Historical Painting Techniques, Materials, and Studio Practice

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poured off each time; such a process is actually a washing process. Water­

soluble 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 fu nction of changing the texture and

the chemical composition of the material, supposing the point of departure

is soluble anhydrite.

Te chnical 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). Unfor­

tunately, this point was not addressed in the otherwise excellent 1954 ex­

amination 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

fo llowing compositions fo r the gesso grosso: 10 0% anhydrite or 75:25 an­

hydrite:dihydrate or 50:50 anhydrite:dihydrate. For the gesso sottile, two ra­

tios: 10 0% 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 fo rm, 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 con­

trolled burning process.

In the samples from Umbria, Latium, the Marches, Venice, and Ferrara show­

ing single-structured grounds, the dihydrate present (100%) is claimed to be

the natural unburned dihydrate (26). The raw gypsum (calcium sulfate di­

hydrate) 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 reconstruc­

tions 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 fa cilitate 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 fa cilitated by first

burning the calcium sulfate to an anhydrite, fo llowed by grinding and pro­

cessing with a final soak in water to produce the dihydrate fo rm. This is the

Historical Painting Techniques, Materials, and Studio Practice