علم سرامیک

2.1. Ceramics and clays Chapter 1 showed that ceramic, or rather ceramics, are varied and complex materials; the study of the structural transformations and physicochemical reorganizations at all stages of their manufacture constitutes a vast field of research that is still vibrant and largely open, since many new ceramic products continue to be developed for the needs of the most advanced technology. But to trace the history of ceramics, we need to reiterate some basic concepts. Brongniart observes in the introduction to his famous Traité des arts céramiques that “clay is undoubtedly the most widespread raw material on the surface of the earth, the easiest to work with immediately and to transform, but also the one that allows the most utilitarian and artistic productions”. This universality and this ease explain why as early as the end of the Stone Age, ceramics gradually became what we can truly call a ubiquitous invention, insofar as it emerged in many human settlements, on all the continents, at extremely different eras. However, when a terra cotta artifact, whether prehistoric, antique and often even more recent, is found, it is always through a comparative analysis of the material and forms associated with a rigorous dating and an exhaustive study of the archaeological context that we can affirm whether this artifact is the creation of a local craftsmanship that emerged and developed in situ, or the result of a foreign Chapter written by Anne BOUQUILLON. 30 Ceramic Materials know-how, established there in the wake of migrations and conquests, or whether it arrived fortuitously through exchanges or expeditions. The location of the birth, evolution and progress of ceramic art in time and space, particularly in the first few millennia of its existence would require the extensive break up of the subject in order to get a sufficiently clear picture. The scope of this chapter would not allow it, and therefore we will limit ourselves to the presentation of some of the most outstanding facts in the history of this material. But first, what is clay? It is a rock resulting either from the disintegration of pre- existent crystalline rocks (granites, gneiss, etc.), or from a formation inside large sedimentary basins. Clays are made up of fine pseudo-hexagonal particles often a few micrometers in size. We can distinguish, from a ceramic point of view, at least two types of deposits: primary clays are found on the site of formation; they are coarse, mixed with residues of original rocks (quartz, flint, micas, feldspars, etc.); and secondary clays, which have generally undergone a long transportation that induced a natural decantation. These deposits contain clays that are much finer, more homogenous. In mineralogical and chemical terms, clays are diverse and as a result have particular properties depending on the family considered: the refractory nature of kaolinite, swelling properties of smectites, propensity to vitrification of illite, high absorptive capacity of vermiculites and palygorskites. However, almost all clays have a layered morphology (phyllo-silicates) and high water content (15% H2O), hence a plasticity that allows easy forming. The addition of water at the time of shaping (approximately 25%) enables the potter to create a shape and to preserve it during drying. To make it permanent, firing is required and the irreversible chemical and crystallographic modifications resulting from the rise in temperature will make it possible to keep the ceramics. These ceramics are fragile and cannot be re-used once they are broken, but the material hardly deteriorates when used or buried; this explains why they are frequently unearthed during excavations. They must be considered as “fossils” embodying great civilizations and their evolution. Primitive man learnt very quickly to take advantage of the plasticity of raw clay; observation, deduction, and the fortuitous contact between this material and heat sources would put them on the track to this vital essential discovery: ceramics. 2.2. The first ceramics: sporadic occurrences as early as the end of the Paleolithic The first ceramics appeared at Dolni Vestonice (in the former Czechoslovakia), as early as 26,000 BC. They were both anthropomorphic figurines and diverse artifacts whose shards were discovered in the thousands. The technology was History of Ceramics 31 rudimentary: use of raw local loesses, shaping and firing in open “horseshoe shaped” kilns at temperatures not exceeding 900°C [VAN 90]. This manifestation at the end of the Stone Age apparently remained very isolated and it would be several thousands of years later that the Japanese Jomon ceramics (more than 12,000 years old and discovered in the Fukui caves, near Nagasaki) marked the beginning of a production that has subsisted until now. As early as this period, there was a more complex use of argillaceous earth, which was prepared by adding, probably voluntarily, organic fibers and mica [HAR 97]. It is interesting to note that the first ceramics appeared sporadically in hunter- gatherer, semi-nomadic societies, in which an elaborate social structure