Archaeological Chemistry

3 Major and Trace Elements in Sasanian Silver

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 5, 2017 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/ba-1974-0138.ch003

PIETER MEYERS The Metropolitan Museum of Art, New York, N.Y. 10028 LAMBERTUS VAN ZELST The Metropolitan Museum of Art, New York, N.Y. 10028 and Brookhaven National Laboratory, Upton, N.Y. 11973 EDWARD V. SAYRE Brookhaven National Laboratory, Upton, N.Y. 11973 and Institute of Fine Arts, New York University, New York, N.Y. 10028 Thermal neutron activation analysis was done on small sam ples from Sasanian silver objects to determine the concen trations of three major elements, Ag, Cu, and Au, and 16 trace elements, Νa, K, Sc, Cr, Mn, Fe, Ni, Co, Zn, As, Br, Sn, Sb, Se, Ir, and Hg. Two microsampling techniques (drilling and rubbing) were used for Ag, Cu, and Au. Analyses for major components in rubbing and drilling samples of the same objects showed the effects of surface enrichment for gold and surface depletion for copper. Drilling samples of ca. 500 μg were used to determine trace element concentrations. Multiplicate analyses were satisfactorily reproducible in the silver alloy for Ir, Zn, Se, As, and Sb, less satisfactory for Sn, Sc, Mn, and Br while large inhomogeneities were observed for Cr, Ni, Fe, Co, Hg, Νa, and K. We attempted to group objects according to trace element compositions and to relate such groups to art historical information. So far gold and iridium concentrations seem to be the most promising cri teria for distinguishing different origins of silver ore. * " p h e e l e m e n t a l c o m p o s i t i o n of a n c i e n t silver objects is a p o t e n t i a l source of i n f o r m a t i o n o n t h e k i n d of ores u s e d to p r o d u c e silver, t h e l o c a t i o n of these ores, t h e ancient m e t a l l u r g y u s e d to extract t h e silver, a n d t h e t r a d e routes t h r o u g h w h i c h they passed.

A s p a r t of a c o m p r e h e n s i v e

s t u d y o n S a s a n i a n silver at t h e M e t r o p o l i t a n M u s e u m of A r t w e u s e d t h e r m a l n e u t r o n a c t i v a t i o n to a n a l y z e s m a l l samples f r o m silver objects 22

Beck; Archaeological Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

3.

MEYERS

E T

AL.

Impurities

in Sasanian

23

Silver

of the S a s a n i a n p e r i o d ( P e r s i a 227-651 A . D . ) as w e l l as f r o m r e l a t e d n e a r eastern c i v i l i z a t i o n s . C o m b i n e d w i t h i n f o r m a t i o n f r o m i n s c r i p t i o n s , f r o m s t y l i s t i c e x a m i n a t i o n , a n d f r o m studies of other p r o p e r t i e s of these objects ( e.g., m e t h o d of m a n u f a c t u r e , t o o l m a r k s ) w e are i n v e s t i g a t i n g to w h a t extent the e l e m e n t a l c o m p o s i t i o n of these objects c a n y i e l d a d d i t i o n a l i n f o r m a t i o n . M o s t l i k e l y the sources of s i l v e r d u r i n g the S a s a n i a n


p e r i o d w e r e argentiferous l e a d ores, p r o b a b l y m a i n l y g a l e n a ( P b S ) a n d p o s s i b l y cerussite ( P b C 0 ). B e c a u s e n a t i v e silver a n d s i l v e r ores r a r e l y 3

o c c u r i n the N e a r E a s t , these sources w e r e m u c h less l i k e l y u s e d for s i l v e r p r o d u c t i o n (1,2,3). g a l e n a i n t w o steps.

S i l v e r m e t a l c a n be o b t a i n e d f r o m s i l v e r - c o n t a i n i n g (1)

T h e l e a d ore is s m e l t e d i n a f u r n a c e

under

p a r t l y o x i d i z i n g c o n d i t i o n s , a n d m o l t e n l e a d is c o l l e c t e d at t h e b o t t o m of the f u r n a c e ; this l e a d contains the silver as w e l l as most of the m e t a l l i c impurities.

(2)

S i l v e r m e t a l is extracted f r o m the l e a d b y c u p e l l a t i o n —

i.e., the l e a d is o x i d i z e d b y heat w i t h a i r i n a porous vessel

(cupel).

M o s t of the less n o b l e metals w i l l also b e o x i d i z e d d u r i n g this process. T h e l e a d o x i d e ( l i t h a r g e ) is a b s o r b e d i n the porous m a t e r i a l s i n or n e a r the b o t t o m of the c u p e l , l e a v i n g the silver m e t a l b e h i n d .

Cupellation

c a n be r e p e a t e d u n t i l the m e t a l has a satisfactory p u r i t y .

