Prékopa Ágnes (szerk.): Ars Decorativa 30. (Budapest, 2016)
Kornélia HAJTÓ: Zsolnay Pyrogranite: Tradition and Fact
Types of Pyrogranite Most authors classify Pyrogranite products as glazed, unglazed and salt glazed.41 The first two categories do indeed exist (some on the outside walls of the Museum of Applied Arts), but the term “salt glazed Pyrogranite” is a consequence of the misunderstandings outlined above. Salt glaze The term “glaze” refers any material that melts during firing and creates a dense waterproof layer on the surface of the ceramic. The use of salt glaze on stoneware is a technique that was developed in the Rhine country in the fifteenth century. The fire clay quarried in that region enabled the manufacture of dense “stoneware”. It was glazed by sprinkling salt (NaCl) into the combustion chamber during firing at the high temperature required to produce dense ceramic. The salt dissociates into sodium and chlorine. The chlorine reacts with the steam in the products of combustion to become hydrogen chloride gas (HC1) and departs with the fumes. The sodium, in the form of sodium oxide (Na20) deposits on the surface of the ceramic and acts as a flux, melting the top layer of the ceramic. The resulting glaze layer (a complex sodium aluminium silicate, varying between Na20 • 0.5Al2O3 • 2.8Si02 and Na20 • Al,Oj • 5.5Si0242) is actually fused stoneware. The salt glaze has a very strong chemical bond with the substrate, giving it a durability and resistance on a par with porcelain glaze (the material of porcelain glaze differs from the clay only in the addition of flux oxides so that the top layer fuses). Porous ceramic does not take up the salt glaze even at high temperature, because the gases required to produce the glaze diffuse into the pores.43 Dense stoneware is thus essential for the use of salt glaze. Stoneware is a dense fired ceramic and was used by Zsolnay to make salt- glazed water pipes and chimney elements. Since Pyrogranite is a porous product, it could not have been made with salt glaze. Plutonit is often mentioned as a version of Pyrogranite. Some see it as a precursor, and others exactly the opposite, a later development. Again, we encounter a confusion surrounding a Zsolnay product. Plutonit is stoneware that was fired at high temperature but not to full density. Its clay consisted of fire clay, relatively coarse chamotte and ground porcelain. It was made to replace stone statues and architectural terracotta and was not glazed. The grey or brownish tone of plutonit was influenced by the iron content of the clay, and the fly ash and flame contact in the course of firing. Pyrogranite is the improved version of plutonit, using similar ingredients but without the ground porcelain. It is made from fire clay and 30-40% ground chamotte of grain size 1-2 mm.44 Summary Pyrogranite was a successful product developed by the Zsolnay company for architectural ornaments. It used a chamotte clay mixture of higher refractoriness than ordinary clay. When fired with proper care, the resulting ceramic had a mechanical strength that enabled the product to withstand adverse weather. The final product is porous and of variable appearance. The published definitions of Pyrogranite contain many contradictions. It is difficult to untangle the threads of inconsistent 135
