OCTOBER, 7934 Vol. 59, No. 703

THE ANALYST

PROCEEDINGS OF THE SOCIETY OF PUBLIC ANALYSTS AND OTHER ANALYTICAL CHEMISTS

Deaths WITH great regret we record the death of

VISCOUNT DEVONPORT, who was made an Honorary Member in recognition of his Parliamentary services to the Society; also of

ALFRED WALTER STOKES, who was one of the earliest Members of the Society, and contributed numerous papers to THE ANALYST in 1881 and the following years, but had left the Society some years before his death.

Old Masters and Modern Forgeries BY A. P. LAURIE, M.A., D.Sc., F.R.S.E.

(Professor of Chemistry to the Royal Academy of Ar ts ) (Lecture delivered at the Summer Meeting, North of England Section,

J u n e 22nd, 1934)

IN my former lecture to this Society (cf. ANALYST, 1930,55, 162) I dealt mainly with the chemical methods of examining the pigments of paintings; to-day I propose to give a more detailed account of the use of other scientific methods that enable one to form a judgment as to the authenticity of a picture. I t will be remembered that the chemical examination consists in removing a minute fragment from the surface of the picture by means of a micro-borer made from a hypodermic needle, and subjecting the particle to a series of microchemical tests in order to identify the pigment or pigments of which it consists.

Systematic examination of a very large number of paintings has established the fact that certain pigments were in use at one time, but not a t another, and that it is therefore often possible to determine within relatively narrow limits the

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658 LAURIE: OLD MASTERS AND MODERN FORGERIES

period within which a picture must have been painted. For example, a very beautiful azurite, owing doubtless to the discovery of a fine deposit, became a fashionable pigment about 1450, even replacing ultramarine, and disappeared altogether from the artist’s palette about 1650. Smalt, again, was not used before the latter part of the sixteenth century, and Prussian blue does not appear in the artist’s palette until the middle of the eighteenth century. The classified results of these chemical tests have been tabulated in a chart (ANALYST, 1930, 55, 176), which shows at a glance the pigments in use a t successive periods, and thus may help to date a pacticular painting.

This microchemical method has also given us a knowledge of the pigments used by the great masters, and this may be a decisive factor in determining whether a doubtful picture has been correctly attributed to a particular artist.

An interesting trial, in which the chemical evidence was an important factor, was the libel action brought against Lord Duveen for saying that Mr. Hahn’s version of the picture “La Belle Ieronniere” was a copy of the picture in the Louvre attributed to Leonard0 da Vinci. There were two versions of this picture in the possession of Louis XIV-one the original and the other a copy. Mr. Hahn claimed that his version was the original. The microscopical examination of the two pictures was most informing. Leonard0 da Vinci has left an account of the pigments he used for painting flesh; these I found in the Louvre picture, whereas the pigments in the Hahn picture, although also old, were quite different.

The owners of “Old Masters” upon which doubt has been thrown are some- times unduly perturbed by the idea that a chemical examination, the result of which will be invisible-to them, will damage the picture, while they accept without demur the injury which may have been done to the painting by destructive restoration ; for the early nineteenth-century restorers were seldom content with restoring flaws in the surface and could not resist the temptation of retouching the painter’s work .

Notwithstanding these considerations, many owners still have a concealed preference for optical methods of examination, and, fortunately, these are frequently very effective.

PRELIMINARY EXAMINATION.-~Then a picture is submitted for scientific examination one of the first steps is to make exact measurements of the size of the panel or canvas. I t was a common practice of the art patrons of the seventeenth and early part of the eighteenth century to keep a favourite painting’at their principal residence and to have replicas of it made to hang a t their other seats, but no care was taken to see that these copies were made to the precise dimensions of the original. Not infrequently a descendant of the first owner of a picture has sold the original and contented himself with the copy, and this, in time, may have come to be accepted as the original.

The dimensions of most of the authenticated works of the old inasters have been recorded, and a copy of one of them claiming to be the original may thus be eliminated at once by not corresponding with the measurements of the record.

A general survey of the picture is then made under a low magnification of about two to three diameters, and for this purpose the Zeiss magnifier is particu- larly useful. This is a binocular magnifier provided with an adjustable band

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LAURIE: OLD MASTERS A N D MODERN FORGERIES 659

which can be passed round the back of the head to keep the glass steady while leaving the hands free to move the picture into different positions.

Among the points to which attention is directed in this survey are’the signature or any other distinguishing mark that may be present, the condition of the canvas, indications of restoration and retouching, the condition of the varnish, etc.

