Historical Painting Techniques, Materials, and Studio Practice

poured off each time; such a process is actually a washing process. Watersoluble
impurities, such as salts, possibly present in the gypsum would be
washed away in this process. Such impurities might cause discoloration of the
ground or efflorescence of salts from it. Apart from this, soaking the burned
gypsum in water obviously also had the function of changing the texture and
the chemical composition of the material, supposing the point of departure
is soluble anhydrite.
Technical evaluation and comparison of results
The recent examination of grounds in Italian paintings by the Laboratoire
de Recherches des Musees de France elaborates on the double structure of
the Italian gesso grounds (22). This double structure was also shown in the
examinations by the National Gallery's laboratories in London (23). Unfortunately,
this point was not addressed in the otherwise excellent 1954 examination
by Gettens and Mrose, which makes their results of somewhat
limited value in this context.
Concluding from the results of examinations of grounds, the gesso grosso
consists of mainly anhydrite, sometimes with dihydrate present. I am here
referring to the French examination, which even states the ratio of anhydrite
to dihydrate (24). The numerous Florentine and Sienese examples show the
following compositions for the gesso grosso: 100% anhydrite or 75:25 anhydrite:dihydrate
or 50:50 anhydrite:dihydrate. For the gesso sottile, two ratios:
100% dihydrate or dihydrate containing 25% anhydrite.
Considering the chemical changes of soluble anhydrite in contact with water
(the binding medium of the ground is animal glue, which always contains a
certain amount of water), it seems puzzling that the gypsum in the layers of
gesso grosso kept its anhydrous fo rm. A possible explanation could be that
the conversion process became slower as the burning temperature increased
(25). As the anhydrite did not change into dihydrate, the evaporation time of
the water in the binding medium must have been shorter than the hydration
time of the anhydrite. It would be extremely improbable that the gesso grosso
material would be a stable anhydrite, which is not able to react with water,
or a dead-burned anhydrite, which is neither form, because the examples
concerned show the double-structured grounds of gesso grosso and gesso
sottile. It would be very unlikely that two different kinds of gypsum would
be employed in the process. The analyses of the sottile layers show dihydrate,
which is the soluble anhydrite soaked in water. It must be assumed that the
point of departure in both the grosso and the sottile is the same compound:
soluble anhydrite. The presence in the gesso grosso layers of a mixture of
anhydrite and dihydrate would be explained by the sometimes poorly controlled
burning process.
In the samples from Umbria, Latium, the Marches, Venice, and Ferrara showing
single-structured grounds, the dihydrate present (100%) is claimed to be
the natural unburned dihydrate (26). The raw gypsum (calcium sulfate dihydrate)
does not differ either in chemical composition or crystal structure
from a dihydrate that has gone through the process applied to a gesso sottile
of burning and soaking in water (27).
The question remains, is it possible that the dihydrate present in the examples
mentioned above could be the processed dihydrate? The author's reconstructions
indicate that the raw calcium sulfate dihydrate can only with great
difficulty be triturated to a degree that will make it usable as a ground. Even
then, the resulting surface will not facilitate a satisfactory base fo r gilding. It
must be admitted, however, that raw gypsum can vary considerably with
regard to texture (28). Considering this, we can exclude the possibility that
raw gypsum could have been pulverized to yield a satisfactory product. It
seems likely that in most cases the process would be greatly facilitated by first
burning the calcium sulfate to an anhydrite, followed by grinding and processing
with a final soak in water to produce the dihydrate form. This is the
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Historical Painting Techniques, Materials, and Studio Practice