectors, a n d dealers b o t h i n t h e U . S . a n d a b r o a d it was possible to take streaks of h u n d r e d s of s u c h silver Sasanian a n d S a s a n i a n - s t y l e d art objects. I n c l u d e d w e r e the entire L e n i n g r a d c o l l e c t i o n a n d also n u merous p r i v a t e l y o w n e d " S a s a n i a n " plates that v i r t u a l l y every art h i s t o r i a n of this p e r i o d has d e s c r i b e d as either d e c i d e d l y suspicious or p r o b a b l y fake.
T h e streak analysis d a t a for those
plates i n v o l v i n g a Sasanian
k i n g - h u n t i n g scene are g i v e n i n F i g u r e 6. T h e s e d a t a are i d e n t i f i e d a c c o r d i n g to the g e n e r a l consensus of art historians a n d curators r e g a r d i n g the a u t h e n t i c i t y of each plate. A s seen i n F i g u r e 6, a l l plates for w h i c h there is u n a n i m o u s or n e a r - u n a n i m o u s agreement a b o u t t h e i r a u t h e n t i c i t y h a v e % A u / % A g
( X l O O ) values i n
excess of 0.60, except t w o . T h e first is a p l a t e i n the H e r m i t a g e m u s e u m (Au in A g =
0.18)
s h o w i n g S h a p u r I I o n horseback
(16);
the style i n
w h i c h the figure, horse, a n d lions are d e p i c t e d differs f r o m that o n other plates i n the L e n i n g r a d c o l l e c t i o n . It is possible that this object has been e i t h e r m i s a t t r i b u t e d t e m p o r a l l y or g e o g r a p h i c a l l y .
T h e second is a p l a t e
8.
GORDUS
AND
Gold Impurity
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7. Gold impurity levels in Mexican, South American and European coinage
(Potosi),
140
ARCHAEOLOGICAL
CHEMISTRY
a n d M e x i c a n ) s i l v e r a v a i l a b l e for c o i n i n g d u r i n g t h e p e r i o d , ( b ) tional complications
addi
arise because the a b u n d a n t a r c h i v a l e v i d e n c e
for
the silver m i n e p r o d u c t i o n at P o t o s i has not yet b e e n e x p l o r e d , a n d
(c)
there is e v i d e n c e that " l e a k a g e " o c c u r r e d silver f r o m
(26)
d i v e r t i n g some A m e r i c a n
its S e v i l l e d e s t i n a t i o n , it has u s u a l l y b e e n assumed
that
P o t o s i s i l v e r i n f i l t r a t e d the currencies of E u r o p e . It is this basic a s s u m p t i o n to w h i c h the p r e l i m i n a r y s t u d y was
addressed.
F o r t u n a t e l y , for o u r purposes, the silver f r o m P o t o s i has a c h a r a c t e r i s t i c a l l y l o w l e v e l of g o l d i m p u r i t y w h i c h a l l o w s i t to be differentiated c l e a r l y f r o m the M e x i c a n - E u r o p e a n silver stock.
As shown in Figure 7
s i l v e r f r o m this latter stock was u s e d i n S p a i n , B a r c e l o n a ,
(27),
and
F r a n c e , before 1570-1580> w h e n the P o t o s i m i n e b e g a n to be e x p l o i t e d o n a large scale a n d its y i e l d s s h i p p e d to S p a i n . P o t o s i s i l v e r was c e r t a i n l y u s e d i n S p a n i s h c o i n a g e ; i n fact, the c r u d e l y struck réaies b e a r i n g the P o t o s i m i n t m a r k w e r e l e g a l tender i n S p a i n a l t h o u g h m u c h of the silver was r e m i n t e d i n t o S p a n i s h réaies of a h i g h e r artistic s t a n d a r d . H o w e v e r , n o n e of the P o t o s i silver appears d i r e c t l y i n the coins of B a r c e l o n a or F r a n c e . scholars h a v e (28),
We
m i g h t h a v e e x p e c t e d , as some
that at least some F r e n c h c o i n a g e c o u l d be m a d e of
P o t o s i silver o n the grounds that so m u c h m o r e m o n e y was m i n t e d i n F r a n c e i n that p e r i o d t h a n before; h o w e v e r , it is n o w clear that e v i d e n c e for a d i r e c t c a u s a l l i n k b e t w e e n P o t o s i silver a n d i n c r e a s e d F r e n c h m i n t o u t p u t cannot be a d d u c e d .
