Energence of Ceramic Technology 1

Chapter 2

The Emergence of Ceramic Technologyand its Evolution as Revealed with the use

of Scientific Techniques

Y. Maniatis1

Introduction

From the earliest stages of his appearance on theearth, a couple of million years ago, man began hiseverlasting effort to comprehend and exploit theenvironment. His aim was initially to secure hissurvival and perpetuation and later the progressiveimprovement of his biotic level for a more com-fortable personal life, and a more mature social life.From his first steps and for hundreds of thousands ofyears man shapes and uses the natural materials hefinds in his surroundings, such as, stone, timber,plants, bones, etc., for making tools, utensils, arms,clothes and lodgings to obtain and prepare his foodbetter and to encounter more comfortably theenvironmental conditions. The release of his handswith the development of the erect position helpedhim in such constructional attempts and at the sametime contributed to the growth of intellectual abilitythat lead him progressively to the manufacture ofmore and more complicated artefacts. In this long

Abstract

In the long human evolutionary course the “technology” is initially limited only in the formingand shaping, of existing natural materials. The production of new materials like the ceramicspresents the first technological revolution in human history which occurred only in the last9000 years. It was most probably the result of the combination of two long-existing independentexperiences; the pyrotehnology for burning limestone, and the molding of raw clay. Thescientific techniques developed and applied in the last 30 years, have lead to remarkablediscoveries about the knowledge of ancient potters. We now know how the pottery technologydeveloped, how the raw materials were selected and treated, how the kiln atmosphere andtemperature was controlled and how the different decoration colours and contrasts wereobtained. This paper is an account of how our understanding about the ancient ceramictechnology developed with the progressive use of scientific techniques and methodologies. Itbegins with the very early attempts of potters to produced desired colours for the body anddecoration and ends with the ingenious manipulation of materials, kiln atmospheres andtemperatures to produce the high technology black and red glosses in the Classical period.

human evolutionary course the “technology” isinitially limited only in the shaping, however elabor-ate, of existing natural materials. The production ofnew materials is a relatively recent development andbegan at about 11,000 years before present with thediscovery of pyrotechnology, which was initiallyapplied, on the manufacture of lime and gypsummortars (Kingery et al 1988).

However, the grand moment in the technologicaldevelopment is the manufacture of the first ceramicsome 9,000 years before present in the Near East anda bit earlier in the Far East (Rice 1987). I believehumans achieved that by combining two long-standing and up to that moment independentexperiences; a) their experience in pyrotehnology forburning limestone at temperatures above 800 oC andb) their experience in molding raw clay. Theymanaged in this way to produce for the first time onearth a new material, the ceramic. The manufactureof this new material constitutes undoubtedly the first

Y. Maniatis2

technological revolution in the human history. Thisis because; man with the deliberate use of hightemperature and long time heating, managed to alterthe physicochemical properties of raw clay and thusto produce a new hard and durable material, theceramic. This was an unprecedented experience anda glorious moment in the course of human evolution.This first technological revolution was followed by acontinuous development during which man manu-factured progressively a lot of new materials. In themillennia that followed the ceramic revolution wewitness the appearance of the vitreous materials(glazed stones, faience) technology, the appearanceof metallurgy, a bit later the glass and so on. Fromthen onwards the technological advancementbecomes progressively faster reaching an extremelyhigh rate of evolution within the last 100 years when

a large range of new materials is manufacturedamongst which, are the polymers and the pure siliconon which the modern electronic industry is based.

Despite the large variety of new materials that arecontinuously produced until the present day, theceramics never stopped to be manufactured andimproved, being strongly associated with the courseof social and technological development until today.

The ceramic technology characterises and reflectsimportant parameters of an ancient cultural society,as: 1) the organisation of the society including thefood preparation and storage, 2) the economy, 3) thetrade and commerce and 4) the connections andcompetitions with other societies. For this reason theinvestigation of the ancient ceramic technology isvery important and its full understanding presents achallenge for Archaeometry

Figure 1: Stages and degrees of human intervention in the production of a ceramic vessel

Energence of Ceramic Technology 3

It is worth refreshing our minds how a ceramic ismanufactured and the degree of human interventionin each stage. The above diagram (Fig. 1) shows thesuccessive stages in the manufacture of a ceramic.The starting material is the clay, a natural sedimentarymaterial that existed long before the appearance ofman on the earth.

The selection of a suitable NATURAL CLAY thatcontains a satisfactory percentage of argillaceousminerals, is relatively fine and has a good degree ofplasticity constitutes the first step in the manufactureof ceramics. The next step involves the removal fromthe clay of the various accessory inactive and coarse-grained stone fragments. This is done by levigation,sieving and suspension in water. This process resultsto an IMPROVED RAW MATERIAL, very rich inargillaceous minerals and quite plastic. The refractoryproperties of this improved raw material need to beadjusted according to the type of ceramic productthat is going to be manufactured. This can be done byadding, if necessary, calculated quantities of aplasticinclusions like quartz sand or crashed pebbles,feldspars, limestone, shells, or vegetal inclusions. Aspecific quantity of water is then added and the mixedmaterial is manipulated either by wedging, kneadingwith hands or foot treading until it is fullyhomogenised and the air pockets removed. Thus thefinal clay composite material ready for making pots isready. We can call this final regulated clay FABRICCLAY (Pilos). The next step involves the molding andshaping of pilos to form the VESSEL. In the earlierperiods, the vessel was constructed by shaping a claylump with the hands or building it with the coiltechnique and later with the help of a wheel. Thevessel could then be painted or decorated. The stageof forming and decoration of a vessel is very important,as in the form and decoration all parameters thatdetermine an ancient society are reflected. Suchparameters are: 1) the dietary habits, the needs forstorage and commerce (cultivation, accumulation ofgoods, exchanges, economy), 2) the needs for socialand artistic practices, 3) the needs of religious warshipand 4) effects that emanate from possible social classdifferentiations (variable accumulation of wealth,specialisation of potters, access to raw materials, etc).The shaping and decoration of the vessel is followedby the firing stage. The firing causes permanentphysicochemical changes to the natural clay material.Initially and at low temperatures (100–200°C) onlythe absorbed water is evolved. As the temperatureincreases (400–800°C) the chemically bound hydroxyl(OH) water is removed; a process associated with thedisorganisation of the clay minerals. With further riseof temperature (800–1000°C) solid-state reactions

begin to take place during which new minerals mayappear with the simultaneous formation of anamorphous phase (vitrification), which consolidatesand cements the particles together. This whole processconverts permanently the natural clay to a CERAMIC.

