Mineralogical Applications of SEM

7/29/2019 Mineralogical Applications of SEM

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Mineralogical appl icat ions of the analyt icalSEM in archaeo logyI . C . F R E E S T O N E A N D A . P . M I D D L E T O N

Brit ish Museum Research Lab ora tory , London W C1B 3D G

Abs t r a c tT h e m o d e r n a n a l y t i c a l S E M , w h i c h c a n p r o v i d e h i g h - q u a l i t y im a g i n g f a c i li ti es t o g e t h e r w i t h q u a n t i t a ti v ee l e m e n t a l a n a l y s i s u s i n g a n e n e r g y - d i s p e r s i v e s p e c t r o m e t e r , is f i n d i n g w i d e a p p l i c a t i o n in t h e i n v e s t i g a t i o no f a r c h a e o l o g i c a l p r o b l e m s . M a n y o f t h es e i n v e s t ig a t i o n s i n v o lv e t h e s t u d y o f s il ic a te a n d c a r b o n a t e - b a s e da r t ef a c ts w h i c h m a y b e r e l a t iv e l y u n m o d i f i e d f r o m t h e i r o r i g i n a l g e o l o g i ca l p a r e n t r a w m a t e r i a l s s o t h a tm i n e r a l o g i c a l l y b a s e d i n t e r p r e t a t i o n s a r e o f t e n a p p r o p r i a t e . I n t h is p a p e r w e p r e s e n t a s e ri es o f e x a m p l e si l lu s t r a t in g t h e r o l e o f th e a n a l y t i c a l S E M i n t h e m i n e r a l o g i c a l in v e s t i g a ti o n o f a r c h a e o l o g i c a l p r o b l e m s ,i n c l u d i n g t h e c h a r a c t e r i z a t i o n a n d p r o v e n a n c i n g o f g e o l o g i ca l r a w m a t e r i a ls , t h e e l u c i d a t i o n o f t h ep r o c e s se s u s e d t o t r a n s f o r m t h o s e r a w m a t e r i a l s in t o u s e f ul o b j e c ts a n d t h e r e c o g n i t i o n a n d c h a r a c t e r iz a -t i o n o f c h an g e s w h i c h a r c h a e o l o g i c a l a rt e f ac t s m a y h a v e u n d e r g o n e d u r i n g b u r i a l o r d u r i n g s t o r a g e .KEYWORDS: analytical scanning electron microscope, archaeology.

In t roduc t i onB E V O RE t h e I n d u s t r i a l R e v o l u t i o n , s o p h i s t i c a t e dr e f i n i n g a n d s e p a r a t i o n t e c h n i q u e s w e r e n o t , i ng e n e r a l , a v a i l a b l e t o t r e a t r a w m a t e r i a l s , w i t h t h er e s u l t t h a t a r c h a e o l o g i c a l a r te f a c t s a r e o f te n c l o s eo r ( a s in t h e c a s e o f s t o n e ) i d e n t i c a l i n c o m p o s i t i o nt o t h e i r g e o l o g i c a l p a r e n t s, F u r t h e r m o r e , t h e g r e a tb u l k o f e x c a v a t e d a r t e f a c t u a l m a t e r i a l i s s i l ic a t e o rc a r b o n a t e - b a s e d . A m i n e r a l o g i c a l i n p u t c a n t h e r e -f o r e b e c r i t i c a l i n t h e i r i n t e r p r e t a t i o n .T h e c o m b i n a t i o n o f h i g h -q u a l it y im a g i n g a n da n a l y t i c a l f a c il i ti e s o f f e r e d b y t h e S E M i s p a r t i c u -l a r l y v a l u a b l e i n t h e s e s t u d i e s b e c a u s e t h e m a t e r i a l sa r e o f t e n f i n e - g r a i n e d . I n a d d i t i o n , t h e i r p a r a -g e n e s i s i s s u c h t h a t t h e y m a y c o n t a i n a s s o c i a t i o n so f p h a s e s w h i c h a r e u n f a m i l i a r i n n a t u r a l c o n t e x t sa n d t h e a n a l y t i c a l f a c i li t y o f f e r e d b y t h e S E Ma l l o w s d i r e c t e l u c i d a t i o n o f t h e se a s s e m b l a g e s . T h ea dv e n t o f e ff i c ie n t ba c k -sc a t t e re d e l e c t ron de t e c t o r s ,w h i c h h a s c o n t r i b u t e d s i g n i f i c a n tl y t o t h e u s e o f t h eS E M i n p e t r o l o g i c a l i n v e s t i g a t i o n s , f o r e x a m p l e int h e s t u d y o f fi n e g r a i n e d s e d i m e n t s ( e.g . K r i n s l e y e tal., 1983 ; W hi t e et al. , 1984) , i s o f subs t a n t i a l be ne f i ti n s u c h w o r k .T h e a r c h a e o l o g i c a l s c ie n t is t m a y b e r e q u i r e d t oa n s w e r a n a r r a y o f q u e s ti o n s r a n g i n g f r o m t h em u n d a n e t o t h e e s o te r i c; f o r t h e p r e s e n t p u r p o s e ,h o w e v e r , w e fo c u s u p o n t h o s e w h i c h a r e p a r t i c u -l a r l y a m e n a b l e t o m i n e r a l o g i c a l so l u t io n s . T h e s eMineralogical Magazine, March 1987 , Vol. 51, pp. 21-31Copyright the Mineralogical Society

