Prékopa Ágnes (szerk.): Ars Decorativa 30. (Budapest, 2016)
Kornélia HAJTÓ: Zsolnay Pyrogranite: Tradition and Fact
dedicated worker with great experience and expertise. It was a highly risky operation and one badly-executed step could spoil a whole kilnful of ware. This caused Vilmos, in the early days, to take personal supervision of the firing. His daughter wrote: “The notebooks also contain records of the firings which he personally supervised, as well as the experiments and recipes. He wrote in the times of ignition and first test pulling, how much and for how long firing was needed, how much coal was burned, and the highest firing temperature. ”34 The ware was bricked up in the ring kiln, leaving only a small opening. Seger cones (see below) and various clay fragments were placed behind the opening, which was closed up with a brick before the kiln was fired up. After some time, the brick was removed to pull out a test shard, showing the state of firing, and adjustments were made accordingly. Jakab Mattya- sovszky, who supervised the manufacture of salt glazed pipes, made the following notebook entry about a firing: “Fired up at 6 pm on 18 December. Test pull at 11 am on 22 December: weak, pale. Second test pull at 4 pm, slightly bright fracture. After more fuelling, the first salting, then a pull at 9 pm. The test piece has a fine gloss and slightly glossy inside too. After consumption of 1000 kg of coal and 60 briquettes, last salting at 8.45. Kiln opening on morning of 30th, unpacking on 1 and 2 January. ”3> Each firing took two weeks. During this time, the kiln had to be heated up to the appropriate temperature at the correct rate and held at maximum temperature for exactly the right time. The atmosphere of the kiln had to be maintained in the appropriate oxidizing or reducing state. The fuel charge and the amount of air admitted influences whether combustion takes place in an excess or deficiency of oxygen, but the oxidizing/ reducing state can change accidentally, and must be monitored. The technique of firing ceramics in a reducing atmosphere has a long history in pottery.36 At lower oxygen content, the metallic atoms are in lower oxidation states and this can cause changes in colour and material properties.37 The kiln was brought to the right temperature using a set of Seger cones, each made of clay with a composition chosen to soften at a specific temperature (the peak of the cone collapses). The sequence in which they melted indicated the rise in temperature.38 The person in charge of the firing observed the cones through an opening. Test bodies were also placed in the kiln where they could be pulled out and examined to determine whether they had fired properly. Refractoriness is still given in terms of Seger cone numbers, with the letters SK (Seger Kegel). The temperatures given for firing temperatures nowadays are:-brick: 920-1000 °C- earthenware: 1100-1250 °C- stoneware, keramit: 1200-1350 °C- porcelain: 1250-1450 °C- refractory ceramics: 1300-1700 °C39 The refractoriness of clay refers to the maximum firing temperature it withstands without deformation. Clay of low refractoriness deforms when overheated, then melts as glass and flows. Firing clay to the 133
