Ancient Gold Solders: What Was Chrysocolla? - Advances in

13 Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 23, 2018 | https://pubs.acs.org Publication Date: July 1, 1989 | doi: 10.1021/ba-1988-0220.ch013

Ancient Gold Solders: What Was Chrysocolla? G . Demortier LARN, Facultés Universitaires Notre-Dame de la Paix 22, Rue Muzet, B-5000 Namur, Belgium

Arguments are presented for reconsidering the meaning of the name "chrysocolla," used in ancient textbooks on metallurgy, and for iden tifying chrysocolla as cadmium sulfide (greenockite), a yellow natural mineral. Several results from thousands of nondestructive analyses by proton-induced X-ray emission (PIXE) of solders on gold artifacts cannot be interpreted if chrysocolla is either green copper carbonate (malachite), as generally assumed until now, or blue hydrated copper silicate, the mineral now called chrysocolla. Several paragraphs of the 33rd book of Natural History of the Elder Pliny (1st century Α.D.) are critically analyzed in light of these analytical results.

EARLY ARTISANS USED SEVERAL RECIPES

to prepare g o l d solders. T h e s e recipes i n c l u d e d natural chrysocolla (from the G r e e k krysos a n d kolla, or g o l d solder) as w e l l as other compositions p r e p a r e d from alloys c o n t a i n i n g metals such as copper, silver, a n d gold. T h e use of chrysocolla was d e s c r i b e d i n considerable d e t a i l i n the 1st c e n t u r y A . D . b y P l i n y the E l d e r i n his Natural History. A s can be expected from its etymological o r i g i n , the n a m e chrysocolla m e a n t i n a n t i q u i t y any material u s e d to j o i n parts of g o l d j e w e l r y . A t the b e g i n n i n g of his 3 3 r d v o l u m e , P l i n y w r o t e , " G o l d is d u g out a n d , w i t h i t , also chrysocolla, w h i c h continues to bear a n a m e d e r i v e d f r o m the t e r m g o l d i n o r d e r that its value may appear greater. " T h i s special attention to chrysocolla indicates its i m p o r t a n c e i n ancient g o l d s m i t h e r y . B r a z i n g alloys that contain c a d m i u m have b e e n u s e d extensively since m i d - 1 9 t h c e n t u r y . A small a m o u n t of c a d m i u m i n a g o l d matrix can l o w e r the m e l t i n g p o i n t b e l o w that obtained b y a d d i n g equivalent concentrations 0065-2393/89/0220-0249$06.00/0 © 1989 A m e r i c a n C h e m i c a l S o c i e t y

Allen; Archaeological Chemistry IV Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 23, 2018 | https://pubs.acs.org Publication Date: July 1, 1989 | doi: 10.1021/ba-1988-0220.ch013

250

ARCHAEOLOGICAL CHEMISTRY

of c o p p e r or s i l v e r to the same gold matrix. It is c o m m o n l y b e l i e v e d that c a d m i u m or c a d m i u m c o m p o u n d s w e r e not u s e d i n a n t i q u i t y ( c a d m i u m ores are i n d e e d rare i n nature), a n d that objects c o n t a i n i n g c a d m i u m , e i t h e r i n the solder or as an i m p u r i t y i n the gold alloy, w e r e r e c e n t l y manufactured or restored. N e v e r t h e l e s s , c a d m i u m was detected i n a solder o n a m o n e t a r y m e d a l l i o n excavated several years ago i n H o u m e a u , F r a n c e b y an official staff of F r e n c h archaeologists (J). Several experts i n ancient j e w e l r y q u e s t i o n the a u t h e n t i c i t y of the gold j e w e l r y that w e analyzed, because the artifacts w e r e not f o u n d d u r i n g official excavations a n d several of the artifacts have b e e n d e c l a r e d b y us as fakes or recent restorations on the basis of our analyses a n d our c r i t e r i a of authentification. A l t h o u g h it is usually present at l o w e r concentrations, c a d m i u m is s o m e times present i n concentrations u p to several percent i n regions w h e r e s o l d e r i n g was necessary to j o i n elements i n pieces of apparently ancient j e w e l r y . W e have shown (2-7) that the a m o u n t of c a d m i u m found i n s u c h j e w e l r y (generally f r o m I r a n , S y r i a , or southern Italy) is related to the amounts of c o p p e r a n d silver present, b u t i n c o m p l e t e l y different proportions from those o b s e r v e d i n m o d e r n s o l d e r i n g alloys. A n a l y t i c a l results (2, 4, 6, 7) collected d u r i n g analyses of n u m e r o u s artifacts, from m u s e u m s as w e l l as from private owners, have l e d us to propose several c r i t e r i a u p o n w h i c h to d i s t i n g u i s h m o d e r n from ancient j o i n i n g procedures that u s e d c a d m i u m based materials a n d to indicate a n e w i n t e r p r e t a t i o n of o l d m e t a l l u r g i c a l descriptions. W e suggest that ancient Iranian goldsmiths s m e l t e d greenockite (a natural y e l l o w c a d m i u m ore) simultaneously w i t h c o p p e r ores a n d gold to obtain an alloy suitable as a solder at l o w temperatures.