P a r t i c u l a r attention was p a i d to coins f r o m
s u c h p o r t cities as M a r s e i l l e s a n d B a y o n n e , to c h e c k b o t h F r e n c h coast lines w h e r e s i l v e r m i g h t h a v e " l e a k e d " i n the sense that these m i g h t h a v e b e e n c o n v e n i e n t places for i l l e g a l transactions.
I n the same w a y , those
m i n t towns o n the route o v e r l a n d t h r o u g h F r a n c e , w h e r e A m e r i c a n silver w a s s h i p p e d b y l a n d to the N e t h e r l a n d s f r o m S p a i n , w e r e also i n v e s t i gated.
O v e r 150 c a r e f u l l y selected
F r e n c h coins ( i n c l u d i n g those
for
w h i c h d a t a are g i v e n i n F i g u r e 7) w e r e a n a l y z e d , a n d no P o t o s i silver was identified. D e s p i t e the l o n g - s t a n d i n g a n t a g o n i s m b e t w e e n C a s t i l e a n d C a t a l o n i a , it m i g h t h a v e b e e n e x p e c t e d that since B a r c e l o n a w a s the p o i n t of
de
p a r t u r e for t h e silver f r o m S p a i n to G e n o a w h e r e it was e x c h a n g e d for g o l d a n d w h e r e business was transacted w i t h G e n o e s e b a n k e r s , B a r c e l o n a m i g h t h a v e r e c e i v e d some P o t o s i silver a n d m i n t e d it. I n fact, it seemed so l i k e l y t h a t o n l y 25 coins of B a r c e l o n a w e r e a n a l y z e d ,
concentrating
o n the likeliest dates, b u t a g a i n no P o t o s i s i l v e r was f o u n d .
H o w e v e r , the
G e n o e s e d i d m i n t some coins m a d e of P o t o s i silver. M i l a n , another b a n k i n g center, a n d p e r h a p s m o r e i m p o r t a n t l y a c i t y n o t e d for arms m a n u f a c t u r e , r e c e i v e d P o t o s i silver, as s h o w n b y 5 of the 40 coins a n a l y z e d . T h i s is s t i l l a v e r y s m a l l a m o u n t .
T h e e n t e r p r i s i n g E n g l i s h seamen
who
c a p t u r e d S p a n i s h galleons b r o u g h t the silver to E n g l a n d , a n d vast q u a n -
8.
GORDUS A N D
GORDUS
Gold Impurity
tities of i t are s u p p o s e d to h a v e
in
141
Silver
a r r i v e d at the T o w e r
mint.
Again,
h o w e v e r , analysis of 60 E l i z a b e t h a n coins c h o s e n to c o i n c i d e w i t h s u c h deliveries r e v e a l no P o t o s i silver. H a m i l t o n a s s u m e d that a large p r o p o r t i o n of the S p a n i s h treasure w e n t to G e r m a n y , p r o b a b l y m o s t l y to p a y the i m p o r t a n t bankers to the S p a n i s h C r o w n , the F u g g e r s of A u g s b u r g .
C o i n s of G e r m a n y are yet to
be a n a l y z e d , b u t , except for this G e r m a n d e s t i n a t i o n , the results so far o b t a i n e d l e a d us to q u e s t i o n w h e t h e r the A m e r i c a n treasure h a d
any
r e a l c o n n e c t i o n at a l l w i t h t h e v e r y r e a l E u r o p e a n i n f l a t i o n . W e do
not
assert that the A m e r i c a n treasure h a d no effect o n the E u r o p e a n e c o n o m y ; w e q u e s t i o n w h e t h e r i t was r e a l s i l v e r w h i c h c a u s e d m o n e y to b e c o m e c h e a p or w h e t h e r i t was the e x p e c t a t i o n of silver.