THE TRAVELLING MIcRoscoPE.-~wing to the large area that has to be traversed when examining an oil painting it is essential to have a rigid microscope stand, which can be used with a binocular magnifier, a Greenough binocular body, or with the “Ultropak illuminator.” The construction of the stand for the examination of hanging pictures differs from that required for examining horizontal pictures on a table; both forms have been designed by Messrs. Leitz (London), Ltd.

The arrangement for suspended pictures is fixed on two wall brackets support- ing a rail, upon which moves on rollers a carriage holding the microscope placed horizontally. This microscope has the usual coarse and fine adjustments, but is also provided with a traversing arrangement working on ball-bearings, which allows of a displacement of 2 inches in either direction; the height is adjusted by raising or lowering the picture, which hangs on pulleys between the two brackets.

For inspection of horizontal pictures not exceeding 48 inches in width, the microscope is mounted on a rail similar to that used with vertical pictures and moves on rollers, and the fine adjustment is made by means of a travelling device on ball-bearings. For the coarse adjustment in either direction the rail rests a t both ends on a carriage, also moved on rollers on rails, between which the picture lies. The microscope tube is provided with an inclined eyepiece to facilitate observation.

Fig. 1 Ultropak with illuminating apparatus screwed on

MICROSCOPE ILLUMINATION.-In a microscopical examination of the Surface of a picture the use of a vertical source of illumination has the drawback that light reflected at the lens surfaces of the objective produces a glare that tends to render the image indistinct. This haze is largely eliminated by the use of a method in which the illuminating rays lie entirely outside the path of the rays of the micro- scope. The “Ultropak illuminator’ ’ (Leitz), which is constructed on this principle, is a device which has a side-tubulure and is screwed on to the body of the micro-

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scope above the objective. The source of light is a small %volt filament lamp which is fitted into a special centring mount, and the lateral rays from which are reflected downwards by an annular mirror fixed at an angle of 45” (see Fig. 1).

Each objective has a special ring condenser which can be rotated so as to obtain a uniformly bright image. This instrument reduces, but does not entirely prevent, the glare from the irregular, varnished surface of a picture, but by the addition of a polarising device the image appears quite sharp, even under a high magnification. In the “Polarising Ultropak” (Fig. 2) the polariser is fitted into the lateral illuminat- ing tube and can be rotated, whilst the analyser is placed just above the objective, where it is attached to the microscope between crossed nicols. When the planes of polarisation of the two prisms are at right angles, all glare from the surface of the object under examination is entirely eliminated.

Fig. 2

Section of Ultropak with device for use with polarised light (about half-natural size). A, analyser; P, polariser; F + G, slot for filters and selenite plate

In front of the lamp is a recess, with cover, intended for light-filters; these usually include a daylight filter (blue), a heat-absorbing filter (pale green) and yellow-green photographic filters, which may be used separately or in combination.

THE PAINTER’S BRuSHwoRK.--Each of the great masters had a style of applying the paint as distinctive as the writing of his signature, and his characteristic brushwork will be found to run throughout his work, especially that of a particular period. The direct brushwork of Romney (Plate, 11), for instance, is very different from the characteristic strokes of Rembrandt, typical examples of which are shown in the Plate (I and 111). The peculiar texture of this brushwork suggests that Rembrandt probably used “stand” oil as his medium. This oil has

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11.

111. IV. Cracks in the varnish on an old painting ~ 4 0 .

Magnified photograph of the nose in the “Portrait of Titus” by Rembrandt (Wallace Collection,

Magnified photograph of part of the face in “The Lady and Child” by Romney (National Gallery,

Brushwork of Rembrandt in the “Woman Bathing.” (National Gallery, No. 54) X2.

No. 29) x2.

No. 1667) x2.

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LAURIE: OLD MASTERS AND MODERN FORGERIES 661

been prepared for centuries in Holland from linseed oil, and I have brought together a good deal of evidence indicating that it may have been the kind of oil used by the early Flemish and Dutch painters.

Bol (1616-1680) and other Dutch painters under Rembrandt’s influence produced a similar surface, but when magnified photographs are compared, the brushwork is seen to be weak and meaningless compared with that of the master. Further application of this method of examination will probably dispose of the claims of many pictures that have long been accepted as “Rembrandts.”

An interesting point in connection with Romney’s brushwork is that he some- times painted on a twilled canvas with hardly any priming, and indications of this surface appear in the finished work.

My idea of studying the brushwork technique of the great painters by means of photomicrographs has frequently proved of the greatest value in deciding who painted a particular picture. One of the earliest instances in which the method was employed was during the Dutch Exhibition, when a press attack was being made on the magnificent Rembrandts in the possession of the Duke of West- minster; it was alleged that these were eighteenth-century forgeries, this judgment being based on a mistaken diagnosis of the wood used in the panels. Photo- micrographs of the brushwork, however, showed unmistakably the characteristic strokes of that master.

