The topic of this paper is the mutual relationship between
historical technology and expertise of works of art. The
possibilities and limits of the application of historical
technology to matters of autheticity can be exemplified
on certain cases that appeared in the National Gallery
The following are the general assumptions that we use
in evaluation: the relevant technology of the period,
which can be copied but never fully applied in the whole
of its complex structure; relevance to changes in time,
i.e. the natural ageing of the material structure and
its consequences, which appear both in the external appearance
and physical qualities. By a combination of these
criteria we can reach the conclusion that will be of great
likelihood. This refers to stipulation of the period,
not attribution. Evaluation on the basis of historical
technology requires a sufficient amount of comparative
As part of our work the National Gallery in Prague provided
us with such material in the course of long-term preparations
of certain exhibitions. Recently our Restoration Atelier
undertook research into some one hundred Italian Gothic
and Renaissance paintings belonging to various collections
in readiness for an exhibition that was assembled by Dr.
O. Pujmanová. All pictures were subjected to X-ray
examination, microscopes we're used to determine the species
of wood, composition of the priming or imprimature, studies
were made of the preparatory drawing, the manner of gilding
and embossing, the system of paint layers and the character
of the brushwork, some types of pigments and, in exceptional
cases, also the character of the paint media.
For the question of authenticity the importance of these
individual technical data is not identical. The type of
wood used is of little avail for identification. A whole
series of forgeries were made on poplar wood, which is
typical of Italian medieval painting. On the other hand,
eight other kinds of wood formed the support for authentic
works. In order of frequency these were: walnut, particularly
popular in Renaissance painting, followed by fir, lindentree,
oak, spruce, pine, beech and pear wood.
The layer of priming reveals something more: natural gypsum
is the main ingredient of grounds in Italian Gothic and
Renaissance painting, even though the addition of chalk
must be conceded, especially in northern Italy. Pure chalk
was found in the case of forgeries and non-Italian picures.
The mixture of brown and red ochre of the bolus type –
natural siliceous clay – did not appear before the end
of the 16th century, and the appearance of such priming
points, at best, to a later copy.
Venetian painter of the 2nd half of the ijth century.
St George – forgery. (collection of Ferdinand d'Este)
In Renaissance painting priming of white
lead is frequent, either pure or faintly tinged with red
and even yellow ochre. The preparatory drawing is difficult
to copy – sometime the forger does not even know about
it – and this is very symptomatic. Typical of Italian
painting is black drawing with a brush, appearing
in varying intensity already during the 14th century.
A special study was made of the manner of gilding and
embossing. The famous forger Federico, llicio Joni used
ingenious complex punches of his own make. Our Western
Bohemian Museum in Plzeň owns an example of his work –
a painted book binding dated 1471. To judge the authenticity
one requires a whole collection of punches, but distinction
is possible as Dr. M. Frinta showed.
Underpainting, in particular the well-known green preparatory
layer of flesh appears often in fakes, too. Among pigments
we traced the kinds with a specific occurrence. Analysis
of blue and yellow pigments proved best in distinguishing
forgeries. Typical medieval blue pigments such as lapis
lazuli and azurite need not be a guarantee of authenticity,
as shown by the fakes of Federico Joni, who used natural
ultramarine, as we found.
The media used in painting were also put to the test.
Laboratory tests of 14th century paintings showed the
classical Italian eggvolk tempera, to which, in all likelihood
in the late 15th century, various high percentages of
oil were added. Typical of early Renaissance painting
is the use of walnut oil.
The study of Baroque painting was of slightly different
character. The criteria for determining the period or
region is complicated by the fact that many qualities
are common for longer periods of time and broad territory.
In research on 17th and 18th century Italian paintings
in our National Gallery and other collections, especially
in Olomouc, we had the opportunity to realise that certain
characteristic details exist that can be applied in evaluating
It is known that the canvases of 17fh century Italian
painting were of strikingly tenuous structure. Throughout
the 17h century a density of a mere 5 or 6 threads
to the centimetre were found, and the thin structure of
up to 10 threads continued to the middle of the 18th century.
But this technical detail is not categorical, and its
absence is no proof against anthenticity.
Another, again rather doubtful criterion is the structure
of the ground layers. According to data we found, typical
of the first half of the 17th century, in Italy, is a brown
ground, macroscopically rather dark, with brown ochre
as its main component accompanied by a siliceous
admixture. The laboratory found also a small percentage
of red ochre and also lead white, probably used as siccative,
for, according to the laboratory analyses, these grounds
contain a large amount of oil. The excessive darkness
of the brown priming is sometimes subdued by a further
layer, a grey one, which is composed of lead white
and black, bound with oil. Neapolitan painting, on the
other hand, preferred very dark, even black layers of
priming. In the second half of the17th century red-brown
to red tones of the ground began to appear, and in the
first half of the 17th century there were mainly clear
red tones, which later, in the second half, reached an
orange yellow tone, especially in the region of Venice.
An important criterion of authenticity is the brushwork
of the painting, clearly visible on a radiograph,
thanks to the fact that the ground layers of baroque paintings
are relatively easy penetrable by the X-rays.
The analysis of pigments, particularly again the blue
and yellow ones, are suitable for a distinction of
17th century painting from those of a later period.
Thus the occurrence of Prussian blue simplified the period
dating into the 18th century, the spectrographic determination
of chrome yellow shifted the dating to the second decade
of the 19th century. The signature is an important part
of an authentic picture. The authenticity of the signature
provides many difficulties, and many ingenious manners
of faking exist. Not even a signature carved into
the wet colour with the handle of the brush need always
be authentic. Luminescence in ultra-violet light showed
in one case that in the place of the signature the forger
had removed the original paint and added fresh paint in
a manner that imitated the original brushwork. The
forgers naturally know the test of relative solubility
and try to carry out the signature in a resisting
technique, for example Indian ink, or they cover the signature
in an insulating layer. The assumption that the artist
used identical medium for the painting and the signature
is not always valid, as shown by the signatures of Julius
Mařák using a pen or Jan Zrzavý using a pencil.
But a different medium for the signature always rouses
suspicion even though it is no proof. A combination of
several criteria is recommended, such as testing the age
with the solvent and, at the same time, judging the course
of the crackles that interrupt the strokes of the lettering.
That involves knowing the laws of origin of crackles and
possessing sufficient comparative material. Sometimes
ultra-violet light is sufficient to prove a faked
signature. This is so in the case when the lettering is
carried out on a non-original layer of varnish which
has the decisive luminescence. False signatures appear
even on authentic pictures of identical artists. In special
cases even on authentic pictures by another artist, less
well known whose signature has been deleted by force.
There even appear false signatures on pictures where the
correct signature escaped attention and survived undisturbed
under the varnish in the dark parts of the picture. Evaluation
from the point of view of historical technology is possible
only when strictly objective criteria are applied. Bui
it can be decisive in dealing with the complex questions
of the authenticity of a work of art.
Mojmír Hamsík, AHVT A 012 (T. G.)