The firing of ceramics requires specific skills forreaching and maintaining firing temperatures in therange 800–1000°C, know-how for building kilns,control of air and it generally reflects thetechnological level of a society at a specific periodand place. It is therefore obvious that the investiga-tion of the ancient ceramic technology with modernscientific methods provides invaluable informationon all the aspects of technological development butalso on aspects that are related with the economic,social, and religious life of man. Methodologies forthe study of ancient ceramic technology began to bedeveloped in the ‘60s, based on Powder X-rayDiffraction (Perinet 1960), Thermal Expansion (Coleand Crook 1962; Tite 1969), optical microscopy(Cowgill and Hutchinson 1969). In the next decade(‘70s), new methods are added, based on MössbauerSpectroscopy (Bouchez et al. 1974; Janot and Delcroix1974), Ceramic Hardness (Fabre and Perinet 1973),Differential Thermal Analysis (Kingery 1974; Slageret al. 1978), Porosity (Morariu et al. 1977), andScanning Electron Microscopy (Tite and Maniatis1975; Maniatis 1976; Maniatis and Tite 1978/9). Thenext decade (‘80s) witnessed the further developmentof the methodology based on the Analytical ScanningElectron Microscopy that was destined to dominatethe ancient ceramic technology studies. With thismethod, a deeper understanding of the effects offiring on the different types of clay used for makingancient pottery (Maniatis and Tite 1981) and informa-tion contributing to the qualitative and economicelements of a society is extracted (Maniatis et al. 1988).Today we understand most of the parameters relatedto ancient ceramic technology and the techniques ofceramic decoration and we are in a position toappreciate the degree of difficulty, the specialisationand the know-how that existed in the manufacture ofmany types of ceramics in various periods andcultures. This is the result of the continuous develop-ment and systematic application of Materials Sciencemethodologies and the progressively accumulatedexperience by dedicated archaeological scientists(Archaeometrists). The investigation of the thermalbehaviour of raw materials in combination with thecultural context and the absolute chronologies of thefindings has provided information not only on theceramic technology itself but also on the conditionsthat preceded this technological revolution.

The opinion I formulated earlier, that man achieved

Y. Maniatis4

the production of the first ceramic when he combined,at a certain moment in time, two of his long-standingprevious experiences; his art in shaping the raw clayand his knowledge in pyrotehnology, is based onresults produced by the application of scientifictechniques. For example, it has been proved that manhad acquired the knowledge to process and moldclay long before the firing of ceramics. In particular,objects of refined, worked and shaped but unfiredclay, identified using X-Ray Diffraction and AnalyticalTechniques, were found in various parts of the world.Some examples are the famous clay figurines foundunfired and subsequently fired (most probablyaccidentally in a destruction fire) from Dolní Vestonicein Czechoslovakia, dating to 30,000 BC (Zimmermanand Huxtable 1971). Also the cylindrical clay rods,dating to 12,000 BC, from the cave Theopetra inThessaly, Greece (Facorellis et al. 2001) which areconsidered the oldest clay objects in the Greek region.It is certain that the evidence would have been muchricher but the clay objects being unfired cannotsurvive to our days. At the same time, it has beenproven that humans, long before the manufacture ofthe first ceramic, possessed the knowledge to obtainand maintain temperatures above 800oC in largevolumes. Indeed, examination of different mortarsusing electron microscopy, from settlements inMesopotamia, dating 10,000–9,000 BC, showed thatthey consisted either of calcium carbonate (CaCO3)or gypsum (CaSO4.x H

2O) particles. The small particle

sizes (a few micrometers) and their characteristicmicrocrystalline forms indicated secondary crystal-lization in-situ on the wall after the application of themortar (Kingery et al. 1988). This obviously impliedcombustion of natural limestone and gypsum rocksat temperatures above 800 oC in order to evoke thedissociation of the natural rocks and the productionof lime (Ca(OH)

2) and anhydrous gypsum (CaSO

4)

from which the corresponding mortars are manu-factured. The firing of limestone and plaster requiressimilar skills and logic with the firing of the ceramics;i.e. the firing temperatures are more or less the same,they should be uniform in large volumes (in speciallybuilt bonfires or kilns) and they should be maintainedconstant for a long time interval (a few hours).

Thus the conquest of the ceramic technology, cameas a reasonable consequence of a clever combinationof the above two long-standing and independentexperiences. For this reason the first ceramics whenthey appear in Mesopotamia, are well made and firedat suitable temperatures (850–950 oC) to produceenough sintering and vitrification for durableceramics. This has been shown by the solid-statereactions that have occurred between the clay

minerals in their body and the development ofamorphous phase, observed by scanning electronmicroscopy and X-ray diffraction (Tite and Maniatis1975; Maniatis 1976).

Firing temperature, microstructure andmechanical properties

There are various scientific methods that can be usedto get an estimate of the temperature at which aceramic has been fired (Heimann and Franklin 1979;Tite 1995). These are based on: 1) mineralogicalchanges occurring in the clay body during firing,monitored with powder X-ray diffraction (XRD)(Maggetti 1982), thermal expansion (Tite 1969),differential thermal analysis (DTA), thermogravi-metric analysis (TGA) (Kingery 1974), Mössbauerspectroscopy (Maniatis et al. 1982; Wagner et al. 1986),infra red spectroscopy (FTIR) (Maniatis et al. 2002),etc, 2) Colour changes (Matson 1971) and 3) sinteringand vitrification, monitored with thin-section opticalmicroscopy, hardness, porosity changes and scanningelectron microscopy, or the combination of the abovemethods. The firing temperature is not a strictlydefined term because the firing rate and soaking timeaffects the mineralogical changes and the degree ofsintering and vitrification. It has been estimated thatfiring at 960 oC with a fast heating rate of about 800 oC/hr followed by rapid cooling, conditions similar tothat obtained in a bonfire (Shepard 1956), wouldcreate the same effect on vitrification as that obtainedwith firing at 900 oC with a slow heating rate of 200 oC/hr and 1 hour soaking time (conditions comparableto firing in a kiln). Thus shortening the total time offiring from a day that is the usual situation with kilnfirings, to just 2 hours increases the effectivetemperature by 60 oC (Maniatis 1976). Similar resultsare obtained when the soaking time at the toptemperature decreases by 5–fold (e.g., from 300 minto 5 min) (Norton and Hodgdon 1931; Maniatis 1976)and when the atmosphere changes from reducing tooxidising (Maniatis and Tite 1981). Comparableresults are also obtained for the mineralogical changesoccurring during firing at lower temperaturesmonitored with FTIR, the equivalent temperaturebeing higher by 50–60 oC when the soaking timedecreases by 5–fold (Maniatis et al. 2002). For thisreason it is better when one refers to firingtemperatures usually refers to the “equivalent firingtemperature”, i.e. equivalent to the heating rates andsoaking times obtained in a kiln (Tite 1999).

