2012 NCECA Journal 2012: Volume 33

Page 58

We were able to map the firing temperature of the Koryǒ body through measurements of the glass chemistry in the fired body, so we were relatively certain that the soaking temperature was 1250°C (2280°F), or the modern equivalent of about Cone 7.3 (Based on the soak time later determined via body-glaze interaction, it is expected this was likely equivalent to Cone 12 or 13.) Our only other variable that would explain such a deep body-glaze interaction was time. To model the effect of time on body-glaze interactions, preliminary experiments were conducted to identify a suitable marker by introduction into either the body or the glaze (or at the original body-glaze interface). Zircon introduced in the body (10% by weight) provided the best chemical marker as it is marginally soluble in the glaze, insoluble in the body, and the particles are small enough to be unlikely to migrate (via settling due to gravity) during firing.4 The glaze was a standard pottery glaze, commonly known as “4321” and was applied to a slip cast commercial body (Porcelain for the People, Matt and Dave’s Clays, Alfred, NY) containing the zircon addition. Both bisque and green tiles were glazed then fired (for twice- and single-fire penetration measurements) at 1300°C for 1, 2, 4, 8, 16, 32, and 64 hours. After firing, the glazed tiles were sectioned perpendicular to the glazed surface, polished, then mapped using WDS (JXA-820OF Electron Probe Microanalyzer, JEOL, Ltd, Tokyo, Japan). The images clearly showed the original body-glaze interface (by the abrupt lack of zirconium) and indicated that the glaze-body interactions followed a log-log relationship

What this plot clearly shows is that in order to obtain the 200 µm penetration depth measured on the Koryǒ Celadon, the piece would have needed to be fired for approximately 96 hours (4 days) at temperature. Some other ancient Korean Celadons exhibited a smaller penetration depth indicative of a two day soak.

Using this Information to Calculate the Glaze Chemistry Shift Based on the ratio of the glaze to the body (after firing) the chemistry shift associated with body dissolution into the glaze can be calculated. For example, if the glaze application thickness was 100 µm and the overall glaze thickness was 200 µm, the final glaze chemistry would be a 50-50 mixture of the body and the glaze. The change in chemistry can be calculated using what is commonly known as a “Rule of Mixture” or ROM. This is illustrated schematically in the graphic below:

Table I. Chemical analysis of Korean Celadon glazes and bodies.5 SiO2 Al2O3 TiO2 FeO Na2O K2O MgO Glazes 57.6 12.4 0.1 2.1 0.7 2.8 4.2 58.1 13.9 0.2 1.4 0.5 2.9 1.8 59.6 14.1 0.1 1.4 0.8 3.8 2.7 Bodies 76.0 17.0 0.8 2.1 0.7 2.5 0.5 73.0 17.5 0.9 2.8 0.8 2.6 0.7 73.0 18.0 1.2 2.5 0.9 3.4 0.5

of penetration depth with time, as illustrated above. Somewhat surprisingly, the data also clearly indicates there is little, if any, difference between single-fire and twice-fire body-glaze interactions with respect to penetration depth. In the case of the ancient Koryǒ Celadon, the original bodyglaze interface would need to be identified. When analyzing the WDS chemistry maps in the body-glaze interaction region, it became evident that there were mullite (3Al2O3-2SiO2) particles imbedded in the glaze. Mullite is a common precipitated phase in a fired body, but it is not possible to crystallize mullite from the glaze during firing (based on the glaze chemistry). Also, Koryǒ Celadons were single-fired systems meaning that mullite would not be a viable crystallization product in the interfacial regions. Therefore, the only way for mullite to be present in the glaze would be if fired body had been crushed after firing and used as grog for new body. Twelfth-century Koreans practiced recycling. Figure 2 shows the mullite grains imbedded in the glaze layer and can be used to identify the original body-glaze interface, thus indicating a 200 µm glaze-body interaction zone.

CaO 17.7 19.9 16.0 0.3 0.2 0.5

Table II. Korean Celadon glazes and bodies in a Seger format using the average compositions of the three compositions listed in Table I. SiO2 Al2O3 TiO2 FeO Na2O K2O MgO CaO Glaze 2.24 0.31 0.00 0.05 0.02 0.08 0.16 0.73 Body 19.8 2.7 0.19 0.49 0.21 0.48 0.23 0.09

Previously published body and glaze chemistries are listed in Table I.5 These chemistries are converted to a Seger format in Table II. Recently measured values are similar. Since Koryǒ Celadons are single-fired systems, we can simply subtract a proportional level of body from the as-fired glaze chemistry to obtain the initial glaze chemistry. If this calculated glaze was applied to a similar body and fired at a similar temperature for a similar time frame (4 days at temperature) a similar glaze character would be obtained. However, modern firings are significantly shorter, usually on the order of 3-4 hours, so if this glaze-body pair were fired using a modern cycle, the glaze would still not be the same due to a significant reduction in dissolved body. Therefore, the glaze for use in modern cycles would need to be modified to accommodate the reduction in bodyglaze interaction.

nceca 57 Journal 2012 • Panels

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