riederer 2004

Hyperfine Interactions 154: 143–158, 2004.© 2004 Kluwer Academic Publishers. Printed in the Netherlands.

143

Thin Section Microscopy Applied to the Studyof Archaeological Ceramics

J. RIEDERERRathgen-Forschungslabor, Schloß-Str. 1a, D 14059 Berlin, Germany

Abstract. For the characterization of archaeological ceramics, the study of thin sections under thepolarizing microscope is a very efficient analytical technique. There are two properties of ceramicswhich can be analysed by thin sections, namely the mineralogical composition and the fabric. Bothfeatures show a considerable variety which permits a very detailed description of ceramic wares.With respect to the mineralogical composition, there is a wide variety of rock forming minerals,of heavy and ore minerals, fragments of stone, fossils, organic inclusions like straw or pieces ofcharcoal as well as artificial inclusions like slag or crushed pottery, which define with the temperof the pottery in much detail. The fabric also shows considerable differences in grain size, in theamount of temper, in the orientation of grains and other features, which provide further quantitativedata on the properties of archaeological ceramics. From this information, the material can be wellcharacterised, and conclusions as to the region of origin and the potter’s techniques can often bedrawn.

Key words: thin section microscopy, archaeological ceramics.

1. Introduction

The study of thin sections of archaeological ceramics is a traditional, but neverthe-less very efficient approach to describe and to characterize the material propertiesand the technique of manufacture of ancient pottery. Thin sections of ceramic mate-rials provide three types of information, which help one to understand how potterywas made in the past. First, thin sections give us precise and detailed information onthe mineralogical composition of the coarse grained temper, which varies consider-ably from place to place, depending on the regional geology as well as the potters’habits and experience. Second, thin sections permit an accurate determination ofthe percentage of temper in the ceramics, of certain properties of the temper likethe grain size and the grain size distribution, and of the number, size and shape ofpores. These data characterize the potters technique to prepare and shape the clay.Third, thin sections may help us to estimate the baking temperature by observingtransformations of minerals at high temperature, like the transformation of calciteinto calcium silicate and of quartz into cristobalite or the formation of vitreouscompounds.

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2. The microscopical techniques

Thin sections of archaeological ceramics will normally be studied under a polariz-ing transmission light microscope. If a detailed identification of the opaque oreminerals is desired, the study of polished thin sections in reflected light is themethod of choice. Moreover, staining techniques may be used, for instance, todistinguish the different species of feldspars.

For the preparation of thin sections, thin slices of pottery, normally cut per-pendicularly to the surface of the vessel, are polished on one side, glued with thispolished side on a glass slide by means of an adhesive, e.g., a thermosetting epoxyresin, and then ground down to a thickness of about 0.03 mm. At this thicknessthe different minerals show the interference colours which can be used to identifythem (Figure 1) under the polarising microscope.

The microscopic study of thin sections of ceramics is done between two po-larizing foils. The foil below the thin section, the polariser, causes the light wavesto oscillate in only one direction. The foil above the thin section, the analyser, isoriented in a crossed position. In this arrangement, light polarized by the lower foildoes not pass through the upper foil to the observer’s eye.

If we remove the analyser and study the cross section only under polarisedlight, we observe the natural colours of the minerals and a peculiar phenomenon,the pleochroism. This is a special property of many coloured minerals like theamphiboles, the pyroxenes or the iron rich micas to show different orientationsof the crystals under the microscope. If the analyser is not inserted, it is possibleto estimate the refractive index of minerals, which is also a criterium for theiridentification. Further the shape of minerals, the porosity, the presence of opaqueinclusions, the structure of surface layers, the transition of dark, reduced areasinto light, oxidized areas is revealed by this kind of observation. If we insert the

Figure 1. Thin sections under the polarizing microscope (left) and with inserted analyzer (right).