There are many applications on a large number ofancient ceramic groups of different periods andlocations, through which estimates, as good as

Energence of Ceramic Technology 5

possible, of the original firing temperatures employedhave been obtained. However, it has been argued(Gosselain 1992) that the firing temperature by itselfdoes not mean very much neither for assessing theancient ceramic technology nor for extracting culturaland behavioural information concerning the pro-duction and use of ceramics. Indeed, the temperatureestimation should be combined with determinationsof the chemical composition, the refractory potteriesand the tempering of the clay used. Only in this waythe level of understanding of the raw materials andtheir behaviour on heating by the ancient potters canbe assessed. A method that was developed since themid seventies and found very interesting applicationsin the study of ancient ceramic technology is theanalytical scanning electron microscope (SEM-EDXA). This method, is based on estimating thedegree of sintering and vitrification (glassy phase)that is observed in the microstructure of a ceramic(Maniatis and Tite 1975; Tite and Maniatis 1975; Titeand Maniatis 1975; Maniatis 1976; Maniatis and Tite1981). For example, Figure 2 shows the micro-morphology of the body of a clay vessel that has notbeen fired, as seen under the SEM and at a magnifica-tion of 2000x. The characteristic flakes of the raw claycan be clearly seen. Figure 3 shows the microstructureof the same clay vessel now fired at 930°C underoxidizing conditions (all vents of kiln open). Theamorphous phase takes the form of wavy strips ofglass developing as a result of sintering and meltingof the edges of the parallel-orientated clay flakes. Theprogressive sintering and vitrification helps theadhesion and cementing of the particles together, aprocess, which converts the natural clay to a ceramic.

The development of scanning electron microscopyin combination with microprobe X-ray analysiscontributed a lot to the understanding of the effect of

firing on natural clays and the factors that influencethe progressive changes in their microstructure. It isnow known that the sintering of clays during firingoccurs with the transfer of material to the contactsurface between the particles (Fig. 4a) through aprocess of “plastic flow” (mobilization of moleculeswithout full melting) (Kingery et al. 1976). Thedevelopment of a glassy phase (vitrification) isindependent of the sintering. However, the appear-ance of the vitreous “liquid” phase increases thesurface tension between the clay particles and thiscreates a lower pressure in the contact surface andhence the appearance of attractive forces that drawthe particles together (Fig. 4b) leading to the familiarcontraction of the ceramics during firing. Vitrificationappears as continuous glass filaments at first, joiningthe edges of the parallel aligned clay particles andlater as wavy glassy strips (Fig. 4b) when the filamentsfrom several clay layers fuse together. This process

Figure 2: SEM backscattered electron image of an unfiredclay vessel (fractured surface). The clay flakes can be clearlyseen.

Figure 3: SEM backscattered electron image of clay vesselfired at 930 oC (fractured surface). The vitrification in theform of smooth glass wavy strips is evident

Figure 4: Layers of clay flakes aligned parallel as in a clayvessel, a) during heating material flows to the contact edgesof the particles, b) melting occurs at the edges creatingwavy strips of glass. The particles are drawn togetherresulting in sintering and contraction.

Y. Maniatis6

can be monitored very precisely with the SEM as canbe seen in Figure 3. The temperatures at which theabove changes occur, the amount of glassy phasedeveloped, the pattern the vitrification exhibits inthe clay matrix and the degree of contraction, dependon the chemical and mineralogical composition, andthe particle sizes. It also depends on the amount,type and size of accessory minerals and aplasticinclusions. Figures 5a and 5b, show the dramaticdifferences, revealed with the SEM, in themicrostructure between a ceramic made of a low incalcium (CaO = 0–2 %) clay and a ceramic made of aclay high in calcium (CaO = 15%), both fired at thesame temperature (1000 oC). The calcareous ceramics,containing CaO > 6% in a fine calcium carbonateform, exhibit a characteristic cellular structure with ahigh porosity (Tite and Maniatis 1975) and in thesame time the vitrification is more restricted andcontrolled up to 1150°C. This characteristic micro-structure remains constant for 200°C (850–1050°C)and above that there is a progressive increase ofvitrification, the glass phase becoming grainy andhighly viscous (Tite and Maniatis 1975; Maniatis1976). Contrary to that, the non-calcareous clays,containing CaO < 6%, produce a much more vitrifiedceramic body with a high density and impermeableto fluids. Furthermore, due to the extensive and rapidvitrification the non-calcareous ceramics collapse attemperatures approaching 1100°C (Maniatis 1976).The calcareous ceramics have a greater resistance tothermal and mechanical shocks due to their highlyporous microstructure (Kingery et al. 1976), as theenergy is absorbed by the voids, but they have lowerresistance to loading and compression, as they areless rigid.

Hence, the examination of the ceramic micro-structure under the analytical SEM provides informa-tion on the degree of vitrification that leads to theestimate of the firing temperature but in the sametime on the chemistry and type of clay used and itsrefractory properties (Maniatis and Tite 1981).Important information is thus extracted on the levelof apprehension by the ancient potters of theproperties of different clays. This in combination withthe raw material availability in a certain region andits use for specific types of ceramic ware leads to adeeper understanding of the level of ancient ceramictechnology and the social and economic implicationsrelated with it (Maniatis et al. 1988; Tite 1999).

