Prékopa Ágnes (szerk.): Ars Decorativa 30. (Budapest, 2016)
Kornélia HAJTÓ: Zsolnay Pyrogranite: Tradition and Fact
fired products such as porcelain and stoneware. The last is made up of fine-pored products, whose pores rapidly take up water when immersed, and when the water freezes, the ice is prevented from sliding out by the high friction in the pores and inevitably cracks the material. Earthenware is an example of such material. The most interesting category is that of potentially frost-resistant ceramics, the best-known examples being roof tiles and pillar bricks. Non-ceramic materials with similar properties include concrete and many porous natural rocks. These materials are frost resistant in practice, because despite their pores, they can withstand multiple freezing even when saturated with water. A building material is usually classed as frost resistant if, when saturated with water, it can withstand being frozen to -20°C and defrosted for twenty-five cycles without damage. Frost resistance depends on the size and shape of the pores as well as their number, and on the strength and elasticity of the material. Ice is malleable, and as it expands upon freezing, the increasing pressure squeezes it out of large, smooth- walled pores. If the material is sufficiently strong to withstand the pressure required to press out the ice, then it will not crack. [...] We can enhance the frost resistance of porous material by a) raising the fracture strength of the material, b) raising the elasticity of the material, such as with a closed pore structure, c) reducing the number of small open pores, d) creating coarse pores to allow the ice to be pressed out even from inner open small pores. The importance of allowing the ice to be pressed out becomes clear when we glaze an ordinary roof tile or pillar brick. Such a tile or brick cracks apart in frost. The glaze does not prevent moisture ingress, but does prevent the exit of ice. Glazed ceramics will only be frost resistant if they meet the above criteria. Pyrogranite is not fired to as dense a state as porcelain, whose fine, compact clay requires much more complicated forming, drying and firing processes than are involved in manufacturing well- proven coarse-tempered ceramic. The firing temperature, however, must be raised to the point that the clay binder fuses with the fine components of the temper and no fine open pores remain. Higher firing also increases mechanical strength. Strength requires the even forming of pieces, without internal flaws or gaps; they must be free of mechanical stresses during drying and firing. ” The aim of using fire clays for architectural ceramic pieces was not to produce dense ceramic but to enable a higher firing temperature that would give the product sufficient mechanical strength. Mattyasovszky-Zsolnay wrote on the composition of Pyrogranite clay: “Pyrogranite clay is similar to the raw material of stoneware for the chemical industry - fire clay containing fine quartz. For unglazed and salt-glazed Pyrogranite, we use clay with 2-3% iron oxide, which fires to a dark colour. For glazed Pyrogranite, we use clay with 0.5-1.2% iron oxide, which fires to a light colour. Old Pyrogranite was made with clay whose location was given as “Zágor” in Croatia and another clay from Skalná in 131
