9 Comparative Analysis of Archaeological Bronzes W. T. CHASE
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Freer Gallery of Art, Smithsonian Institution, Washington, D.C. 20560 While thousands of analyses of archaeological bronzes have been reported in the literature, the basis for comparing them, especially those from different laboratories, is shaky. A round-robin project of chemical analyses was attempted to improve the situation. Two ancient bronze objects were milled to afinepowder, sieved, and mixed to a homogeneous mass. Samples of 500 mg each drawn randomly from this mass were circulated, and results were returned from 21 laboratories. Forty-eight elements were analyzed; some laboratories did only one element, some did as many as 42. The coefficient of variance (or relative standard deviation) ranges from 4% for Cu up to over 200% for some trace elements. The results are tabulated, and methods are suggested to narrow the spread of results in the next run of this program. '"p'he A
comparative
analysis p r o g r a m b e g a n
at t h e t i m e " T h e F r e e r
C h i n e s e B r o n z e s " (1, 2) w a s b e i n g w r i t t e n ; R . J . G e t t e n s r e a l i z e d , b y
s t u d y i n g t h e l i t e r a t u r e c a r e f u l l y , that m a n y analyses h a d b e e n a c c e p t e d a n d p u b l i s h e d w i t h o u t c r i t i c a l a p p r a i s a l (3).
T h e v a l i d i t y a n d u t i l i t y of
m a n y s u c h analyses s e e m e d o p e n to q u e s t i o n .
A s Gettens s a i d i n t h e
earliest draft of t h e p r o p o s a l for t h e c o m p a r a t i v e analysis p r o g r a m ( A p r i l 1965)
(4):
M o s t c e r t a i n l y , w e a l l ask ourselves w h e n r e a d i n g t h e results of a n analysis of a n a n c i e n t m e t a l object, " W o u l d I h a v e gotten a c o m p a r a b l e result i f I h a d s a m p l e d a n d a n a l y z e d that object b y m y o w n t e c h n i q u e s ? " T h i s q u e s t i o n m a y seem s i m p l e to answer, b u t t h e p r o b l e m has m a n y aspects w h i c h m u s t b e c o n s i d e r e d .
Variation i n the techniques a n d
m e t h o d s o f s a m p l i n g is t h e first aspect.
W e d e c i d e d to e l i m i n a t e this
v a r i a t i o n i n o u r i n i t i a l study b y t a k i n g t h e samples ourselves ( see b e l o w ). T h u s , this p a r t o f t h e p r o g r a m concentrates o n a n a l y t i c a l t e c h n i q u e s . 148 In Archaeological Chemistry; Beck, C.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
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9.
CHASE
Comparative Analysis of Bronzes
149
Even among analytical techniques there is great variance. In our study, 10 different analytical methods were used (see Table II), and as many different, separate techniques as there are laboratories. In fact, some laboratories employed more than one technique within a given method (—e.g., laboratory 13, which used optical emission spectrometry by both the photoplate and the direct-reading method). Other variations among laboratories exist—e.g., sample size, number of replicates, elements determined, detection limits, and reporting conventions. T o approach the problem of comparable results i n the simplest way possible, we took two ancient bronze objects of different composition, reduced half of each object to homogeneous filings, and circulated samples drawn from these filings to various laboratories which had routinely analyzed archaeological bronzes or wanted to participate in the program. The results were tabulated and reported. O u r goal was to test the comparability of the results from each laboratory