FRESCO-BUONO PROCESS--COLOURS--PREPARATION OF THE WALL--METHOD OF
EXECUTION
Fresco-buono, or true fresco, is so called because the painting is
executed “a fresco,” that is, directly on the fresh, or wet plaster,
which forms the painting surface of the wall. This plaster, or mortar,
is a mixture of lime and sand, and the colours used in the painting are
such as will remain unchanged in hue when in contact or in mixture with
the lime. When all necessary details relating to the method of procedure
connected with the work are carefully carried out, true fresco paintings
may be said to have a higher degree of permanence and durability than
those executed by any other method or medium. The fine surface texture
and luminous quality of buon fresco pre-eminently distinguishes it as
the most beautiful colour finish for mural decoration.
The great permanence of fresco paintings is due to the formation of
carbonates, and sometimes silicates, of lime on the surface of the
plaster, which takes place during the drying of the latter. The
carbonic acid contained in the natural limestone is driven out of it by
the process of burning in the kiln, and after the burnt lime has been
slaked into lime “putty,” it has then become what is known as a hydrate
of lime. As long as the plaster is wet on the wall the lime in the
plaster exists in the state of a hydrate, and the applied colours of the
painting become saturated with this form of lime; but during the process
of drying, the lime on the surface, and to a slight depth below the
surface of the plaster, rapidly absorbs carbonic acid gas from the
atmosphere, and becomes a carbonate of lime, which is formed as a hard
and crystalline skin, or surface covering, under which the colours are
locked up, and so protected from any atmospheric influences. In addition
to this carbonate surface a silicate of lime is sometimes formed,
especially when in the painting some of the more earthy colours are used
which may have silica in their composition. These thin coverings of
carbonates and silicates of lime render fresco paintings impervious to
wet or damp on the surface, so that they may be occasionally washed
without injury; but bad air, such as sulphuretted hydrogen gases, or the
sulphur products given off by gas and coal combustion, will in time
convert the carbonate covering into a sulphate of lime, a substance
which disintegrates and destroys, not only the colours, but the plaster
surface also. This would suggest that in buildings which contain fresco
paintings coal fires and gas should not be used for heating and
lighting purposes, and also that the buildings should be properly
ventilated.
PREPARATION OF THE WALL IN BUON-FRESCO:
The wall on which a work in buon-fresco is to be painted should be of
good brick, or if a stone wall, it should be lined with brick on its
inner face, or, better still, it should be what is known as a hollow
brick wall--that is, one having an air-space between the outer and inner
linings. The joints between the bricks should be scored out, so as to
leave a key for the first coating of rough plaster; this should consist
of old lime and coarse, gritty, well-washed river sand, mixed with
ox-hair, or white asbestos cut into small bits, so as to bind the
mixture more effectually. For this first coating of plaster the usual
proportions are one part of lime and two parts of the gritty sand; it
should be about three-quarters of an inch in thickness. This coating
should have a roughened surface, made by scratching it with a
coarse-toothed kind of wooden comb, and should be left for the best part
of a year, so that it may thoroughly harden, before it receives the
second, and last, coat of plaster, that on which the painting is to be
executed, which is known as the _intonaco_. This plaster mixture must be
prepared with great care, and more than sufficient to cover the whole
wall space should be made before beginning the painting, so as to get
the whole surface evenly tempered, and of the same mixture; for if
different mixtures were made, and at different times, the lime and sand
might vary in proportions, which would possibly affect the colours of
the painting, and prevent them drying uniformly, as to tint or tone.
The proportions of lime and sand for the intonaco is, one part of lime
and three parts of fine and well-washed river sand. Pit sand must not be
used, on account of the clay and earthy matter which it contains; nor
sea sand, which of course contains a certain amount of salt that would
attract damp and cause the lime to perish. The lime must be well slaked,
and must be fairly old in the “putty” state, so that there may be no
fear of its blistering and blowing off here and there in round flakes on
the finished surface, which it will do if it is too new or not properly
slaked, even six or eight months after the plaster has been spread on
the wall.
