Gypsum and glue ground preparation
The operative precepts are extensively described by Cennino Cennini ( The Book of the Art , late Fourteenth Century) who, specifying the various fundamental phases for a preparation with gypsum and glue, passes on customs typical of Mediterranean countries.
How to fasten linen on panels.
Having thus applied the size, get some linen
cloth, old, fine, and white, and free from all grease.
Take your best glue, cut or tear large or small strips
of this linen, soak these in the glue, and spread them valle
with your hands over the surface of the panel ;
remove the seams, and spread the strips out with
the palms of the hands, and leave them to dry for
two days. And remember it is best to use glue when
the weather is dry and windy. Glue is stronger in
the winter than in summer, and in winter gilding
must be done in damp and rainy weather.
How to lay grounds of gesso grosso
on the flat surface of a picture with a spatula.
When the panel is quite dry, take the point of a
knife like a file (inella, rasp?) which rasps well,
and search over the surface to find any little knots,
or any seams, and remove them. Then take some
gesso grosso, that is to say volteranno (plaster of
Paris) purified (purgato) and sifted like flour. Put a
porringer-full on the porphyry slab, grind it well
with this size by hand as you would grind colours,
collect it with a spatula, and put it on the surface
of the panel, and with a very smooth and rather
large spatula (stecca) cover the whole surface, and
whenever you can use the spatula, do so. Then
take some of this ground plaster (gesso), warm it,
take a soft hog’s-hair brush and give it a coat on
the cornices and foliage, as on the even surface
with the spatula. Give three or four coats on the
other parts of the cornices; but on the level parts
you cannot put on too much. Leave it to dry
for two or three days. Then take the iron rasp
(mesella) and scrape the flat surface; procure some
small iron tools, which are called raffiette, such as
you will find at the painters’ ; there are several kinds
of them. Pick out all the cornices and foliage if
not well done, that they may not be choked up, and
generally take care that all defects of the flat surface
or cornices are remedied by this grounding of plaster.
How to prepare gesso sot tile (slaked
plaster of Paris) for grounding panels.
You must now prepare a plaster for fine grounds,
called gesso sottile. This is made from the same
plaster as the last, but it must be well purified
(purgato), and kept moist in a large tub for at least
a month ; renew the water every day until it almost
rots, and is completely slaked, and all fiery heat
goes out of it, and it becomes as soft as silk.
Throw away the water, make it into cakes, and
let it dry; and this gesso is sold by the druggists
to our painters. 1 It is used for ‘grounding, for gilding,
for working in relief, and other fine works.
(1) To make sure that the plaster cannot set, about a gallon of water must be put to each pound of plaster. When ready, the water can be strained off through a tammy or linen sieve.
How a panel is grounded with gesso
sottile, and how it is to be tempered.
Having laid on the gesso grosso, rubbed it smooth,
and levelled it well and delicately, take some of this
gesso sottile, and put it cake by cake into a pipkin
of clean water, and let it absorb as much as it will.
Put it little by little on the porphyry slab, and with-
out adding any more water to it, grind it perfectly-
Put it then on a piece of linen cloth, strong and
white, and go on till you have as much as a loaf,
then fold it up in this cloth, and wring it well to
get as much water out as possible. When you
have ground as much of it as you want, for you
must consider what quantity you will want, that
you may neither have to make two portions of
tempered plaster, nor to throw away any good
plaster, take some of the same size with which you
tempered the gesso grosso. You must make sufficient
at one time to temper both kinds of gesso. The
gesso sottile requires less tempering than the gesso
grosso ; the reason ? that the gesso grosso is the
foundation of all your work, and you must also
reflect, that howsoever much you press the gesso
sottile a little water will still remain in it. For this
reason, diligently make the same size for both.
Take a new pipkin which is free from grease, and
if it is glazed, so much the better. Take a cake
of this gesso sottile and scrape it fine with a knife,
as you would cheese, and put it into the pipkin.
Put some of the size on it and work the gesso
with the hand, as you would a paste for making
fritters, smoothly and dexterously, so that it may
not froth at all. Have a cauldron of water, and
make it very hot, and put into it the pipkin ‘con-
taining the tempered gesso. This will keep the
gesso warm, and it will not boil ; for if it should
boil it would be spoiled. When it is warm, take
your panel, and with a large and very soft brush
of hog’s bristles, dip in the pipkin and take some in
moderate quantity, neither too much nor too little,
and spread it evenly over the level surfaces, the
cornices, and the carved foliage. It is true that in
doing this the first time you should spread and rub
the gesso with your fingers and hand, round and
round, and this will incorporate the gesso grosso
with the gesso sottile. When you have done this,
begin again, and lay on one coat with the brush
without rubbing it in with the hand. Let it rest
a little, but not so as to dry thoroughly; then go
over again in the other direction also with the brush,
and let it dry as usual, then give another coat in
the reverse direction; and in this manner, always
keeping your gesso warm, give the flat surfaces
eight coats at least. Foliage- and other reliefs do
with less, but you cannot put too much on the
flat. This is on account of the rasping or rubbing
down, which is done next.
How to prepare grounds of gesso sottile not having previously laid on a ground with
Small and delicate works may, as I told you before,
be glued two or three times, and then give them as
many coats of gesso sottile only as you find from
experience they will require.