appeared. For the moment, in the current state of discoveries, these very early appearances of terra cotta in the form of statuettes are rare and do not seem to last (except in the case of Jomons). They are reported in Siberia towards 12,000 BC, then in China in the Jiangxi province 1,000 years later. In Mesopotamia, on the Mureybet site, ceramic fragments dating from circa 8,000 BC have been found; they come from small coarsely modeled artifacts [MAR 91]. They are very few, not well fired, but nonetheless constitute a proof of the early appearance of terra cotta in the Middle East. They are referred to as intermittent ceramics [MAR 91] and they all belong to a phase that may also be called “pre-ceramic” (without potteries) of the Neolithic era. The other civilizations used, particularly as containers, stone, fired clay, sometimes dried in the sun or possibly rubefied holes fired in the heat of a hearth. More interesting for the history of pyrotechnology, and therefore of ceramics, are these “white wares”, these floor and wall tiles and these lime and gypsum sculptures found on sites in Palestine first (Beidha, 8,300–7,600 BC) then in Iraq, Syria, Anatolia and Jericho [KIN 92]. 2.3. The Neolithic era: the true beginning The explosion of the use of this material would be observed in Asia as well as in the Near and Middle East and in Europe circa 7,000–6,000 BC. It resulted from an important change in lifestyles, needs and beliefs in the wake of sedentarization, cultivation and cattle-raising. This is generally referred to by the term neolithization [RIC 90]. Some, however, believe that the first ceramics had a more symbolic than utilitarian role (Figure 2.1) [PER 94]. From a technical point of view, all ceramics must be classified under soft pastes, i.e. pastes with high porosity, fired at a temperature lower than 1,000°C. In the first few millennia that followed the invention of ceramics, pastes were often coarse, and 32 Ceramic Materials potters understood very quickly the need to modify the intrinsic properties of raw earth to obtain better resistance to drying and firing. For this purpose, they added to the raw material non-plastic particles (tempers), which to some extent constitute the “skeleton” of the artifact. These tempers are of various types: mineral, organic, natural or anthropic. Their precise characterization is essential in archeometric studies to specify the function of a ceramic, determine the know-how, define a culture or establish an origin. Figure 2.1. Anthropomorphic vase of the final Neolithic (© National Museum of History, Sofia) A few examples: – to determine the function: in the Neolithic era, the uses of ceramic vases were manifold and each of them largely determined the technique; the so-called “provision” vases were large earthenware jars made of porous paste, with coarse tempers, intended to protect grains, fruits, etc. from insects, rodents or moisture; other less porous vases could hold thick liquids, dairy products, oils, etc. The vases with more meticulous surface treatment could contain drinks and, finally, others undoubtedly show specific residues from culinary preparations or materials such as birch pitch used as an adhesive [REG in press], [REG 03]. Researches undertaken on the Clairvaux sites in the Jura or Chalain in the Savoy have brought to light a fine History of Ceramics 33 diversity of these ceramics in the limited context of villages. The addition of plant matter to the paste was not insignificant; in fact, these materials do not resist heating, except very rarely, and they leave large pores when they are burnt. Ceramics with vegetal temper in our regions often identify artifacts used as coolers, or that had to undergo violent and repeated thermal shocks which were mitigated by the pores [RYE 89]; – to determine the know-how: the use of Grog as temper (fragment of crushed fired ceramic) is the most logical (Figure 2.2); in fact, these non-plastic particles exhibit the best properties: perfect compatibility with clay, optimum expansion/shrinkage coefficient (the earth has already been fired) and reuse of wastes and wasters. This is found very early, in the Neolithic era, on the megalithic site of Bougon (Deux-Sèvres) for example; Figure 2.2. Grog temper of a ceramic in Northern France – petrographic microscopy on thin section (© C2RMF – photo: A. Leclaire) – to define a culture: in the Neolithic era, ceramic pastes of the so-called “Cerny” culture are characterized by a temper made of splinters of bone [BRI 90]. It has been sometimes said that this temper had a ritual significance; – to establish an origin: in Brittany, the campaniform (bell-shaped) culture developed during the Chalcolithic age; characterized by bell-shaped vases, this culture was found all along the Atlantic coastline, in Spain, Portugal, etc. The presence on the sites of the Southern Finistere of some shards containing fragments of volcanic rocks among ceramics tempered with fragments of granitic rocks typical 34 Ceramic Materials of the region has revealed probable imports from the Rhine areas in this period [CON 98]. 