a r e (1 ) t h e c h a r a c t e r i z a t i o n o f t h e m a t e r i a l f r o mw h i c h a n o b j e c t w a s m a d e ( 2 ) t h e r e c o n s t r u c t i o n o ft h e t e c h n o l o g y i n v o l v e d i n i ts m a n u f a c t u r e , ( 3) t h ei n f e r en c e o f t h e p l a c e o f m a n u f a c t u r e o r s o u r c e o fr a w m a t e r i a l s a n d ( 4) t h e c h a n g e s t h a t h a v e o c c u r r e di n th e o b j e c t d u r i n g b u r i a l o r s t o r a g e . S u c h i n -f o r m a t i o n c a n b e u s e fu l in t h e e v a l u a t i o n o f c u l t u r a la n d e c o n o m i c a s p e c t s o f p a s t s o c i e ti e s . T h e r o l e o ft h e m i n e r a l o g i s t in s o u r c i n g a r t e f a c ts s h a p e d i ns t o n e h a s b e en t h o r o u g h l y r e v ie w e d b y K e m p e a n dH a r v e y ( 1 98 3) , a n d i n t h e p r e s e n t p a p e r , w e d w e l lm a i n l y u p o n s y n t h e t i c m a t e r i a l s , i.e . t h o s e i n w h i c ht h e c h e m i c a l a n d / o r p h a s e c o m p o s i t i o n h a s b e e nm o d i f i e d b y M a n . T h e r e i s a v e r y c lo s e p a r a l l e lb e t w e e n s u c h s t u d ie s a n d t h o s e o f m i n e r a l a n d r o c kp a r a g e n e s i s ; i n b o t h c a s e s th e a n a l y s i s o f a n e x i s t in ga s s e m b l a g e is c a r r i e d o u t w i t h a v i e w t o t h ee s t a b l i s h m e n t o f t h e p r o c e s s e s w h i c h g a v e r i se t o i t .I n t h e f o l lo w i n g p a r a g r a p h s a n u m b e r o fe x a m p l es a r e g r o u p e d b y m a t e ri a l o r a p p r o a c h t oi l l u s t r a t e t h e r a n g e o f a p p l i c a t i o n s a n d t h e b e n e f i tsc o n f e r re d b y t h e S E M . T h e s t u d ie s s u m m a r i z e d a r ed r a w n f r o m w o r k c a r r i e d o u t b y t h e a u t h o r s a n dt h e i r c o l l e a g u e s a t t h e B r i t i s h M u s e u m .

Me t h o d sT h e e a r l ie r o f o u r o w n s t u d ie s w e r e c a r r i ed o u tu s i n g a C a m b r i d g e $ 6 0 0 S E M f i tt e d w i t h a q u a l i t a -t i v e e n e r g y - d i s p e r s i v e s p e c t r o m e t e r . T h e d a t a s o


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22 I. C. FREE STON E AND A. P. MID DLE TONobtained were supplemented with quantitativeanalyses obtained using a Cambridge Geoscanmicroprobe fitted with a Link Systems energy-dispersive spectrometer. In more recent studies wehave used a JEOL JSM840 SEM fitted with a LinkSystems spectrometer which provides analyticaldata of comparable quality together with high-quality imaging facilities.Where possible and appropriate, samples wereprepared as polished thin-sections. However, manyof the objects we have examined are of high artisticand/o r historical value and only the smallest pos-sible sample may be removed; duplicate sampling isin general unacceptable. In such cases small samplesare set in blocks of epoxy resin and polished.

The effect of temperature on themicrostructure of clayClay artefacts are ubiquitous from the Neolithicperiod and an understanding of the level of claytechnology is impor tant to an appreciation of thegeneral technical sophistication of a culture. Thetemperatures at which ceramics were fired in thepre-industrial world varied over the range 600-1300 ~ depend ing on the type of clay used and thetypes of kiln available. The use of clay materials inmetallurgical processes extended this range up to1400 ~ The temperature to which a ceramic hasbeen subjected may be estimated by the examina-

tion of its microstructure in the SEM. This ap-proach has been explored particularly by Tite andManiat is (1975), Mania tis and Tire (1981) and Titeet al . (1982a).A photomicrograph of a low-fired (c. 800~poorly-refined earthenware clay is shown in Fig. I a.The individual mineral particles are readily distin-guished and there is only limited grain-to-graincontact. The source rocks for this particular claywere Precambri an slates, reflected in the quart z andmica-rich assemblage. With increase in tempera-ture localized melting occurs and the ceramicbegins to vitrify. The degree of interconnection ofthe particles increases and the porosity decreases(Fig. lb; T = c. 1000-1050~ Finally , the poresare isolated and the trapped gases expand, causingthe formation of rounded vesicles (Fig. lc; T = c.1100-1150 ~ and at higher temperatures (c. 1200-1250 ~ macroscopic bloating.The various stages in the vitrification of aceramic may be characterized and, by retiring testpieces of the ceramic in the labor atory over a rangeof temperatures, this sequence of changes can becalibrated for temperature. In order to use such acalibration to determine the temperature at whicha ceramic has been fired, it is important to test tirea piece of the same ceramic because the behaviour

FIG. 1. Micrographs of polished thin-sections showingprogressive changes in the degree and texture of vitrifica-tion from the cool portion to a position close to the hotsurface of a fragment from a zinc distillation furnace atZawar. (a) T = 800 ~ (b) T = c. 1000-1050~ (c) T =c. 1100-1150~ BSE micrographs; operating voltage25 kV. The scale bars represent 100 #m.

with temperature is very dependent on composit ion(Maniatis and Tite, 1981). Analytical data, deter-mined by EDXA, are very useful as they enable thefiring behaviour to be matched with that of pre-viously analysed samples of similar composition,allowing the extrapolat ion of results from refiringsat only one, or perhaps two, temperatures. Further-more, environmental contami nation of the ceramic


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MINERALOGICAL APPLICATIONS IN ARCHAEOLOGYby fluxes such as sodium chloride may also berecognized and taken into account when interpret-ing retiring data.