The mechanical and thermal properties ofceramics can be modified strongly by introducingaplastic inclusions whose concentration and sizeaffects strongly these properties. Such inclusions maybe fragments of quartz, feldspars, limestone, seashellsetc. Their role is quite important in preventingextensive cracking by the stresses developing duringthe drying shrinkage, especially in thick walledvessels like pithoi, but also increase the toughness(prevent breakage by cracking) of the ceramic duringloading. Figure 6a and 6b show in diagrammatic formhow the cracks developing initially in the vicinity ofan inclusion (Fig. 6a) are widened during drying (Fig.6b). The energy of the widened cracks is absorbed inthe void of the inclusion, which in this way preventsthe cracks extended from one surface of the vessel tothe other. In the same way a crack propagating in aceramic, containing quartz inclusions, by bending isabsorbed and its propagation stopped by theinclusions (Fig. 7). The role of inclusions in themechanical and thermal properties of ceramics has

Figure 5 (a): SEM backscattered electron image of a non-calcareous ceramic fired at 1000 oC (fractured surface) – Totallyvitrified matrix. (b): SEM backscattered electron image of a calcareous ceramic fired at 1000 oC (fractured surface) – Anopen cellular porous matrix

Energence of Ceramic Technology 7

been the subject of extensive research in recent years(Kilikoglou et al. 1998; Vekinis and Kilikoglou 1998;Tite et al. 2001).

Colours of body and paint in oxdisingconditions

The systematic applications of microanalysis,mineralogical studies and Mössbauer spectroscopyin combination with magnetic measurements con-tributed greatly to the better understanding of theparameters controlling the colour differencesbetween the different ceramics and the utilization ofthese properties by the ancient potters to produce

various aesthetic and functional results. It is nowknown that the colour exhibited by a ceramic is aresult of the chemical composition of the clay and thefiring conditions (temperature and atmosphere). Theiron oxides play a very important role in the colourof the fired ceramic and influence also the colour ofthe natural raw clay. Initially in the raw clay, iron is inthe form of iron hydroxides, such as, FeO(OH), thatare orange or brown in colour and fine iron trioxide(Fe

2O

3), which is red/brown in colour. Some Fe is also

bound in the form of ions Fe3+ or Fe2+ in the structureof the clay minerals that are colourless. The com-bination of the different forms and quantities of ironexisting in a clay together with the quantity of organicmaterial contained gives a variety of colours to theraw clays which range from grey, beige, brown,orange, or red.

When a clay is fired the colours change dependingon the temperature and atmosphere, but the presenceof fine calcium carbonate in the raw clay plays again,as in the development of micromorphology, a veryessential role to the final colour. In non-calcareousclays fired at oxidising atmosphere (all vents of kilnopen) there is a progressive crystallisation of Fe inthe form of alpha-Fe

2O

3 (hematite), which is red in

colour. These oxides grow in size and quantity as thefiring temperature increases above 700°C at theexpense of the Fe-hydroxides and the Fe-ions in theclay mineral lattice, which begin to disorganise anddissociate above that temperature liberating Fe ions(Maniatis et al. 1981; Maniatis et al. 1982; Maniatis etal. 1984). As a result, the non-calcareous clays fired atoxidising atmospheres exhibit red colours, whichbecome more intense as the firing temperatureincreases. Contrary to that, the reactions occurringduring firing in the calcareous clays (fine CaO > 6%)are quite different. The CaO that appears from thedissociation of calcium carbonate above about 750–800 oC, reacts strongly with the iron oxides and breaksthem down. This leads to the decrease in the size andamount of Fe-oxide particles and hence to thebleaching of the red colour to pink, cream or evenwhitish as the temperature increases above 850°C andaccording to the original amount of calcium carbon-ate in the clay. The Fe which is liberated from thedissociation of iron oxides participates in the crystal-lisation of new calcium aluminosilicate minerals(Maniatis et al. 1981) which stabilise the micro-structure of the calcareous clays for 200°C (850–1050°C). These new minerals are colourless. Excep-tions to this general behaviour do occur. For examplethere are calcareous clays that they are red from thebeginning and remain red after firing, because theoriginal amount and particle size of the iron oxides

Figure 6: Crack propagation, a) cracks propagate to theinclusion, b) cracks become wider during drying but theirenergy is absorbed around the void of the inclusion.

Figure 7: Crack created and propogating (from left to rightunder mechanical bending in a ceramic containing quartzinclusions.

Y. Maniatis8

present is so large that the CaO is not enough to reactto a considerable degree as to bleach the colour(Maniatis et al. 1981), equally there are some non-calcareous clays that fire to whitish colours becausethe initial amount of iron oxides they contain is verysmall. The latter are usually the high refractorykaolinitic clays (Maniatis and Tite 1978/9).

These colour differences between the various clayswere cleverly utilised by the ancient potters from theNeolithic times. They used these different clayproperties in order to produce decorative colourcontrasts with a single firing. An example is shownin Figure 8 where a beautiful red coloured decorationhas been applied on a buff coloured body. This resultwas achieved by using a highly calcareous clay forthe body and a non-calcareous fine clay for thedecoration paint and fired in an oxidising atmosphereat a temperature of 900°C. Judging from the finalfired colours and based on recent experience accu-mulated on various raw clays one can say with a fairdegree of certainty that the initial raw colours for thetwo clays used for this vase in Figure 8 must havebeen; light grey for the body and orange or red forthe paint. The manufacturing of such a vase requireda selection and refinement of raw materials, as wellas uniform oxidising firing, facts that are associatedwith production of high quality pottery. Pottery ofthe same kind, with red “flame-like” decoration onbuff body, was found in the Middle Neolithic periodat Sesklo, Thessaly among other monochrome vases.The scientific investigation using analytical SEM,showed that this pottery was indeed of high qualityrequiring the use of a special calcareous clay that wasnot available in the immediate vicinity of the site anda specially treated non-calcareous clay for the red

decoration. Furthermore, this pottery was fired atsuch conditions as to take full advantage of thespecific clay properties. The same was not true forthe rest of the pottery found at the site or in a nearbysite (Sesklo B) that was dominated by the mono-chrome lower quality pottery (Maniatis et al. 1988).Therefore, this high quality pottery required skilfuland specialized potters most perhaps not producingtheir own food, probably implying a central economicsystem that distributed the wealth (Kotsakis 1983).