Figure 8. Percent silver having a low gold level (0.001, 0.004, or 0.06% Au in Ag for each of the three curves) in a mixture with silver having 0.15% Au in Ag impurity as a function of the resultant gold impurity level of the mixture T h i s s t u d y has thus f a r b e e n b a s e d o n the analysis of a single d i a g nostic e l e m e n t : the g o l d i m p u r i t y l e v e l . A s a result, w e are not able to d i s t i n g u i s h M e x i c a n a n d E u r o p e a n silver. E q u a l l y i m p o r t a n t , w e are not a b l e to d e t e r m i n e to a h i g h degree of c e r t a i n t y w h i c h , i f a n y , of
the
E u r o p e a n coins w e a n a l y z e d m i g h t be m a d e of m i x t u r e s of P o t o s i a n d other silver. It s h o u l d be e m p h a s i z e d that because of the l o w g o l d i m p u r i t y l e v e l i n P o t o s i silver, the a d d i t i o n of s m a l l amounts of s i l v e r h a v i n g large g o l d i m p u r i t y levels c o u l d m a r k e d l y c h a n g e the o v e r a l l g o l d l e v e l of the m i x t u r e . A s s h o w n i n F i g u r e 8, a m i x t u r e m a d e , for e x a m p l e , of 2 0 % E u r o p e a n silver ( g o l d l e v e l =
0 . 1 5 0 % ) a n d 8 0 % P o t o s i silver ( g o l d l e v e l
142
ARCHAEOLOGICAL CHEMISTRY
== 0 . 0 0 4 % ) has a n o v e r a l l g o l d i m p u r i t y l e v e l of 0 . 0 3 3 % . T h e r e f o r e , some of the E u r o p e a n post-1550 coins c o u l d p o s s i b l y i n v o l v e m i x t u r e s of w h i c h P o t o s i s i l v e r is a p r e d o m i n a n t c o m p o n e n t .
One such coin, a F r e n c h
T e s t o n of H e n r i I I ( 1 5 4 7 - 1 5 5 9 ) h a v i n g a g o l d l e v e l of 0.024 ( d a t a p o i n t s h o w n i n F i g u r e 7 ) , c o u l d b e of this t y p e . C l e a r l y , the use of g o l d as the o n l y d i a g n o s t i c e l e m e n t is i n a d e q u a t e to d i s t i n g u i s h p u r e P o t o s i s i l v e r a n d s o m e m i x t u r e s c o n t a i n i n g P o t o s i silver a n d does not a l l o w us to d i s t i n g u i s h p u r e M e x i c a n a n d p u r e E u r o p e a n silver. D a t a for other elements, w h i c h c a n serve i n the same m a n n e r as g o l d , are n e e d e d to d i s t i n g u i s h these a l t e r n a t i v e p o s s i b i l i t i e s . T o ac c o m p l i s h this, m e t h o d s are p r e s e n t l y u n d e r i n v e s t i g a t i o n w h e r e b y
the
streak s a m p l e w o u l d b e d i s s o l v e d f o l l o w i n g n e u t r o n a c t i v a t i o n analysis a n d the d i s s o l v e d s a m p l e w o u l d be s u b j e c t e d to flameless a t o m i c a b s o r p t i o n s p e c t r o m e t r y . T h e i n i t i a l results i n d i c a t e t h a t the l e a d a n d b i s m u t h contents of the d i s s o l v e d streak samples c a n be d e t e c t e d easily, a n d these a d d i t i o n a l e l e m e n t a l d a t a m a y serve p a r t i a l l y for d i a g n o s t i c d i f f e r e n t i a tion.