23

Pottery and porcelain technologyFiring temperatures of pottery can be used toevaluate the sophist ication of the firing process; forexample, whether in an open bonfire (typically lessthan 850 ~ or in a simple updraft kiln (T = 850-ll0 0~ The nature and development of theceramic microstructure as a function of tempera-ture can be related to the composition of the claysource exploited. For example, calcareous clays(CaO > 5 wt. %) were widely used in the Mediter-ranean area in antiquity and have a particularlystable microstructure over the temperature range850-1050 ~ There is thus a considerable marginfor error in firing, and such clays would guaranteea consistent product quality relative to the low-refractory, non-calcareous clays which were avail-able as alternatives. Chronological development inclay exploitation and ceramic microstructure hasbeen traced using SEM and EDXA by Noll and hisco-workers (e.g. Noll, 1978, 1982; Letsch and Noll,1983).The analytical SEM has made possible thedetailed examinat ion of the inorganic surface coat-ings and pigments which were applied to muchancien t pottery. Attention has focussed particularly

on the fine, glossy coatings on classical fine waressuch as Greek Attic, Roman Samian wares andtheir derivatives and precedents (Noll, 1981; Noll etal., 1974, 1975; Tite et al., 1982b, 1982c; Maggetiand Galetti, 1981). These K-, Fe- and Al-richcoatings (shown in cross-section in Fig. 2) arethought to have been prepared from suspensions ofvery fine illitic clay particles. The clay plateletsoriented fiat against the surface of the pots as theslip dried, resulting in a high-quality glossy surface.A study by Middleton (in press) on applied redcoatings on certain Late Bronze Age pottery fromSouthern England illustrates a number of aspects ofsuch studies. This type of pottery has generally beentermed 'hematite-coated ware' because of its dis-tinctive, bright red, often burnished, finish. Detailedexaminat ion of the coatings on sherds from about adozen archaeological sites in southern England hasshown that several techniques were used to producethe red finishes. Some sherds showed evidence forthe application of crushed hematite or ochre coat-ings (e.g. Fig. 3 which shows a cross-section througha coated sherd), while the surfaces on others hadevidently been produced by the application ofvarious clay slips or even ust burnished without theapplication of any coating material. Variation inthe detailed character of both the crushed hematite

FrG. 2. Red, high-gloss surface finish on a sherd of RomanSamian ware. Note the lack of apalastic inclusions andvery fine-grained iron oxide particles (white) in thecoating (arrowed). BSE micrograph of a cross-section;operating voltage 15 kV. The scale bar represents 10 #m.

and clay slip coatings was interpreted as reflectinglocal variation in available raw materials andtechnique. Thus, in the production of these wares,the potters were trying to imitate a t ype- -the y werenot sharing techniques. This study showed that theuse of the term 'hematite-coated' could be verymisleading.Against the soft, low-fired bodies and coatings ofthe pottery of Bronze Age Brita in we may counter-pose the highly vitrified, glazed porcelains of China,which represent a level of technical ability andcontrol that western ceramicists were striving toemulate a millenn ium after their inception in theEast. The availability of appropriate clays and

FIG. 3. Sherd of Late Bronze Age 'hematite-coated' warefrom Kimmeridge, Dorset showing the use of crushedhematite or ochre to produce a red surface coating(arrowed). BSE micrograph of a cross-section; operatingvoltage 15 kV. The scale bar represents 40 #m.


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24 I . C . F R E E S T O N E A N D A . P . M I D D L E T O Nfluxes for the pr oduc t ion of porcela in was one of thefac tors accounting for the precociousness of theCh inese ce ramic indus t ry , and examina t ion wi ththe SEM gives some ins ight in to the nature ofthe raw ma te r ial s . The mic ros t ruc tu re o f Ch inesepo rce la in may be cha rac te r ized in t e rms o f fou rbas ic morpho log ica l un i ts (T i te et al., 1984; Tite an dFreeston e , 1986): (1) round ed, re l ic t quar tz gra ins ,(2 ) a ma t r ix composed o f va ry ing p ropo r t ions o fvery f ine-gra ined mull i te in g lass , (3) ragged, la th-shaped aggregates of mull i te and (4) g lassy poolscon ta in ing ran domly -o r ien ted m u l l i te need le s (F ig .4). Coup led with m orpho logy, A 1203/SiO 2 ra t iossuggest tha t the la th- l ike aggregates are pseud o-morphs o f muscov i te and tha t the g la s sy poo ls a rethe resul t of the melt ing of fe ldspar; K /N a ra t ios arecons tan t th ro ughou t due to the mob i l i ty of thesecomponen ts .

FIG. 4. Etched section of underglaze-blue porcelain show-ing relict grains o f quartz ( dark grey, q) in glassy matri xcontaining ragged, lath-shaped aggregates of mullite andglassy pool s containing fine mullite needles (g). BSEmicrograph; operating voltage 15 kV. The scale barrepresents 20 #m.

Exam ina t ion o f the c la ss ic 'y ingq ing ' and b lueand white porcela ins of J ingdezhen, southern China ,dat ing to the Yuan dynasty (13-14th centur ies)sugges ts tha t m ica was a m a jo r com ponen t o f thebodies and w as genera l ly dom inan t over fe ldspar.Th is co r re sponds we l l wi th ou r unde rs tand ing o fthe p roduc t ion o f po rce la in a t J ingdezhen today ,when a gre isenized 'porce la in s tone ' or 'pe tunt se ' ismixed wi th kao l in to p rod uce the ce ramic body . Incon t ra s t to Eu ropean ha rd po rce la in , where feld -spar is t rad i t io nal ly used as a f lux , in Chi na acr i t ica l com pon ent was ser ic i t ic mica which noton ly p rov ided K 20 a s a f lux bu t a l so con t r ibu ted tothe p la s t ic i ty o f the body , no do ub t an im por tan tfac tor in the reper to ire of ceramic forms produc ed.