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upper polarizing foil, the analyser, the minerals show characteristic interferencecolours, which are an essential property for their identification. Since the interfer-ence colours depend among other factors on the orientation of the mineral in thethin section, peculiar structures due to different orientation of intergrown grains,like twinning phenomena or an undulatory extinction due to a deformation bystress become visible. The measurement of the direction of the optical axis ofa mineral or the observations of the angle of extinction permit a differentiationbetween closely related minerals, like the members of plagioclase, amphibole orpyroxene family [1–8].

3. The mineralogical composition of ancient ceramics

To describe the mineralogical composition of ancient ceramics, the material of thefine grained clay matrix has to be distinguished from the comparatively coarsegrained temper, which was either added to the clay to improve its properties forshaping and baking or which has already been part of the clay as a residue fromthe original rock from which the clay developed by weathering. Sometimes it isdifficult to distinguish whether coarse grains have been added to the clay intention-ally as a temper, or whether they were already part of the clay which the pottergot from a clay deposit. Usually, all particles that are larger than 0.01 mm andhence still clearly visible under high magnifications in the optical microscope areconsidered as temper, whatever their origin.

The microscopic study of the clay matrix is difficult, since usually the parti-cles are too small for a reliable identification. The scanning electron microscopeprovides better information on the shape of the clay minerals and their alterationduring firing. But even under the limited useful magnification of the optical mi-croscope the properties of the matrix can be characterized, e.g., by distinguishingbetween dense, flaky or granular clay fabrics.

The important information on the mineralogical composition of ancient ce-ramics are obtained by the study of the coarse minerals of the temper. They areextremely manifold both with respect to the materials which occur as a temper andwith respect to the properties of these materials. Five main groups of materials usedas a temper can be distinguished: monomineralogical particles, fragments of stone,artificial components, fossils and plant residues [9].

3.1. THE TEMPER OF ANCIENT CERAMICS

The coarser components of ceramics, either added intentionally by the potter asa temper or already part of the clay used for manufacturing pottery, have grainsizes which are in the same range as the grain sizes of minerals in natural rocks,from which the clays formed by weathering. Among the minerals which are partof the temper of ceramics, quartz predominates, since it is resistant against chemi-cal transformation and mechanical attrition. Second in importance are feldspars, a

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group of minerals with different compositions and varying crystallographic proper-ties depending on their origin, thus providing information on the rocks from whichthey stem. Furthermore, there is a big variety of other silicates, like micas, am-phiboles and pyroxenes, each of which again represents a family of minerals witha large number of members with a considerable diversity of properties. Calcite isquite abundant as a temper in clays which formed by weathering of limestones andmarls as well as in artificially tempered clays in regions where limestones or marbleare available. Other monomineralic compounds may be of local importance, likegraphite, which occurs in certain types of early pottery from Bavaria where thismineral was already mined in prehistoric times. Finally there are a variety of heavyminerals, like garnets, apatite, titanite, or zircon, which may be very characteristicfor their place of origin and hence well suited for determining the provenience ofarchaeological ceramics.

3.1.1. Quartz

Quartz grains provide a large number of features which are useful to characterizea ceramic material (Figure 2). Their shape, their inclusions and their structure arefeatures that deserve attention. The shape varies from exactly globular particlesto angular grains with sharp edges and depends on the origin of the quartz. For

Figure 2. Varieties of quartz inclusions in archaeological pottery (2a quartz with round edges,2b quartz with sharp edges, 2c quartz with undulatory extinction, 2d quartz with large fields, 2e quartzwith small fields, 2f quartz with inclusions of rutile).


instance, quartzes in Egyptian ceramics are well rounded, since they have beentransported by the desert wind [10]. Quartz in ceramics from Austria and southernBavaria have sharp edges since they remained close to the place where their rock oforigin decomposed by weathering [11] and thus were not rounded during transport.Ceramics for which crushed quartz has been used as a temper, like for the stoneware of the Rhine area and the area of Colgone, contain these angular quartz as aconsequence of their production technique [12].