It s h o u l d also b e p o s s i b l e to detect C d , F e , Z n , A l , a n d p e r h a p s
other metals b y this p r o c e d u r e . The Silver Coinage of Zapata,
1914-1915
T h e M e x i c a n R e v o l u t i o n c a u s e d enormous e c o n o m i c d i s l o c a t i o n , one aspect of w h i c h w a s t h e collapse of the m o n e t a r y system.
The Mexico
C i t y m i n t , w h i c h h a d s u s p e n d e d g o l d coinage i n 1910, ceased to strike the pesos fuertes i n M a r c h 1914 a n d the s u b s i d i a r y s i l v e r i n S e p t e m b e r . F e d e r a l g o l d a n d silver coins soon d i s a p p e a r e d f r o m c i r c u l a t i o n . T h r o u g h out the c o u n t r y p a p e r m o n e y v a l u e was issued.
of i r r e g u l a r c i r c u l a t i o n a n d p r e c a r i o u s
It was w i d e l y c o u n t e r f e i t e d , a n d e v e n the
p a p e r m i g h t be d e c l a r e d i n v a l i d o v e r n i g h t .
S o m e coins w e r e
genuine produced
l o c a l l y a n d u n o f f i c i a l l y — a s f a r as the f e d e r a l g o v e r n m e n t was c o n c e r n e d — i n a n a t t e m p t to s t a b i l i z e values a n d p r o v i d e s m a l l change.
These coin
ages exist i n great q u a n t i t y a n d d i s p a r i t y , d i f f e r i n g f r o m the F e d e r a l a n d f r o m e a c h other i n a l l o y , w e i g h t , types, legends, a n d d e n o m i n a t i o n . Z a p a t i s t a s i l v e r one- a n d t w o - p e s o pieces, F i g u r e 9, w e r e first struck i n G u e r r e r o i n 1914; f r a c t i o n a l b r o n z e d i d not a p p e a r u n t i l the f o l l o w i n g year.
T h e t w o silver d e n o m i n a t i o n s w e r e p r o p o r t i o n a t e l y s m a l l e r a n d
c o n t a i n e d less w e i g h t of s i l v e r t h a n t h e p r e v i o u s f e d e r a l coinage, b u t t h e i r a c c e p t a n c e w a s e n c o u r a g e d b y the a n n o u n c e d presence of a stated a m o u n t of g o l d i n each. T h e peso bears the l e g e n d O R O : 0,300 { g r a m s } , " the t w o pesos, O R O : 0,595." A s t u d y of this c o i n a g e , w h i c h w a s i n i t i a t e d b y T . V . B u t t r e y , has b e e n d e s c r i b e d (29,
30).
B y c a r e f u l e x a m i n a t i o n of a n u m b e r of
the
e x i s t i n g coins B u t t r e y w a s a b l e to d e t e r m i n e that almost a l l of t h e dies
8.
GORDUS
A N D
GORDUS
Gold Impurity
143
in Silver
Figure 9. Zapatista one- and two-peso coins issued in 1914 and 1915 and patterned after the previous federal coinage
u s e d to m i n t the coins w e r e c u t b y the same h a n d . B y a r r a n g i n g t h e coins i n terms of i m p e r f e c t i o n s i n the l e t t e r i n g c a u s e d b y w e a r a n d p a r t i a l b r e a k i n g of the letter fonts u s e d to p r e p a r e the dies, he w a s a b l e to arrange s e q u e n t i a l l y the o r d e r i n w h i c h the 1914 a n d 1915 coins w e r e m i n t e d . C o m b i n e d n e u t r o n H o w i t z e r a n d streak analyses of 125 one- a n d t w o - p e s o coins not o n l y r e v e a l the r e a l g o l d content of these issues b u t h e l p to establish the s i l v e r s t a n d a r d , w h i c h w a s p r o b a b l y a i m e d at 900 thousands fine ( 9 0 % thousands ( 8 0 % )
s i l v e r ) , a c r e d i t a b l e a l l o y a n d better t h a n the 800
of t h e f e d e r a l s u b s i d i a r y coinage.