Colours in reducing conditions

Firing pottery in reducing conditions (lack of oxygenand presence of reducing gasses, such as CO insmaller or larger amounts) produces different results.The Fe-hydroxides and Fe-oxides existing in the rawclay dissociate during firing under reducing con-ditions above about 700°C and by liberating oxygenthey are progressively converted to magnetite (Fe

3O

4)

or wustite (FeO) or even rarely to metallic irondepending on the intensity of the reducing conditionsand the firing temperature. Both magnetite andwustite are black in colour while metallic iron has adark metallic shade. Furthermore, wustite is a verystrong flux and its presence in the clay matrix attemperatures above 800°C can lead to intense solid-state reactions with the argillaceous clay minerals.This results to an increased vitrification and rapiddrop of the viscosity of the glass phase. The droppingglass viscosity in combination with the liberation ofoxygen from the dissociating iron oxides createsbloating in the microstructure. The bloating poresclearly seen with the SEM (Maniatis and Tite 1975)increase in size with the firing temperature and thisleads very soon to the swelling and deformation ofthe vase and finally to its collapsing. Thus theconversion of iron oxides from their oxidized form tothe reduced one produces in general dark ceramicsand glass with low viscosity and bloating.

However, this is not always the case. The presenceof fine calcium carbonate in the clay is again veryimportant for the properties of the ceramic such asmicrostructure, vitrification, bloating and the finalcolour. In the non-calcareous clays firing in a reducingatmosphere produces dark grey to even black colourceramics, while in the calcareous ones reducing firingproduces light grey to even whitish colours. Thedifference is due to the fact that in the calcareousclays the CaO reacts with the Fe-oxides and as in thecase of the oxidising conditions forms new Ca-Fe-aluminosilicate minerals (Maniatis et al. 1983). Thisabsorption of Fe into new minerals does not favourthe crystallisation of the reduced black iron oxides

Figure 8: Red on buff decoration on a Neolithic Periodvessel from Thessaly, Greece.

Energence of Ceramic Technology 9

(magnetite and wustite), and so the colours of thecalcareous ceramics fired in reducing conditionsrange from pale grey to whitish (CaO > 15–20%)according to the initial amount of calcium carbonateand the firing temperature. The decrease in theproduction of FeO in the calcareous ceramics, apartfrom the colour, has an important effect in theproperties of these ceramics. The uncontrollableproduction of low viscosity glass in the micro-structure is stopped and the bloating is largelyreduced (Maniatis and Tite 1975) preventing in thisway the deformation and the collapsing of the vessel,even at higher temperatures.

For the colour of ceramics fired in reducingconditions, it should be noted that the differencesbetween calcareous and non-calcareous clays emergeonly above 800°C, because the dissociation of CaCO

3

and the appearance of the reactive CaO in the claymatrix occurs at about this temperature. Firing attemperatures below 800°C does not produce anydifference in the colour or the properties of theceramics. The colours in this case are all dark greyirrespective of the clay chemistry. Figure 9 shows anindicative diagram for the colours developing incalcareous and non-calcareous ceramics during firingin a reducing atmosphere. The curve of non-calcareous clays is only theoretically extended to1100°C, because as discussed above such a ceramiccannot survive above 950°C in reducing conditions.However, when a non-calcareous clay is heavily

tempered it can resist higher temperatures inreducing conditions. An example are the high density,strong and impermeable bodies of a class of darkcoloured Punic Amphorae found at Corinth (5thcentury BC), containing quartz, feldspar, mica schistbut also some limestone inclusions. (Maniatis et al.1984).

The reducing conditions during firing of theceramics can be created by closing all the openingsof a kiln and throwing some fresh wood in the firecompartment or by firing in ground pits and coveringall the vessels with straw and wood or with largervessels (pithoi). Sometimes reducing conditions canbe created at a certain point inside a kiln or a bonfireaccidentally due to bad air circulation. A lot of darkcoloured ceramics, some nicely burnished, areobserved in the Neolithic and Early Bronze AgePeriod in many parts of the world and it is the resultof firing in ground pits covered with straw, woodand perhaps soil on top. These vessels are typicallymade of non-calcarous clays and fired at about 800°Cor even sometimes of calcareous clays but fired below800°C so that the calcium carbonate does not dis-sociate. The reducing firing is surely intentional andthe dark coloured ceramics represent a certaintradition. However, the firing conditions are not easilycontrolled and for this reason a lot of ancient ceramicsare partially grey and partially red (Fig. 10). At aboutthe Middle Bronze age in Greece (1900 BC) greyuniform ceramics fired in reducing atmosphere but

Figure 9: Diagram indicating the development of colours in non-calcareous and calcareous ceramics fired under reducingconditions.

Y. Maniatis10

certainly in kilns are produced in many parts ofGreece. Typical examples are the so-called “GreyMinyan” ceramics (Fig. 11), the best of which aremade of a highly calcareous clay, fired at about 950oC. They have a uniform light grey colour and acharacteristic soapy feel due to the very finecalcareous clay from which they are made.

Black decoration on a light background

The black decoration on a light body (red, pink, crèmeor white) represents a further advancement inceramic technology. In order to produce a red or light

colour body the ceramic needs to be fired underoxidising conditions. In this case however, if thedecoration paint were made of a fine clayish materialits colour after an oxidising firing would be red orpink, like the example of Figure 8. Thus, in order toachieve a black decoration on a reddish or whitishbody a material other than clay should be used. Thismaterial must be black and should stay black afterfiring in an oxidising atmosphere.

Typical materials for this technique are the naturalmanganese ores used during the Neolithic and Earlyand Middle Bronze Age (Noll 1982). Such ores arethe MnO

2 either in pure crystalline form called

pyrolusite or in a colloidal form known aspsilomelane, the latter sometimes containing alsobarite (BaSO

4). These manganese oxides are originally

black and remain black after firing in oxidisingconditions. Sometimes Fe-Mn ores are used whichalso contain small quantities of clay (Noll et al. 1975;Aloupi and Maniatis 1990; Kilikoglou et al. 1990),which helps to adherence better the paint on thevessel’s surface. In this case the colour of the decor-ation at higher temperatures can come out darkbrown rather than black. Using therefore manganeseoxide pigments for the decoration it is relatively easyto obtain the contrast of a black or dark browndecoration on a light body. The body can be made ofcalcareous clay and the vessel fired in oxidisingconditions at temperatures in the range 800–900 oC.Using the manganese oxides polychrome decoratedvessels could also be obtained (Fig. 12). In this casemanganese oxide is used for the black decoration,non-calcareous refined clay for the red decorationand calcareous clay for the body (Aloupi andManiatis 1990; Kilikoglou et al. 1990). A single firingat an oxidising atmosphere is enough to produce thispolychrome result. However, the black coloureddecoration obtained with manganese pigments is notin general of so good quality, because it is relativelycoarse and does not sinter in the usual firing tempera-ture ranges (850–1050°C). As a result the paint has amat appearance without any sheen, despite thepolishing efforts obvious in some cases and has a badadherence with the body being rubbed off by thehand quite easily.