A certain

a m o u n t of c o p p e r , g e n e r a l l y a b o u t 5 % , was d e l i b e r a t e l y a l l o y e d w i t h t h e silver thus o b t a i n e d to increase its strength a n d m a l l e a b i l i t y . I m p u r i t y concentrations i n s i l v e r objects d e p e n d o n : ( 1 ) I n i t i a l c o n c e n t r a t i o n of e a c h element, r e l a t i v e to t h e c o n c e n t r a t i o n of s i l v e r i n the ore (2) Contamination b y materials added d u r i n g smelting a n d cupella tion—i.e., f u e l , flux ( 3 ) E v a p o r a t i o n of v o l a t i l e metals or m e t a l c o m p o u n d s smelting, cupellation, casting, a n d annealing

during

(4)

S e p a r a t i o n of less n o b l e metals f r o m silver d u r i n g c u p e l l a t i o n

(5)

C o n t a m i n a t i o n ( f o r some elements)

b y the a d d i t i o n of c o p p e r

( 6 ) U s e of " s c r a p " s i l v e r , e s p e c i a l l y w h e n o b t a i n e d f r o m l e a d ores

different

( 7 ) I n h o m o g e n e o u s d i s t r i b u t i o n of the i m p u r i t i e s i n b o t h t h e l e a d ore a n d the s i l v e r m e t a l . S i n c e so m a n y factors affect the final c o m p o s i t i o n of a n c i e n t silver a n d since t h e i r effects o n t h e final c o m p o s i t i o n are v i r t u a l l y u n k n o w n , i n t e r p r e t a t i o n of a n y a n a l y t i c a l d a t a c o u l d b e v e r y difficult. H o w e v e r , p r e v i o u s analyses of silver coins, e s p e c i a l l y those b y K r a a y a n d E m e l e u s (4)

and

b y G o r d u s ( 5 ) , h a v e s h o w n t h a t the c o n c e n t r a t i o n of at least one element — g o l d — i n silver c a n b e i n d i c a t i v e of the s i l v e r - b e a r i n g ore used.

Concen

t r a t i o n levels of other n o b l e metals r e l a t i v e to silver, s u c h as i r i d i u m , c a n b e e x p e c t e d to r e m a i n constant or almost constant d u r i n g s i l v e r p r o d u c t i o n . T h e s e elements, l i k e g o l d , c o u l d b e u s e d to d i s t i n g u i s h s i l v e r o b t a i n e d


24

ARCHAEOLOGICAL CHEMISTRY

f r o m l e a d ores of different g e o g r a p h i c

origins.

O t h e r elements m i g h t

c h a n g e t h e i r c o n c e n t r a t i o n levels r e l a t i v e to s i l v e r . N e v e r t h e l e s s , these concentrations c o u l d s t i l l h e l p to " f i n g e r p r i n t " t h e s i l v e r or to y i e l d m o r e i n f o r m a t i o n o n one or m o r e of the processes u s e d to p r o d u c e silver. I n this r e p o r t w e d e s c r i b e our q u a n t i t a t i v e analyses for three m a j o r c o m p o n e n t s a n d for 16 t r a c e elements present f r o m parts p e r b i l l i o n ( p p b )


to parts p e r m i l l i o n ( p p m ) i n S a s a n i a n s i l v e r b y t h e r m a l n e u t r o n a c t i v a t i o n analysis. T h e a n a l y t i c a l d a t a are d i s c u s s e d element b y

element.

S o m e of the conclusions w h i c h c a n b e d r a w n f r o m the objects are d i s cussed. S i n c e the a m o u n t of c o m p a r a t i v e d a t a a v a i l a b l e is too s m a l l for g e n e r a l i z a t i o n s , m a n y of o u r c o n c l u s i o n s a n d assumptions are subject to change w h e n more data and information become available. Analytical Techniques I n o r d e r not to disfigure v a l u a b l e a n c i e n t s i l v e r objects, o n l y m i n u t e specimens c a n b e r e m o v e d f o r analysis. S a m p l e size is thus a m a j o r r e s t r i c t i o n o n the a n a l y t i c a l m e t h o d chosen. T h e r m a l n e u t r o n a c t i v a t i o n analysis p r o v i d e s excellent o p p o r t u n i t i e s f o r q u a n t i t a t i v e m u l t i - e l e m e n t analysis of these s m a l l samples. T h e q u e s t i o n as to h o w a c c u r a t e l y these samples represent t h e entire s p e c i m e n is c o n s i d e r e d i n d e t a i l later. T w o m i c r o s a m p l i n g t e c h n i q u e s a p p l i c a b l e to a n c i e n t s i l v e r a n d other m e t a l objects w e r e d e v e l o p e d at 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 : ( 1 ) surface r u b b i n g s o n g r o u n d q u a r t z supports (also c a l l e d s t r e a k s ) , a n d ( 2 ) m i c r o d r i l l i n g s t h a t penetrate t h e i n t e r i o r of t h e object. T h e s e s a m p l i n g t e c h n i q u e s , together w i t h a n a l y t i c a l m e t h o d s for the m a j o r c o m p o n e n t s ( A g , C u , a n d A u ) b o t h i n r u b b i n g s a n d d r i l l i n g s a n d for 16 t r a c e elements ( N a , K , Sc, C r , M n , F e , N i , C o , Z n , A s , B r , S n , S b , Se, Ir, and H g ) i n drillings, have been reported i n detail (6). Therefore, a n a l y t i c a l t e c h n i q u e s are d e s c r i b e d o n l y briefly. G o r d u s ( 5 ) also has m a d e extensive use of the r u b b i n g ( s t r e a k ) t e c h n i q u e . T h e r u b b i n g s , either o n q u a r t z plates or q u a r t z t u b i n g , e a c h w r a p p e d separately i n h i g h p u r i t y a l u m i n u m f o i l , w e r e i r r a d i a t e d f o r 30 m i n to gether w i t h r u b b i n g s of s i l v e r standards of k n o w n c o m p o s i t i o n i n the B r o o k h a v e n H i g h F l u x B e a m reactor ( H F B R ) at a flux of 1.8 Χ 1 0 n e u t r o n s / c m / s e c . T h e g a m m a - r a y energy spectra of e a c h s a m p l e w e r e r e c o r d e d , s t a r t i n g a p p r o x i m a t e l y 3 hrs after a c t i v a t i o n , u s i n g a n a u t o m a t i c s a m p l e c h a n g i n g system a n d a 40-cc G e ( L i d r i f t e d ) s e m i c o n d u c t o r g a m m a r a y detector c o u p l e d to a 3200 or 4096 c h a n n e l p u l s e h e i g h t a n a l y z e r . T h e g a m m a - r a y s p e c t r a w e r e c o m p u t e r a n a l y z e d for p e a k energies a n d p e a k areas. B y c o m p a r i n g p e a k areas at 412 k e V ( e m i t t e d i n the d e c a y of 2.7 d A u ) , 511 k e V (12.8 h C u ) , a n d 658 k e V (245 d A g ) i n s p e c t r a for samples f r o m objects a n d f r o m standards, the w e i g h t ratios of s i l v e r , c o p p e r , a n d g o l d c o u l d b e d e t e r m i n e d . T h e a s s u m p t i o n t h a t these three elements are t h e o n l y ones present i n significant concentrations a l l o w s us to c a l c u l a t e the a p p r o x i m a t e c o m p o s i t i o n s of the s i l v e r samples. T h i s a s s u m p t i o n e l i m i n a t e s the n e e d to k n o w the s a m p l e w e i g h t w h i c h is diffi c u l t to d e t e r m i n e for r u b b i n g s . L i t t l e e r r o r is i n t r o d u c e d b y a s s u m i n g t h a t s i l v e r , c o p p e r , a n d g o l d a d d u p to 1 0 0 % since other elements, t h e m a j o r one b e i n g l e a d , are r a r e l y present i n t o t a l concentrations over 1 - 2 % . 1 4