It w a s
therefore
p o s s i b l e to estimate f a i r l y closely the t h e o r e t i c a l c o m p o s i t i o n a n d v a l u e of the Z a p a t i s t a silver coinage. T h e coins r e v e a l c e r t a i n aspects of Z a p a t i s t a m o n e t a r y p o l i c y a n d control.
First, an unprecedented alloy was created from w h i c h , it was
c a l c u l a t e d , coins e q u a l i n i n t r i n s i c v a l u e to the f e d e r a l c o u l d b e p r o d u c e d . H o w e v e r , this t h e o r e t i c a l system w a s not consistently p r a c t i c e d .
The
m a j o r 1914 issues of t h e t w o pesos g e n e r a l l y k e p t to a h i g h g o l d content, b u t t w o m i n o r m i n t s a n d a l l m i n t s i n 1915 d i d not. A s s h o w n i n F i g u r e 10, the average s i l v e r a n d g o l d contents, w h i l e e q u a l to or e v e n e x c e e d i n g the peso v a l u e i n the early c o i n , s h o w e d m a r k e d d e b a s e m e n t or o u t r i g h t m a l f e a s a n c e i n the m i n t i n g of s o m e of the later issues, a n d e s p e c i a l l y i n the one-peso issues.
144
ARCHAEOLOGICAL CHEMISTRY
T h e p r o c l a i m e d g o l d content represents o n l y a b o u t 2 %
g o l d since
the v a l u e - r a t i o of 1 g r a m of g o l d vs. 1 g r a m of s i l v e r w a s a b o u t 2 7 : 1 . A s n o t e d a b o v e , m a n y s i l v e r sources c o n t a i n g o l d as a n i m p u r i t y .
F o r ex
a m p l e , the d a t a for sixteenth-seventeenth c e n t u r y M e x i c a n s i l v e r piecesof-eight a n d l o w e r d e n o m i n a t i o n réaies s h o w n i n F i g u r e 7 i n d i c a t e a b o u t 0.07-0.3%
A u i n A g . T h e r e f o r e , the Z a p a t a coins h a v i n g h i g h e r g o l d
contents t h a t the p r o c l a i m e d values w e r e p r o b a b l y m a d e b y u s i n g n e w silver w h i c h a l r e a d y c o n t a i n e d some g o l d i m p u r i t y ; its presence, h o w e v e r , w a s not r e a l i z e d b y the m i n t e r s , a n d t h e a c t u a l p r o c l a i m e d a m o u n t of g o l d w a s a d d e d , i n effect, to the g o l d a l r e a d y present i n the silver. O n t h e other h a n d , those one-peso pieces h a v i n g g o l d contents m u c h l o w e r t h a n O R O : 0,300 m u s t also h a v e b e e n m a d e of n e w silver. H o w e v e r , i n I PESO COINS
ζ ο ο
2 PESO COiNS
ACTUAL
VALUE:
0
SILVER
S
GOLD
PROCLAIMED GOLD
'
CONTENT
III!
1914
1915
DATE
1914
1915 DATE
Figure 10. Proclaimed gold contents (in peso equivalents) and actual average silver and gold content values for Zapata one- and two-peso coins this case the i n t e n t w a s to d e c e i v e a n d , i n fact, n o g o l d w a s a c t u a l l y a d d e d to the silver. T h e detected g o l d l e v e l is s i m p l y the i m p u r i t y i n the s i l v e r u s e d f o r t h e coinage. I t w o u l d b e easy to d r a w a c y n i c a l c o n c l u s i o n f r o m these d a t a w h i c h s h o w that coins of l o w e r d e n o m i n a t i o n — n a m e l y those most l i k e l y to b e u s e d b y the p o o r e r p e o p l e — w e r e
d e b a s e d to a greater degree t h a n the
t w o - p e s o pieces w h i c h w e r e u s e d i n m a j o r transactions b y t h e r e v o l u t i o n a r y groups a n d w e r e k n o w n i n the A m e r i c a n b o r d e r states as the G u e r r e r o d o l l a r . B u t care s h o u l d b e t a k e n to dissociate these a p p a r e n t l y s e l f - s e r v i n g e c o n o m i c m a n i p u l a t i o n s as s h o w n t h r o u g h the c o i n a g e f r o m the m a i n t h r u s t of the Z a p a t i s t a r e v o l u t i o n a r i e s . Zapata's c o a l i t i o n was
8.