During the end of the Middle Bronze Age butespecially during the Late Bronze Age, a black paintof a much superior quality appears produced withthe so-called iron reduction technique involving thereduction of iron oxides in the paint. The under-standing and deliberate use of this technique by theancient potters represents a big step forward in theceramic technology evolution. For the production ofthis black paint a very good control of the kiln is

Figure 10: Neolithic dark coloured (dark burnished) vesselmade of non-calcareous clay fired in reducing conditions atabout 800°C. The bottom part is either intentional oraccidentally oxidized.

Figure 11: Middle Bronze Age ‘Minyan pottery” made ofa highly calcareous clay fired in reducing conditions atabout 950°C. Compare the light grey colour due to thepresence of CaO with the dark of figure 11.

Energence of Ceramic Technology 11

required with a precise closing and opening of thevents in order to change the atmosphere in the kilnfrom oxidising to reducing and then back to oxidisingat specific temperatures and times. In this way a blackglossy decoration is produced on a light body back-ground. With the application of scientific techniquesit was made possible to reveal the complicatedtechnology involved in the manufacturing of theblack gloss decoration, using the iron reductiontechnique (Hofmann 1962; Noll et al. 1975; Tite et al.1982; Aloupi and Maniatis 1990; Kingery 1991; Aloupi1993; Maniatis et al. 1993), and to establish that thistechnique reached an extremely high level of tech-nology during the Classical Period in Attica. This wasthe period when the famous Attic Black Figured andRed Figured vases (Fig. 13) were manufactured andacquired a very high artistic and commercial valuebeing traded all over the ancient world of that period.The analytical scanning electron microscopy provedonce again a powerful tool for the extraction ofinvaluable technological information. Figure 14shows the micromorphology of the black gloss paintand the ceramic body of an Attic vessel at a crosssection. The black gloss layer exhibits a high degreeof uniformity and sintering. The amount of vitrifica-tion is optimum so that the layer is quite dense andperfectly bonded to the body but at the same timedoes not show any deformation or bloating. Theceramic body exhibits an open cellular micro-structure characteristic of a calcareous clay fired inthe range 850–1050°C. In the next figure (Fig. 15) onecan see the micromorphology of black paint fromearlier periods (LBA), when the first attempts toproduce black paint on light body using the ironreduction technique begun (Aloupi and Maniatis1990). The differences with the attic black gloss areapparent. The vitrification of the LBA black layer isexcessive; the viscosity of the melted paint hasdropped dangerously with the simultaneous appear-ance of bloating and deformation, resulting to a lowerquality product. Actually, this outcome is the mostfrequent result when there is no absolute control onthe firing conditions in the kiln (atmosphere andtemperature) or/and on the selection and treatmentof the raw materials.

A comparison of the chemistry between blackglosses of different periods, from Early Bornze Ageto the Classical period (Table 1) shows the improvedrefinement of the paint material with time as wit-nessed by the progressive removal of Ca and theincreased ration of Al/Si, from 0.45, to 0.61 and finallyto 0.67 in the classical period.

For the high quality black gloss paint, like the oneon the Attic vases, a carefully selected and thoroughly

Figure 12: Vessel from Akrotiri, Thera bearing Mn-blackdecoration. The red decoration is made of fine non-calcareous clay. The body is made of a highly calcareousclay. It is fired in an oxidizing atmosphere.

Figure 13: Red figured attic vase. The black paint isproduced with the iron reduction technique.

Y. Maniatis12

treated clay is needed, as well as, a fully controlledthree-stage firing. The raw material to be used for thepaint should be practically free of calcite (CaO < 1%),enriched in fine clay minerals, potash and iron oxides.This can be achieved by the selection of a fine illiticand calcium free clay and then by a persistentsuspension in water for months. During the pro-longed suspension the aplastic and coarser particles,like quartz, feldspars, aggregates etc., are removedand the suspended fraction is enriched in very fineargillaceous minerals (mainly illite) and very fine ironhydroxides or/and iron oxides that give to the raw

paint an orange to red initial colour. The selectionand treatment of the raw materials can be seen inFigure 16 that shows the iron concentration;important for the colour, against the Al/Si ratio thatindicates degree of refinement. The consistency inchemistry and treatment is remarkable, particularlyfor the black gloss paint.

The examination of the black gloss paint on atticvases with the transmission electron microscope(TEM), at high magnifications, gave importantinformation on the grain sizes and the firing con-ditions (Maniatis et al. 1993). The black paint seemsto contain mainly magnetite (Fe

3O

4) crystals, as was

verified with electron diffraction, that are dispersedinside an amorphous matrix (Fig. 17). Magnetite hasa black colour and its presence gives the paint itscharacteristic black colour. As was discussed earlier,magnetite does not exist in the raw clay but it isproduced from the dissociation of hematite duringfiring in reducing conditions. It is therefore clear thatat some stage of the firing the atmosphere in the kilnwas reducing. Furthermore, the affluent presence ofmagnetite suggests that the reduction stage was mildand controlled so that the dissociation of hematitepractically stopped after conversion to magnetite anddid not progress further to produce large quantitiesof FeO. The latter is very reactive and if present wouldlead to rapid vitrification with inevitable bloating anddeformation as in Figure 15. Contrary to the reducingfiring verified for the black paint, the ceramic body isfired in oxidising conditions as is deduced from itspink colour. These results clearly indicate an alterna-tion in the kiln’s atmosphere from oxidising toreducing and vice-versa. As far as the grain sizes ofthe paint material is concerned, the greatest size oforiginal particles typically found in the attic blackgloss are some rare titanomagnetite particles (a rathercommon aplastic accessory mineral in a lot of clays)of dimensions 0.0003 mm (Maniatis et al. 1993). Thus,from the TEM examination it is confirmed that therefinement of the clay used for the paint wasextremely persistent.