using its usual method under routine conditions. N o w that we have the results, how do they compare? T o answer this question fully we have tabulated a detailed, element-by-element examination of the analytical findings below (Table III). In general, the results are fairly good, but discrepancies exist. A careful statistical treatment of the results is very difficult because of several factors (see below). The disparities in the results and the doubts that they raise could be reduced by circulating standards of known composition for bronze analysis and by adopting standard methods for analyzing archaeological bronzes. A t present the adoption of standard methods seems impractical because of the differences in equipment and techniques favored by each laboratory. The acquisition and circulation of standard reference materials for bronze analysis are, however, possible and should narrow the spread of results. The program could then be repeated using more samples and a careful statistical design, and sources of variation could be assessed more accurately than can now be done. Of course, the problem of sampling actual objects must also be attacked. The importance of complete reporting (5) when publishing analyses in the field of archaeological chemistry cannot be overstressed. Procedure The objects chosen and the procedure used in comminuting them are outlined in our Document No. 2 ( 6 ) and detailed below. Sample Preparation. T w o bronze objects of limited value as museum display pieces were given to the Freer Gallery by an antique dealer ( Simon Kriger, Washington, D . C . ) . Both pieces were photographed before sectioning. The first object, SC528 or SC-B-1, is a fragment of a Shang Dynasty Chinese bronze ku, or beaker, dating from ca. 1000 B . C . (Figure
In Archaeological Chemistry; Beck, C.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
150
ARCHAEOLOGICAL CHEMISTRY
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1 ) . It was sectioned i n h a l f l o n g i t u d i n a l l y w i t h a d i a m o n d saw, a n d o n e - h a l f w a s r e t a i n e d for c o n t r o l a n d later reference. T h e other h a l f w a s r e d u c e d to fine t u r n i n g s w i t h a r o t a r y file i n a n electric h a n d d r i l l ; before this the outer a n d i n n e r surfaces w e r e g r o u n d to c l e a n m e t a l w i t h another r o t a r y file. T h e c o r r o s i o n m a t e r i a l was s a v e d b u t not m i x e d w i t h the m e t a l s a m p l e . N o a t t e m p t was m a d e to separate i r o n w i t h a m a g n e t or to r e m o v e c o r r o s i o n f r o m the m e t a l s a m p l e b y p i c k i n g it out u n d e r the m i c r o s c o p e . T h e s a m p l e was passed t h r o u g h a 30-mesh screen, a n d coarser c h i p s w e r e r e t a i n e d separately. N i n e t y - n i n e grams of c o m m i n u t e d s a m p l e w e r e p r o d u c e d . T h i s was b o t t l e d i n glass a n d s h a k e n to ensure h o m o g e n e i t y . A f t e r s h a k i n g , p a r t of the s a m p l e was r e b o t t l e d i n 0.5-gram ( 500 m g ) lots i n glass for d i s t r i b u t i o n .
Figure 1. Top: Shang Dynasty Chinese bronze k u (wine beaker) in fragmentary condition. Freer Gallery Study Collection No. SC528 or SC-B-1, 13 cm high, before sectioning and comminution. Bottom: after sectioning.
In Archaeological Chemistry; Beck, C.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
9.
CHASE
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Figure 2.
Comparative
Analysis
of
151
Bronzes
Luristan bronze spearpoint, Freer Gallery Study Collection SC615 or SC-B-59 before comminution
No.