GORDUS
AND
Gold Impurity
GORDUS
in
145
Silver
c o m p o s e d of t w o s o c i o - e c o n o m i c groups (31).
T h e first was c o m p o s e d of
disaffected i n t e l l e c t u a l s w h o s u p p o s e d l y h a d u r b a n ties; it is most u n l i k e l y that these i n t e l l e c t u a l s w e r e m i n t e r s .
T h e other forces w e r e
peasants
w h o s e g r o u p resources a l l o w e d t h e m to p r o m o t e t h e i r o w n s p e c i a l p o l i t i c a l interests. H o w e v e r , e v e n a r e l a t i v e l y affluent peasant is u n l i k e l y to h a v e e n g a g e d i n the financial p a r t of the r e v o l u t i o n . I n fact, the l i m i t a t i o n of the Z a p a t i s t a m o v e m e n t w a s that it was a peasant m o v e m e n t ; troops m a r c h e d out f r o m villages for c o m b a t a n d r e t u r n e d to farms after the fray. If w e are to j u d g e w h a t appears to b e a b e t r a y a l of the interest of the p e o p l e b y the h i g h e r eschelons i n Zapata's r e v o l u t i o n a r y c a d r e , w e m u s t r e c a l l that w e are d e a l i n g w i t h a m o n e t a r i l y u n s o p h i s t i c a t e d g r o u p t h a t m a y v e r y w e l l h a v e e n l i s t e d the services of moneyers not o r i g i n a l l y Zapatistas a n d that r e m a i n e d u n a w a r e of the debasement of the coins. W h e n w e consider t h a t the Zapatistas h a d h o m e m a d e
arms a n d n o
o r g a n i z e d system of s u p p l i e s , it seems u n l i k e l y t h a t m a n y of t h e m r e a l i z e d their m o n e y was d e b a s e d .
C l e a r l y , t h e y never h a d m u c h of i t for w h e n
t h e y m a d e t h e i r v i c t o r i o u s e n t r y i n t o M e x i c o C i t y , the Zapatistas h a d n o f o o d a n d h a d to b e g f r o m
householders.
T h e s t u d y of r e v o l u t i o n s has a l w a y s p r o d u c e d instances of c o u n t e r r e v o l u t i o n a r y a n d o p p o r t u n i s t i c b e h a v i o r w h i c h are t h e n u s e d as e v i d e n c e for the b a d f a i t h of a l l the p a r t i c i p a n t s or at least t h e i r leaders. T h e w o r s t charge i n the l i g h t of the e v i d e n c e presented here (as w e l l as the other sources c i t e d ) w h i c h c a n be l e v e l e d at Z a p a t a , or for that matter at his N o r t h e r n c o u n t e r p a r t , P a n c h o V i l l a , is that n e i t h e r h a d sufficient p o l i t i c a l or e c o n o m i c s k i l l to g o v e r n the c o u n t r y .
B o t h a d a m a n t l y refused
the
office of p r e s i d e n t a n d felt themselves i l l - e q u i p p e d to p r o v i d e a n y t h i n g m o r e t h a n a t e m p o r a r y caretaker arrangement.