The above scientific conclusions inferred from theexamination and analysis of the attic black gloss weretested thoroughly with simulation experiments in thelaboratory (Aloupi 1993). These experiments sug-gested that the optimum three-stage firing cycle musthad been as follows:

Stage 1: Initial firing in oxidising conditions up toabout 900 oC and soaking at top temperature foran hour or more.Stage 2: At the top temperature creating reducingconditions by closing all the vents of the kiln andfeeding the fire with fresh wood, most probably

Figure 14: Micromorphology of the black gloss of an atticred-figured vase (6th cent. BC) at a cross section near thesurface. The unique homogeneity of the sintered andcompact black gloss layer adhering nicely to the porouscalcareous body is evident. SEM image at backscatteredelectron mode.

Figure 15: Micromorphology of a much earlier (1600–1500BC, Thera) black paint produced with the iron reductiontechnique, at a cross section near the surface. The blacklayer is grainy, with bloating porous and deforming. Thebody is again calcareous and porous. SEM image atbackscattered electron mode.

Energence of Ceramic Technology 13

wet. Inevitably and wilfully the temperature dropsa couple of hundred degrees to about 700–750 oC.Stage 3: Opening again all the kiln’s vents and newincrease of temperature up to about 850 oC for anhour and then cooling, maintaining the oxidisingconditions until the end.

In the first oxidising stage, the body of the ceramicbecomes pink since it is made of calcareous clay. Thepaint becomes intense red as it contains no calciumcarbonate and it is rich in iron, the latter formingwell-crystallised hematite particles. At this stage acontrolled sintering and densification occurs in thepaint layer.

In the second stage that is reducing, hematite istransformed to magnetite and as a result the colourof the paint becomes intense black. The body colourbecomes grey as all calcareous clays (see diagram ofFig. 9). This stage is very critical because if the controlon temperature or atmosphere is lost to more intense

Table 1: Comparison of average chemical compositions of black gloss in Early Bronze Age and Attic Pottery. Concentrationsexpressed as % oxides (Fe as FeO) (Aloupi and Maniatis 1990; Aloupi 1993; Maniatis et al. 1993).

Black gloss Na Mg Al Si P S Cl K Ca Ti Mn Fe Al/Si

EBA, Thera 1.4 2.6 22.0 48.7 0.5 0.3 0.1 5.9 4.4 0.5 0.3 18.3 0.45

Late Geometric, Naxos 1.9 2.9 26.9 44.0 0.3 0.2 0.3 6.2 1.9 0.6 0.2 14.7 0.61

Archaic Period, Attic 1.0 2.6 28.9 45.4 0.2 - 0.1 6.1 1.0 0.7 0.1 14.2 0.64

Classical Period, Attic 0.7 1.9 30.3 45.2 0.4 0.1 0.1 5.3 0.6 0.7 0.1 14.9 0.67

Figure 16: Fe concentration plotted against the ratio Al/Si for the body and the black gloss of a number of attic pottery ofthe 6th and 5th century BC.

Figure 17: Micromorphology of attic black gloss under thetransmission electron microscope at high mignifications(75,000 x). The black grains are magnetite or hercyniteparticles, their typical electron diffraction pattern is shownon the top right-hand corner (Maniatis et al. 1993).

Y. Maniatis14

reducing conditions, magnetite may dissociatefurther to wustite leading to rapid vitrification andmelting of the paint layer as discussed earlier. On theother hand if the atmosphere swings to partiallyoxidising hematite would not be fully converted tomagnetite resulting to brown rather than blackcolours. There are a lot of examples of both cases offailed black gloss in antiquity. The controlled reduc-tion at this stage produces the optimum degree ofsintering and vitrification so that the paint layerbecomes compact and impermeable to gasses,excluding in this way any diffusion of oxygen into itand proxibiting the re-oxidation of magnetite at thefinal oxidising stage.

At the third stage, a re-oxidation of the porousbody occurs and its colour is reinstated to a greatdegree to the initial pink/reddish colour of the firststage. However the paint layer is impermeable andcannot be re-oxidised (if the reducing conditions areright, as explained above), remaining black and stableuntil the end of the firing cycle.

Figure 18 shows approximately the colour changesoccurring in the body and paint during the variousfiring stages for producing a black glossy decorationon a pink or light red body, as that observed on thered-figured or black-figured attic vases. The surfaceof the vessel made of a calcareous clay is wiped witha wet sponge to make the surface smooth. The colourof the raw body clay was almost certainly grey, as allthe natural fine calcareous clays that can be found inGreece. The decoration that is going to come out blackis painted with the specially refined clay, as discussedearlier. The initial raw colour of the paint materialwas either orange or red due to its enrichment in ironoxides. The difference in colour of the raw claysbetween body and paint helped the artist to paint thedecoration details, which in some cases wereextremely fine. The rest of the surface was not coveredwith any other slip or material in order to remainporous and facilitate its re-oxidation during the finaloxidising firing stage.

Concluding for the black gloss paint on the Atticvases, the selection and extremely high refinement ofthe raw materials and the control of the kiln duringthe complicated three-stage firing is reflecting a veryhigh technology level. It surely denotes an importanttechnological achievement at the end of a progressivedevelopment for about 1000 years from the date itwas first tried in MBA about 6000 years after the firstappearance of ceramics. This manufacturing know-how in conjunction with the beautiful in most casesangiographies gave these vases a very high tradingvalue.

Red gloss in conjunction with black gloss

There are some Attic vessels, although very rare,bearing simultaneously black and red glossdecoration on a pink or light-red body. The term redgloss refers to a red sintered and glossy paint havingmore or less the same appearance as the black glossexcept that it is red in colour. This paint wassometimes called “intentional red glaze” (Farnsworthand Wisely 1958) in order to signify the fact that itwas a paint designed to come out of the firing redand glossy, and it is not a failed black gloss or abackground red colour. The intentional red glossshould not be confused with the so-called “accessoryred” which is a coarse matt paint of a purple colour.Given the fact that the red gloss paint layer is sinteredand may block the entry of oxygen at the lastoxidising firing stage, its presence on a ceramicsurface together with the black gloss presents achallenging technological achievement and makes itsscientific investigation quite interesting.

Some researchers (Richter 1951) had suggested thatthe paint for the red gloss was applied after the firstthree-stage firing that produces the black gloss and asecond purely oxidising firing was necessary for thered gloss. One of the earliest scientific investigationsof the intentional red gloss combined also withreproduction experiments was in 1958 (Farnsworthand Wisely 1958). They suggested that a second firingwas not necessary. The intentional red was made fromthe same clay used for the black gloss by adding to itsome quantity of very fine ochre prepared in asuspension. The added ochre delays sintering and

Figure 18: Diagram showing the colour changes duringthe three-stage firing for the production of the attic blackgloss.

