علم سرامیک

above- mentioned methods could not offer. The first tin opacification “tests” date back to the 9th century AD in Syria and then in Egypt. Tin was added to a glaze often using a mixture of lead and tin called calcine. On firing, lead and tin dissociated and tin oxidized to produce tiny cassiterite grains. 10% of cassiterite was sufficient to opacify a glaze and to give it a perfect white color. 2.3.2.3.5. Western faience: emergence in the 13th century The tradition of ceramics with stanniferous glaze developed first in the entire Mediterranean Basin and thereafter across the Western world. In the 13th century, there was a substantial production of ceramics decorated with tin oxide in Spain. They would be exported in large quantities to the entire Mediterranean Basin, particularly to Italy and Southern France. It was at this time that the first centers for the production of faience were set up in Italy and shortly thereafter in Marseilles [COL 95]. Expansion was also favored by the recruitment of Spanish artists to work on royal building sites where they made use of their know-how by adapting it to local materials. Artisans from Saragossa, Jehan de Valence and Jehan-le-Voleur are known on the royal building sites of Mehun sur Yèvre, where the first faience tiles have been found [BON 90]. During this period, the production of majolica began in Italy and many large centers were established: Faenza of course, but also Urbino. Each of these centers developed an iconography and a specific type of decoration, but all shared the same technique, described by Piccolpasso in his work Three Books of the Potter’s Art published in 1548. The body of the ceramic is worked with fine marly earth; it is fired first at about 950°C; thereafter several stages are necessary for the production of the glaze: preparation of the calcine (a mixture of lead and tin); preparation of a sand frit, calcine, wine dreg (KNO3); firing and crushing of the frit which is added to water and addition, if necessary, of coloring metallic pigments. This mixture is applied on the piece to be decorated and the whole is reheated again at about 950°C. Later, there could even be more than three firings when colors other than the high fire colors (cobalt blue, copper green, manganese purple, antimony yellow, etc.) are used, i.e. low fire colors (pink, green, red, etc.). 2.3.2.3.6. Productions of the Renaissance Towards the 15th century, it was in Spain with luster and in Italy with majolicas that earthenware experienced their most spectacular growth. The greatest Italian centers were Faenza, Deruta, Gubbio, Urbino [PAD 03] and Casteldurante [GIA 35], but also Florence, particularly with the productions of Della Robbia. This family of sculptors and ceramists used glazed terra cotta as a new material for the sculpture of busts, retables, decorative tiles, vases, etc. They succeeded in achieving such a 42 Ceramic Materials degree of perfection (see Figure 2.6) with respect to both sculpture and colors that these productions immediately sparked off a great interest everywhere in Europe. It is even said that Palissy, on seeing the works of Della Robbia, relentlessly sought to unveil the secret of their so perfect marmoreal white. Figure 2.6. Bust of young man attributed to Della Robbia – inv. OA1932 Department of sculptures – Louvre Museum (© C2RMF – photo: D. Bagault) We cannot speak about Renaissance ceramics without mentioning Palissy and his achievements, both in the domains of marbled earth and earthenware with rubble. According to his writings and also all the material found in his workshop, he was the first ceramist to experiment so much in order to achieve the desired color and effect. Some have even attributed to him a prestigious production (Figure 3.7), known as Henry II faiences, or Saint Porchaire wares, which constitutes the beginnings of hard pastes. Less than 60 specimens in the whole world are known and the esthetic quality, the great technical mastery both in shaping and in the production of the decorations have given rise to varied interpretations, put forward even recently [COL 97]: was this a production of Palissy himself? Is it a Parisian production reserved for the king and nobles? Is this a production of Saintonge? History of Ceramics 43 Figure 2.7. St Porchaire ewer – inv. Ec83 – National Museum of the Renaissance Ecouen (© C2RMF – photo: D. Bagault) The skills of the Italian artists in particular, in the wake of the travels of artisans and the disclosure of trade secrets, inspired the creation in the 16th century in France of the greatest earthenware makers still operating today. First in Lyon, then especially in Nevers, Rouen, Strasbourg, etc.; it is in the 16th and 17th centuries that stanniferous earthenware would evolve, as a high quality production was demanded. However, from the French Revolution onwards, a