T h e s e c o n d s a m p l e is a L u r i s t a n spear p o i n t ( S C 6 1 5 or S C - B - 5 9 ) , a b o u t 12 inches l o n g , d a t i n g f r o m ca. 800 B . C . ( F i g u r e 2 ) . T h e c o r r o s i o n was g r o u n d f r o m b o t h sides of h a l f of the b l a d e , f r o m the b l a d e e d g e to the spine. T h i s h a l f was c o m m i n u t e d as a b o v e a n d sieved to pass 30-mesh. S e v e n t y - n i n e grams of s a m p l e w e r e p r o d u c e d ; they w e r e b o t t l e d , s h a k e n , a n d r e w e i g h e d i n t o 5 0 0 - m g lots as above. T h e s h a k i n g , s i e v i n g , a n d r e p e a t e d s h a k i n g w e r e d o n e to m a k e the samples as h o m o g e n e o u s as possible. T h e b o t t l e d 5 0 0 - m g samples w e r e p l a c e d i n a box a n d d r a w n r a n d o m l y for c i r c u l a t i o n . A n o t h e r s a m p l e , s a m p l e 3, c o n s i s t i n g of 5 0 0 - m g lots of the U . S . N a t i o n a l B u r e a u of S t a n d ards S . R . M . 124c ( o u n c e m e t a l ) , w a s sent to laboratories w h o s e results p a r t i c u l a r l y interested us. O n reflection, w e t h i n k that s a m p l e 3 s h o u l d h a v e b e e n sent to a l l l a b o r a t o r i e s , a n d t o w a r d the e n d of t h e p r o g r a m it was sent to a n y l a b o r a t o r y that r e q u e s t e d samples. W e t r i e d to m i n i m i z e the n u m b e r of laboratories r e c e i v i n g s a m p l e 3 at first because o u r stock of S . R . M . 124c is v e r y l o w . T h e use of other s t a n d a r d reference m a t e r i a l s , b o t h as controls a n d for c a l i b r a t i o n , w i l l b e a n i m p o r t a n t feature of the next phase of the p r o g r a m . W h e n the samples w e r e sent out, a p o s t c a r d for a c k n o w l e d g i n g safe r e c e i p t w a s i n c l u d e d . W e u s e d the dates o n these postcards to m a r k the start of analysis. O n the q u e s t i o n n a i r e ( o u r D o c u m e n t 3, q u e s t i o n 7 ) a space w a s s u p p l i e d for the l e n g t h of t i m e r e q u i r e d for analysis. F e w l a b o ratories p e r f o r m e d t h e i r analyses a n d r e t u r n e d results w i t h i n the t i m e t h e y i n d i c a t e d o n the q u e s t i o n n a i r e . T h i s is not s u r p r i s i n g . I n fact, the l e n g t h of t i m e r e q u i r e d w a s r e q u e s t e d so that w e w o u l d k n o w w h e n to w r i t e the l a b o r a t o r y a n d ask for t h e i r results. Results S a m p l e s w e r e sent to 36 l a b o r a t o r i e s , a n d results w e r e r e c e i v e d f r o m 21. I n a l l , 48 elements w e r e a n a l y z e d . T h e results f o r m a c o m p l i c a t e d p i c t u r e w h i c h w e h a v e t r i e d to c l a r i f y , w h i l e g i v i n g a l l the necessary i n f o r m a t i o n , b y b r e a k i n g d o w n the results i n t o a series of tables. T a b l e I shows the laboratories w h i c h r e s p o n d e d w i t h results; these are l i s t e d a l p h a b e t i c a l l y b y n a m e of the p r i n c i p a l r e p o r t e r (as r e c e i v e d f r o m the laboratory).