Iranian Goldsmithery from the 4th Century B.C. T h e h i g h l e v e l of w o r k m a n s h i p of ancient goldsmiths can scarcely be m a t c h e d e v e n today. A m o n g the several h u n d r e d items of Iranian a n d Syrian j e w e l r y that w e have s t u d i e d (2-7), the w o n d e r f u l A c h e m e n i d e pendant of the 4 t h c e n t u r y B . C . , w h i c h belongs to the D e p a r t m e n t of Iranian A n t i q u i t i e s of the Museé d u L o u v r e , clearly illustrates the skill of ancient Iranian g o l d smiths. T h e p e n d a n t is s h o w n i n F i g u r e 1 a n d is accepted as genuine (8). T h e total w i d t h of the disk decorated i n repoussé is 5.5 c m . T h e s u r r o u n d i n g ornaments w e r e made w i t h 28 i d e n t i c a l motifs. E a c h of t h e m i n c l u d e s a h o l l o w c y l i n d e r a n d two h e m i s p h e r i c a l caps. T h e d e t a i l of the top r i g h t part ( F i g u r e 2) shows clearly v i s i b l e solders. T h e s e areas w e r e n o n d e s t r u c t i v e l y analyzed for c o m p o s i t i o n , a n d the results are s u m m a r i z e d i n Table I. A l l measurements w e r e p e r f o r m e d w i t h p r o t o n - i n d u c e d X - r a y emission ( P I X E ) i n a m i c r o p r o b e assembly (9). O u r P I X E m i c r o p r o b e facility, the L a b o r a t o i r e d'Analyse par Réactions N u c l i a i r e s ( L A R N ) , allows the i r r a d i a t i o n of s m a l l regions of a sample i n a v a c u u m . T h e v a c u u m c h a m b e r i n w h i c h the artifact is p l a c e d was d e s i g n e d



13.

DEMORTIER

Ancient Gold Solders;

Chrysocolla

251

Figure 1. Achemenide pendant of the 4th century B.C.

Figure 2. Detail of the Achemenide

pendant showing regions of analysis.

to accept pieces of j e w e l r y whose dimensions may reach 30 c m . T h e i t e m is fixed o n an X - Y frame that can be m o v e d i n the p r o t o n b e a m b y s t e p p i n g motors. A small c o m p u t e r controls m e c h a n i c a l d i s p l a c e m e n t (with a r e p r o d u c i b i l i t y better than 2.5 μπι after a translation of several centimeters) a n d c o l l e c t i o n of data from the p h o t o n a n d e l e c t r o n detectors a n d carries out


252


Table I. Composition of Different Regions of the Achemenide Pendant Cu %

Region

4

5.6 ± 0.3 f 9.8 ± 0.6 5.2 ± 0.3 Î 7 . 5 ± 0.5

5 6

4 3.1

1 2


3

5.7

0.3

7.0 ± 0.4

7

8 9

± 0.4 ±

Î

12.7

± 0.7

7.2

± 0.4

Ag %

Au %

11.6 ± 0.6 f 15.6 ± 0.8 12.6 ± 0.7 f 13.9 ± 0.8

82.8 ± 1.2 I 74.6 ± 1.5

11.7

i6.8 16.6 i 14.6 15.6

± 0.7

± ± ± ±

0.3 0.9 0.8 0.9

82.2

Description

± 1.0

i 78.6 ± 1.1 82.6 ± 0.9 Î90.1 ± 1.5 76.4 ± 1.3 4 7 2 . 7 ± 1.3 77.2

± 1.4

repoussé area brazing vertical sheet brazing cylinder local melting internal hemisphere copper diffusion external hemisphere

NOTE: T h e arrows indicate increased (or decreased) concentrations at solders

n u m e r o u s calculations. A t present, u p to 128 simultaneous images can b e c o l l e c t e d , each i n v o l v i n g u p to 1600 analyzed points. T h e t y p i c a l a c c u m u lation t i m e is 1-5 s p e r point. F o u r joins w e r e made from regions 2 to 8 of F i g u r e 2, over a distance e x t e n d i n g less than 5 m m . T h e e l e m e n t a l compositions at solder sites 2, 4 , 6, a n d 8 show that three different j o i n i n g procedures w e r e p e r f o r m e d . A t site 6, the g o l d content is significantly greater t h a n i n the n e i g h b o r i n g regions a n d indicates that the j o i n was made b y w e l d i n g w i t h o u t the a d d i t i o n of external material. T h e e n d of the c y l i n d e r a n d the b o t t o m o f the first h e m i s p h e r i c a l cap w e r e simultaneously heated to an early stage o f fusion. I n this process, metals l i k e c o p p e r a n d silver w e r e selectively e l i m i n a t e d , m a i n l y b y oxidation, so that gold content was enhanced. T h e t e m p e r a t u r e was a r o u n d 1000 °C. A t site 8, an increase i n c o p p e r concentration was o b s e r v e d s i m u l t a neously w i t h a decrease i n g o l d a n d silver concentrations. T h e two caps, p r e v i o u s l y w e l l - f i t t e d , w e r e j o i n e d b y the process k n o w n as solid-state diffusion b o n d i n g w i t h c o p p e r salts i n a r e d u c i n g atmosphere. T h e process took place at about 890 °C (10). T h e r e l a t i v e l y l o w t e m p e r a t u r e p e r m i t t e d j o i n i n g at site 8 w i t h o u t d e s o l d e r i n g the j o i n at site 6, o n l y 1.5 m m distant. T h i s j o i n i n g m e t h o d was w i d e l y u s e d b y the E t r u s c a n s i n t h e i r famous granulation work. A n increase i n c o p p e r a n d s i l v e r a n d a decrease i n g o l d w e r e s i m u l t a neously o b s e r v e d at sites 2 a n d 4 , a result i n d i c a t i n g that at b o t h sites a b r a z i n g alloy h a d b e e n used. T h e l o w e r concentration of g o l d i n d i c a t e d that the t e m p e r a t u r e of fusion of the alloy used at site 2 was l o w e r than that u s e d at site 4 (possibly a r o u n d 820 a n d 860 °C, respectively). T h e s e temperatures are close to 890 °C, the s o l d e r i n g t e m p e r a t u r e at site 8; h o w e v e r , the latter j o i n d i d not desolder. T h e process of diîusion b o n d i n g produces a j o i n that cannot b e desoldered. R e h e a t i n g after j o i n i n g results i n f u r t h e r c o p p e r diffusion a n d gives a local decrease i n c o p p e r content w i t h a consequent increase i n local m e l t i n g t e m p e r a t u r e . T h e t e m p e r a t u r e difference of sites 2 a n d 4