The method of preparing lime for fresco work is, first to select the
best white variety which has been properly calcined. The lime is put
into a large wooden trough with sufficient clean water to slake and
dissolve it into a thin creamy consistency. The mixture is then strained
through a fine sieve into a brick-lined pit, roofed over to keep out the
wet and dust. A thick coating of clean river sand should be put over the
lime when it has cooled down and thickened into a paste, and has become
what is known as “lime putty.”
There are many tales and legends concerning the extreme old age of lime
putty before it has been used in the plaster of wall surfaces, or as a
painting material in fresco.
It has been stated that for some of the Italian frescos the lime used
was eighty years old. As a matter of fact it is quite ripe for use about
one year after it has been slaked. The lime used by Sir Edward J.
Poynter in his fresco in St. Stephen’s Church at Dulwich was about two
years old, and that used in the Houses of Parliament frescos was three
years. The quality of the lime, however, is of more consequence than its
age. The common grey lime used by the London builders should be avoided;
the best English variety is the pure white limestone, which is quarried
and burnt near Buxton in Derbyshire.
Before describing the method of laying on the last plaster ground, or
intonaco, a few words must be said about the preparation of the design
or subject to be painted. A finished coloured drawing or painting of the
design, to a smaller, or even to the same scale as the fresco painting,
must be prepared, and the artist should endeavour to make an exact copy
of this on the plaster surface, as there is no time to experiment in
colour schemes, or to make alterations from the original coloured design
in the short period that is at the disposal of the painter when he is at
work on the wet plaster. If the colour sketch of the work is prepared on
a small scale it will be also necessary that a cartoon in light and
shade drawing should be made to the full scale of the fresco painting,
before beginning to paint on the wall.
As the colours, and everything else that may be required, must be in
readiness, and close at hand before laying on the plaster ground and
before commencing the painting, it will be as well to indicate here the
proper colours which may be safely used on the lime plaster.
Although the palette in buon-fresco painting is very simple and
restricted as to the number of the colours which may be trusted to
withstand the caustic action of the lime, at the same time a fairly rich
and luminous colour scheme may be obtained, notwithstanding the limited
range of the palette. The following list may be safely relied upon:--
WHITE. Lime white (_hydrate of lime_).
YELLOW. Raw sienna (_a ferruginous earth_).
“ . Cadmium yellow (_cadmium sulphide_).
RED. Vermilion (_sulphide of mercury_).
“ . Light red (_calcined Oxford ochre_).
“ . Indian red (_ferric peroxide_).
BLUE. Cobalt blue (_phosphate of cobalt and alumina_).
GREEN. Oxide of chromium (_anhydrous sesquioxide of chromium_).
“ . Emerald oxide of chromium (_hydrated oxide of chromium and
borax_).
“ . Cobalt green (_oxides of cobalt and zinc_).
ORANGE. Burnt sienna (_raw sienna calcined_).
BROWN. Raw umber (_oxides of iron, manganese, and clay_).
“ . Burnt umber (_raw umber calcined_).
BLACK. Ivory black (_charred bones_).
There are other pigments that might be used, but are really not
necessary, and some of them are doubtful as to their permanence. One
colour, not in our list, is the genuine ultramarine, a splendid blue,
and thoroughly permanent in buon-fresco, but its great price prohibits
its use, except in very small quantities, or on small portions of the
work. French, or factitious, ultramarine is also very permanent, and
withstands the action of the lime, but at the same time it is a most
harsh and disagreeable colour when used in lime or in any other kind of
fresco; when used alone it destroys by its intensity of hue the proper
values of other colours in the scheme; on the other hand, if any attempt
is made to tone it down by mixing it with another pigment, or by glazing
another colour over it, the result is cold and muddy in the extreme, so
it is best left out of the list, as it is almost unmanageable in fresco.