The structure of the quartzes is of particular importance. They may be com-pletely homogeneous, for instance when they were part of volcanic rocks, but theymay also show obvious signs of mechanical stress due to pressure experiencedduring the formation of metamorphic rocks like schists. Quartz may be free ofinclusions, but frequently they contain, again as a consequence of their formation,needles of rutile or flakes of mica.

3.2. FELDSPARS

Feldspars, a group of potassium, calcium and sodium silicates, are quite charac-teristic for their origin (Figure 3). The subgroup of the potassium feldspars, whichcan be distinguished by their crystallographic properties and their morphology,was formed under completely different conditions than the calcium and sodiumfeldspars. The potassium feldspars comprise a variety of species: Microcline andorthoclase occur in magmatic and metamorphic rocks, sanidine is found only involcanic rocks and adular was formed at relatively low temperatures in alpine veins.In the calcium-sodium feldspars, also called plagioclases, the calcium-to-sodiumratio, which can be deduced under the microscope from the optical properties,depends on the type of original rock, mainly its alcalinity and hence its origin.It could be shown that in larger geological units the optical and crystallographicproperties of potassium feldspars change continuously over a certain area, so thatby determining the direction of the optical ax angle or the triclinity, the provenanceof the feldspars within a larger massive of magmatic or metamorphic rocks can bedetermined [13]. The potassium feldspars as well as the plagioclases show a widevariety of optical features, which again can be used to characterize them and insome cases also to find out the rocks of origin. These features are twinning accord-ing to different rules, zoning which is typical for volcanic feldspars, intergrowthslike myrmecitic or rhopalophyric structures, or peculiar stages of transformationin secondary minerals [13]. Properties of this kind are closely related to specialproperties of the original rocks and hence often permit one to get an idea about therocks which included the feldspars before weathering. For the analysis of ceramicsfrom northern Germany, the detailed description of the potassium feldspars, whichwere part of the glacial deposits of magmatic rocks from Scandinavia, was one ofthe most useful criteria for defining local materials [14–16].

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Figure 3. Varieties of feldspars in archaeological pottery (3a microcline with grid of twinning,3b orthoclase with perthitic inclusions, 3c twinning of microcline, 3d quartz-feldspars intergrowth,3e twinning of plagioclase, 3f zoning of plagioclase).

3.3. MICAS

Like the feldspars, the micas form a relatively large group of silicates with relatedproperties, first of all an excellent cleavage along one crystallographic plane. Dueto minute differences in their composition there are micas of different colours. Themost important species are the black biotite, the green chlorite and glauconite andthe colourless muscovite. Their size, inclusions or transformations in weatheringproducts may help to identify the region of origin [17]. The study of muscovitewas of importance in studies of ceramics from the northeastern part of UpperBavaria, i.e., Celtic ceramics from Manching and early medieval ceramics froma cementary at Straubing, where local wares had to be distinguished from importedwares. In both cases, the abundance and the peculiar shape of muscovite, occur-ring in the tertiary sediments of this region, was a convincing argument for theirprovenance [17].

3.4. AMPHIBOLES AND PYROXENES

Two other large groups of silicates are amphiboles and pyroxenes, which bothinclude dozens of defined minerals. They occur in all types of volcanic, intrusiveand metamorphic rocks, but always as well defined species. If they can be found


in thin sections of ancient ceramics, quite precise hints on the origin of the clay ortemper are possible.

For their precise identification a key system has been developed. The systemclassifies related minerals first according to their colour or their pleochroism, whichusually covers a very broad range between colourless to dark green, dark brown orbluish hues even in mineralogically very similar species. Minerals with similarcolours are subdivided by optical properties, like the extinction angle or the opticalaxe angle. This key for the precise identification of defined minerals within a groupof related minerals has been published by several authors like Tröger [18], whodistinguished species of amphiboles or species of pyroxenes, which all are commonrock forming minerals and can therefore be expected as a component of weatheredrocks in ceramics.

