Prékopa Ágnes (szerk.): Ars Decorativa 30. (Budapest, 2016)
Kornélia HAJTÓ: Zsolnay Pyrogranite: Tradition and Fact
terracotta or glazed tiles. Here we investigate what lies behind this erroneous but long-held belief. The roof ornaments on the Museum of Applied Arts have gone through many repairs and replacements since the building was completed in 1896. The replacements made at various times are visibly different from the originals, even though all of them were made by Zsolnay. (Fig. 2) One striking difference is in the colour of the ceramic itself: the original is red, while later copies are white. There are two fundamental factors that determine the properties of ceramic products. One is the composition of the raw material, and the other is the manufacturing technology, above all the firing. We give a simple overview of these here. Raw material There is a wide variety in the composition of the clay from which ceramics are made. Clay forms by the degradation of feldspar- containing rock. Its composition is firstly primarily determined by the parent rock, which may be sodium feldspar (NaAl- Si3Og), potassium feldspar (KAlSi3Og) or calcium feldspar (CaAl2Si208). Degradation occurs through the action of atmospheric moisture and carbon dioxide. The other main factor affecting the properties and usefulness of clay as a raw material is the route the degraded feldspar takes to the deposit site. Deposits of clay in the purest form, kaolin (Al203-2Si0,-2H20), occur where the degradation product remains at the point of formation and does not mix with other material. This is the raw material of porcelain. Usually, however, the degraded rock is carried away by water and deposited at a remote secondary spot. On the way, some components of the clay are washed out and foreign components are added, causing changes in grain size and composition. A clay deposit may also contain undegraded or partially transformed components of the parent rock. These factors all take effect on the properties of the eventual ceramic, and particularly variations in refractoriness and melting point make clays suitable for different products. Refractory clays have a similar chemical composition to that of kaolin, but with some foreign minerals mixed in. Their melting point is above 1580 °C. Their colour after firing depends on the proportion of added iron oxides and the firing temperature and varies from white through yellowish brown to red. Earthenware clays fire to yellow or red, and soften at temperatures less than 1350 °C (some as low as 1100 °C). There is no sharp dividing line between the two categories, but they serve as an easy way of classifying products. Obtaining the right raw material is therefore clearly of primary importance in ceramics manufacture. The addition of tempering materials to the clay (such as sand or ground-up fired ceramic, known as chamotte) is particularly important in the manufacture of large ceramic products. These degrade the elasticity of the clay but greatly reduce the incidence of faults as it shrinks during drying and firing. The type and grain size of the tempering material also affects the properties of the finished product.3 Natural clays already contain some tempering material, so that their elasticity is highly variable. Vilmos Zsolnay was quick to recognize the potential of clays of different qualities. This is clear from his notebooks. His daughter, Teréz Zsolnay, described a notebook which he started on 1 May 1866: 119
