–→ Table of Contents 30 Introduction d) Excess material protruding above upper edge of mold is scraped off Most of the information in this section is based on interviews that were conducted with former porcelain craftsmen in the late 20th century. Some of these interviews still exist as audio material or in written form. The in-depth discussions that were recorded with Gerd Leib, former chief modeler of the Porzellanmanufactur Schierholz Plaue, and with specialists from other porcelain factories, proved to be especially informative and full of useful details. These interviews made it clear that the individual manufacturers used quite different production methods. Preparing the porcelain body The classic body used in the production of porcelain lithophanes is made from a mixture of the naturally occurring materials china clay (kaolin), feldspar, and soapstone. The first steps of the body preparation involve the elutriation of the china clay and the crushing and fine grinding of the feldspar and soapstone. The resulting mixture requires thorough cleaning. Sieves remove coarse impurities, and magnets pull out any remaining particles of iron. The final cleaning is performed by pumping the slurry, which can contain up to seventy percent water, into a filter press to remove the entrapped air and any remaining ultra-fine impurities. This process also reduces the moisture content to approx. twenty percent. The result is a plastic body with good workability. In the days when manufacturers were still producing their own mixtures, the additional “aging” played a significant role. This process of allowing the organic material in the clay to break down by storing it for a long period of time improved the plasticity of the body, making it easier to shape. The porcelain body used in the production of lithophanes can be either a dense and highly plastic clay (for hand-pressing), with a water content of 24 to 27 percent, or a slip (for pouring) with a water content of 34 to 37 percent. Before the body was used, a craftsman known as a “clay wedger” would pound the block of clay, cut it in half, slam the pieces back together, and then pound it again. This process was repeated for several hours. The resulting high density of the clay was associated with a high degree of homogeneity, which made the body especially well suited for use in the production of lithophanes. However, today’s industry requires a level of efficiency that no longer allows for such time-consuming manual labor, and the processes have since been largely simplified and automated. Many companies buy ready-made porcelain bodies on the international market that have been pre-mixed in accordance with their specifications. The bodies can then be modified by the respective factories, i.e. adapted to their special needs. Today, France (Limoges) and China are important suppliers of these raw materials. When G. F. C. Frick from KPM Berlin had the idea for the production of lithophanes, he also spent several years developing an especially wellsuited porcelain body, the Neue Berliner Lichtschirm-Masse (literally “new light-screen body from Berlin”), which was a biscuit material with an exceptionally fine-grained structure and a high degree of translucency. The biscuit porcelain body that had already been in use since the 18th century had a high percentage of feldspar and a low quartz content. It was well suited for the production of coins, medallions, figures, vessels, and other pieces, as well as for making ornamentation for cups, vases, etc., but did not satisfy the new demands. Frick was the first to successfully adapt this body for the casting of sufficiently thin pieces that would not crack during firing. According to the surviving documentation, the new recipe (arcanum) had the following composition: “930 pounds of finely washed (geschlämmte) Sennewitz kaolin (= 65.90%),64 420 pounds of finely ground and washed feldspar (= 29.76%), and most importantly 61.25 pounds of finely ground and washed soapstone (= 4.34%).”65 Although the basic recipe did not remain a secret for long, no competitor was able to match the quality of the KPM body because Frick apparently never revealed the true arcanum. The Chinese already knew about the silky sheen and translucency that could be achieved by adding soapstone (talc, steatite) to the body. Bourgoing reported having used an additional platinum bath (one gram of platinum to 20 liters of water) “to achieve a beautiful translucency.”66 In Fig. 26 a–h Lithophane production process Seltmann Weiden (Schierholz Plaue). The first photo shows wax carver Martina Zapfe 1998/99 (photo by Seltmann Weiden) a) The wax model is produced b) One-piece plaster casting molds c) The porcelain slip is poured in
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