2

1 9 8

6 4

1 1 0

m


3.

Impurities

MEYERS ET AL.

in Sasanian

25

Silver


D r i l l e d samples, 2 0 0 - 1 0 0 0 μg a n d sealed i n p u r e q u a r t z v i a l s , are activated a n d analyzed i n a similar w a y for silver, copper, a n d gold con tent. D r i l l bits of t u n g s t e n c a r b i d e a n d stainless steel, m a t e r i a l s m u c h h a r d e r t h a n s i l v e r , w e r e u s e d i n t a k i n g samples. S i n c e no significant differences i n concentrations of t h e m a j o r elements of the d r i l l b i t m a terials c o u l d be f o u n d i n samples t a k e n w i t h either of t h e m , p o s s i b l e c o n t a m i n a t i o n b y the d r i l l bits c a n b e n e g l e c t e d . T h e h i g h intensities of t h e g a m m a rays e m i t t e d i n t h e d e c a y of t h e r a d i o a c t i v e silver, c o p p e r , a n d g o l d isotopes p r e v e n t the o b s e r v a t i o n of g a m m a rays f r o m most other r a d i o a c t i v e isotopes present i n i r r a d i a t e d silver samples. T h e r e f o r e r a d i o a c t i v e silver, c o p p e r , a n d g o l d w e r e s e p a r a t e d c h e m i c a l l y f r o m v i r t u a l l y a l l other elements. T h e p r o c e d u r e for d e t e r m i n i n g 16 trace elements, p r e v i o u s l y d e s c r i b e d i n d e t a i l ( 6 ) , is as f o l l o w s . T w o sets of d r i l l e d samples w e r e w e i g h e d ( 2 0 0 - 1 0 0 0 μg) a n d sealed i n d i v i d u a l l y i n s m a l l q u a r t z vials. O n e set, to b e a n a l y z e d for r a d i o n u c l i d e s w i t h h a l f - l i v e s shorter t h a n 7 d a y s , w a s i r r a d i a t e d f o r 16 hrs at a flux of 1.8 Χ 1 0 n e u t r o n s / c m / s e c i n t h e H F B R at 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 . T h e second set, to b e a n a l y z e d f o r r a d i o n u c l i d e s w i t h h a l f - l i v e s l o n g e r t h a n 7 d a y s , w a s i r r a d i a t e d for 7 days i n the same facility. 1 4

2

S t a r t i n g 2 hrs after i r r a d i a t i o n , samples of t h e first set w e r e d i s s o l v e d i n n i t r i c a c i d . R a d i o a c t i v e silver, c o p p e r , a n d g o l d isotopes w e r e t h e n q u a n t i t a t i v e l y r e m o v e d f r o m s o l u t i o n b y isotope e x c h a n g e w i t h excess freshly p r e p a r e d c u p r o u s i o d i d e w h i c h w a s m i x e d w i t h the s o l u t i o n . G a m m a - r a y e n e r g y spectra of t h e solutions m e a s u r e d d u r i n g 100 m i n w i t h a 40-cc G e ( L i - d r i f t e d ) s e m i c o n d u c t o r detector c o u p l e d to a 4096 c h a n n e l p u l s e h e i g h t a n a l y z e r s h o w e d p h o t o p e a k s of N a (t = 15 h , E = 1369 k e V ) , K (t — 12.4 h , E = 1524 k e V ) , M n (t — 2.6 h , E = 847 k e V ) , N i (t = 2.6 h , E = 1482 k e V ) , Z n (t — 13.8 h , E = 439 k e V ) , As (t = 26 h , E — 559 k e V ) , B r (t — 35 h , E — 776 keV), and S b (t = 2.7 d , E = 564 k e V ) . It w a s e x p e r i m e n t a l l y d e t e r m i n e d that the s e p a r a t i o n y i e l d for a l l of these elements w a s at least 90%. 2 4