3.5. CALCITE

Calcite appears much more frequently in thin sections of ancient ceramics thanexpected, since it should transform into caustic lime at temperatures around 700◦C.In spite of this, calcite is found in a considerable variety. There are isolated particlesof calcite, rhomboedral fragments of large crystals of marble, pieces of limestone,fragments of shells and a large variety of microfossils. For this reason, each occur-rence of calcite in thin sections provides important information on the provenienceof clay and temper.

3.6. THE HIGH TEMPERATURE PHASES DIOPSIDE, CRISTOBALITE AND

MULLITE

With increasing firing temperatures, some constituents of ceramics become unsta-ble and transform into high temperature phases. At 850◦C, for instance, calcitereacts with silica to form the calcium silicate diopside. This transformation fromcalcite to diopside is clearly visible under the microscope. Mullite, an aluminiumsilicate, is formed at 950◦C and quartz changes to cristobalite at 1050◦C. Theformation of cristobalite and mullite, which are rare in archaeological ceramics,is difficult to observe under the microscope, but quite obvious in X-ray diffractiondiagrams. The observation of these transformations provide reliable evidence onfiring temperatures.

3.7. HEAVY MINERALS

Natural rocks contain a large number of accessory heavy minerals as minute inclu-sions, notably zircon, apatite, garnet, tourmaline, titanite, monazite, and xenotime,which have been extensively studied in petrography, since they provide essentialinformation on the formation of rocks and are quite characteristic for a certain typeof a magmatic or metamorphic rock. Though they are not rare in clays used for the

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manufacture of pottery since they resist weathering, they are often neglected in thestudy of archaeological pottery, be it by thin sections as by other techniques, likethe separation of heavy minerals.

3.8. ORE MINERALS

In ancient ceramics ore minerals, like hematite, magnetite, or ilmenite are abun-dant, but difficult to identify in thin sections, since they are opaque and theiroptical properties cannot be detected under the transmission microscope. Until nowthere are hardly any approaches to characterize them under reflected light by thetechniques of ore microscopy.

3.9. EXCEPTIONAL COMPONENTS

In some regions locally occurring minerals are added to improve the properties ofpottery. One example is the use of graphite to produce a black pottery in southernGermany, where this mineral occurs in large deposits in metamorphic rocks [19].Prehistory graphite was used locally, but also traded over considerable distancesfor use in pottery making.

3.10. VOLCANIC GLASSES

In some groups of Precolumbian Peruvian and Bolivian pottery tiny flakes of glasscould be detected under the microscope together with other minerals from volcanicrocks [20]. These flakes are therefore thought to be a weathering product of vol-canic ashes or tuffs which contained them as inclusions, rather than stemming fromthe remains of the working of obsidian by man.

3.11. FOSSILS

As already mentioned, fossils are quite abundant in archaeological ceramics, sincemarls have been frequently used as a raw material for pottery. All the light potteryof Egypt is made from marl, which is rich in microfossils (Figure 4). They havebeen studied in detail to find out the quarries where marl was mined in antiq-uity [21, 22]. But not only microfossils occur in ancient pottery. One also findsfragments of shells, snails or spikes of sea urchins [11].

Clays in southern Germany, which were deposited in a very cold climate atthe end of the diluvial period, contain high amounts of diatomaceous earth. Sincepottery made of this clay in prehistory was traded over long distances, the identifi-cation of the diatom microfossils is an excellent tool to establish their provenience.

Roman amphorae found in southern Germany were found to contain micro-fossils from Eocene limestones occurring in southern Italy. This shows that theamphorae were not locally made, but brought from Italy to southern Germany [23].


Figure 4. Fossils and inclusions of plants in archaeological ceramics (4a–4d inclusions of fossils,4e–4f inclusions of carbonized plants).

The study of microfossils also turned out to be of particular importance in studiesof sherds of amphorae from the coast of Sicily. There, in the shallow sea closeto Ognina, ships sank and lost their cargo of amphorae during centuries in antiq-uity. The provenance of these amphorae could be accurately established by theidentification of microfossils [24].