An important trial in which photomicrographs of the painter’s brushwork were material in deciding the issue was the famous Romney case. A picture had been sold in all good faith to an American collector as a painting by Romney of Mrs. Siddons and Miss Fanny Kemble, but was later condemned by one of the leading art experts, and an action was brought by the purchaser to recover the price he had paid. There was no doubt that the picture belonged to the period, and the question at issue was-who painted i t ? A comparison of photomicro- graphs of this painting with those of pictures by Romney of about the same date proved beyond doubt that it was not the work of Romney, for, instead of his clear, decisive masterly strokes (cf. Plate, 11), it showed the uncertain brushwork of a less competent artist. Other evidence supporting this conclusion was obtained by a study of the representation of the facial shadows and of the folds of the drapery in the picture.

The solution of the problem was found by a search of the catalogues of the Royal Academy, which showed that a picture of the two Miss Waldegraves, agreeing with the disputed picture, had been painted by Ozias Humphrey for Horace Walpole. This picture had not met with the approval of Walpole and had been left on the artist’s hands. On his death it passed to his son, and its later history could be traced until 1850, when it disappeared. Twenty years afterwards the picture appeared in a private collection as “Mrs. Siddons and Miss Fanny Kemble by Reynolds,” in an eighteenth-century frame with a gold plaque, once inscribed with the name of Romney. Ozias Humphrey, who was well known as a miniature painter, took up portrait painting when his eyesight began to fail. He was a friend of Romney, and adopted his style of painting.

Even the artist’s brushwork is no longer proof against the ingenious devices of the modern forger. Copies of actual pictures are not made, but pictures are

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built up by photographing parts of a number of genuine paintings-a bit from one and a bit from another-f, say, Rembrandt, reproducing these on sensitised gelatin, and impressing the gelatin stamp on the prepared soft paint surface. Exact facsimiles of the original brushwork are thus actually present on the panel; the forger paints round these, covers the whole with an “antique” varnish suitably cracked, and finally scrapes off and “restores” the picture in certain places.

CRACKS IN OIL PAINTINGS.-The cracks which frequently cover the whole surface of old paintings are often due to the shrinkage of the layer of paint above the priming, which produces cracks with straight sides. The character of these cracks is clearly shown in a photomicrograph (c j . Plate, IV). It is very difficult for a forger to produce cracks that will stand examination under the microscope, although the “cracks” of an old picture on a panel can be imitated so skilfully as to escape detection under an ordinary magnifying glass.

The cracks that occur in mastic varnish are quite different in character from those occurring in oil-paint. They are due to movement in the oil film and show desiccation of the various pigments incorporated therein. Mastic cracks tend to have a web-like form and do not show the long, straight-sided fissures of the paint cracks.

The surface of old pictures varies greatly according to the practice of the painter. Sir Joshua Reynolds was constantly making experiments with mixtures of waxes and balsams, with the result that many of his pictures are in very bad condition, full of cracks. Romney, on the other hand, usually employed only a limited number of pigments ground up with oil, and most of his pictures are in far better condition than those of Reynolds. Very exceptionally, Romney added a little asphaltum to his paint, and the paintings for which it was used are all in bad condition.

The low tone of some of Rembrandt’s paintings is due to the surface-varnish having turned yellow; the paint beneath the varnish shows no material signs of deterioration.

REFRACTIVE INDEx.-The refractive index of a minute particle of dried oil is determined by the Becke method, as described by Chamot and Mason (Handbook of Chemical MicroscoPy, Vol. I, p. 369). In this method the fragment is immersed in a liquid of known refractive index, and the movement of the bright peripheral line (known as the Becke line) is observed when the objective is slightly raised from the position of sharp focus. This line appears to move into the medium of higher refractive index, whether this is the particle or the surrounding liquid. By substituting liquids of different refractive index one will be found which yields no Becke line when the objective is raised, thus indicating that this value for the particle and the liquid are identical. The refractive index of the standard liquid is then checked with an Abbe refractometer, as usual. In this way it is possible to determine, with an accuracy of about 0.002, the refractive index of the dried oil particle. Since the refractive index of liquids changes slightly with variation in temperature it is advisable to use the standard temperature at which the liquids have been calibrated for the determination; the temperature of 20” C. is usually adopted.

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LAURIE: OLD MASTERS AND MODERN FORGERIES 663

Fresh linseed oil, as is well known, has a refractive index of about 1.480. This has been found to rise slowly with age, and it seems probable that a refractive index of about 1-600 represents an age of about 600 years-an estimate which is borne out by the examination of known pictures of the thirteenth and other centuries and determination of the refractive index of the oil thereon. Obviously, the method requires prolonged investigation, but it seems to promise valuable confirmatory evidence in assessing the age of a picture. The solvent or liquid applied for the determination of the refractive index does not appear to influence the result, but here again, further work which is required is in progress, as also are tests on the refractive index of egg and other substances that were used as mediums in painting.