4 2

1/2

6 5

1/2

76

1/2

m

1/2

y

1/2

8 2

y

1 2 2

1/2

6 9

y

1/2

y

5 6

y

y

1/2

y

y

T h e s e c o n d set of samples w a s c o o l e d f o r 1 0 - 1 4 days after i r r a d i a t i o n . T h e s i l v e r samples w e r e t r e a t e d l i k e those of the first set except that silver i o d i d e w a s u s e d for the isotope exchange. S i l v e r i o d i d e is s l i g h t l y better t h a n c u p r o u s i o d i d e for this process b u t does n o t r e m o v e r a d i o a c t i v e c o p p e r isotopes f r o m s o l u t i o n . H o w e v e r , the h a l f - l i f e of C u , 12.8 h r s , is so short that its a c t i v i t y l e v e l has sufficiently decreased d u r i n g c o o l i n g a n d thus does n o t interfere w i t h the o b s e r v a t i o n of other r a d i o a c t i v e n u c l i d e s . T h e g a m m a - r a y energy spectra of the solutions a c q u i r e d i n 2 0 0 - m i n c o u n t i n g p e r i o d s , s h o w e d m e a s u r a b l y intense p h o t o p e a k s of S c (t _ 84 d , E — 889 a n d 1120 k e V ) , C r (t = 28 d , E = 320 k e V ) , 59Fe (t — 45 d , E = 1099 a n d 1291 k e V ) , C o (t = 5.3 y, E — 1173 a n d 1333 k e V ) , Z n (t — 245 d , E = 1115 k e V ) , S e (t = 120 d , E — 136 k e V ) , I n ( d a u g h t e r of Sn, t = 115 d ) (E = 391 keV), S b (t — 60 d , E — 1691 k e V ) , I r (t = 74 d , E = 316 K e V ) , and H g (t = 47 d , E = 279 k e V , c o r r e c t e d for interference f r o m S e ) . A g a i n the q u a n t i t a t i v e y i e l d s of these elements w e r e satis factory. 6 4

4 6

1/2

5 1

y

1/2

1/2

6 0

y

6 5

1/2

1 1 3

y

1 2 4

1/2

2 0 3

1/2

m

y

7 5

y

1 1 3

1/2

1 9 2

y

1/2

y

1/2

y

1/2

y

y

7 5



26


Q u a n t i t a t i v e estimates of the c o n c e n t r a t i o n levels of these 16 elements c o u l d b e o b t a i n e d b y u s i n g s i l v e r standards c o n t a i n i n g k n o w n amounts of the elements of interest. T w o standards w e r e p r e p a r e d at 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 l t h o u g h w e h a v e e s t a b l i s h e d that the elements i n these standards are h o m o g e n e o u s l y d i s t r i b u t e d , w e h a v e not c o m p l e t e d t h e i r c a l i b r a t i o n s . M o r e o v e r t h e standards w e r e not u s e f u l for s c a n d i u m , t i n , a n d i r i d i u m . F o r these elements the concentrations w e r e c a l c u l a t e d r e l a t i v e to those of elements for w h i c h g o o d standards w e r e a v a i l a b l e , u s i n g n u c l e a r cross-sections a n d d e c a y constants f r o m the l i t e r a t u r e ( 7 ). Discussion Major Components: Silver, Copper, and Gold. B e f o r e e v a l u a t i n g a n y a n a l y t i c a l d a t a it is i m p o r t a n t to s t u d y t h e a c c u r a c y a n d r e p r o d u c i b i l i t y of e a c h s a m p l i n g t e c h n i q u e used—i.e., r u b b i n g a n d d r i l l i n g . Is a d r i l l i n g s a m p l e of 2 0 0 - 5 0 0 ^ g representative of t h e entire object? D u p l i c a t e a n a l y ses o n d r i l l i n g samples f r o m different areas of the same ( a n c i e n t s i l v e r ) object gave consistent results for g o l d ( T a b l e I ). T h e v a r i a t i o n s are w i t h i n e x p e r i m e n t a l error. C o p p e r analyses s h o w a g r o u p s t a n d a r d d e v i a t i o n of t h e f r a c t i o n a l d e v i a t i o n s f r o m the m e a n of 4 . 5 % for c o p p e r contents less t h a n 6%

a n d of 1 5 % for c o p p e r contents m o r e t h a n 6 % .

indicate an inhomogeneous

These variations

d i s t r i b u t i o n of the c o p p e r i n the s i l v e r w h i c h

c a n b e c a u s e d b y p h a s e s e p a r a t i o n w h i c h for p u r e c o p p e r - s i l v e r alloys starts at a b o u t the 5 % c o p p e r l e v e l , p o o r m i x i n g or l o c a l d e p l e t i o n a r i s i n g from

corrosion. Table I.

Percent Group Standard Deviation from the Mean in Multiple Analyses of D r i l l i n g Samples Cu ~6% 4 15

T h e r u b b i n g t e c h n i q u e y i e l d s o n l y surface samples.