3.12. FRAGMENTS OF STONE

The temper in pottery does not consist exclusively of isolated minerals but alsoof fragments of stone which either escaped conversion into clay minerals duringweathering, or were added by the potters (Figure 5). Depending on the region oforigin, practically all known types of rock can be detected under the microscope asa temper of pottery clay and this again provides an excellent possibility to localisethe deposits, either of the clay or of the intentionally added temper.

3.13. ARTIFICIAL TEMPER

There are a few groups of ancient ceramics which contain higher amounts ofcrushed pottery or slags. This kind of temper is not very common, since it doesnot have better properties and since common sand and natural temper is usuallyavailable everywhere.

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Figure 5. Fragments of stone and artificial temper in archaeological ceramics (5a granite, 5b basalte,5c vitreous volcanic tuff, 5d sandstone, 5e fragment of pottery, 5f slag inclusion).

Since the clay for ceramic products usuall is prepared and shaped close to theareas where it is baked, the ground might be covered with remains of the burntwood. This can explain the occasional presence of charcoal particles in pottery.

3.14. ORGANIC MATERIALS

In the manufacture of archaeological pottery it was quite common that straw orfragments of plants were added to the clay in order to increase the porosity of thesherd owing to the voids left behind after the combustion of the organic matterduring firing. Since these fragments are enclosed by clay, they are often carbonisedrather than oxidised completely, particularly when the firing was performed underreducing conditions. In any case, the remaining voids retain the shape of the orig-inal plant material. Hence studies of the void structure permits an identification ofthe parts of a plant used, like leaves, stalks, seeds or roots. In some cases also thetype of plant can also be identified, like the grass from a dry region or the leavesfrom trees in a tropical region.

4. The fabric of pottery

The characterization of the fabric of archaeological ceramics is as important as thedescription of the mineralogical composition. The fabric first of all comprises the


properties of the grains of temper and the description of the pore structure. Themain properties of the mineral inclusions, which largely determine the fabric of apiece of pottery are the grain size, the number of grains of a certain type, the grainsize distribution, the percentage of temper and the orientation of the grains [9].

4.1. THE GRAIN SIZE

It is obvious that the grain size is a very characteristic property of a ceramicobject. In archaeological pottery the grain size may vary between grains below0.02 mm, which are no longer clearly visible in the optical microscope and whichoccur mostly in dense potteries like terra sigillata (Samian ware) to grains withsizes up to 5 or even 10 mm. The latter are, for instance, not uncommon in ce-ramics from northern Germany, which often contain large inclusions of feldsparsfrom weathered Scandinavian granites. For the description of the grain sizes thepetrographic nomenclature may be used, although it does not subdivide the finergrain sizes, which form an important part of the temper of pottery. According tothe petrological nomenclature grain sizes below 0.1 mm are called dense, thosebetween 0.1 and 0.33 mm are called fine grained, those between 0.33 and 1 mmsmall grained, those between 1 and 3.3 mm medium grained, and those between3.3 and 10 mm coarse grained.

4.2. THE GRAIN NUMBER

The grain number is a valueable parameter, which is as useful for the characteri-zation of pottery as the grain size. It describes the number of grains on a certainarea, usually one mm2. The grain number for pottery commonly varies between 1 to2000 mm−2. This considerable variability is a very descriptive figure to characterisethe properties of ceramics.

4.3. THE GRAIN-SIZE DISTRIBUTION

Normally the temper does not consist of grains of just one size. Rather, grains ofall possible sizes, between 0.01 and 1 mm may be detected in an average archae-ological pottery under the optical microscope. For an accurate description, it istherefore necessary to determine the percentage of grains in different size fractions,for instance in the size ranges 0.01–0.05, 0.05–0.1, 0.1–0.5, 0.5–1, and >1 mm.The size ranges have to be adjusted to the properties of the ceramic material; a finegrained sherd, for instance, needs a more detailed subdivision in the fine range thana coarse grained ware.