period of durable recession would follow, consecutive to a fall in demand, of course but, more importantly, along with the rise in prices of wood and raw materials, tin, lead and the arrival on the market of a remarkable English production: fine earthenware. Only a few great centers would resist and succeed in the industrial, technical and stylistic changes necessary for their survival: for example, the use of coal as fuel, discoveries of certain processes (pouncing patterns, etc.) that accelerated the various phases of decoration, the use of a more complete pallet of colors, thanks to the development of so-called “low fire” colors [ROS 91] and the diversification of productions. However, this type of ceramic would soon fall into disuse. 2.4. Chinese stoneware and porcelains: millenniums ahead We have talked so far about relatively porous ceramics, fired at about 1,000– 1,050°C maximum. With stoneware and porcelains, porosity decreases (less than 5% 44 Ceramic Materials for stoneware, less than 1% for porcelain) and vitrification becomes increasingly significant. Firing temperatures exceed 1,200°C for stoneware and 1,300°C for porcelain. These characteristics have two corollaries: firstly the need for specific clays or mixtures that allow melting and more importantly kilns for reaching such temperatures. These two conditions were met in China as early as 1,000 BC for stoneware under the Shang dynasty in South China and towards 600 AD for porcelains in North China. 2.4.1. Stoneware Stoneware clays have particular characteristics. They are in general very siliceous, aluminous and contain rather significant proportions of potassium, which acts as flux. These clays have the property of vitrifying gradually with the rise in temperature without becoming deformed; they yield an opaque material, often brown in color, variable according to the impurities contained in the initial mixture. Archeologists have discovered in the Harappan sites of the Indus valley, dating back to the third millennium BC [VID 90] a specific production of “stoneware” bracelets. They were made up of very fine clays, fired according to very sophisticated processes, in a reducing atmosphere and inside special containers. This is not exactly stoneware in the meaning that this term has today, but it needs to be mentioned in this context. As we mentioned above, the first attested stoneware are Chinese artifacts dating from the Shang dynasty (1,500–1,050 BC). This stoneware is covered with a glaze very rich in calcium, composed of a mixture of sand and vegetal ashes. Later, ashes would be replaced by a substantial addition of mineral carbonates [WOO 99]. The stoneware tradition would last a long time in China and masterpieces would be created during the following centuries (celadons, Yue, etc.). Stoneware would appear 200 years later in Japan and Korea, but the first stoneware would be manufactured in Europe only in the 10th century. In France, the important regions of stoneware production relied on the presence of Sparnacian clays which were so plastic that a high content of quartz had to be added to avoid very high shrinkage. The glazes, fired at the same time as the paste, were of three types: sea salt glazes, which give a beautiful varnish (see Figure 2.8), blast-kiln slag glazes as in Puisaye and ash glazes. Right from its first appearance, stoneware became a huge success and its production has never ceased. History of Ceramics 45 Figure 2.8. Salt glazed stoneware vase. Martainvill (© O. Leconte) 2.4.2. Porcelains Porcelains were developed in China. As we mentioned earlier, the concurrence of several factors led the Chinese to develop these products: specific raw materials, mastery of firing conditions and the possibility of firing at high temperatures (Figure 2.9). China has numerous kaolin deposits, which were exploited very early on. These fireclays fire white. Depending on the geographical area in question, Northern or Southern China, the composition of these kaolins is a little different. In the North, clays were associated with coal deposits: they were rich in alumina (approximately 30%) and low in flux elements (alkaline, alkaline-earths) and iron. It was therefore necessary, in order to fire ceramics, to reach temperatures estimated at 1,200– 1,350°C [HAR 98]. In the South, on the other hand, kaolins resulted from the deterioration of igneous rocks and as a result they were enriched with flux elements; they could be fired at about 1,200°C. As early as the end of the Neolithic era, Chinese kilns were very sophisticated. The ovoid kilns of Jingdezhen are often cited. The sizes of these kilns, their firing chamber being in the form of an egg, made it possible to reach more than 1,350°C everywhere in the kiln. Temperature control, essential for performing the firing, was done by an ingenious system of windows. The fuel used was made up of small branches and pinewood [HUL 97]. The ceramics were placed in saggers, a kind of small refractory terra cotta boxes which insulated them and which also allowed better heat distribution. 