Historical Painting Techniques, Materials, and Studio Practice
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a 3500 3000<br />
Transmittance I Wavenumber (cm-1)<br />
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Transmittance I Wavenumber (em-1)<br />
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c 3500<br />
Transmittance I<br />
Wavenumber<br />
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Figure 1. Fourier transform infrared spectra of calcium sulfa te Lilith d!fJerent amounts if crystal water:<br />
(a) anhydrite; (b) hemihydrite, dihydrate baked at 135 °Cfor two hOLm; (c) dihydrite.<br />
It is even possible, as suggested by Gettens <strong>and</strong> Mrose, that if the lumps were<br />
fairly large, that the outside would have been burned to anhydrite while the<br />
inside would be only partially dehydrated (17). Some of the inside of the<br />
gypsum lumps in the primitive kiln stack could have been heated below 100<br />
°C so that the gypsum remained hydrated. It is interesting to note here that<br />
Theophilus gives the following instructions: " ... take some gypsum burned<br />
in the fashion of lime ...." which must be understood to have been burned<br />
at very high temperatures, that is, above 900 °C (18). The resulting substance<br />
will be quicklime (calcium oxide), which is slaked in water <strong>and</strong> transformed<br />
into lime (calcium hydroxide).<br />
Gettens describes another kind of kiln that worked in the same way as ancient<br />
bread-baking ovens; after a fire had preheated the inside of the brick oven<br />
until the interior was red hot, the fire was extinguished, the coals removed,<br />
<strong>and</strong> the bread-or, in this case, lumps of gypsum-were placed in the oven<br />
overnight. This kind of operation allowed for a more uniform heating. This<br />
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<strong>Historical</strong> <strong>Painting</strong> <strong>Techniques</strong>, <strong>Materials</strong>, <strong>and</strong> <strong>Studio</strong> <strong>Practice</strong>