13.

DEMORTIER

Ancient Gold Solders:

Chrysocolla

253

seems to indicate that the 28 attached elements w e r e s o l d e r e d to the g o l d sheet before the sheet was j o i n e d to the decorated disk, because the alloy u s e d at site 4 h a d a h i g h e r m e l t i n g p o i n t t h a n that at site 2. T h e h i g h degree of w o r k m a n s h i p d i s p l a y e d i n a p i e c e as c o m p l e x as the A c h e m e n i d e p e n d a n t shows that the g o l d s m i t h of 2500 years ago r e c o g n i z e d a n d u s e d different temperatures for various j o i n i n g processes.


Later Iranian Goldsmithery I n any h u m a n a c t i v i t y , alternate means are sought to achieve a g i v e n e n d . E a r l y goldsmiths m u s t have tested a n d u s e d a n u m b e r of different j o i n i n g materials a n d methods. A m o n g Iranian a n d S y r i a n items f r o m the 1st to the 9 t h c e n t u r y A . D . , w e have, at t i m e s , f o u n d significant amounts of c a d m i u m i n solders. F o r most p e o p l e c o n c e r n e d w i t h archaeological j e w e l r y , the p r e s ence o f c a d m i u m suggests forgery or m o d e r n repair. H o w e v e r , i n several investigations c o n d u c t e d since 1983, w e have shown that ancient solders c o n t a i n i n g c a d m i u m may sometimes be differentiated from m o d e r n solders (4, 6, 9, 11). I f several closely spaced joins are present o n a p i e c e of j e w e l r y , solders w i t h different m e l t i n g points m a y b e necessary, d e p e n d i n g o n the o r d e r of construction. M o d e r n c o m m e r c i a l solders are characterized b y compositions i n w h i c h silver a n d c o p p e r concentrations are r e t a i n e d i n a constant p r o p o r t i o n (direct correlation) d u r i n g the s o l d e r i n g p r o c e d u r e ( F i g u r e 3). O n the other h a n d , for ancient solders, a strong correlation exists b e t w e e n c o p p e r a n d c a d m i u m concentrations ( F i g u r e 4). O u r p r i n c i p a l c r i t e r i o n b y w h i c h w e d i s t i n g u i s h m o d e r n from ancient solders c o n t a i n i n g c a d m i u m a p plies o n l y to j e w e l r y artifacts that show several regions of solders. T h e artifact is r e c o g n i z e d as ancient i f relative concentrations o f c o p p e r , s i l v e r , a n d c a d m i u m i n a l l solders are c o r r e l a t e d l i k e those i n F i g u r e 4. T h e artifact is c o n s i d e r e d to b e a forgery or a restored p i e c e i f these relative concentrations are correlated l i k e those i n F i g u r e 3. N o c r i t e r i o n for a u t h e n t i c i t y is available i f the n u m b e r of solders is not sufficient to d r a w a l i n e across the p o i n t s . O t h e r c r i t e r i a m a y be u s e d to d i s t i n g u i s h ancient from m o d e r n c a d m i u m - c o n t a i n i n g solders. T h e s e c r i t e r i a i n c l u d e a. T h e frequent presence of i r o n i n ancient solders. M o d e r n b r a z i n g alloys (for fine g o l d jewelry) sold b y precious m e t a l suppliers or made b y j e w e l r y craftsmen i n t h e i r o w n shops contain o n l y g o l d , s i l v e r , c o p p e r , a n d sometimes z i n c a n d c a d m i u m . T h e s e metals are a l l o y e d from p u r e m e t a l ingots, whereas ancient solders w e r e made w i t h r o u g h materials, g e n e r a l l y b y d i r e c t s m e l t i n g o f o r e s l i k e c h a l c o p y r i t e (an i r o n - c o p p e r ore w i t h a color e l c _ to that o f gold). I r o n is therefore expected to b e present i n ancient artifacts b u t not in modern jewelry.