So the f a m o u s s i g n over
the P a l a c i o N a c i o n a l a d v e r t i s i n g the l a c k of a p o t e n t i a l p r e s i d e n t of a h i g h c a l i b e r : " W a n t e d — a n honest m a n " (32)
s h o u l d not b e t a k e n to
gether w i t h this m o n e t a r y e v i d e n c e to demonstrate m o r a l b a n k r u p t c y of the M e x i c a n R e v o l u t i o n . If a lesson m u s t be d r a w n , it s h o u l d be that m e n of s u c h l i m i t e d means c o u l d c h a n g e the face of a n a t i o n a n d yet r e m a i n h u m b l e e n o u g h to r e a l i z e t h e i r o w n shortcomings, a n d e v e n m a k e this s e l f - a p p r a i s a l k n o w n . Acknowledgment N o n e of the studies r e p o r t e d here w o u l d h a v e b e e n possible w i t h o u t the gracious a n d w i l l i n g c o o p e r a t i o n of curators b o t h i n the U n i t e d States and
Europe.
George
C . M i l e s of the A m e r i c a n N u m i s m a t i c Society,
T . C h a s e of the F r e e r G a l l e r y , P . H a r p e r of the M e t r o p o l i t a n M u s e u m , D.
M . M e t c a l f of the A s h m o l e a n M u s e u m , N . L o w i c k of the
British
M u s e u m , R . C u r i e l of the Bibliothèque N a t i o n a l e - C a b i n e t des Médailles, and
V . L u k o n i n , Β. M a r s h a k , a n d Ε. Z e y m a l of the State H e r m i t a g e
146
ARCHAEOLOGICAL CHEMISTRY
M u s e u m , L e n i n g r a d , a r e a m o n g those w h o a l l o w e d us to take streaks f r o m coins a n d a r t objects i n t h e i r collections.
T h e U n i v e r s i t y of M i c h i
gan, through the Department of Chemistry a n d the M i c h i g a n M e m o r i a l P h o e n i x Project, as w e l l as t h e U . S. A t o m i c E n e r g y C o m m i s s i o n , t h r o u g h its D i v i s i o n of R e s e a r c h ( o f w h i c h this is r e p o r t C O O - 9 1 2 - 2 7 ) , p r o v i d e d t h e r e s e a r c h f u n d s w h i c h m a d e this s t u d y possible.
T h r o u g h t h e years
n u m e r o u s u n d e r g r a d u a t e science students assisted i n t h e analyses, a n d w i t h o u t t h e i r a i d these thousands
o f analyses w o u l d n o t h a v e
been
possible. Literature
Cited
1. Gordus, Α. Α., Griffin, J. B., Wright, G. Α., "Activation Analysis Investiga tion of the Geologic Origins of Prehistoric Obsidian Artifacts," Chap. 16, pp. 222-234, "Science and Archaeology," R. Brill, Ed., MIT Press, Cam bridge, 1971. 2. Gordus, Α. Α., "Quantitative Non-Destructive Neutron Activation Analysis of Silver in Coins," Archaeometry (1967) 10, 78-86. 3. Gordus, Α. Α., "Rapid Non-Destructive Activation Analysis of Silver in Coins," Chap. 10, pp. 145-155, "Science and Archaeology," R. Brill, Ed., MIT Press, Cambridge, 1971. 4. Gordus, Α. Α., "Neutron Activation Analysis of Coins and Coin-Streaks," "Methods of Chemical and Metallurgical Investigation of Ancient Coin age," Royal Numismatic Society, Spec. Publ. 8 (1972) 127-148 + Plate XV. 5. McKerrell, H., Stevenson, R. Β. K., "Some Analyses of Chemical and Metallurgical Investigation of Ancient Coinage," Royal Numismatic So ciety, Spec. Publ. 8 (1972) 195-210. 6. Wertime, Τ. Α., "A Metallurgical Expedition through the Persian Desert," Science (1968) 159, 927-935. 