4.4. THE PERCENTAGE OF TEMPER

The quantity of coarser grains in the clay is quite an important criterion for thetechnical properties of the ceramic material. As already mentioned, it is almost

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impossible to distinguish between an artificial temper added by the potter or nat-ural coarser grained constituents of a clay. But often the amount of temper is ofmore interest than the question whether it was intentionally added or not. Usually,the percentage of all grains larger than 0.02 mm, the size still clearly visible un-der the optical microscope at usual magnifications, is considered as temper. Thequantity of temper varies between almost none and amounts up to 75%. Since thetemper largely consists of transparent minerals and hence appears light under thepolarising microscope and contrasts well with the darker clay matrix, an automaticmeasurement is quite reliable and may be more convenient than manual countingtechniques with a point counter or similar devices.

5. The types of pottery fabrics

Since the fabric of pottery is determined by three parameters, namely the grain size,the grain size distribution, and the amount of temper, the structural properties ofceramics cannot be depicted in a two dimensional diagram. However, a diagram ofonly the grain number versus the amount of temper already describes the fabricof ceramics quite well. For the discussion of the results of the thin section analy-sis of ceramics, an array like that shown in Figure 6 may be useful. This arrayshows the most common fabrics arranged in columns representing increasing grainnumbers from left to right, while the grain size decreases from top to bottom. Thecolumns are designated 1–4, the rows A–F. One can use this array to characterizeceramics by types describing the fabric: Type A1 in the upper left corner has fewand big grains, while at the opposite corner, F4 represents fabrics with a highamount of small grains.

5.1. THE ORIENTATION OF GRAINS

Clays usually contain sheet minerals like micas, which often show a more or lessdistinct orientation parallel to the surface of a piece of pottery, depending from thetechnique of manufacture. This orientation can be measured or just be describedqualitatively as one of the characteristic features of pottery.

Since the orientation of minerals in rocks is of a primary importance for theirclassification, techniques have been developed to record this property by means ofa turntable, mounted on the microscope. With this device, which permits to turna thin section in all directions, the optical axis of the mineral can be brought intoa vertical position. By that the orientation of a large number of grains in one thinsection can be plotted and treated with statistical methods, to find out for instance,which percentage of micas is oriented parallel to the wall of a ceramic vessel,which reveals the technique of shaping a pot or the intensity of shaping the pot onthe potters’ wheel.


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5.2. THE PORE STRUCTURE

Like the grain structure, the pore structure also provides information on the man-ufacture of ceramic objects [25]. The number of pores, their size, their shape andtheir orientation again are properties which vary considerably and thus are fea-tures which should be mentioned when archaeological objects are studied. Theproperties related to pores can be treated quantitatively in a similar manner as theproperties of grains.

6. Conclusions

This short survey over the coarser materials that can be detected by means ofthe polarizing optical microscope in archaeological ceramics shows that a widevariety of natural and artificial, inorganic and organic, materials occur as temper.These materials first of all reflect the properties of the local geology of the place ofmanufacture of the pottery, but also the potters’ habits, which largely determinedthe use of particular kinds of temper and the addition of artificial components, inan effort to improve the properties of the clay and to impart particular properties tothe pottery. Thus the precise description of the coarse grained inclusions of potterycontributes to an accurate and detailed characterization of the ceramic material.The purpose of the study of ceramics by thin sections is the precise identificationof the components of the temper, the determination of quantitative properties, likegrain size or amount of temper, to define the fabric of the ceramic, to study thetechnique of preparing peculiar surfaces, for instance by applying paints or glazes.A special advantage of microscopic techniques is the precise identification of thenature of mineral compounds, even in a single grain. The identification of the kindof temper provides information on the place of manufacture. The precise definitionof existing minerals is of importance for the interpretation of Mössbauer spectra,since, apart from the amount of iron in a mineral, also its valence state and theposition of iron ions in the crystal lattice is known. Areas in a sherd where differentatmospheres developed during the baking process, and the relation between thesezones can clearly be distinguished. Further, under the microscope transformationsof minerals by temperature in the range of the baking temperature of pottery canbe observed, contribution by that to the technique of manufacture of a certain typeof pottery.

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