254


Figure 3. Ternary composition diagram for Cu-Ag-Cd in modern soldering alloys. Analytical results indicate that when the cadmium amount vanes, the ratio of copper on silver remains constant. b. A l u m i n e s c e n c e can b e i n d u c e d b y the P I X E p r o t o n b e a m o n ancient solders. T h i s effect is p r o b a b l y due to the presence o f m i c r o i n c l u s i o n s of slag r e t a i n e d b y the m e t a l p r o d u c e d b y a c r u d e s m e l t i n g t e c h n i q u e . T h i s optical l u m i n e s c e n c e is i n d u c e d o n l y i n insulators, not i n metals, a n d does not appear i n m o d e r n g o l d s m i t h e r y i n w h i c h o n l y metals are i n v o l v e d . c. Differences i n relative concentrations of z i n c a n d c a d m i u m d. T h e presence o f sulfur A p p l i c a t i o n of these c r i t e r i a is illustrated i n the f o l l o w i n g study o f a h o l l o w g o l d p e a r l ( F i g u r e 5) f o u n d i n S y r i a along w i t h other m o r e prestigious objects (necklaces, p e n d a n t s , phials) s t u d i e d e a r l i e r (3, 4, 6). M o s t of these artifacts w e r e f o u n d d u r i n g excavations i n H a u r a n for the d r a i n i n g of L a k e Orantes i n the 1950s. Some of this j e w e l r y was a c q u i r e d b y m u s e u m s , such as the Staatlishe M u s e e n i n B e r l i n . S e v e r a l pieces of the treasure, first c o n s i d e r e d as authentic b y curators a n d archaeologists, w e r e t h e n classified as fakes or restored items after c a d m i u m h a d b e e n d e t e c t e d



13.

DEMORTIER

Ancient Gold Solders: Chrysocolla

255

Figure 4. Ternary composition diagram for Cu-Ag-Cd in regions of solders on an Iranian necklace of the 1st century Α.Ό. (See ref. 2 for details). Here, the copper concentration increases with the cadmium concentration.

Figure 5. Hollow pearl of the 6th century A.D. excavated with other more prestigious objects of Byzantine jewelry. Total width is 8.5 mm.



256


b y o u r P I X E m e t h o d a n d a n a l y z e d according to o u r c r i t e r i a to d i s t i n g u i s h m o d e r n from ancient pieces (6, 7). T h i s identification of m o d e r n w o r k m a n ship l e d to suspicion of other g e n u i n e artifacts of the treasure. A n e l e c t r o n m i c r o g r a p h ( F i g u r e 6) of a r e g i o n near two s o l d e r e d granules clearly shows that a b r a z i n g p r o c e d u r e h a d b e e n used to j o i n b o t h granules to a t h i n g o l d base that was t h e n soldered to the m a i n h o l l o w sphere. Scans across large areas w e r e p e r f o r m e d w i t h the L A R N p r o t o n p r o b e (50 μηι wide); n a r r o w e r scans i n characteristic regions w e r e also o b t a i n e d (II). M a p s of c o p p e r , g o l d , s i l v e r , a n d c a d m i u m ( F i g u r e 7) clearly indicate the presence of c a d m i u m o n l y i n solders b e t w e e n the granules a n d b e t w e e n the m a i n h o l l o w sphere a n d each granule. T h e local decrease of s i l v e r a n d g o l d at each site w h e r e c a d m i u m was present, a n d a slight increase of c o p p e r at that site, p o i n t out a d i r e c t correlation b e t w e e n c o p p e r a n d c a d m i u m c o n tents. T h e c o r r e s p o n d i n g t e r n a r y diagram indicates that the results are l o cated i n the left b o t t o m c o r n e r , far away from the regions characteristic of m o d e r n j e w e l r y (see F i g u r e 3). W h e n a solder s u c h as the one j u s t m e n t i o n e d is i r r a d i a t e d i n a c a d m i u m rich r e g i o n w i t h 1 . 9 - M e V deuterons, the p r o t o n s p e c t r u m indicates traces of sulfur (around 30 p p m ) (12). T h e presence of sulfur suggests that c a d m i u m sulfide c o u l d have b e e n u s e d as a c o m p o n e n t of the b r a z i n g material. T h i s possibility has b e e n e x a m i n e d i n o u r laboratory a n d has b e e n c o n f i r m e d (13, 14). A d d i t i o n a l solder characterization was a c c o m p l i s h e d b y u s i n g the se-

Figure 6. Electron micrograph of detail on the pearl in Figure 5. Brazing procedures are clearly indicated.



13.

DEMORTIER


Chrysocolla

257

Figure 7. Maps of Cu, Ag, Au, and Cd obtained by PIXE microprobe analysis. Correlation of copper and cadmium is obvious. lective excitation of Z n Κ X - r a y s w i t h o u t excitation of gold L X - r a y s . It was f o u n d that z i n c is also present, at trace levels (about 300 p p m ) (15). Sulfur a n d z i n c determinations may be a d d e d to the m a i n c r i t e r i a of characterization of ancient solders: c o p p e r - c a d m i u m correlation a n d p r e s ence of i r o n , to give a total of four differences b e t w e e n solders o n the ancient h o l l o w p e a r l a n d solders m a d e w i t h c o m m e r c i a l b r a z i n g alloys c o n t a i n i n g c a d m i u m . Sulfur at a 3 0 - p p m l e v e l i n a r e g i o n w h e r e c a d m i u m is present at 3 % indicates that, i f C d S h a d b e e n u s e d as a l l o y i n g m a t e r i a l w i t h g o l d , at least 9 9 % of the sulfur w o u l d have b e e n e l i m i n a t e d d u r i n g the a l l o y i n g process. T h i s p r o p o r t i o n was also o b s e r v e d i n our e x p e r i m e n t a l archaeolog ical tests (15, 19), w h i c h are s u m m a r i z e d i n the next section. F i n e m o d e r n c a d m i u m - b a s e d b r a z i n g alloys are zinc-free or contain z i n c a n d c a d m i u m i n a ratio of about 1:5. W h e n C u - A g - Z n - C d - A u alloys are m e l t e d to make solders, c a d m i u m is selectively e l i m i n a t e d because of its h i g h volatility. T h e final relative concentration of z i n c to c a d m i u m i n m o d e r n joins is t h e n still zero or h i g h e r than 1:5. I n the h o l l o w p e a r l (and also i n other jewelry) this ratio lies b e t w e e n 1:25 a n d 1:300. T h e range of these ratios is e i t h e r too l o w to be a t t r i b u t e d to the use of m o d e r n alloys c o n t a i n i n g z i n c or too h i g h to be i n t e r p r e t e d as i m p u r i t i e s i n metals u s e d for a l l o y i n g . T h e s e amounts o f z i n c i n ancient j e w e l r y items are understandable because greenockite (a natural c a d m i u m ore) appears as a y e l l o w coating o n z i n c b l e n d e s . T h e z i n c content i n ancient s o l d e r i n g processes is a t t r i b u t e d to the i m p e r f e c t p r o c e d u r e of separation of greenockite from the z i n c b l e n d e .