9 2.2 T o establish

the extent to w h i c h these are t r u l y representative of the entire object w e c o m p a r e d ( 1 ) analyses of r u b b i n g s f r o m 30 s i l v e r objects, s a m p l e d a n d analyzed by Gordus (8)

to ( 2 )

analyses of b o t h r u b b i n g s a n d d r i l l i n g s

f r o m the same objects, s a m p l e d a n d a n a l y z e d b y the authors. B e c a u s e of n a t u r a l c o r r o s i o n a n d c l e a n i n g operations, c o m p o s i t i o n a l changes at t h e surface c a n b e expected.

Copper

p l e t e d at t h e surface w h i l e g o l d

(less n o b l e t h a n s i l v e r ) w i l l b e ( m o r e n o b l e ) w i l l be e n r i c h e d .

de The

influence of these surface effects w i l l d e p e n d , a m o n g other factors, o n the d e p t h at w h i c h the r u b b i n g is t a k e n . S i n c e surface effects m i g h t b e s t r o n g l y r e l a t e d to the fineness of t h e silver, o n l y objects are c o n s i d e r e d


3.

MEYERS ET AL.

Impurities

in Sasanian

27

Silver

w i t h a s i l v e r content b e t w e e n 90 a n d 9 8 % w h i c h is t h e most f r e q u e n t l y encountered

fineness

of S a s a n i a n silver. T h e results o f this c o m p a r i s o n ,

expressed i n average ratios of the concentrations of g o l d a n d c o p p e r i n the r u b b i n g s a n d t h e c o r r e s p o n d i n g concentrations i n d r i l l i n g s , are s h o w n i n T a b l e I I . S u r f a c e effects are c l e a r l y a p p r e c i a b l e i n t h e r u b b i n g samples. I n the results of G o r d u s one observes a surface d e p l e t i o n r e l a t i v e to s i l v e r Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 5, 2017 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/ba-1974-0138.ch003

o n the average of 1 2 % for c o p p e r a n d a n e n r i c h m e n t o n the average of 1 2 % f o r g o l d . F o r o u r w o r k these figures are 1 7 % d e p l e t i o n for c o p p e r a n d 2 2 % e n r i c h m e n t for g o l d .

T h e better results of G o r d u s c a n b e at

t r i b u t e d to a s l i g h t l y different s a m p l i n g t e c h n i q u e .

G o r d u s cleans

s a m p l i n g area w i t h e m e r y p a p e r , t h e r e b y r e m o v i n g the u p p e r

the

surface

layer. W e c l e a n this area first u s i n g a cotton s w a b s o a k e d w i t h w a t e r a n d acetone.

T h e n , s e v e r a l r u b b i n g s are t a k e n , a n d the first three or f o u r are

discarded.

O u r less d e s t r u c t i v e r u b b i n g t e c h n i q u e p r o v i d e s samples at

less d e p t h w h i c h are therefore less r e p r e s e n t a t i v e of the i n t e r i o r . Table II. Average Ratios of Copper and Gold Concentrations in Rubbings to those in Drillings with their Group Standard Deviation Copper Ratios Gordus rubbings" Our rubbings Objects sampled 0

0.87 =b 0.21 0.83 =b 0.33 27

Gold

Ratios

1.12 =b 0.09 1.22 ± 0.23 30

° Relative to our drillings. D e s p i t e a n a p p r e c i a b l e i n f l u e n c e of surface effects o n the a c c u r a c y of analysis of r u b b i n g samples, this s a m p l i n g t e c h n i q u e is s t i l l v e r y u s e f u l , e s p e c i a l l y w h e r e a h i g h l y a c c u r a t e analysis is not r e q u i r e d or

where

d r i l l i n g c a n n o t b e p e r m i t t e d . H o w e v e r f r o m s i l v e r objects w i t h a n i n t e r n a l c o p p e r c o n c e n t r a t i o n a b o v e 1 0 % , r u b b i n g samples often c o n t a i n o n l y a s m a l l f r a c t i o n of this h i g h c o p p e r content.

D r i l l i n g samples ( i f possible

m u l t i p l e d r i l l i n g s ) are p r e f e r r e d for objects w i t h h i g h c o p p e r contents. Trace Elements. A s i n t h e case of the major components, the r e p r o d u c i b i l i t y of the analysis was tested for e a c h element b y a n a l y z i n g t w o or m o r e samples f r o m different areas of the same object.

T h e results,

p r e s e n t e d i n T a b l e I I I , s h o w for e a c h element t h e g r o u p s t a n d a r d d e v i a t i o n of m u l t i p l e analyses a v e r a g e d o v e r a n u m b e r of objects. T h e r e p r o d u c i b i l i t y is satisfactory for i r i d i u m , z i n c , s e l e n i u m , arsenic, a n d a n t i m o n y ; less satisfactory for t i n , s c a n d i u m , manganese, a n d b r o m i n e ; a n d l a r g e v a r i a t i o n s are o b s e r v e d i n d u p l i c a t e analyses f o r c h r o m i u m , n i c k e l , i r o n , cobalt, m e r c u r y , s o d i u m , a n d p o t a s s i u m .

( A l t h o u g h the v a r i a t i o n s r e

p o r t e d here for n i c k e l are r e l a t i v e l y s m a l l , a d d i t i o n a l d a t a s h o w that this element s h o u l d b e l o n g i n this g r o u p . )

T h e large v a r i a t i o n s for t h e ele

ments of the last g r o u p i n d i c a t e that t h e y are too i n h o m o g e n e o u s l y


dis-

28

ARCHAEOLOGICAL

Table III.