Indeed, there is ev idence tha t , a t one s tage , only as ingle raw mater ia l , a kaol in ized gre isen , wasut i l ized to pro duce some of the ear l ie r porce la inbodies (Ti te et al., 1984) . The SE M is a lso provingextremely usefu l in the s t udy of Chinese g lazes andoverlay enamels (Kingery and Vandiver , 1986;Vandiver and Kingery , 1985) .Provenance studies

Thin -sec t ion pe t rog raphy has p roved pa r t i cu -lar ly usefu l in a l lowing the ident if ica t ion of thesources of raw mater ia ls of s tone and ceramicar tefacts to be es tabl ished (Kem pe and H arvey,1983). Such information is of value for the insightsi t p rovides in to the economies o f pas t cu l tures andthe deg ree o f cohes ion wi th in them and con tac tbetween them.In such cases the use of the analy t ica l SEMand /o r mic rop robe can sha rpen the in fo rma t ionp rov ided by op t ica l pe t rog raphy . As an example weci te the case of a group of ceramic f lagons , ja rs andpla t ters of Ro man s ty le found on a number of s i tesin southeastern Bri ta in da t ing to the Late Pre-Roman Iron Age (Rigby and Frees tone , 1983,1986). In thin section these wares are found to bere la t ive ly f ine gra ined , conta in ing 10 20~o poo rly-sor ted , very f ine sand an d coarse s i l t in a matr ix off i red c lay . The c las ts a re predominantly fe ldsparand qua r tz wi th common muscov i te and b io t i t e ,

and oc casional heavy minera ls such as amphibole ,c l inopyroxene, o l iv ine and sphene. Rock fragmentsare rare , but in re la t ive ly coarse-gra in ed examplesof the fabr ic , occasional f ragmen ts o f ac id p lu tonic ,sch i s to se me tamorph ic and vo lcan ic rocks a reobserved . Unfor tunate ly the f requency of occur-rence of these rock fragments is so low as toprec lude any inferences of the geologica l source ofthe c lay f rom their presence . Therefore the mine ra lf ragments in the pot tery were analysed to de ter-mine the i r env i ronmen t o f fo rma t ion .Using the appro ach o f Treve na and N ash (1979,1981), feldspars were found to be of at least twotypes , volcanic and p lu tonic /me tam orph ic (Fig. 5).Amphiboles form two c lear analy t ica l groups , onetend ing to kae rsu t i t i c compos i t ions and a g roup o fmagnesio-hornblendes (Fig . 6) . Cl inopyroxeneswere not very informative , t ransgress ing thebounda ries be tween tec tonic group ings (e.g. Nisbetand Pearce , 1977) . However , a s ingle o l iv ine had acalc ium concentra t ion charac ter is t ic of volcanicolivines (Simkin and Smith, 1970). These resultswere considered to support the l imited evidencefrom the rock fragments tha t the sand was amixture of mater ia l f rom at lea s t two sources :vo lcan ic rocks and p lu ton ic /me tamorph ic rocks .The presence of f resh , volcanic o l iv ine indica tes


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MINERALOGICAL APPLICATIONS IN ARCHAEOLOGYA n

A b O r

0"6TiO' b

0"4

0"3

0- 2

0 " I 9

K a e r s u t i t e s

25

Ma g n e s i o - h o r n b l e n d e sQO 9

i i0"3 0'4 0"5 0-6 0"7 0"8 09

NaFIGS. 5 and 6. FIG. 5 l e f t ) . Composition of feldspars from Roman-stylepottery of late, pre-Roman, Iron Age date, foundin southeastern Britain. The boundaries between the feldspars of different geological origins are drawn after Trevenaand Nash (1979, 1981): v = volcanic, p = plutonic, m = metamorphic. FIG. 6 ( r i g h t ) . Diagram (atoms per 23 oxygens)illustrating the occurrence of two compositional groups of amphiboles in late pre-Roman, Iron Age pottery fromsoutheastern Britain. (See text for discussion.)

that at least the volcanic signature is unlikely to bethe result of sedimentary recycling. The Na-, Ti-richamphibole group (Fig. 6) is suggestive of an alkalinevolcanic rock source, and occasional volcanic rockclasts appear to be trachytic in composition. Con-siderat ion of the geology of the most likely sourcesof Roman pottery in Western Europe and theMediterranean area led to the suggestion that theMassif Central, France, was the most likely sourcefor the pot tery (Freestone, 1982; Rigby and Free-stone, 1983) and recent examinat ion of the typologyof the pottery in that area has confirmed it as theprobable source (Rigby and Freestone, 1986).A second example concerns a small bronzestatuette of the Roman goddess Minerva (Fig. 7)which has been in the collections of the BritishMuseum since the last century, but for which thereis no record of its origin. Small, black, prismaticcrystals found embedded in the corrosion productsof the bronze were shown by SEM and EDXA to besalitic pyroxenes with well-preserved crystalmorphology (Fig. 8). Melt inclusions in the pyro-xenes are of potassic composition. The statuetteappears to have been buried in a pyroclastic depositassociated with potassic magmatism. Given theRoman association of the object an I talian originwas clearly indicated, most probably the ashesassociated with the catastrophic er uption of Vesu-vius in A.D. 79. Exa mination of the corrosionproducts on bronzes known to have been excavatedfrom Pompeii or its environs revealed, in somecases, corrosion products a nd clinopyroxene in-clusions similar to those on the Minerva, corro-

borating the inferred source area (Freestone e t a l . ,1984).M e ta ll u r g i ca l b y -p r od u c ts

The discarded refractories and slag which may befound in copious quantities on metal extraction andmetalworking sites often represent our principalsource of information on the metallurgical tech-nologies and it is vital to extract from them themaximum possible information. At the most ele-mentary level it may s imply be a matter of identify-ing the particular metal which was being worked byEDXA analysis of metal inclusions in the slaggedsurface of a crucible (Tite e t a l . , 1985). Moredetailed studies involve the determinat ion of para-meters such as the temperature, duration andreducing conditions during smelting, the process-ing of ores and the preparation of refractorymaterials.An example which illustrates many of theseaspects of investigation is that of zinc and s ilver-lead smelting at Zawar, Rajasthan. The local ore isa stratabound, sphalerite-galena deposit in animpure dolomitic host. Radiocarbon dating sug-gests that these deposits have been exploited forlead and silver for at least two thousand years andthat the extraction of zinc metal began some time inthe Medieval period. This was a major technologi-cal advance. Zinc metal boils at 913 ~ which isbelow the temperatu re required to reduce the oxidein the presence of carbon. Thus the metal isproduced as a gas and it must be distilled rather


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26 I. C. FRE EST ONE AND A. P. MI DD LE TO Nthan smelted in the normal way. Excavations atZawar (Craddock et al., 1985) have revealed intactzinc distill ation furnaces still conta ining the retortsin which the zinc was reduced (Fig. 9).