Rudimentary Preparations of Low-Melting Brazing Alloys E a r l y attempts to p r e p a r e l o w - m e l t i n g alloys w i t h c a d m i u m sulfide (its nat ural f o r m , greenockite, is o r a n g e - y e l l o w ) encouraged further investigation


258


to check w h e t h e r this process m i g h t have b e e n u s e d i n antiquity. P r e l i m i n a r y results (13) indicate that the p r o d u c t i o n o f b r a z i n g alloys b y a p r o c e d u r e o f "soft m e t a l l u r g y , " w h i c h can give a s o l d e r i n g whose c o m p o s i t i o n can b e m o r e easily c o n t r o l l e d than the o n e frequently u s e d b y m o d e r n j e w e l e r s


(14). A few m i l l i g r a m s o f gold was m e l t e d to form a small sphere. P o w d e r e d c a d m i u m sulfide was t h e n p o u r e d onto the m e l t e d gold. T h e p o w d e r d i s solved r a p i d l y a n d gave a s m a l l sphere o f alloy that was t h e n a n a l y z e d microscopically. E a c h sphere of alloy appeared approximately homogeneous. T h e centers o f the spheres contained less c a d m i u m than the surfaces, b u t the difference i n concentration b e t w e e n the center a n d the surface was less than 2 0 % . D e p e n d i n g o n several conditions (i.e., t e m p e r a t u r e , c r u c i b l e shape, a d d i t i o n o f c o p p e r a n d i r o n ores), the s o l u b i l i t y o f c a d m i u m f r o m c a d m i u m sulfide r e a c h e d saturation w h e n the a l l o y e d c a d m i u m content was b e t w e e n 1% a n d 10%. I n a l l cases, the a d d i t i o n o f c o p p e r o r c o p p e r ores to m o l t e n gold e n h a n c e d the s o l u b i l i t y o f c a d m i u m . T h i s observation leads to a d i r e c t correlation b e t w e e n c o p p e r and c a d m i u m contents. W h e n p o w d e r e d c a d m i u m sulfide is m i x e d w i t h chips o f gold a n d the m i x t u r e is h e a t e d to m e l t i n g , t h e dissolution takes longer than w h e n greenockite is a d d e d to l i q u i d gold. F u r t h e r m o r e , the alloy o b t a i n e d b y this last p r o c e d u r e is less h o m o g e n e o u s than the alloy m a d e w i t h c a d m i u m sulfide p o u r e d onto the m o l t e n gold. T h e sphere o f g o l d - c a d m i u m alloy sometimes appeared to consist o f two different parts: a g o l d - r i c h alloy f o r m e d at the b o t t o m o f the c r u c i b l e a n d a black coating that appeared at the top ( F i g u r e 8). C a d m i u m a n d c o p p e r correlatipns w e r e o b s e r v e d i n the g o l d - r i c h m a t e r i a l as w e l l as i n the black earthy coating (see ref. 13). C a d m i u m sulfide was u s e d i n this w o r k as part o f an archaeological argument: t h e m i x t u r e b y ancients o f materials o f nearly the same color. D i r e c t a l l o y i n g of m e t a l l i c c a d m i u m w i t h m e l t e d gold, a m o d e r n p r o c e d u r e , results i n a v i o l e n t i n t e r a c t i o n w h e n the m e t a l l i c c a d m i u m is p l a c e d i n the m e l t because the b o i l i n g p o i n t o f c a d m i u m (765 °C) is far b e l o w the m e l t i n g p o i n t o f g o l d (1063 °C). T h e m e l t i n g p o i n t o f c a d m i u m sulfide is so h i g h (above 1750 °C) that its i n c o r p o r a t i o n i n the m o l t e n gold is not v i o l e n t . N o c a d m i u m vapor is p r o d u c e d w h e n the a m o u n t o f greenockite a d d e d to gold is b e l o w the saturation concentration. A b o v e this l i m i t , wisps w i t h the color of c a d m i u m sulfide appear floating above the c r u c i b l e .