F r o m T a b l e I I o n , the laboratories are r e p r e s e n t e d
In Archaeological Chemistry; Beck, C.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
by
152
ARCHAEOLOGICAL
Table I. Principal
CHEMISTRY
Laboratories and Reporters Represented in Report"
Reporter
Address
H . Barker
T h e B r i t i s h M u s e u m Research Laboratory, London W C 1 B 3 D G , England
I. V . Bene
T h e F r e e r G a l l e r y of A r t T e c h n i c a l L a b o r a t o r y ,
L . A . Carrara
M a s s a c h u s e t t s I n s t i t u t e of T e c h n o l o g y , C e n t r a l A n a l y t i c a l F a c i l i t y ( R o o m 13-4153) C a m b r i d g e , M a s s . 02139 Analytical Laboratory, Henry Francis D u P o n t Winterthur M u s e u m , W i n t e r t h u r , D e l . 19735
W a s h i n g t o n , D . C . 20560
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V . F . Hanson
A. Hartmann
W . W . Harrison
Wiirttembergisches Landesmuseum Stuttgart, Chemisch-Physikalisches Labor, D 7 0 0 0 S t u t t g a r t 1, G e r m a n y
Β. B . J o h n s o n
U n i v e r s i t y of V i r g i n i a , Charlottesville, V a . L a b o r a t o r y of A n a l y t i c a l C h e m i s t r y , F a c u l t y of E n g i n e e r i n g , N a g o y a U n i v e r s i t y , Furocho, Chikusaku, Nagoya, Japan
F. J . Langmyhr
Conservation Center, L o s Angeles C o u n t y M u s e u m of A r t , 5905 W i l s h i r e B l v d . , L o s Angeles, C a l . 90036
D . Ishii
L. Maes
E. J . Maienthal
F. Michel
R . M . Organ
C . Patterson and E. Bingham
Rehmatullah J . Riederer
D e p a r t m e n t of A n a l y t i c a l C h e m i s t r y , U n i v e r s i t y of O s l o , Oslo 3, N o r w a y Institut R o y a l du Patrimoine Artistique, 1 P a r c d u C i n q u a n t e n a i r e 1040, Brussels, B e l g i u m N a t i o n a l B u r e a u of S t a n d a r d s , Micro-chemical Analysis Section, R o o m B331 Chemistry, W a s h i n g t o n , D . C . 20234 L a b o r a t o i r e de R e c h e r c h e des Musées de F r a n c e , Palais d u Louvre, Place d u Carrousel, 75 P a r i s (1er), F r a n c e Conservation-Analytical Laboratory, U n i t e d States N a t i o n a l M u s e u m , Smithsonian Institution, W a s h i n g t o n , D . C . 20560 Spectrographic L a b o r a t o r y , D i v i s i o n of G e o l o g i c a l a n d P l a n e t a r y Sciences, C a l i f o r n i a I n s t i t u t e of T e c h n o l o g y , P a s a d e n a , C a l . 91109 Central Archaeological Laboratory, Old Fort, Lahore, West Pakistan D o e r n e r — I n s t i t u t , D 8 0 0 0 M u n c h e n 2, Meiserstrasse 10, G e r m a n y
In Archaeological Chemistry; Beck, C.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
9.
Comparative
CHASE
Analysis Table I .
Principal
B . A . Thompson
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C. P . W a n and Li Chi K . Yamasaki
C. M . Yang
a
Continued Address
Reporter
R . C. Rooney
153
of Bronzes
Rooney and W a r d L t d . , Blackwater Station Estate, Camberley, Surrey, England Analytical Chemistry Division, N a t i o n a l B u r e a u of S t a n d a r d s , W a s h i n g t o n , D . C . 20234 I n s t i t u t e of H i s t o r y a n d P h i l o l o g y , Academia Sinica, N a n - k a n g , Taipei, T a i w a n , R e p u b l i c of C h i n a D e p a r t m e n t of C h e m i s t r y , F a c u l t y of Science, N a g o y a U n i v e r s i t y , N a g o y a 464, J a p a n D e p a r t m e n t of N u c l e a r E n g i n e e r i n g , National Tsing-hua University, H s i n c h u , T a i w a n , R e p u b l i c of C h i n a
Arranged alphabetically by principal investigator.
n u m b e r o n l y to preserve a n o n y m i t y a n d thus ensure c o o p e r a t i o n i n f u t u r e studies.
E a c h l a b o r a t o r y was a p p r i s e d of its n u m b e r b y p r i v a t e c o m
munication. T a b l e I I shows the m e t h o d s u s e d b y e a c h l a b o r a t o r y , the s a m p l e sizes u s e d , w h e t h e r the results c a m e b a c k o n o u r r e p o r t f o r m or not, a n d a n y other p e r t i n e n t r e m a r k s b y procedure.
the l a b o r a t o r y about the samples
or
T w o laboratories (01 a n d 04) c h a n g e d t h e i r p r o c e d u r e as a
result of o u r c o m m e n t s o n t h e i r first analyses. A f t e r these changes
were
m a d e , e a c h l a b o r a t o r y s u b m i t t e d n e w results. I n b o t h cases w e r e t a i n e d the o l d a n d n e w results a n d u s e d the n e w results for the statistics b e l o w .