2.3.1. Forming and firing 2.3.1.1. Forming If the composition of pastes varies from one region to another, techniques of shaping vases follow an identical evolution: besides simple modeling, primitive traditional methods include coiling (wads of clay are shaped then placed on a base depending on the desired shape), assembly by juxtaposition of clay plates shaped beforehand and the forming by mining or digging – by pinching or drawing a ball or a thick plate of clay. These techniques are still used today. The greatest revolution in the field of forming was the use of the wheel. The ceramic is placed in the center of a table revolving at 50-150 rotations/minute, caused by the potter himself. This technique, which probably appeared in Asia around the fourth millennium BC [ROU 98], a little later in the Middle East and was apparently unknown in America until the arrival of the conquistadors, changed ceramic techniques considerably. In fact, turning ceramics implies a very special know-how and a mastery that can be acquired only after a long period of training: the table must be turned with regularity to thin the walls symmetrically. The generalization of the use of the wheel necessarily required a modification of the pastes: these must be markedly finer and more homogenous. Turned ceramics are fine, the shapes are regular and more elaborate and, especially for a skillful craftsman, it takes less time to produce pottery. The time was ripe to enter the era of mass production. Most of the time, modeled and turned wares continued to be manufactured simultaneously, sometimes on the same sites and it is not always easy to affirm that a particular ceramic was manufactured using the wheel; in fact, the typical traces left on the ground by this method are sometimes ambiguous, and we must not mistake them for the marks of a hand or a tool intended to smooth and refine the surface of the potteries made by coiling or by another technique described above, but worked with a turntable, the predecessor of the wheel. Later, other methods would come to enrich this ancient technology of terra cotta: so, for example, molding using earthen or plaster molds would become frequent, as can be seen particularly in Gallo-Roman terra sigillata wares. We note here that these sketches of the history of ceramics before the 19th century focus especially on pottery itself. To review all the techniques of manufacture, assembly and shaping that were born out of terra cotta arts, we must also take into account all the other types of productions: floor tiles, architectural elements, more important statuettes and sculptures, various artifacts, etc. The scope of this book does not allow us to delve into this subject in detail. History of Ceramics 35 2.3.1.2. Kilns Since the most primitive kilns, comprising a hole in the ground with pots and hearths in a single structure covered with branches or earth, the design of kilns has changed significantly, making it possible to have a better control of the temperatures and atmospheres, to fire a greater quantity of objects in a single batch and a more efficient use of fuel. These kilns were also reusable after one firing. As early as the 4th century BC, Greek kilns were very sophisticated, easily enabling a change from an oxidizing atmosphere to a reducing atmosphere. On certain sites of the major Gallo-Roman workshops (Gueugnon, Lezoux, Graufesenque, etc.), up to 12 different types of kilns have been discovered, each one dedicated to a particular type of production: from the half-buried single chamber in which the pots to be fired were placed directly on two hearths especially used to fire the dark common potteries of Gallic tradition, to the more sophisticated kiln dedicated to the famous terra sigillata, built in such a manner that the vases to be fired were protected from the products of combustion of the hearth, which were located under a hearth. Heat was distributed by a system of pipes present both in the floor and the wall nearest to the kiln chamber. The most widespread kilns were, however, simpler in design with a half-buried “laboratory” separated from the fuel by a pierced floor. An opening, often made on the top of the kiln, helped control the temperature and, more importantly, the atmosphere of the kiln [DUF 96]. 2.3.2. Decorations This know-how and these techniques would unleash creative imagination through the millennia, to the four corners of the world: a universe burgeoning with shapes and decors. The shapes, from the simple useful object to the prestigious work of art, are innumerable, so varied from one culture to another. As for decorations, for reasons of accuracy, we will now present the techniques of the most characteristic and most universal decorations. 2.3.2.1. The very first decorations As early as the first potteries, decoration was immediately an essential element, a symbolic system with which a whole culture identified itself. The simplest shapes are incisions, nail marks, scratches, etc. An example taken from the old Neolithic era [MOH 98] will illustrate the importance of the study of decoration in the knowledge of cultures. Along the Mediterranean, around 6,000 BC, ceramics with cardial decorations flourished: we can trace the expansion of the culture between 5,000 and 4,500 BC towards the Massif Central and the Atlantic coasts by discovering the presence of ceramics in the excavated sites. 