Analysis of the Elder Pliny s Natural History N o n e o f the o r i g i n a l manuscripts o f P l i n y is n o w available. T h e m o d e r n translation of P l i n y ' s book often refers to the Bambergensis m a n u s c r i p t (10th century) a n d less often to t h e Parisinus L a t i n u s (12th century). T h e t w o versions are v e r y similar, b u t several terms are different. A l l c o m m e n t s i n the E n g l i s h (16) a n d F r e n c h (17) translations refer to t h e B a m b e r g e n s i s



13.

DEMORTIER


Chrysocolla

259

Figure 8. Gold-cadmium-copper alloy obtained by "dissolution" of CdS in molten Au-Cu alloy. The black earthy deposit is easily separated from the golden region.

m a n u s c r i p t , for w h i c h the E n g l i s h chemist K . C . B a i l e y gave t e c h n i c a l c o m ments o n a l l scientific subjects (18). T h e 26th paragraph of the 3 3 r d book i n b o t h versions (Bambergensis a n d Parisinus L a t i n u s ) describes the o r i g i n of chrysocolla: G o l d solder is a l i q u i d f o u n d i n the shafts w e spoke of, flowing d o w n along a v e i n of gold, w i t h a s l i m e that is solidified b y the c o l d of w i n t e r e v e n to the hardness of p u m i c e stone. A m o r e h i g h l y spoken of v a r i e t y of the same m e t a l has b e e n ascertained to be f o r m e d i n c o p p e r m i n e s , a n d the next best i n silver m i n e s . A less valuable sort also w i t h an e l e m e n t of g o l d is also f o u n d i n l e a d m i n e s . I n a l l these m i n e s h o w e v e r an artificial v a r i e t y is p r o d u c e d that is m u c h inferior to the natural k i n d r e f e r r e d to: the m e t h o d is to i n t r o d u c e a gentle flow of water into the v e i n all w i n t e r a n d go o n t i l l the b e g i n n i n g of J u n e a n d t h e n to d r y it off i n J u n e a n d J u l y , clearly s h o w i n g that g o l d solder is n o t h i n g else than the putrefaction of a v e i n of m e t a l T h e next sentence i n the B a m b e r g e n s i s is, i n the L a t i n v e r s i o n , "Nativa duritia maxime distat; uvam vocant" w h i c h is translated as " N a t u r a l g o l d solder, k n o w n as 'grapes' differs v e r y greatly from the artificial i n h a r d n e s s "


260


(16). O n the other h a n d , i n the Parisinus L a t i n u s one finds: "Nativa duritia maxime distat, luteam vocant" w h i c h is n o w translated as " N a t u r a l g o l d solder is the best as far as its hardness is c o n c e r n e d ; it is c a l l e d yellow

chrysocolla."


T h e next sentence i n b o t h versions is "Et tamen ilia quoque herba, quam lutum appellant, tingitur" w h i c h is translated as "Nevertheless it is still dyed w i t h a p l a n t c a l l e d yellow-weed" If, i n the translation of the B a m b e r g e n s i s , uvam refers to the shape of grapes, one cannot u n d e r s t a n d w h y P l i n y ' s text continues w i t h Et tamen (Nevertheless). T h i s sentence refers to the color of the m a t e r i a l , a n d " n e v e r theless" does not make sense, i f the reference to grapes concerns shape. T h i s i n t e r p r e t a t i o n has l e d translators a n d c h e m i c a l archaeologists (18) to identify chrysocolla w i t h g r e e n malachite, w h i c h sometimes occurs as g r e e n "grapes." I n the Parisinus L a t i n u s v e r s i o n , uvam becomes luteam, a n d luteam means yellow, y e l l o w l i k e g o l d (19). W e t h i n k that i n the B a m b e r g e n s i s v e r s i o n , " u v a m " can be c o n n e c t e d not w i t h the shape b u t w i t h the color of grapes; specifically the color of Italian grapes (muscat), w h i c h are g o l d e n y e l l o w i n full m a t u r i t y . C h r y s o c o l l a is t h e n y e l l o w (like gold). C o l o r seems to b e one of the reasons for the use of chrysocolla not o n l y i n g o l d j e w e l r y , b u t also as a dye a n d i n m e d i c i n e . T h i s use of a y e l l o w p l a n t to i m p r o v e the y e l l o w color of an alloy is difficult to u n d e r s t a n d w i t h o u r present k n o w l e d g e of materials science. E v e n ancient goldsmiths h a d o b served that vegetables s u b m i t t e d to t e m p e r a t u r e treatments w o u l d u n d e r g o c o m p l e t e change i n t h e i r shape a n d color. T h e usefulness of this y e l l o w w e e d i n the s o l d e r i n g p r o c e d u r e may p r o b a b l y be b e t t e r u n d e r s t o o d i n the context of the f o l l o w i n g paragraph of P l i n y ' s d e s c r i p t i o n of u r i n e b e i n g a d d e d to a recipe of g o l d s m i t h e r y . Reasons for a d d i n g u r i n e may have b e e n

a. to liberate carbon as r e d u c t o r b . to liberate n i t r o g e n , w h i c h increases the solubility of c a d m i u m f r o m C d S i n g o l d as o b s e r v e d i n a previous w o r k (13) c. to e m i t a y e l l o w substance i n n o n m e t a l l u r g i c a l applications (27th a n d 2 8 t h paragraphs of P l i n y ' s book).

