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 in Prague.

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 material.

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 origin.

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.)