36 Ceramic Materials At the same time, in Central Europe, around 4,750 BC, another culture developed, that of ceramics with linear decoration, where the decorations are ribbons, volutes, horseshoes, etc. covering the Danube valley and the Rhine valley. Later, in the West, the civilization of megaliths would develop, characterized, as regards ceramics, by shapes that are not decorated or with a few gripping buttons. Painted ceramics Painted ceramics also became very quickly a developed means of expression, as early as the first ceramic periods. Paintings, generally of mineral origin, slip (very watered-down fine clay) of red metallic oxides (hematite, goethite or red clays like ochres), black metallic oxides (manganese), kaolinite or calcite for white, are executed either before firing on dried ceramics or after firing. This technique would produce remarkable works in various sites on all the continents. Speaking about antique ceramics, we can cite among many others the ceramics of Susa, pre-Indus ceramics (fourth and third millennia BC) at the Nausharo site (Figure 2.3), those of the Banshan culture in the third and second millennia BC, the art of the Cyclades, Cretan ceramics, vases with geometrical decorations of the first millennium BC in Greece and painted potteries of the Iron Age discovered in Champagne. Figure 2.3. Painted vase of Mehrgarh (Indus-Pakistan valley), pre-Indus period (© C. Jarrige) History of Ceramics 37 Added decorations Here again, the diversity is remarkable: the addition of small clay buttons like on the Carn ceramics, the addition of fine clots on certain Jomon potteries from a more recent period or on some large earthenware jars of Cnossos. Subsequently, finer decorations would be obtained by molding (see Figure 2.4) or stamping on finely engraved molds or carved punches. This type of decorations is found on Gallo-Roman terra sigillata ceramics. “Scenes” so finely represented on the bodies of vases are obtained by molding. An artist first creates a hollow matrix. The finer the details are, the finer the paste must be, obtained by decanting a basic argillaceous earth a number of times. These decanting procedures were developed by making the clay pass through a series of basins. Figure 2.4. Tanagra statuette – inv. 556 – Louvre Museum – Department of Greek, Etruscan and Roman Antiquities (© C2RMF – photo: G. Koatz) 2.3.2.2. Ceramic of classical antiquity Decorative effects could also be achieved by the play of the oxidations/reductions of certain metallic compounds. But for this, firing had to be controlled. As early as the third millennium BC, artisans already knew how to fire or smoke potteries in a 38 Ceramic Materials reducing atmosphere by adding grass or green wood, stopping all the air intakes or, on the contrary, in an oxidizing atmosphere. The Egyptians had first fired two-tone ceramics with a red base and blackened neck, called black-top, by designing half- buried kilns; the ceramic was half-buried in the sand, the protected part fired red, the other part, directly in contact with the atmosphere of the kiln, fired black [NOB 88]. But it was undoubtedly with Greek potteries featuring black Attic decorations that we can best see the remarkable mastery of the potters of the 6th century BC in Greece (Figure 2.5): this technique relied on a precise preparation of the clay of the paste and the decoration as well as on a perfect mastery of the various phases of firing. Figure 2.5. Attic crater inv. MNE938 – Department of Greek, Etruscan and Roman Antiquities PAS 00 (© C2RMF photo: D. Vigears) Attic clay naturally contains iron, therefore fired dark red in the normal oxidizing conditions of the kilns of the time. The potters shaped the ceramic on the wheel, allowed it to dry in the sun and then painted the decoration scenes with a slip that consisted of a water suspension of the same clay used for the body but very decanted. This slip was applied on all the areas that had to be black. Firing was done only once but had three phases: – phase 1: oxidizing firing at 900°C: the entire ceramic is red; iron is in the form of Fe2O3; – phase 2: the kiln is closed; the addition of green wood as fuel creates a reducing atmosphere; the entire ceramic becomes black under the effect of the reduction of iron in the form of FeO or Fe3O4; History of Ceramics 39 – phase 3: the kiln is opened again; oxygen is reintroduced: only those areas of the paste that are still porous will be able to change to red again; the areas covered with slip are vitrified (effect of the potassium in the clays that acts as flux on very fine particles); the process of the reoxidation of iron cannot take place. Thus the famous Attic “black varnish” was created; the technique was rediscovered only in 1948 by Schumann [SCH 48]. The red varnishes of terra sigillata ceramics or Italic ceramics were achieved in the same way but in a single phase of oxidizing firing. 