7. LeStrange, G., "The Lands of the Eastern Caliphate," University Press, Cambridge, 1905. 8. al-Kazvini, Η. A. M., "The Geographical Part of the Nuzhat-al-Qulub," tr. by G. LeStrange, E. J. W. Gibb Mem. Ser. (1919) XXIII.2., 193. 9. Walker, J., "A Catalog of Arab-Byzantine and Post-Reform Umaiyad Coins," p. lxxxviii, British Museum, London, 1956. 10. Bacharach, J. L., Gordus, Α. Α., "The Purity of Sasanian Silver Coins," J. Amer. Oriental Soc. (1972) 92.2, 280-283. 11. Gordus, Α. Α., "Non-Destructive Analysis of Parthian, Sasanian, and Umay yad Silver Coins," in "Near Eastern Numismatics, Iconography, Epi graphy, and History: Studies in Honor of George C. Miles," pp. 141-162, D. K. Kouymjian, Ed., American University of Beirut, Lebanon, 1974. 12. Göbl, R., "Sasanian Numismatics," Klinkhardt and Niermann, Braun schweig, (English ed.), 1971. 13. Lukonin, V. G., "Archaeologia Mundi: Persia II," Nagel Publishers, Geneva, 1967. 14. Ghirshman, R., "Persian Art: 249 B.C.-A.D. 651," Golden Press, New York, 1962. 15. Orbeli, J., Trever, C., "Orfeverie Sassanide," Moscow and Leningrad, 1935. 16. Lukonin, V. G., op. cit., Figures 136, 138; Ghirsham, R., op. cit., Figure 253; Orbeli, J., Trever, C., op. cit., Figure 6. 17. Ghirsham, R., op. cit., Figures 248, 249, 250. 18. Ibid., Figure 252.
8.
GORDUS
A N D GORDUS
Gold Impurity
in Silver
147
19. Gordus, Α. Α., Metcalf, D. M., "The Alloy of the Byzantine Miliaresion and the Question of the Reminting of Islamic Silver," in press. 20. Porteous, J., "Coins in History," pp. 170-171, Plate 194, Weidenfeld and Nicolson, London, 1969. 21. For valuable elaborations on the Spanish economic theoreticians, see: Grice -Hutchinson, M., "The School of Salamanca," Oxford University Press, 1952, and Vilar, P., "Les primitifs espagnols de la pensée economiques," in "Mélanges offerts à Marcel Bataillon," Colin, Bordeaux, 1963. 22. Bodin, J., "Response à M. Malestroit," Henri Hauser, Ed., Colin, Paris, 1932. 23. Hamilton, E. J., "American Treasure and the Price Revolution in Spain," Harvard University Press, Cambridge, Mass., 1934. 24. Hammarström, I., "The Price Revolution of the Sixteenth Century, Some Swedish Evidence," in "The Price Revolution in Sixteenth Century Eng land," collected by P. H. Ramsey, Methuen, London, 1971. 25. Wiebe, G., "Zür Geshichte der Preisrevolution des XVI und XVII Jahrhunderts," Vol. II, Leipzig, 1895. 26. Braudel, F., "The Mediterranean and the Mediterranean World in the Reign of Philip II," Vol. I, p. 476, transl. by S. Reynolds, Collins, Lon don, 1972. 27. Gordus, Α. Α., Gordus, J., LaDurie, E. L., Richet, D., "Le Potosi et la Physique Nucleaire," Ann.: Econ. Soc., Civilis. (1972) 1235-1256. 28. Spooner, F. C., "L'Economie Mondiale et les Frappes Monetaires en France, 1493-1689," C.N.R.S., Paris, 1960. 29. Buttrey, T. V., Gordus, Α. Α., "The Silver Coinage of Zapata: 1914-1915," Hispanic Amer. Hist. Rev. (1972) 52, 456-462. 30. Buttrey, T. V., Gordus, Α. Α., "A Neutron Activation Analysis of the Silver Coinage of Zapata, 1914-1915," Amer. Numismatic Soc. Museum Notes, in press. 31. Wolf, E . R., "Peasant Wars of the Twentieth Century," pp. 29-35, Harper and Row, New York, 1971. 32. Pinchon, E., "Zapata the Unconquerable," p. 306, Doubleday, New York, 1941. RECEIVED July 9,
1973.