Energence of Ceramic Technology 15

vitrification, thus, both paints were applied on thevessels when the body was in a leather-hard state anda single three-stage firing was performed. During thisfiring the black sinters and becomes a coherent solidmass as discussed before, while the ochre-containingglaze remains porous and is easily reoxidised at thelast oxidising stage. More recent work (Tite et al. 1982)using scanning electron microscopy suggested thataddition of ochre to the red gloss cannot be verifiedby analysis and microscopic examination. However,the more porous texture of the red gloss that wouldallow reoxidation during the final oxidation firingstage was verified. This according to the authors couldbe obtained by collecting a fine but slightly coarserfraction from the suspension during the refiningprocess to prepare the black paint. Using these twofractions the black and red gloss can be producedsimultaneously at a single three-stage firing.

New analysis and examination of the intentionalred gloss on attic vases of the 6th and 5th centurywhich is under progress at the Laboratory ofArchaeometry, NCSR “Demokritos” is providinginteresting but a bit puzzling new evidence. Theintentional red gloss seems to have at least twoversions. The 5th century sample, called also “coral

red” contains a higher amount and particle sizedistribution of iron oxides and the degree ofrefinement of the original clay (Al/Si = 0.60) is lessthan that of the black gloss (Table 1). This could wellhad been produced by adding ochre to the paintprepared for the black gloss. It is most probably thetype of “intentional red” examined by Fansworth andWisely (1958), so their suggestion makes sense. Onthe contrary, our 6th century sample is of a verydifferent nature. The chemistry and microstructureare very similar between black and red (Table 2). Thisagrees more with the samples examined by Tite et al(1982), but only as far as the similarity in chemistrybetween black and red gloss is concerned. As far asthe microstructure is concerned, in our case they areidentical between black and red (Kavoussanaki 2002).Figures 19 and 20 show SEM micrographs of polishedsections of black gloss and red gloss on the same atticvessel. They exhibit the same size and distribution ofiron oxide particles (white spots) and same degree ofsintering. This similarity makes it very difficult tounderstand how the red paint layer could have beenre-oxidised but the black could not. One wanders ifthere is any minute porosity differences, undetectablewith the SEM that could perhaps explain the re-

Figure 19: Black glass of attic vessel – polished section.SEM backscattered image. White bar corresponds to0.01mm.

Figure 20: Red gloss – polished section the same attic vesselas Figure 19. SEM backscattered image. White barcorresponds to 0.01mm.

Sample Na2O MgO Al2O3 SiO2 K2O TiO2 FeO Al/Si

Black Gloss 0.74 1.40 31.14 46.48 4.78 0.49 14.99 0.67

Red Gloss 0.77 1.49 31.62 45.37 4.63 0.55 15.00 0.69

Coral Red 1.77 1.89 27.68 46.29 3.47 1.25 16.88 0.60

Accessory Red (Purple) - - 5.88 34.34 2.39 0.56 54.05 0.17

Table 2: Examples of the basic chemistry of various Attic red paints co-existing with black gloss.

Y. Maniatis16

oxidation of the red paint at the final firing stagehowever, it has to be proven by further investigation.The possibility of a second firing in oxidisingatmosphere just for the red, using exactly the samepaint material as for the black, must be excluded asthe optical microscope examination reveals a blackzone in the red gloss paint at its innermost side (Fig.21). This black zone is clear evidence that this painthas undergone a reducing cycle at an intermediatestage, the density of which been such that it was notfully re-oxidised, down to the deepest layer close tothe vase body, during the final oxidizing stage. Apaper presenting these new experimental results isunder preparation.

Conclusions

I hope this paper has shown the invaluable con-tribution of scientific techniques in the study ofancient ceramic technology. This was the result ofsystematic and dedicated work by several archae-ometrists during the last few decades, one of thembeing undoubtedly M.S. Tite. Important informationon various aspects of ceramic technology has beenextracted from the moment of its emergence till recenttimes. We now know the role of clay chemistry andfiring properties of the different natural clays and theeffect of treatment of the raw materials. We also knowthat ancient potters progressively understood betterand deeper the parameters influencing the physical

and chemical properties and quality of the finalproduct. Through the selection of suitable materials,teh modulation of their properties by ingenioustreatment and clever manipulation of the kiln con-ditions the ancient potters reached an extremely highlevel of ceramic technology in classical times. Thiswas followed by the development of transparentglazes and porcelain. The scientific investigation ofancient ceramic technology has allowed the under-standing of the technological solutions adopted ineach period and place and has shed light on thetechniques used for the production of most knowntypes of ceramics all over the ancient world.

Archaeologists have realised the importance ofthese new developments and collaborate systematic-ally with archaeological scientists for ancient ceramictechnology studies. A number of them are engagedthemselves in scientific examination and haveacquired experience in reading and interpreting thescientific results. We have gone far beyond the earliertimes when archaeologists described the pottery onlyby body form and style of decoration and the timeswhen they believed the different ceramic coloursobserved were solely due to different clays used; beentotally unaware of the dramatic effects on the colourof the firing temperature and atmosphere.

It is now clear that the full understanding ofancient ceramic technology and its social and eco-nomic implications can only be obtained through anintegrated approach. This involves the investigationof the system: Clay selection and refinement –refractory properties – firing temperature and atmo-sphere – mechanical and thermal properties –decoration technique – availability and provenanceof raw material – social context and use.

There is still a lot to be learned, many differenttypes of ceramics to be investigated but the mostimportant goal has been already achieved. This is theset of unique developed methodologies and back-ground knowledge obtained by the previous archae-ometric generation. These are now inherited to theyounger researchers of the field and allow theintegrated study of any group of ceramics and theassessment of the level of technology and know howat any period or area.

Note

1 Laboratory of Archaeometry, Institute of Materials Science,NCSR “Demokritos”, 153 10 Aghia Paraskavi, Attiki, Greece,e-mail: maniatis@ims.demokritos.gr

Figure 21: Optical microscope picture of polished crosssection of intentional red gloss. The inner part of the paintlayer is black indicating a preceding reduction stage andnot full reoxidation during the final firing stage(magnification 100 x).

Energence of Ceramic Technology 17

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