An interesting technical question associated with this rise in the refractive index of the oil in oil paintings is the effect of this change upon the translucency of the pigments. Thus a transparent blue, if applied in a thick layer, eventually turns black, and a thin layer of white lead painted over black, gradually becomes transparent and allows the black to become visible again. Whilst this may be due, in part, to slow inter-penetration of the pigment by the oil, with the formation of more transparent soaps, there is no doubt that the increase with time in the refractive index of the oil increases the transparency of the pigment. It has been proved experimentally that pigments which become degraded in tone through a rise in the refractive index of the oil retain their brilliance if painted on a bright under-surface.

The Flemish painter, Van Eyck, and his followers must have understood the optical properties of their pigments and mediums.

X-RAY EXAMINATION.-The value of X-ray photography in the examination of old paintings is that it brings out the structure of the picture, showing the graining of panels, joints glued to the back, nails in the wood, and the difference between the priming and the surface painting. In my former lecture (Zoc. c i t . , p. 179) I described the method of taking X-ray photographs, and cited several instances in which the method had shown that certain pictures had been retouched at later periods. One of the most striking of these was the discovery of Dr. de Wild, who found that the Frans Hals picture, “The Toper,” in the National Gallery of Scotland, had been retouched at a date later than the original painting. In this case the exact details of the original work indicated by the X-ray photograph were supplied by the discovery of an etching by J. van de Velde (about 1568-1623); this is a portrait of Verdonck, who was a friend of Frans Hals, and it was believed to have been made from a missing painting by Frans Hals. The X-rays thus proved “The Toper’’ to be the original “Portrait of Verdonck.”

An ingenious device of a forger was detected when a suspected “Frans Hals” was subjected to scientific examination. The forger had used an old panel strengthened by battens which had been nailed on from the front, and since the X-rays revealed that modern wire nails had been used, and that the heads of these were below the paint, the picture was a self-evident forgery. This was confirmed by the chemical examination, which showed that the pigments were modern. The painter of the picture was afterwards discovered, and is supposed to have

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painted his own portrait and to have added the pointed beard and rakish hat required to produce a Frans Hals “gentleman.”

THE USE OF ULTRA-VIOLET LIGHT.-MoSt of the dark inorganic pigments do not fluoresce, and thus dark brown appears black in ultra-violet light. On the other hand, the addition of organic lakes to red lead or to vermilion may often be detected by their distinctive fluorescences. White pigments may also be differentiated; thus, white lead that has been exposed to the air gives a pinkish- brown; zinc white, a yellow or yellowish-green; and titanium white, a deep violet- blue fluorescence. Results obtained with mixtures are more uncertain than those given by the pure pigments alone, and the shade of the fluorescence may be materially affected by the presence of traces of impurities. The fluorescence of the surface varnish or the medium with which the paints were originally mixed may also mask the fluorescence of the pigments.

For these reasons the examination of pictures in ultra-violet light is of value only in exceptional cases, as, for example, when it reveals the fact that parts of the surface have been retouched at a later period or with different pigments. A good example of this was its use in the examination of certain pictures in the Louvre, the authenticity of which had been questioned. The over-painting and retouching revealed by the application of the ultra-violet rays were obvious, and proved that these pictures were not genuine.

Ultra-violet light has also been of service in deciphering the artist’s signature on a picture, as is exemplified by a case in which it proved that a doubtful signature of Rubens was really his.

Hence the method should always form part of the routine examination of a picture. The instrument I use is a portable quartz mercury-vapour lamp, the rays from which are projected horizontally on to the picture, which is fixed in a vertical position.

INFRA-RED PHoToGRAPHY.-Although paint pigments show considerable differences in their absorption of infra-red rays, the method has not proved of much value for distinguishing between them in a painting. On the other hand, the penetrative power of the rays was effective in revealing the details of a signature of Rembfandt of an earlier period, and thus in helping to establish the approximate date of a picture.

In this outline of the application of scientific methods to the examination of the ‘‘ Old Masters” I have dealt with only some of the lines of investigation still in progress, for the subject is a wide and ever-growing one. To-day there are journals in Germany and the United States of America devoted to the scientific study of the various problems connected with pictures, old and modern. Harvard has a special laboratory devoted to the subject, and the Louvre has also a magnificent one, while in this country the Courtauld Institute is engaged in fitting up a laboratory, and a brilliant young physicist has been added to the staff of the National Gallery.

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