::::: i84

Fro. 9. Excavated zinc smelting furnaces at Zawar show-ing arrays of inverted ceramic retorts, supported byperforated plates (which divide the hot, upper chambercontaining the retorts from the cool, condensationchamber below).

Petrography of the ceramic retort bodies andfurnace walls shows that they were made from localclays (Freestone et al., 1985a). Analysis of theceramic matrices using EDXA in the SEM revealsthat they contain around 15 wt. ~o fluxes (FeO,MgO, CaO, NazO, K2 0 ). Thus the ceramics arecloser to house bricks than to modern refractoriesin composition, a characteristic typical of mostmetallurgical furnaces up to the end of Medievaltimes (Freestone and Tite, 1986). Examination ofthe vitrification textures of the Zawar materials (e.g.Figs. la-c) and comparison with retired examplesshows that temperatures of c. 1200-1250 ~ wereatta ined during smelting. These were at the limits oftolerance of the clays and slumped and distortedretorts are common in the field. The furnace wallsshow a gradient in vitrification structure from theinterior (high temperature, highly vitrified) to theexterior (low degree of vitrification). This can beinterpreted in terms of a temperature gradientwhich developed in the furnace wall during smelt-ing and which can be estimated by comparing themicromorphologies at series of points with those ofa series of retired examples. The gradient dependsFIGS. 7 and 8. FIG. 7 (top). Statuette of Minerva (height0.10 m). The arrow indicates the area of corrosioncontaining small pyroxene crystals. FIG. 8 (bottom).Pyroxene crystal observed in corrosion products onstatuette of Minerva. SE image; operating voltage 25 kV.The scale bar represents 400/~m.


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M I N E R A L O G I C A L A P P L I C A T I O N S I N A R C H A E O L O G Yo n t h e d u r a t i o n o f t h e s m e l t a s w e ll a s t h e t e m p e r a -t u r e a c h i e v e d i n t h e f u r n a c e , s o t h a t , u s i n g a s i m p l eh e a t c o n d u c t i o n e q u a t i o n , t h e t i m e f o r w h i c h af u r n a c e w a s a t h i g h t e m p e r a t u r e m a y b e e s t i m a t e d(T i t e e t a l . , 1982a , 1985) . F o r t he z i nc sm e l t i ngp r o c e s s a t Z a w a r , d u r a t i o n s o f a r o u n d 5 h r a r eo b t a i n e d .

T~8 0 0 9 0 0 1 0 0 0 11000 F - - - r - ~ - - - r - - - - - , v

-log pO

1 2 0 0 1 3 0 0F

FIG . 10. Tem perature/PO2 relationships for selectedreactions of interest (see text for discussion). Th e pr ob abl econditions within the zinc retorts during firing andapproximate condit ions for some other pyrotechnicalprocesses are indicated.

A n a l y s i s o f t h e c o n t e n t s o f th e z i n c s m e l t i n gr e t o r t s r e v e a l s t h a t a c r u s h e d , s i l i c e o u s d o l o m i t ew a s u s e d a s a c a r r i e r m a t e r i a l f o r t h e r o a s t e ds p h a l e r i t e ( F r e e s t o n e e t a l . , 1985a ) . In some c a se st h is h a s p a r t i a l l y m e l t e d t o g i v e a n a s s e m b l a g e o fg l as s , Z n - b e a r i n g d i o p s i d e a n d m e t a l l ic ir o n . U s i n gt h e c o m p o s i t i o n o f th e g l a s s d e r i v e d f r o m E D X A ,a n d a s s u m i n g t h a t i t w a s i n e q u i l ib r i u m w i t hm e t a l l i c F e a n d Z n , a n a i v e ( i d e a l - m i x i n g ) m o d e l f o rt h e a c t iv i t ie s o f Z n O a n d F e O i n t h e g la s s w a s u s e dt o e s t i m a t e t h e P O 2 i n t h e r e t o r t d u r i n g s m e l t i n g .T h e r e s u l t s fo r i r o n a n d z i n c w e r e f a ir l y c o n s i s t e n t( s ee F ig . 1 0 ) . T h e c o n d i t i o n s a t t a i n e d d u r i n g z i n cs m e l t i n g w e re e x t r e m e r e l a t i v e t o o t h e r e a r l y p y r o -t e c h n o l o g i e s , e m p h a s i z i n g t h e s o p h i s t i c a t i o n o f t h ep r o c e s s .A b u n d a n t e v i d e n c e i s a l s o f o u n d a t Z a w a r f o rs i l v e r - l e a d s m e l t i n g , i n t h e f o r m o f s i li c a t e s l a g sw h i c h h a d b e e n s e p a r a t e d f r o m a r g e n t if e r o u s l e a d