A c c o r d i n g to the foregoing i n t e r p r e t a t i o n , chrysocolla o f ancient times is n e i t h e r malachite n o r a b l u e c o p p e r silicate (the m i n e r a l n o w c a l l e d c h r y socolla) b u t is a y e l l o w substance, possibly the y e l l o w m i n e r a l c a d m i u m sulfide, w h i c h appears as a coating on other m i n e r a l s , chiefly z i n c sulfide. T h i s d e s c r i p t i o n fits P l i n y ' s text, w h i c h describes g o l d solder (chrysocolla) as a l i q u i d that "flows from several mines to give a s o l i d d e p o s i t . "


13.

DEMORTIER


Chrysocolla

261

T h e 29th paragraph of P l i n y ' s book deals w i t h the use of chrysocolla b y


goldsmiths. I n the B a m b e r g e n s i s v e r s i o n : "Chrysocollam et aurifices sibi vindicant adglutinando auro, et inde omnes appeUatas similiter virentes dicunt", or " T h e goldsmiths also use a special g o l d solder of t h e i r o w n for s o l d e r i n g g o l d , a n d according to t h e m it is from this that a l l the other substances w i t h a similar green color take the n a m e . " T h e translation of " v i r e n t e s " as g r e e n is the most e v i d e n t , b u t " v i r e n t e s " may be also u n d e r s t o o d not as a color, b u t as the p r o p e r t y of brightness that can be a t t r i b u t e d to a l l metals that reflect light. I n the Parisinus L a t i n u s , this sentence is "Chrysocolhm et aurifices sibi vindicant adglutinando auro: et inde omnes appellatam similiter utentes dicunt" or " T h e goldsmiths also use g o l d solder for s o l d e r i n g g o l d , a n d all the users [goldsmiths] call w i t h the same n a m e any substance suitable for this p u r p o s e . " T h i s sentence gives no n e w i n f o r m a t i o n , b u t rather s i m p l y recalls that the m e a n i n g of chrysocolla refers to its use as g o l d solder. O n this use of chrysocolla b y goldsmiths, b o t h versions are i d e n t i c a l at the e n d of the paragraph: Temperatur autem ea cypria aerugine, et pueri impubis urina, addito nitro. Teritur cyprio aere in cypriis mortariis: santernam vocant nostri. Ita ferruminatur aurum> quod argentosum vocant. Signumque est, si addita santerna nitescit. Ε diverso aerosum contrahit se, hebetaturque, et difficulter ferruminatur. Ad id glutinumfit, auro, et septima parte argenti ad supradicta additis, unaque contritis" w h i c h is translated as T h e y [goldsmiths] m a k e the mixture with copper verdigris a n d w i t h u r i n e of a boy w h o has not yet reached p u b e r t y a n d some soda [sodium carbonate]. It is g r o u n d w i t h a c o p p e r pestle i n a c o p p e r mortar. T h e y call this p r e p a r a t i o n santerna. In this way

they can solder argenteous gold. A sign of its having been so treated is if the application of santerna gives a brilliant colour. On the other hand c o p p e r y gold shrinks in size and becomes dull a n d is difficult to solder. F o r this purpose a solder is m a d e b y a d d i n g g o l d w i t h one seventh of silver into the above m a t e r i a l a n d they g r i n d t h e m together. S u r p r i s i n g l y , w e have f o u n d no c o m m e n t o n this sentence. I f chrysocolla is malachite (copper carbonate), w h y can cuprous g o l d not be s o l d e r e d w i t h a b r a z i n g alloy c o n t a i n i n g c o p p e r , w h e n c o p p e r can be dissolved i n g o l d i n


262



any p r o p o r t i o n ? I f chrysocolla is malachite, w h y this d u a l use of the c o p p e r minerals v e r d i g r i s a n d malachite? I f chrysocolla is greenockite ( c a d m i u m sulfide); h o w e v e r , this situation is e x p l a i n e d b y our results o n s o l d e r i n g alloys p r e p a r e d w i t h c o p p e r salts, c a d m i u m sulfide, a n d gold s h o w i n g a black earthy deposit i n a d d i t i o n to the g o l d e n alloy (13, 14). E x c e p t for o u r investigations, no systematic study of c a d m i u m i n j e w e l r y has b e e n u n d e r t a k e n . S e v e r a l relevant experiments b y other investigators u s i n g the e l e c t r o n m i c r o p r o b e d e m o n s t r a t e d an increase of c o p p e r a n d s o m e times s i l v e r at solders (20). S i l v e r was d e t e r m i n e d w i t h L X - r a y lines. W e have d e m o n s t r a t e d , h o w e v e r , that c a d m i u m cannot be d e t e r m i n e d w i t h an e l e c t r o n m i c r o p r o b e (21) i n the presence of a h i g h concentration of s i l v e r ; thus, an actual simultaneous increase i n c o p p e r a n d c a d m i u m i n solders m a y b e i n t e r p r e t e d as a s i m p l e increase i n c o p p e r i n experiments w h e r e c a d m i u m cannot be d e t e r m i n e d . Results of electron m i c r o p r o b e studies n e i t h e r c o n firm n o r r u l e out the supposition that c a d m i u m means a m o d e r n o r i g i n .