Percent Group Standard Deviation from the Mean in Multiple Analyses of D r i l l i n g Samples

Percent Standard Deviation

Element


CHEMISTRY

Number of Multiple A

Ir Zn Se As Sb

11 15 19 14 12

12 11 7 6 13

Sn Sc Mn Br

26 37 47 48

4 3 5 4

Cr Ni Fe Co Hg Na Κ

81 13 63 53 61 79 60

3 2 2 7 9 5 5

t r i b u t e d i n t h e s i l v e r for the s a m p l i n g t e c h n i q u e s to b e representative. A m a j o r cause of p o o r r e p r o d u c i b i l i t y for s o d i u m a n d p o s s i b l y p o t a s s i u m is p r o b a b l y c o n t a m i n a t i o n .

M o r e analyses are necessary for t h e second

g r o u p of elements to o b t a i n a m o r e a c c u r a t e q u a n t i t a t i v e estimate of t h e r e p r o d u c i b i l i t y of t h e i r d e t e r m i n a t i o n . Archaeological

Implications

of the Results

W e n o w h a v e p r e l i m i n a r y d a t a o n a b o u t 70 objects f r o m t h e c o l l e c tions of

t h e 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 , T h e M e t r o p o l i t a n

M u s e u m of A r t , N e w Y o r k a n d L e M u s é e d u L o u v r e , P a r i s .

A typical

e x a m p l e of a Sassanian s i l v e r object is s h o w n i n F i g u r e 1. A t t h i s p o i n t a l l a n a l y t i c a l w o r k is n o t c o m p l e t e d ; o u r s i l v e r standards f o r trace ele ments h a v e y e t to b e c a l i b r a t e d . I n a d d i t i o n , a p r o p e r f u l l i n t e r p r e t a t i o n of these results w o u l d o n l y b e feasible i n c o m b i n a t i o n w i t h a c r i t i c a l e v a l u a t i o n of s t y l i s t i c p r o p e r t i e s , m e t h o d of m a n u f a c t u r e , m e t a l l u r g i c a l e x a m i n a t i o n , t o o l m a r k s , etc., w h i c h does not seem to b e a p p r o p r i a t e for this v o l u m e .

T h e r e f o r e , w e confine o u r i n t e r p r e t a t i o n of the a n a l y t i c a l

d a t a to a f e w g e n e r a l observations.

W e w i l l n o t y e t c o n s i d e r those ele

ments w h i c h d i d not y i e l d satisfactory results o n d u p l i c a t e d e t e r m i n a t i o n s a l t h o u g h t h e i r c o n c e n t r a t i o n levels m i g h t be of interest a n d the c o n c e n t r a t i o n ratios b e t w e e n some of t h e m significant. M a n y of t h e objects s t u d i e d consist of separate parts j o i n e d together. F o r e x a m p l e , most of t h e plates h a v e a foot w h i c h w a s f a b r i c a t e d as a


3.

MEYERS

E T

AL.

Impurities

in Sasanian

separate p i e c e f r o m t h e p l a t e shell.

29

Silver

A n a l y s e s of foot a n d p l a t e s h e l l

samples c a n i n d i c a t e i f these parts are m a d e f r o m t h e same or r e l a t e d silver. A g r e e m e n t b e t w e e n the t w o pieces i n the c o n c e n t r a t i o n of a l l of the elements w h i c h c a n b e d e t e r m i n e d r e l i a b l y w o u l d i n d i c a t e a h i g h p r o b a b i l i t y that t h e y w e r e f o r m e d f r o m the same b a t c h of silver. If there is a m a r k e d difference i n the concentrations o f those t r a c e elements f o r


w h i c h c o n s i d e r a b l e v a r i a t i o n is l i k e l y d u r i n g t h e w i n n i n g , a l l o y i n g , a n d treatment of the m e t a l , b u t s t i l l agreement b e t w e e n g o l d a n d i r i d i u m w h i c h l i k e l y r e t a i n t h e i r c o n c e n t r a t i o n levels r e l a t i v e to s i l v e r d u r i n g s u c h p r o c e s s i n g , i t w o u l d s t i l l b e a g o o d i n d i c a t i o n t h a t the same source of s i l v e r w a s u s e d for the alloys of t h e t w o pieces b u t t h a t t h e silver w a s p r e p a r e d i n different batches.

T o t a l d i s a g r e e m e n t , of course,

suggests

q u i t e different silver. O f a t o t a l of 23 objects f o r w h i c h c o m p a r a t i v e analyses for different parts are a v a i l a b l e , seven are m a t c h i n g i n a l l r e l i a b l e elements d e t e r m i n e d

Figure 1. Sasanian silver gilt plate showing a king hunting ibexes, late 5th or 6th century A.D. (Metropolitan Museum of Art, Fletcher Fund, 1934)