In
l a b o r a t o r y 01 t h e m a t e r i a l was g i v e n to a n a n a l y s t r e l a t i v e l y i n e x p e r i e n c e d i n t h e t e c h n i q u e o n the first r u n . I n l a b o r a t o r y 04, a d e q u a t e
standards
for i n s t r u m e n t c a l i b r a t i o n w e r e not a v a i l a b l e for t h e first r u n . N o other laboratories h a d a second
c h a n c e at these samples, b u t w e h o p e that
w h e n this p r o g r a m is r e p e a t e d , this sort of i n t e r a c t i o n b e t w e e n the c e n t r a l d a t a c o l l e c t i o n f a c i l i t y a n d the v a r i o u s laboratories w i l l b e p a r t of t h e procedure. T h e m e t h o d s g i v e n are p r e s e n t e d i n T a b l e I I exactly as t h e y w e r e r e p o r t e d b y the laboratories ( as n e a r l y as possible ). T h e d e t a i l i n w h i c h m e t h o d s are r e p o r t e d varies f r o m elaborate to scanty. N o l i t e r a t u r e ref erences w e r e g i v e n b y a n y l a b o r a t o r y . I n one case ( l a b o r a t o r y 02 ) results c a m e b a c k o n a f o r m m a r k e d " r e p o r t of c h e m i c a l a n a l y s i s , " a n d w e h a d to go b a c k to the q u e s t i o n n a i r e to find that w e t c h e m i s t r y w a s t h e m e t h o d used.
A b b r e v i a t i o n s are u s e d to r e d u c e space i n the m e t h o d
In Archaeological Chemistry; Beck, C.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
column.
154
ARCHAEOLOGICAL CHEMISTRY
S a m p l e sizes reflect the a m o u n t of s a m p l e for a single d e t e r m i n a t i o n (see
note, T a b l e I I ) a n d not the a m o u n t of s a m p l e c o n s u m e d b y the
laboratory. T h e c o l u m n l a b e l e d r e p o r t f o r m u s e d is i n t e r e s t i n g . O f 23 reports ( 01 a n d 04 e a c h gave t w o ), o n l y six u s e d the report f o r m p r o v i d e d . T h i s is w h y so f e w
s a m p l e sizes are listed.
W h i l e the report f o r m itself
( our D o c u m e n t 4 ) c o u l d h a v e b e e n i m p r o v e d a n d w h i l e i t d i d not h a v e space for m u c h i n f o r m a t i o n , each l a b o r a t o r y s h o u l d h a v e filled i t i n a n d t h e n p r o v i d e d us w i t h details o n a d d i t i o n a l sheets.
I n fact, results w e r e
m u c h m o r e difficult to c o m p i l e b e c a u s e so m a n y r e p o r t i n g forms
were
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u s e d ; some reports w e r e e v e n h a n d w r i t t e n o n slips of p a p e r . T h e c o l u m n l a b e l e d other r e m a r k s is also i n t e r e s t i n g ; m a n y l a b o r a tories n o t e d sample
1.
corrosion
and inhomogeneity
S o m e laboratories selected
i n the
sound
samples,
m e t a l for
Table II. Lab. No. 01 (old)
especially
the analysis.