46 Ceramic Materials Figure 2.9. White-blue ewer of Yuan era (1335) – inv. MA 5657 Guimet Museum (© C2RMF – photo: D. Bagault) In the North, the kilns were dug directly into the mountains, on the hillside, sometimes at more than 100 m [WOO 99] with a slope of about 15 to 20°. These “dragon kilns” were already extremely sophisticated, as early as the Song period. Firing started at the base of the kiln. The upper part then served as a pre-heating chamber for the ceramics that were placed there inside saggers. When the firing temperature was achieved in the lower zone, the chimney of the following zone was blocked using branches, in order for the heat to be propagated in this zone, and so on until it reached the top. It is obvious that this system resulted in many wasters, but it also made it possible to fire thousands of pieces in a single batch. The porcelains thus obtained are characterized by a vitrified paste which contains generally high mullite concentrations in microcrystals, mullite being derived from the high temperature treatment of kaolin. All these components (glass, microcrystals, bubbles) gave the much desired translucidity and hardness. 2.5. The quest for porcelains in the East and the West The arrival of Chinese porcelains of the Yuan period, first on the Islamic markets in the 9th century, then later on the European markets in the wake of the voyages of History of Ceramics 47 Marco Polo, triggered an unrestrained quest to uncover the secrets of this matter to which all virtues were attributed, even that of detecting poisoned substances [ROS 95]. Even if, at least initially, it was the esthetic qualities of porcelains that people sought the most: whiteness, translucidity, etc., soon their properties of hardness, resistance to thermal impact and also savings in terms of firing time gave an impetus to the research. Those who battled with the problem explored two essential directions: glass frit pastes and faience fine. 2.5.1. Siliceous pastes and glass frit pastes By refining the recipe of “archaeological earthenware” already known in the fourth millennium BC in Egypt and Mesopotamia, the artisans of the Islamic period added to variable quantities of plastic clays, quartz and glass frits a synthetic material made up primarily of sand and fluxes. This paste had a two-fold advantage: firstly an esthetic one, since it was very white and slightly translucent and then a technical one, since it expanded highly on heating just like the alkaline glazes that decorated it. These pastes were abundant as early as the 9th century AD and widespread in the entire Islamic and Hispano-Moorish world as support for luster or wall tiles. In the West, the first successes are attributed to the Italians in the time of Francesco de’ Medici. Under the impetus of the Renaissance artists and with the protection of the Grand Duke, artisans developed around 1570 a white paste, fired at 1,100°C, whose recipe was an ingenious mixture of Islamic siliceous pastes and Italian majolica traditions. In fact, the paste was made up of a “frit” (marzacotta) prepared with silica, wine dregs and various salts, crushed and added to a white clay enriched with quartz. The resulting paste was very white, porous and exhibited an important vitreous phase. A lead glaze covered a decoration drawn in cobalt blue. This glaze, slightly under- fired, produced an artificial effect of translucidity thanks to the combined effect of thousands of microbubbles, incompletely molten grains of quartz, feldspars and calcium phosphates. Only a small number of artifacts exist today and the production ceased after a few years. Soft-paste porcelain is one of the most beautiful achievements of the 18th century, especially in France and England. Designed on the same principle as the Medici porcelain, soft-paste porcelain combines translucidity and intricacy of shapes made possible by the smoothness and plasticity of the paste. A transparent lead glaze made the subtle combinations of colors possible, but it was fragile and easily 48 Ceramic Materials scratched; moreover, this porcelain was not very resistant to thermal shocks. Several workshops manufactured it, initially Rouen in 1673, then Saint-Cloud, Chantilly, etc. The Vincennes production [PRE 91], representative of the compositions of the French soft-paste porcelains, reveals the complexity of the pastes. A frit was prepared from saltpeter (KNO3), salt, alum, soda, gypsum and sand. After firing and fine crushing, chalk and marl from Argenteuil, a very plastic illitic clay, were added to it. The paste thus obtained was turned, molded or sculpted directly and then fired in oxidizing atmosphere in a kiln at