60
70
60
50
40a 3500 3000
Transmittance I Wavenumber (cm-1)
80
b 3500 3000
Transmittance I Wavenumber (em-1)
60
40
2
c 3500 II 3000
Transmittance I Wavenumber (em-1)
2500 2000 1500 1000 500
2500 2000 1500 1000 500
(^2500 2000 1500 1000 500)
Figure 1. Fourier transform infrared spectra of calcium sulfa te Lilith d!fJerent amounts if crystal water:
(a) anhydrite; (b) hemihydrite, dihydrate baked at 135 °Cfor two hOLm; (c) dihydrite.
It is even possible, as suggested by Gettens and Mrose, that if the lumps were
fairly large, that the outside would have been burned to anhydrite while the
inside would be only partially dehydrated (17). Some of the inside of the
gypsum lumps in the primitive kiln stack could have been heated below 100
°C so that the gypsum remained hydrated. It is interesting to note here that
Theophilus gives the fo llowing instructions: " ... take some gypsum burned
in the fashion of lime .... " which must be understood to have been burned
at very high temperatures, that is, above 900 °C (18). The resulting substance
will be quicklime (calcium oxide), which is slaked in water and transformed
into lime (calcium hydroxide).
Gettens describes another kind of kiln that worked in the same way as ancient
bread-baking ovens; after a fire had preheated the inside of the brick oven
until the interior was red hot, the fire was extinguished, the coals removed,
and the bread-or, in this case, lumps of gypsum-were placed in the oven
overnight. This kind of operation allowed fo r a more uniform heating. This
Historical Painting Tech niques, Materials, and Studio Practice