27a s i m m i s c i b l e l i q u i d s a t h i g h t e m p e r a t u r e s . T h e s es la g s c a n b e e i th e r e a r l ie r o r c o n t e m p o r a r y w i t h t h ez i n c s m e l t i n g d e b r i s . M o r p h o l o g i c a l l y t h e s l a g s c a nb e d i v i d e d i n t o t w o g r o u p s i n t h e f i e ld : g l a s s y s la g sw h i c h a r e n o t a s s o c i a t e d w i t h w i t h z i n c d i s t i l l a t i o nd e b r i s a n d c r y s t a l l i n e s l a g s w h i c h a r e g e n e r a l l yf o u n d a l o n g s i d e d e p o s i t s o f d i s c a r d e d z i n c r e t o r t s .I n t h e S E M i t i s o b s e r v e d t h a t t h e c r y s t a l l i n e s l a g sc o n t a i n a b u n d a n t f a y a l it e a n d / o r i r o n - r ic h c l in o -p y r o x e n e , w h i l e t h e g l a s s y sl a g s c o n t a i n o n l y s p a r s ej c r y s t a l l in e p h a s e s s u c h a s z i n c i a n m e l i l it e , M g - r i c hc l i n o p y r o x e n e a n d z i n c i a n o l i v i n e ( F ig . 1 1 ) . T h i s i sa r e f l e c t i o n o f t h e i r c h e m i s t r y ; t h e c r y s t a l l i n e( fa ya li t ic ) s l a gs ha ve h i gh l e ve ls o f i ron (18 32 wt . %F e ), b u t l o w z i n c ( < 3 % Z n ) , w h e r e a s t h e g l a s s ys l a g s h a v e l o w i r o n c o n t e n t s ( 8 - 1 0 % ) , b u t h i g h e rz i n c ( 7 -1 1 % ) . I t i s in f e r r e d t h a t t h e g l a s s y s la g s w e r ea p r o d u c t o f t h e l e ad s m e l t i n g a c ti v i t y t h a t p r e c e d e dt h e i n t r o d u c t i o n o f z i n c s m e l t in g . A t t h i s s t a g e ah i g h v a l u e w a s n o t a t t r i b u t e d t o s p h a l e r i t e a n d i tw a s n o t e f f i c ie n t ly s e p a r a t e d f r o m t h e g a l e n a . W i t ht h e o n s e t o f z i n c s m e l t in g , h o w e v e r , i t b e c a m ei m p o r t a n t t o m a x i m i z e th e r e c o v e r y o f z i n c s o t h a ts p h a l e r i t e w a s c a r e f u l ly r e m o v e d f r o m t h e l e a d o r e s ;t h e r e s u l t i n g r e d u c t i o n o f t h e f l u x c o n t e x t o f t h e s l a gw a s c o m p e n s a t e d b y t h e a d d i t i o n o f m o r e i r o n ,p o s s i b l y a s p y r i t e , w h i c h p r e c i p i t a t e d a s f a y a l i t e( F r e e s t o n e e t a L , 1986).

V i treo us m a ter i a l sA s u s e d h e r e , 'v i t r e o u s m a t e r i a l s ' is a c a t c h - a l lp h r a s e c o i n e d t o e n c o m p a s s , i n a d d i t i o n t o g l a s si ts e lf , a r a n g e o f c e r a m i c s a n d p i g m e n t s i n w h i c h ag l a s sy p h a s e i s i m p o r t a n t , b u t e x c l u d in g c l a y - b a s e dc e r am i c s . T h e S E M h a s p r o v e d i n v a l u a b l e in t h ec h a r a c t e r i z a t i o n a n d i n t e r p r e t a t i o n o f t h e s em a t e r i a l s .' E g y p t i a n f a i e n c e ' i s a c r u s h e d s i l ic a b o d y h e l dt o g e t h e r b y v a r i a b l e a m o u n t s o f i n t e r s t i t i a l g l a s sa n d a n a l k a l i n e g l a z e . B e a d s , f i g u r i n e s a n d s m a l lv e s s el s m a d e f r o m t h i s m a t e r i a l w e r e w i d e l y u s e d i n

E g y p t a n d t h e N e a r E a s t b e f o r e t h e R o m a n p e r i o d .A t l e a s t t h r e e m e t h o d s w e r e u s e d t o p r o d u c ef a ie n c e, a n d t h e s e c a n b e d i s t i n g u i s h e d b y e x a m i n a -t i o n o f t h e d i s t r i b u t i o n o f g la s s in t h e b o d y u s i n g t h eS E M ( T i t e e t a l . , 1983 ; T i t e a nd B i mson , 1986) .' E g y p t i a n b l u e ' i s a p i g m e n t f o r m e d b y f l i tt i n gq u a r t z , c al c it e a n d a c o p p e r s a l t t o p r o d u c e t h em i n e r a l c u p r o r i v a i t e , C a C u S i 4 O l o w h i c h h a s av i v id b l u e co l o u r . T h e p i g m e n t w a s u s e d i n p a i n t -i n gs , a n d c o u l d b e m o u l d e d t o f o r m s m a l l o b je c t ss u c h a s b e a d s , s c a r a b s a n d s t a t u e t t e s . T i t e e t a l .( 19 84 ) e x a m i n e d s a m p l e s o f E g y p t i a n b l u e d a t i n gf r o m t h e 1 4 th c e n t u r y B . c . t o t h e 3 r d c e n t u r y A . D . I tw a s s h o w n t h a t , i n a d d i t i o n t o c u p r o r i v a i te , u n -r e a c t e d q u a r t z a n d c o p p e r s i l i c a t e g l a s s w e r e a l s o


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28 I. C. FREESTONE AND A. P. MIDDLETON(Bimson and Freestone, 1983) and the enamels ofthe Medieval Royal Gold Cup (Bimson and Free-stone, 1985) have been characterized.Detailed technological studies may involvedetermination of the relat ionships between the sizeand morphology of opacifiers and/or devitrifica-tion products, the properties of the glass and itsbulk composition. For example, Freestone (1986)has examined a group of opaque, red glasses datingto the first and second millennia B.c. from Egyptand the Near East. The colourant /opacifier in theseglasses was cuprite, Cu20, and SEM analysisreveals that the range of shades available to thecraftsmen depended on the size of the cupriteparticles, which in tu rn depended on the concentra-tion of Cu20 and P bO in the glass. Thus brilliantred glasses contain extensive dendritic cuprite (Fig.13), while duller, brownish reds and orange-redscontain very fine blebs of cuprite.

FIGS. 11 and 12. FIG. 11 (top). Lead-smelting slag fromZawar, illustrating the development of spinifex zincianolivine black) and box-likecrystals of melilite dark grey),in a glassy matrix. BSE micrograph; operating voltage 15kV. The scale bar represents 100 ~tm. FIG. 12 (bot tom).'Egyptian Blue', showing the development of prismaticcrystals of the bright-blue mineral cuprorivaite (white).Pools of copper silicate glass (light grey), relict grains ofquartz (dark grey) and voids (black) are also visible.BSEimage; operating voltage 15 kV. The scale bar represents100 pm.

present (Fig. 12). It was possible to relate the colourand hardness of Egyptian blue samples to thegrain-size of the cuprorivaite and the amoun t anddist ribu tion of the glass phase, and thus to the bulkcomposition and manufacturing technique.Much of the glass of antiquity was opaque andcoloured, and was used as a substitute for semi-precious stones such as lapis lazuli, carnelian andturquoise. The aim of the examination of thesematerials is to identify spatial and chronologicalvariations in composition and technique whichmay be culturally related. The introduction of theSEM has me ant t hat many previously inaccessibleobjects may now be sampled, due to the very smallsample size required. Thus the glass of uniqueobjects such as the Roman cameo Portland Vase

FIG. 13. Opaque red glass from Nimrud, dating to thefourth century 8.c. Dendrites of cuprite (light grey) givethe glass its bright red colour. The bright white grains arecopper. BSE micrograph; operating voltage 15 kV. Thescale bar represents 10 pm.