Laboratories,
Method E S — e m i s s i o n spectrography P O L — D a v i s d i f f e r e n t i a l cathode r a y p o l a r o g r a p h y A A — a t o m i c a b s o r p t i o n spectroscopy
01 (new)
P O L — c a t h o d e ray polarography using D a v i s differential cathode r a y p o l a r o g r a p h , m a d e b y S o u t h e r n Analytical A A — a t o m i c absorption on a Hilger & W a t t s Atomspek A A spectrophotometer
02
E S — e m i s s i o n s p e c t r o g r a p h y , samples t a k e n as a n d a r c e d , q u a l i t a t i v e ( v i s u a l estimate) E S — s a m p l e s compared with N B S standards powder visually, semiquantitative
received and
SQ
E S — v a l u e s represent w t % — s a m p l e s c o m p a r e d w i t h N B S s t a n d a r d s b y densitometer m e a s u r e m e n t (averages of 2 - 4 analyses), s e m i q u a n t i t a t i v e W E T — w e t chemistry 04(old)
X R F — n o n - d i s p e r s i v e x - r a y fluorescence s p e c t r o m e t r y , K e v e x
04 (new)
XRF—same
05
E S — o p t i c a l emission s p e c t r o m e t r y , u s i n g C u as a n i n t e r n a l standard
In Archaeological Chemistry; Beck, C.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
9.
CHASE
Comparative
Analysis
of
155
Bronzes
O t h e r s a p p a r e n t l y d i d not i n s p e c t the s a m p l e m i c r o s c o p i c a l l y . T h e
fleck
of g o l d that was d i s c o v e r e d i n s a m p l e 2 b y l a b o r a t o r y 31 is i n t e r e s t i n g ( a n d difficult for us to e x p l a i n ). T a b l e I I shows, i n some d e t a i l , the c o m p l e x i t y a n d v a r i a b i l i t y of the p o p u l a t i o n i n v o l v e d i n this s t u d y ( l a b o r a t o r i e s a n a l y z i n g a r c h a e o l o g i c a l m a t e r i a l s ) a n d of the r e p o r t i n g p r o c e d u r e s .
E v e n t h e elements
a n a l y z e d b y e a c h l a b o r a t o r y are not c o m p l e t e l y clear.
F o r example,
l a b o r a t o r y 02 r e p o r t e d C r i n t r a c e amounts ( 0 . 0 0 0 1 - 0 . 0 0 1 % ) i n samples 1 a n d 2 b u t gave no report for s a m p l e 3 ( t h e space is b l a n k ) . T h u s , w e assume t h a t they d i d n o t look for C r i n s a m p l e 3 a l t h o u g h i t seems e q u a l l y Downloaded by MONASH UNIV on November 14, 2015 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/ba-1974-0138.ch009
l i k e l y that t h e y l o o k e d for it a n d it was not present. T h e p r o p e r use of the t e r m not d e t e c t e d , " the consistent r e p o r t i n g of d e t e c t i o n l i m i t s ( d o n e 4
b y f e w l a b o r a t o r i e s ) , a n d t h e c h o i c e of elements to b e d e t e r m i n e d i n a r c h a e o l o g i c a l bronzes are a l l o p e n questions.
Methods, and Sample Sizes" Report Sample Size NG
6
Form
Used
Other Remarks from
Laboratory
no
A l l samples corroded, especially N o . 1
no
Sample
NG NG ca. 10 m g
1
corroded,
thus
analyses
.17 .14
Av.
Av. Av.
.10 .15 .16 .14 .15 (IVB)< not determined .26 .16 .16 .2 .166 .201 .184 .1 .13
4.3 4.2 4.3 4.3 ± 0 . 1 C '
4.38
Pt (Platinum) not detected
not detected
Rb (Rubidium) not detected
not deteected
S (Sulfur) 08
MS
24
MS
Av. Av.
.005 .004 .0045 ± 5 0 % .016 .024 .020 (IVB) c
A v. Av.
.5 .54 .52 ± 5 0 % .082 .066 .074 Sb (Antimony)
02
VFT
ES
Tc c
C
T
04 (old) 04 (new) 05
XRF XRF ES
08
MS
09 13
ES ES*
15 17 19
ES AA ΝΑ
24
MS
31 33 34
ES AA ΝΑ
15
ES
.35 .1 trace