Conclusions C o p p e r m i n e r a l s w e r e u s e d b y early goldsmiths to p e r f o r m intricate s o l d e r i n g operations o n j e w e l r y . O t h e r procedures, perhaps i n v o l v i n g c a d m i u m sulfide, m a y have b e e n u s e d later. C a d m i u m sulfide seems to have b e e n u s e d o n l y i n I r a n a n d S y r i a , f r o m the 1st to the 9 t h c e n t u r y A . D . W e a n a l y z e d about 100 pieces of j e w e l r y f r o m those regions, a n d of that p e r i o d . A b o u t 30 of t h e m contain c a d m i u m . T h e c r i t e r i a to d i s t i n g u i s h restorations a n d fakes from ancient processes can o n l y be a p p l i e d o n about 20 of those items. T h e other items contain c a d m i u m i n too few solders to identify t h e i r a u t h e n t i c i t y b y the appropriate t e r n a r y C u - A g - C d diagram. A b o u t 150 other items of R o m a n , G r e e k , M e r o v i n g i a n , C e l t i c , B y z a n t i n e , a n d A m e r i c a n j e w e l r y from v a r i e d periods also w e r e investigated w i t h the same analytical m e t h o d , b u t c a d m i u m , e v e n at trace l e v e l , was n e v e r detected i n any of t h e m . U n t i l n o w , o n l y a s m a l l part of the results o n these pieces w e r e p u b l i s h e d (22, 23). F u r t h e r study is i n d i c a t e d , especially u n d e r e x p e r i m e n t a l conditions that allow the detection of c a d m i u m i n the presence of h i g h concentrations of silver. T h e controversy s u r r o u n d i n g the p o s s i b i l i t y of the use of c a d m i u m compounds b y goldsmiths of a n t i q u i t y c o u l d m o r e easily be e l u c i d a t e d i f other analytical research teams w e r e i n v o l v e d i n analyses of j e w e l r y of u n questionable a u t h e n t i c i t y from the same p e r i o d and o r i g i n as those r e p o r t e d here. A m o r e extensive study of ancient textbooks b y b o t h linguists a n d scientists w o u l d p r o b a b l y s h e d a d d i t i o n a l l i g h t o n the subject.

References 1. Flouret, G.; Nicolini, G.; Metzger, C. Gallia 1981, 39, 85-101. 2. Demortier, G.; Hackens, T. Nucl. Instrum. Methods 1982, 197, 223-236.


13. 3. 4. 5. 6.


7. 8. 9. 10. 11. 12. 13. 14.

15. 16. 17. 18. 19. 20. 21. 22.

23.

DEMORTIER


Chrysocolla

263

Demortier, G . Archéologia 1983, 176, 41-51. Demortier, G . Gold Bull. 1984, 17, 27-38. Demortier, G . Nucl. Instrum. Methods Phys. Res., Sect. Β 1986, 14, 152-155. Demortier, G . In Proceedings of the International Precious Metals Institute, Brussels; Vermeylen, G . ; Verbeeck, R, E d s . ; I P M I Press: Allentown, PA, 1987; pp 55-73. Demortier, G . Archaeometry, 1987, 29(2), 275-288. Ghirshman, R. Perse: Ρroto-Iraniens, - Mèdes, Achéménides; Gallimard: Paris, 1963; p 264. Demortier, G . In Applications of Nuclear Microprobes; Watt, F.; Grime G . W., Eds.; Adam Hilger: Bristol (UK), 1987; pp 334-376. Littledale, H. A . P. U . K . Patent 415 181, 1936. Piette, M.; Demortier, G . ; Bodart, F. In Microbeam Analysis; Romig, J r . , A . D.; Chambers W. F., Eds; San Francisco Press: San Francisco, 1986; pp 333-336. Demortier, G . ; Gilson, A . Nucl. Instrum. Methods Phys. Res., Sect. Β 1987, 18, 286-290. Demortier, G . ; Van Oystaeyen, B . ; Boullar, A . Nucl. Instrum. Methods Phys. Res., Sect. Β 1984, 3, 399-403. Demortier, G . ; Decroupet, D . ; Mathot, S. In Proceedings of the International Precious Metals Institute, Brussels; Vermeylen, G . ; Verbeeck, R, Eds.; I P M I Press: Allentown, PA, 1987; pp 335-352. Demortier, G . In Proceedings of the XIth ICXOM Conference, London (Canada); University of Western Ontario Press: London, Ontario, 1987; pp 147-150. Racklam, H. Pliny's Natural History, Loeb ed., Book 33; Heinemann: London, 1968; pp 4-72. Zehnacker, H. Pline l'Ancien, Histoire Naturelle, Livre 33; Les Belles Lettres: Paris, 1983; pp 47-85. Bailey, Κ. C. The Elder Pliny's Chapter on Chemical Subjects; Arnold: London, 1929; pp 59-109; 175-209. Oxford Latin Dictionary; Glare, P. G . N., Ed.; Clarendon: Oxford, 1976. Parrini, P.; Formigli, E.; Mello, E . Am. J. Archeol. 1982, 86, 118-121. Demortier, G.; Houbion, Y. Oxford J. Archaeol. 1987, 6(1), 109-114. Demortier, G . In Gold Jewelry—Craft, Style and Meaning from Mycenae to Constantinopolis; Hackens, T.; Winkes, R., Eds.; UCL Press: Louvain-la-Neuve, 1983; pp 215-227. D e Cuyper, F.; Demortier, G.; Dumoulin, J.; Pycke, J . La croix Byzantine du Trésor de la Cathédrale de Tournai; U C L Press: Louvain-la-Neuve, 1987; pp 1-88.

RECEIVED for review June 11, 1987. A C C E P T E D revised manuscript September 6, 1988.


Ancient Gold Solders: What Was Chrysocolla? - Advances in

Recommend Documents