approximately 1,100°C. The glaze was transparent, made up of lead, alkaline, sand and calcined flints. The decoration was very delicate to execute and the pallet of colors evolved progressively with discoveries of flux compositions and the development of continuous special kilns. Other types of soft-paste porcelains developed (Figure 2.10), particularly in England where the most famous were porcelains containing bone ashes, kaolin and Cornish stone (feldspathic rock used as flux). Figure 2.10. Bone porcelain of Minton 1872 (© O. Leconte) 2.5.2. Faience fine In this case, it was not translucidity, obtained by adding glass, that people were seeking. Through the working of the paste, they were looking for brilliancy, density and sonority, whiteness, as well as ease of mass productions. It was under the influence of the English (the most famous being Wedgwood) that this new type of History of Ceramics 49 production, faience fine, would be born, and then spread out primarily in the wake of the Industrial Revolution. Faience fine was characterized by an opaque paste made up of very fine plastic clay, mixed with flint, fine quartz and grog. The clay was often kaolin, used as white firing element associated with one or more plastic clays. A few feldspars or limestone played the role of fluxing and tempering agents. The glaze was transparent. The classification of soft-pastes was in fact based on the various compositions of the pastes [MUN 54]. In France, the main production centers were in Northern (Douai) and Eastern France (Lunéville) and then the Paris area (Montereau, Creil) where clay deposits were abundant and accessible [GIA 35]. It is important to stress that these products truly marked the beginning of mechanical processes in the preparation of the paste and the decorations and that the establishment of the first manufactures at least was contingent on the proximity of large deposits of coal or raw material in order to overcome constraints of economic profitability. This faience fine would be appreciated by the middle classes, in whose homes they would eventually replace traditional stanniferous faience. It would soon compete with porcelains. 2.5.3. The first veritable porcelains in Europe In 1709 Böttger, working in the Meissen manufacture in Germany, revealed that he had for the first time succeeded in recreating genuine porcelain, although he achieved this by using a recipe very different from the ones normally used which were based on kaolin. With this new method, these ceramics were fired at high temperatures; they were siliceous and aluminous of course, but contained, in the first stages of development, large quantities of calcium (nearly 5%) which conferred on the matter great resistance to thermal shocks. Later, when the recipes evolved after the death of Böttger, lime would be replaced by feldspars. In France, it was in Sèvres, under the aegis of the Academy of Sciences and under the impetus given by Macquer and de Dufour, that the first and the finest successes of hard-paste porcelain would be created. A. d’Albis recently published a work on the “conquest of porcelain in Sèvres” [ALB 99]. He describes in detail the atmosphere of competition, betrayals, and the ceaseless research of the chemists. Three great stages marked this research: the discovery of substantial kaolin deposits at St Yrieix, in 1767; a new model of cylindrical kiln with two superimposed chambers which were capable, in particular, of reaching an extreme phase of reduction necessary for the perfect whiteness and translucidity of the paste; and especially the development, from 1778, of a recipe of impeccable paste and glaze by adding feldspar to them in the form of pegmatite. This recipe has given the Sèvres manufacture an uncontested supremacy until today (Figure 2.11). 50 Ceramic Materials Figure 2.11. Vase of Sèvres – 19th century – Museum of Amiens (© C2RMF – photo: O. Leconte) 2.6. Conclusion: the beginnings of industrialization The Industrial Revolution would introduce into the field of ceramics radical and ceaseless changes during the entire 20th century, in the modes of preparation, manufacture, decorations, coloring and firing and especially with the use of the electric power, advances in chemistry and material sciences. New applications that take advantage of the resistance of the material to thermal shocks (insulators, heat shields, etc.) as well as the inalterability and harmlessness of bioceramics would be implemented. However, we can affirm that all the great traditions of ceramic art are still alive! A material that is so close, so flexible, so faithful will always express the innermost, the most immediate and the most elated preoccupations of man; thousands of examples spread across the entire history of humanity testify to this in such a striking manner. 2.7. 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