Prékopa Ágnes (szerk.): Ars Decorativa 30. (Budapest, 2016)
Kornélia HAJTÓ: Zsolnay Pyrogranite: Tradition and Fact
Czechoslovakia. The iron content is high in Zágor clay and low in Skalná clay. Pe- tény clay has intermediate iron content, and can be made into darker coloured Pyrogranite with the addition of iron- containing clay. Before use, the clay has to be ground fine or sedimented. It is tempered with 20—40% grog (refractory clay fired to shrinking point and ground) of grain size of 1-2 mm. The chamotte must be intimately mixed with the clay. If the clay is not processed by sedimentation but only ground in the dry state, the mixture must be soaked for at least a week and then put through a pug mill, rested for at least another week and then put through the pug again. ”31 Besides simplicity of manufacture, an advantage of porous architectural ceramics was their better adhesion to cementitious lime mortar, making the pieces easier to fix in place on the façade. Tamás Mattyasovszky-Zsolnay, second great-grandson of Vilmos, graduated as a mechanical engineer. He worked in the Zsolnay factory in Zugló and eventually went into retirement as research fellow of the Central Silicate Industry Research Institute, where he passionately researched buildings clad with Zsolnay ceramics. He characterised Pyrogranite as follows: “The outcome of experiments performed over the years is fire clay terracotta with low water absorption, non-conchoidal fracture, high weather resistance and higher refractoriness than ordinary ceramic. ”32 The “low” water absorbency was in fact 10-15%, an unusual categorization considering that the figure for ordinary pottery is 16-17%. Firing the clay We have seen the importance of raw material composition for the properties of ceramic products. The clay has to be quarried, graded, cleaned, sedimented and then formed. The forming process may involve hand modelling, mechanical pressing, moulding, press moulding, hand throwing, mould throwing and jiggering. Architectural ceramics were formed by pressing the clay into plaster moulds and then manually inserting stiffening ribs. The technique remains the same today. The moulded clay is then dried and fired. Firing induces a complex process of metamorphosis that makes the material strong and durable and unsusceptible to softening when it absorbs moisture. “Firing is the stage in the manufacture of ceramic products which converts shaped raw clay into actual ceramic. The processes taking place in the clay as it is fired -previously thought to be purely chemical - include solid state reactions among the components, disintegration of crystalline material, formation of new crystals, melting, shrinking, high temperature melting, and internal modification of components. These changes can be influenced by controlling the firing - rate of heating, firing temperature, duration at highest temperature and rate of cooling. The kiln atmosphere also plays a major role. ”33 The properties of the final product are therefore dependent on the firing technique. In the nineteenth century, heating and handling the kiln was the province of a 132