Vermilion is rendered durable in fresco by pouring lime-water over the
powdered colour, and then draining this water off, without disturbing
the colour. This washing is repeated four or five times, before the
vermilion is ready for use. Permanent white, which is a barium sulphate,
and tin white, or tin binoxide, may both be used in lime frescoes, but
they are unnecessary, and not so good for the purpose as lime white.
Lemon yellow, Naples yellow, aureolin, Venetian red, and terre-verte may
be used with lime, but it all depends on the manufacture of these
colours as to whether or not they may be used with safety. For example,
the Naples yellow, Venetian red, and terre-verte pigments of to-day are
quite different in their chemical constituents to the same named colours
of the fourteenth and fifteenth centuries. Yellow ochre, though it has
been used very much by the old Italian frescanti, is not to be depended
upon when used in lime fresco in this country, as it consists of a
mixture or combination of hydrate of iron and clay; it therefore
attracts the damp that is nearly always present in our atmosphere. Of
course, it can be safely used in very dry climates, but it may be
mentioned that it has been one of the pigments which has largely
perished in some of the frescos of the Houses of Parliament. Another
objection to its use in fresco is, that the caustic nature of the lime
is apt to change yellow ochre to a light red, an after effect which may
not be altogether desirable. Raw sienna, however, more than takes the
place of yellow ochre, and is one of the most useful colours in fresco
painting. We should say that the three most permanent and beautiful
colours for use on the lime plaster are raw sienna, burnt umber, and
Indian red, and whether used in transparent washes or in solid tints, as
when mixed with lime-white, or in the full strength of their hues, they
always dry out luminous and brilliant. All the colours should be ground
very finely in water, and kept in covered jars or wide-mouthed glass
bottles. The lime-white should be strained through muslin and kept
covered. A bone or ivory palette knife must be used, and the palette
should be made of tinned metal, having a series of small circular wells
to hold the various colours. Brushes are of the ordinary kind, hog-hair,
and a few long sable riggers. Hog-hair brushes are best for use when
they are of a flat shape, but of a roundish finish at the end, like an
old or half-worn brush, so that when in use they will not disturb the
surface of the wet plaster, which a new or square-ended brush is apt to
do.
When everything is in readiness to commence work, a portion of the rough
wall surface should be thoroughly saturated with water, and the plaster
ground, or intonaco, should be laid on with a wooden trowel, beginning
at the top and at an angle of the wall, and large enough in area to
constitute a day’s work, or, rather, as much as can be done in about
five hours; for after that time it is not safe to work on the new
plaster, as it then becomes too dry: if allowed to get in this condition
the colours will not properly incorporate with the body of the plaster,
for if any painting is done when the plaster is in a half-dry state the
colours will not adhere permanently to the wall, but will scale off the
surface, or may be brushed off loosely when dry. When any portion of the
plaster ground becomes too dry to work upon.
[Illustration]
it must be cut right away and a new coating of plaster applied; it is of
no avail to syringe it with water in order to keep it damp for painting
on, as the water only weakens the nature of the lime in the plaster. The
chemical action between the atmosphere and the lime, necessary for the
firm locking up of the colours, will have taken place after four or five
hours’ time, according to condition of the atmosphere; and this action
will not repeat itself or be brought about by any application of water
after the period named.
When the day’s work is finished, any part of the surface not painted on
must be cut away, and the plaster all around the edge must be under-cut,
the cut sloping well inwards, so as to form a key for the new piece of
plaster for the next day’s work. This coating of plaster should be
rather more than half-an-inch in thickness, and should be finished to a
level, but not too smooth, a surface, with the wooden trowel, or
“float.” An iron trowel must not be used. After the plaster is laid on
it should remain about ten minutes or so before painting, so as to allow
it to set. It should then be coated with a tint, that would be a deep
vellum colour when dry, made of a mixture of lime-white and a little raw
sienna, about the thickness of cream; this will give a slightly smoother
surface to the plaster and will act as a luminous ground for the
subsequent colouring, besides acting a very important part as an
under-coating on which the superimposed colours will “bear out” more
effectively. The water used should be distilled or boiled, or rain water
collected in clean vessels, as hard water generally contains a quantity
of chalk, and for this reason it is not so powerful a solvent of the
lime as soft or distilled water.