2.3.2.3. Glazes These black and red varnishes were often compared with glazes, but incorrectly; in fact, glaze, discovered approximately 7,000 years ago simultaneously in Egypt, in Mesopotamia and in the Indus valley is a glass made up of a mixture of sand, fluxes (vegetal ashes, natron, natural sodium carbonate or lead compounds) and coloring or opacifying oxides. Such a vitreous glaze has a two-fold advantage: waterproofing as well as coloring and decorating a porous terra cotta. 2.3.2.3.1. First alkaline glazes: the middle of the second millennium BC The first glazes were found on stones (quartz or steatite) and it was much later, about the second millennium BC, that the first glazed ceramics appeared. This delay is explained by the great technical difficulties encountered in the production of a glazed ceramic: the mixture had to be free from impurities, have a composition such that the melting point was compatible with the kilns of the time and its thermal behavior on heating and cooling (expansion/shrinkage) had to be compatible with that of the underlying paste to avoid frequent and permanent firing accidents (crazing, bursting, etc.) [DAY 85]. Recent research has revealed that the appearance of the first glazed ceramics dates back to 1,600–1,500 BC in Northern Iraq, in Alalakh in particular, and Northeast Syria [HED 82]. This technique would be used very quickly in the Middle East for architectural decoration, statues and vases. It is interesting to note that in Egypt it was not until the Islamic era in the 8th century AD that the first glazes appeared. In the beginning, glazes were alkaline, often monochromic and blue or blue-green, colored by copper oxides. Polychromy developed later, in the first half of the first millennium BC. 2.3.2.3.2. Appearance of lead-glazes: the Roman era 1,500 years later, in the 1st century AD, the first glazes appeared in the Roman world, in England and in Asia Minor. Their main flux was lead oxide [HAT 94]. It was a timid appearance: only a few specimens have been found. The technical features are good, low point melting, good adherence, iridescent colors, etc. However, this type of glaze would be abundantly used by the Byzantine artisans. In Europe, after the fall of the Roman Empire, ceramics became less sophisticated, 40 Ceramic Materials often modeled, little decorated, without or almost without glazes until the 6th or 7th century. [ENC 98]. Lead potteries would make a comeback around the 9th century and they would have a long history. 2.3.2.3.3. The glossy decorations of the Islamic world: lusters This technique, directly derived from the traditions of goldsmiths, made it possible to apply metallic salts (gold, silver, copper, etc.) on the vitreous support of an opaque glaze. The first examples were produced at the court of the Abbasid sovereigns in Baghdad, but it was primarily Egyptian artisans, as early as the 9th century AD under the reign of the Fatimides, who honed this technique to perfection. In the wake of the migrations of artisans or recipes, luster was introduced into the entire Islamic world as far as Spain, where it flourished during the entire Hispanic-Moorish period [DAR 05]. How was a luster produced? The body of the pottery, made up of a siliceous or a clayey paste, was covered with a transparent or an opaque alkaline-lead glaze. A painting containing metallic salts was placed on the glaze to execute the decoration. This painting was very complex and contained two types of principal components: metallic salts, of course, and a non-reactive “binder” that helped to apply the painting in a regular way. Metallic salts were mixed according to the ancient recipes of Abul Qasim [ALL 73] with vinegar to form acetates. The whole was fired again at low temperatures (about 240°C) in a reducing atmosphere kiln. A two-fold phenomenon occurred: there was a slight diffusion of the metal in the glaze and a reduction and precipitation of this metal on the surface. A quick polishing after firing highlighted the metallic aspect by the play of the refractions/diffusions of light. The colors of the luster varied considerably and in general depend on the size, the concentration of the particles, the nature of the metal used and the control of the last firing [KIN 86]. 2.3.2.3.4. The opacification of the glazes by the addition of tin: an innovation of the Islamic artisans Opacifying a glaze was an important milestone for the artisans to cross. An opaque glaze has many advantages including the obvious one of hiding a not very esthetic paste color and allowing greater freedom and greater possibilities of decoration. There are several ways to do this: the presence of gas bubbles in large quantities diffuses the light and gives an “opalescent” appearance; the persistence within the vitreous matrix of large-sized grains, generally non-molten quartz or feldspar grains, yield an opaque glaze. Finally, the growth of secondary crystals at the interface between the paste and the glaze also gives an opaque aspect to the glaze [MAS 97]. We should point out that the Egyptian antique white or antique yellow, opaque glasses or glazes were obtained by adding calcium antimoniate