Historical Painting Techniques, Materials, and Studio Practice

that is, plaster of Paris, which has been purified and sifted like flour" (8, 9).
The Swedish translation says: "Tag sedan grovgips, det vill saga Volterra-gips,
som er renad och siktad som mj ol. ..." [Then take coarse gypsum, that is to
say Volterra gypsum that has been purified and sifted like flour. ... J
The question is, apart from the obvious fact that the gypsum mentioned was
mined in the quarries at Volterra, what was then understood by this statement?
Initial attempts by the author at reconstructions were based on
Thompson's translation. The use of his "plaster of Paris," a gypsum burned
to the hemihydrate fo rm, resulted in immediate setting in water (10).
The next chapter, which describes the preparation of the gesso sottile, reveals
details about the gesso volteriano not mentioned in Chapter CXV Chapter
CXVI says, "Ora si uuole chettu abbi dun giesso elquale sichiama giesso
sottile elquale e diques to medesimo giesso mae purghato perbene unmese
tenuto in molle innun mastello rinuoua ogni di laqua chesquasi siinarsiscie
edesciene fuori ogni fo chor di fuocho e uiene morbido chome seta ...." (11).
In Lindberg's translation: "Nu will man att du skall ha en gips som man kallar
fingips, vilken besrar av denna samma gips, men den ar renad i gott och val
en manad, lagd i blot i en balja. Byt varje dag vattnet tills den ar nastan torr,
och varje glad av eld gar ut den, och den blir mj uk som silke ...." [Now
you should have a gesso, which is called fine gesso, which is this same gesso,
but it has been purified for a good month, soaked in a basin. Change the
water every day until it is almost dry, and every glow of fire leaves it, and it
will be soft as silk ....
J
There are two things to be noticed here: Lindberg argues that the remark
about the glow of fire leaving the gypsum-the same gypsum that was used
for the gesso grosso-can only be understood in the sense that the gypsum
was indeed burned. The question is, what form resulted from the process?
The remark about the soaking in water constitutes another important point:
What kind of procedure is meant? The existing translations are not very clear
about this point. It seems quite conceivable that a double purpose-a washing
process and a process of changing the morphology and chemical composition
of the material-was served by this treatment.
The preparation of gesso grounds
The naturally occurring gypsum, calcium sulfate dihydrate CaS04'2H20, can
be burned at various temperatures. Burning at 128 °C produces CaS04'H20,
the hemihydrate form. Burning at 130-160 °c creates an anhydrite and hemihydrate
mixture (12). This is the so-called plaster of Paris or stucco plaster,
which sets quickly with water and thus returns to the dihydrate form. Between
163 °C and 300 DC, soluble anhydrite, CaS04, is formed, which also
reacts quickly with water. According to Mora, et aI., the dihydrate form will,
at temperatures above 250 °C, turn into insoluble anhydrite that is no longer
able to set with water (13). Experiments at the School of Conservation have
shown, however, that anhydrite burned at 300 °C, 400 DC, and even 500 °C
is still able to react with water, forming dihydrate again (Figs. 1 , 2) (14). The
Merck Index even gives 650 °C as the limit above which the insoluble anhydrite
is formed (15). Above 900 DC, the so-called Estrich gypsum (a combination
of anhydrite and calcium oxide) is fo rmed; this material sets very
slowly with water and becomes extremely hard. Gypsum can also occur in
nature as insoluble anhydrite, CaS04.
In Cennini's time, the mined gypsum was burned in rather primitive kilns.
Gettens describes a very ancient kiln with little temperature control, in which
blocks of gypsum are stacked and a fire lighted at the base of the kiln (16).
The result of this process must have been a mixture of anhydrite, hemihydrate,
and even dihydrate forms. Overburned or dead-burned insoluble anhydrite
lumps must have been prevalent at the bottom of the stack near the source
of heat. In the upper tiers, the lumps burned at lower temperatures, hemihydrate/
anhydrite forms must have been present in larger relative amounts.
Federspiel 59