30


500


200

lOOh

~

20

Figure 2. Correlation plot of gold and iridium concentrations relative to silver of all objects analyzed. Analyses of different parts of an object are represented separately. i n the separate parts. T h e s e parts, therefore, a p p e a r to b e f r o m t h e same b a t c h of silver. S i x other objects s h o w m a t c h i n g concentrations of g o l d a n d i r i d i u m b u t not of t h e less n o b l e metals. T h e separate parts of these objects, therefore, c o u l d h a v e b e e n m a d e of s i l v e r f r o m a c o m m o n

ore

source b u t not f r o m a single b a t c h . I n a s u r p r i s i n g l y l a r g e n u m b e r of the objects, 10 i n a l l , there is no m a t c h at a l l i n the m i n o r a n d trace elements. P o s s i b l e explanations for this d i s a g r e e m e n t b e t w e e n the separate pieces of these objects ( u s u a l l y the shells of plates a n d the s u p p o r t i n g feet) m i g h t be m a n u f a c t u r i n g t r a d i t i o n , that is, t h e feet w e r e p r o d u c e d separately as s t a n d a r d items for a t t a c h m e n t to plates or that feet w e r e r e p l a c e d i n later r e p a i r s . B e c a u s e of the extent to w h i c h t h e g o l d a n d i r i d i u m concentrations seem r e l a t e d to ore sources, objects w e r e g r o u p e d a c c o r d i n g to these c o n -


3.

MEYERS

Impurities

E T A L .

in Sasanian

31

Silver

centrations o n a l o g - l o g scale ( F i g u r e 2 ) . A h i s t o g r a m of t h e i r i d i u m c o n c e n t r a t i o n s i n a l l o f the v a r i o u s specimens a n a l y z e d is s h o w n i n F i g u r e 3. T o a v o i d the effect o f d i l u t i o n r e s u l t i n g f r o m the d e l i b e r a t e a d d i t i o n of copper, the concentrations have been n o r m a l i z e d to proportions relative to silver r a t h e r t h a n to t o t a l a l l o y . O n e observes a scattered o c c u r r e n c e of l o w g o l d a n d l o w i r i d i u m v a l u e s , a cluster of i r i d i u m c o n c e n t r a t i o n s b e


t w e e n 4 0 a n d 100 p p b , a n d a n u m b e r of i r i d i u m concentrations a b o v e 100 p p b — t h e last t w o groups w i t h g o l d values m a i n l y b e t w e e n 0.5 a n d 1 . 0 % . T h e objects w i t h l o w i r i d i u m contents, i n most cases c o u p l e d to a l o w g o l d content, are, w i t h a f e w exceptions, s t y l i s t i c a l l y different f r o m " C e n t r a l S a s a n i a n " s i l v e r objects ( 7 ) . I t is n o t y e t p o s s i b l e to i n d i c a t e locations of ores u s e d f o r these objects except t h a t t h i s l o w i r i d i u m g r o u p c o n t a i n s m o s t l y objects j u d g e d to b e f r o m areas east o f t h e " C e n t r a l S a s a n i a n " area.

_

1

-

π ππ Iι I

1

I

1

π

,

,

2

4

1

1

1

1

I

_

UmÉ! II 10

,

20

ppb IRIDIUM

ι, 40

. 11 — II ι• 1 100 200 400 1000

IN SILVER

Figure 3. Histogram of iridium concentrations rela tive to silver in all silver objects analyzed. The dis tribution of iridium concentrations in shells of plates with external rim lines only are represented by dark columns. W e o b s e r v e d at t h e b a c k of a n u m b e r of plates, close to t h e r i m , a c h a s e d o r e n g r a v e d l i n e , p a r a l l e l t o the r i m . Sometimes a s i m i l a r l i n e is v i s i b l e o n t h e foot. purpose.

T h e s e lines d o n o t h a v e a n o b v i o u s

decorative

T h i s g r o u p o f plates contains objects f r o m a w i d e t i m e p e r i o d

a n d v a r i o u s styles. W i t h o n l y o n e e x c e p t i o n a l l plates w i t h these Unes (16 i n t o t a l ) h a v e i r i d i u m contents b e t w e e n 4 5 a n d 85 p p b ( s h o w n i n b l a c k i n F i g u r e 3 ) w h i l e plates w i t h o u t these lines ( 9 i n t o t a l ) also w i t h one e x c e p t i o n h a v e i r i d i u m contents either l o w e r t h a n 4 5 p p b o r h i g h e r t h a n 85 p p b . I t is v e r y u n l i k e l y t h a t this c o r r e l a t i o n is a c c i d e n t a l .

These

s i m i l a r l y l i n e d plates p r o b a b l y h a d a c o m m o n o r i g i n at least t o t h e extent of h a v i n g b e e n m a d e f r o m s i l v e r f r o m a single source.

This particular

source does n o t seem t o h a v e b e e n u s e d f o r t h e other plates a n a l y z e d . F u r t h e r g r o u p i n g m i g h t b e p o s s i b l e o n t h e basis of t h e a n a l y t i c a l d a t a


32


of S a s a n i a n a n d U m a y y a d s i l v e r coins.

B a s e d o n the g o l d content

coins f r o m v a r i o u s m i n t cities, G o r d u s ( 5 )

of

could distinguish a number

of different s i l v e r sources u s e d d u r i n g the S a s a n i a n a n d p o s t - S a s a n i a n p e r i o d . H o w e v e r , the g o l d content a l o n e does not seem sufficient at this


p o i n t to s u b d i v i d e o u r groups of S a s a n i a n silver objects.