WeatheringThe physical and chemical decomposition ofarchaeological materials is of considerable im-port ance in that it (1) affects their integrity asunique and pe rmanent records of past cultures and(2) it can affect the resul ts of scientific examinationsuch as dating and sourcing by elemental analysiswhich require the bulk chemistry of the object toremain constant after it has been discarded.The decay of limestone in a museum storageenvi ronment is very variable in nature a nd degree:some objects may show only min imal damage withpitti ng of fni she d surfaces while others showextensive loss of surface detail an d even 'structural


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MINERALOGICAL APPLICATIONS IN ARCHAEOLOGYfailure' by the flaking off of large fragments of stone.The reasons for these differences are not wellunderstood and a study by Bradley and Middleton(in progress) has sought to understand these for agroup of Egyptian limestones. The decayed samples,all considered to require some conservation treat-ment in order to correct deterioration, fall intothree groups on the basis of the nature of theirdecay and their macroscopic appearance. Thesegroups coincide precisely with those made on thebasis of the observation of textural features andidentification of non-car bonat e phases in the SEM.In particular, the badly decayed sculptures exhibit-ing flaking contain significant amoun ts of a paly-gorskite clay which was not present in thosesculptures that showed only decay by localizedpitting. In these latter sculptures, by contrast, thenon-ca rbonat e fraction was either insignificant (agroup of white, chalky limestones) or consistedmain ly oflepispheres of opal-CT (Fig. 14; a group of

2 9

the range 2512 wt. ~o PzO5 in the clay matrix, whileunmodified clays and soils typically contain lessthan 0-5 wt. ~. Calcium phosphate is added tomodern ceramics e.g. 'bone china' to act as a fluxand vitrify the body, but ancient ceramics with highP2 0 5 tend to have non-vitrified bodies, indicatingthat phosphate was not added for this purpose.Determination of compositional variation acrossthe ceramics using EDXA shows that PzOs, CaOand FeO have U-shaped profiles in some cases (Fig.15), indicating that these elements were mobilewhilst the pottery was buried and that the highP205 contents are the result of environmentalcontaminat ion (Freestone et al. , 1985b). Fur the r-more, linear correlations between P205 and CaO(Fig. 16) suggest that a phosphate compound ofrelatively fixed stoichiometry is present which isdistinct from the more familiar minerals such as

8 -

FIG. 14. Fracture surface of a fragment from an Egyptianlimestone sculpture from the Cairo area, showing lepi-spheres of opal-CT (arrowed). SE image; operatingvoltage 15 kV. The scale bar represents 1 #m.

fine-grained, buff-coloured limestones). The exten-sive decay of the former group of limestones ispossibly the result of the modifications caused totheir pore-size distribution by the palygorskite,which grew authigenically, or possibly due to localplanes of weakness due to the presence of the clay.A further point of interest arising from this workis that the groupings defined above correlate wellwith the ascribed provenance of the objects, prob-ably reflecting the use of locally available stone andalso implying the possibility of using observationsof this nature to provenance objects of unknownsource.In our studies of ancient pottery we have com-monly encountered concentrations of phosphate, in

7 -

6 -

W t .p e r c e n t

o x i d e5 -

2 I I i I I [ J6 4 2 0

D i s t a n c e f r o mSu r f a c e ( m m )

FIG. 15. Compositional change across a late Iron Agesherd, excavated at St Albans, illustrating the develop-ment of U-shaped profiles for P205, CaO and FeO. Theheavy vertical ine indicates the 'inner'surface of the sherd,which had a glossy slip coating.


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30 1. C . F R E E S T O N E A N D A . P . M I D D L E T O Na p a t i t e o r b ru s h i te . D i s c r e t e p h o s p h a t e p h a s e sc o u l d n o t b e i s o l a t e d w i t h t h e S E M , i n d i c a t in g t h a tt h e v e r y f i n e - g r a in e d p h o s p h a t e m a y h a v e b e e na b s o r b e d i n t h e f i r s t i n s t a n c e o n t h e f i r e d c l a yp l a t e le t s . I t i s w e l l k n o w n t h a t d i s o r d e r e d c l a ym i n e r a l s s u c h a s a l l o p h a n e h a v e a h i g h c a p a c i t y t or e t a i n p h o s p h a t e ; t h e d i s o r d e r i n d u c e d i n c l a y s b yf i ri n g a t t e m p e r a t u r e s b e l o w t h o s e s u f f i ci e n t t oc a u s e v i t r i fi c a t i o n a n d r e c r y s t a l l iz a t i o n a p p e a r s t og r e a t l y i n c r e a s e t h i s p r o p e r t y ( F r e e s t o n e e t a l . ,1 98 5b ). I n v i e w o f t h e c a p a c i t y o f p h o s p h a t em i n e r a l s t o i n c o r p o r a t e o r a d s o r b t r a c e e l e m e n t s( M u r r a y e t a l . , 1 9 8 3 ; E d w a r d e t a l . , 1984) , th i s mayh a v e i m p o r t a n t i m p l i c at i o n s f o r p r o v e n a n c es t u d i e s u s i n g t r a c e e l e m e n t d a t a .

s t r a t i g r a p h i c c l a y g ro u p s , i s n o t a v a i l ab l e . G i v e nt h a t t h e a r c h a e o l o g i c a l r e q u i r e m e n t s d o n o t o f te np r e c i se l y m a t c h t h o s e o f th e m i n e r a l o g i s t i n t e r m so f t h e d a t a r e q u i r e d , w e c a n e x p e c t t h e g e n e r a t i o n o fs u c h f u n d a m e n t a l i n f o r m a t i o n t o o c c u r i n c re a s in g l yw i t h i n t h e a r c h a e o l o g i c a l c o m m u n i t y .