The portion of the design selected for the day’s painting is traced from
the cartoon, and is transferred to the soft plaster through the holes,
which are pricked through the tracing paper, by means of pouncing with
powdered charcoal contained in a muslin bag, or it may have the outline
traced through with a sharp-pointed wooden stile; this will leave a
slightly depressed line on the soft plaster, and is a cleaner method
than the charcoal pounce, and on the whole the more preferable one.
We have now arrived at the stage of the work when the painting may be
proceeded with. It may be here mentioned that, in order to try the hues
of the colours and various tints, an ordinary brick having a thick
coating of plaster of Paris, that has been allowed to become thoroughly
dry, should be at hand. If a touch from the brush, dipped in any tint,
be put on this plastered brick it will dry immediately, and show at once
the actual shade of colour that such a tint will be when, after some
days, it has dried out on the plaster surface. It generally takes three
or four days, according to the season of the year, or heat of the room,
before the wall surface and the applied colours finally dry out, and
all
[Illustration:
[_To face p. 23._
PLATE 4.--TRIAL PIECE OF FRESCO PAINTING
_G. F. Watts, R.A., Victoria and Albert Museum_]
colours, more particularly the lighter tints and half-tones, dry
eventually ever so much lighter than they appear when first laid on. It
is only the very darkest tones, or pigments used in their full strength,
those that have no lime mixed with them, that dry anything near the full
strength of their wet state, and then there are exceptions to this; so
it follows that in buon-fresco painting the artist must paint in a much
darker key than the work is intended to appear when it has dried out.
As regards the method of execution in painting it may be urged that the
artist will work according to his own feeling or temperament: for
example, some may decide to paint in thin transparent washes or
glazings, as in water-colour painting; others may prefer to use the
colours in a thick impasto method, as in oil painting. Either method may
be adopted with success, but we should say that a judicious mixture of
both methods, in the same work, will obtain the clearest and most
luminous results, for, as a matter of technique, fresco lends itself
admirably to either methods of painting. The general rule is to model
the shades, half-tones and lights, broadly at first and in the order
named, with brushes rather large than small, and then to finish off by
strengthening the shadows with lesser touches, or by a series of
delicately hatched lines, and brightening the lights in the same way,
using for these purposes sable rigger brushes. The reason for this is
that every touch tells, especially when using a sable brush; you cannot
press or lean on your brush on the soft plaster without disturbing the
underneath colours, and possibly destroying the surface of the plaster
as well. It will be seen from this that the fresco painter must
cultivate a light hand for his work; he cannot indulge, for instance, in
that dexterity of handling that may be accomplished in oil or in, say,
spirit-fresco painting, simply because of the danger of working up the
soft and wet plaster ground: he must know what he has to do, and must do
it frankly and at once; for, although to a certain extent the artist may
be able to paint over parts and so correct occasional mistakes, it is
not advisable to do so, as the corrections will more or less show when
the work has dried out, by looking muddy, and consequently less luminous
than they ought to be. The only alternative, when a correction is
necessary, is to cut the piece out, and lay on a fresh plaster ground.
All this shows how important it is to have a full-sized cartoon in light
and shade, and also a colour scheme previously prepared, from which the
fresco painting may be almost copied directly on to the wet plaster.
[Illustration:
[_To face p. 25._
PLATE 5.--TRIAL PIECE OF FRESCO PAINTING
_G. F. Watts, R.A., Victoria and Albert Museum_]