Conclusions C o m p a r i s o n of m u l t i p l i c a t e analysis of m i c r o samples, 2 0 0 - 1 0 0 0 of c a r e f u l l y h o m o g e n i z e d

μg,

s i l v e r standards has e s t a b l i s h e d that the a n a

l y t i c a l t e c h n i q u e s d e v e l o p e d are a c c e p t a b l y precise for s i l v e r , g o l d , c o p per, s c a n d i u m , c h r o m i u m , manganese, i r o n , n i c k e l , c o b a l t , z i n c , arsenic, selenium, bromine, tin, antimony, iridium, and mercury. A d d i t i o n a l com p a r i s o n b e t w e e n analyses of s i m i l a r m i c r o d r i l l i n g s t a k e n f r o m t h e i n t e r i o r of S a s a n i a n s i l v e r objects h a v e s h o w n t h a t s u c h s a m p l i n g results i n d a t a w i t h r e a s o n a b l y g o o d p r e c i s i o n for s i l v e r , c o p p e r , g o l d , z i n c , arsenic, s e l e n i u m , a n t i m o n y , a n d i r i d i u m ; f a i r p r e c i s i o n for t i n , s c a n d i u m , m a n ganese, a n d b r o m i n e ; a n d p o o r p r e c i s i o n for s o d i u m , p o t a s s i u m , c h r o m i u m , i r o n , n i c k e l , c o b a l t , a n d m e r c u r y . A p p a r e n t l y these last elements are too i n h o m o g e n e o u s l y d i s t r i b u t e d i n the a n c i e n t s i l v e r for s u c h m i c r o s a m p l i n g to be representative. D r i l l i n g - s a m p l e d a t a f r o m 30 S a s a n i a n silver objects w e r e

compared

w i t h t w o sets of r u b b i n g - s a m p l e d a t a f r o m the same objects. T h e r u b b i n g samples consistently h a v e a h i g h e r g o l d content b y a n average

factor

of 1.17 a n d a l o w e r c o p p e r content b y a n average f a c t o r of 0.85.

There

fore, r u b b i n g samples are u n d o u b t e d l y b i a s e d b y surface effects.

The

extent of this bias, h o w e v e r , does not i n v a l i d a t e the r u b b i n g - s a m p l e t e c h n i q u e , w h i c h is often t h e o n l y one p e r m i s s i b l e for c e r t a i n objects. I f the e s t a b l i s h e d bias is k e p t i n m i n d , the results are close e n o u g h to the a c t u a l i n t e r i o r c o m p o s i t i o n to a l l o w m u c h r e l i a b l e inference. O f the 19 elements d e t e r m i n e d i n S a s a n i a n silver objects, o n l y g o l d a n d i r i d i u m seem to i n d i c a t e the s i l v e r source u s e d .

Indirectly through

t h e i r c o n c e n t r a t i o n ratios z i n c , t i n , arsenic, a n t i m o n y , a n d s e l e n i u m m i g h t p r o v i d e some a d d i t i o n a l i n f o r m a t i o n o n the s i l v e r source u s e d , a n d t h e i r absolute concentrations, together w i t h those of other elements m i g h t be useful i n obtaining information on metallurgical techniques used. C o n t i n u e d analysis of S a s a n i a n s i l v e r objects w i l l b e d i r e c t e d

to

w a r d s ( 1 ) a d e t a i l e d s t a t i s t i c a l s t u d y of the d a t a , ( 2 ) analysis of S a s a n i a n a n d U m a y y a d coins, ( 3 ) d e t e r m i n a t i o n of isotope ratios of l e a d e x t r a c t e d f r o m the s i l v e r , ( 4 ) c o r r e l a t i o n b e t w e e n t h e a n a l y t i c a l d a t a a n d stylistic i n f o r m a t i o n o b t a i n e d f r o m a n art h i s t o r i c a l a n d a r c h a e o l o g i c a l a n d w i t h i n f o r m a t i o n o n m e t h o d s of m a n u f a c t u r e , t o o l m a r k , etc.


study

3.

MEYERS

E T A L .

Impurities

in Sasanian

Silver

33

Acknowledgment T h e authors express t h e i r g r a t i t u d e to P r u d e n c e O . H a r p e r , associate c u r a t o r f o r t h e A n c i e n t N e a r E a s t D e p a r t m e n t of t h e M e t r o p o l i t a n M u s e u m of A r t f o r h e r m a n y v a l u a b l e suggestions, h e r expert e v a l u a t i o n of


the objects a n a l y z e d , a n d h e r c o n t i n u o u s interest i n this project. Literature

1. 2. 3. 4. 5. 6. 7. 8. 9.

Cited

Wertime, Th. Α., Science (1968) 159, 927. Tylecote, R. F., Metals Mater. (1970) 4, 285. Ladame, G., Schweiz. Mineral. Petrogr. Mitt. (1945) 25. Kraay, C. M., Emeleus, V. M., "The Composition of Greek Silver Coins: Analysis by Neutron Activation," Oxford University Press, Oxford, Eng land, 1962. Gordus, Α. Α., in "Methods of Chemical and Metallurgical Investigation of Ancient Coinage," Royal Numismatic Society Spec. Publ. 8, 127, London (1972). Meyers, P., van Zelst, L., Sayre, Ε. V., J. Radioanal. Chem. (1973) 16, 67-78. Lederer, C. M., Hollander, Y. M., Perlman, I., "Table of Isotopes," 6th ed., Wiley, New York, 1968. Gordus, Α. Α., private communication. Harper, P. O., private communication.

RECEIVED July 9, 1973. Research performed partly under the auspices of the U.S. Atomic Energy Commission.


Archaeological Chemistry

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