Acknow l edgemen t sWe are part icularly grateful to our colleagues in theBri t ish Museum, especial ly our collaborators in many ofthese projects , M. Bimson, S. Bradley, P. T. CraddockN. D. M eeks and M. S . T i te and to the mem bers o f thean t iqu i t ies depar tmen ts who m ade avai lab le fo r examina-t ion ob jec ts in the i r care. W e thank a l so the Depar tm en to f Minera logy , Bri ti sh Muse um (Natu ra l His to ry ) fo r useof the i r microprobes dur ing the ear l ie r s tages o f th i s work .

CaO

~ T" . . o 9o ~ ~ ~/-G~-

e e

, , . . ,O 1 4 1 5 7

9 1 7 7 3 8

e

1 2 3 4 s 6 7 tt 9 10 11 12w e i g h t p e r c e n t P 2 05

FIG. 16. Rela t ionsh ip be tween C aO and P2 0 5 fo r someLate I ron Age po tsherds excavated in sou thern Br i ta in(see text for discussion).

Conc lu s i on sT h e r e i s a w i d e r a n g e o f p o t e n t i a l a p p l i c a t io n s f o rt h e a n a l y t i c a l S E M i n t h e f i el d o f a r c h a e o l o g i c a lm a t e r i a ls . M a n y o f t h e a p p r o a c h e s a r e d ir e c t l y

d e r i v a ti v e fr o m m i n e r a l o g y a n d p e t r o l o g y a n d t h eq u e s t i o n s a s k e d o f t h e o b j e c ts , f o r e x a m p l e o nd u r a t i o n s a n d t e m p e r a t u r e s o f p r o c es s e s, o n p r o -v e n a n c e , w e a t h e r i n g , e tc . a r e p a r a l l e l t o t h o s e a s k e do f n a t u r a l m a t e r i a l s . T h e t i m e s c al e s o f in t e re s t ,c o u p l e d w i t h t h e t e c h n o l o g i e s u s e d , r e s u l t s i ng e n e r a l ly f i n e - g r a i n e d m a t e r i a l s, s o t h a t t h e S E Mh a s a p a r t i c u l a r l y u s e f u l r o l e t o p la y , e v e n i n t h ei n v e s t i g a t io n o f r e la t i v e ly s i m p l e p r o b l e m s .T h i s t y p e o f s t u d y o f t e n d e p e n d s o n t h e a v a i l -a b i l it y o f su i t a b le g e o l o g i c a l a n d m i n e r a l o g i c a ld a t a a n d r e f e re n c e m a t e r i a l s w h i c h c a n b e u s e d t op l a c e t h e a r c h a e o l o g i c a l m a t e r i a l s i n c o n t e x t . Am a j o r l i m i t a t i o n t o a s tu d y c a n b e t h a t t h e r e q u i r e dd a t a b a s e , w h e t h e r i t i s a p h a s e d i a g r a m , t h er e l a ti o n s h ip b e t w e e n m i n e r a l c o m p o s i t i o n a n d g e o -l o g i c a l s o u r c e , o r th e e l e c t r o n p e t r o g r a p h y o f m a j o r

Reference sBimson, M ., and Freeston e, I . C. (1983) J . Glass Stud. 25 ,55-64.- - - - ( 1 9 8 5 ) In A nn . 9e C onor . A s soc . I n t . pourl ' h i s t o i r e du V er re , Nancy 1983, 209-22.Craddock, P. T. , Freestone, I . C. , Gurjar, L. K., Hegde,K. T. M., and Sonawane, V. H. (1985) M i n e r a l . M a g .48, 45 52.Edward, J . , Fossey, J . M., and Yaffe, L. (1984) J . F i e l dA rchaeo l . 11, 37-46.Freestone, I . C. (1982) A r c h a e o m . 24, 99-116.- - ( 1 9 8 6 ) In E a r l y V i tr e o u s M a t e r i a l s (M. Bimson andI. C. Freestone , eds.) Bri tish M useu m O ccasion al Pape r

No. 56 (in press).- - a n d T i te , M. S . (1986) In C eram i cs and C i v i li sa t ion 3:H i gh - t echno l og y ce ram ics , pa s t , p re sen t and f u t ur e(W. D. Kingery , ed . ) Am erican C eramic Socie ty ,Columbus, Ohio (in press).- - La Niece , S . C . , and Meeks , N. D. (1984) M A S C A J .3, 10-2.- - Cra ddoc k, P. T. , Hegde, K. T. M., Hugh es, M. J . , andPaliwal , H. V. (1985a) In F u r n a c e s a n d S m e l ti n 9 T e c h -n o l o o y i n A n t i q u i t y (P . T . Craddock and M. J . Hughes ,eds.) Bri t ish Muse um O ccasiona l Pape r N o. 48, 229-44.Meeks, N. D., and Middleton, A. P. (1985b)A r c h a e o m . 27, 161-77.- - Cradd ock , P . T . , Gur jar , L . K. , Hegde, K. T . M. , andPaliwal, H. V. (1986) J . A rchaeo l . C hem . (Hyderabad)(in press).Kempe, D. R. C., and Harvey, A. P. , eds. (1983) T h epetro looy o f archaeological ar teJacts . Clarendon Press ,Oxford .Kinge ry, W. D., and Vandiver, P. (1986) In C eram i cs andCivi l i sat ion 2: T e c h n o l o g y a n d S t y l e (W. D. Kingery,ed.) Am. Ceram. Soc. , Columbus, Ohio, 363 74.Krinsley, D . H, Pye, K., and K earsley, A. T. (1983) Geol.M a 9 . 120, 109-14.Letsch, J., and No ll, W. (1983) N e u e s J a h r b . M i n e r a l. A b h .147, 109 46.Maggett i , M., and Galet t i , G. (1981) A r c h a e o m . 23, 199-207.Maniat is , Y., and Tite, M. S. (1981) J . Archaeol . Se i . 8 ,59-76.


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Mineralogical Applications of SEM

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