The following notes are compilations of lectures note acquired from the M.A Conservation of Fine Art modules Northumbria University, Newcastle. The original author of this notes are Jean Brown, Anne Bacon and Nicky Grimaldi.
TECHNICAL EXAMINATION OF PAINTINGS
There are many forms of technical/scientific examination currently available to conservators. This notes will cover the most commonly used methods, raking light, transmitted light, microscopy, ultraviolet fluorescence, infra red and Xradiography. These techniques are all known as nondestructive forms of examination as they do not require any physical 'intervention' with the object itself e.g. the removal of a paint sample. They are extremely useful methods of assessing the art work’s condition. They are also used to document the condition of an object before, during and after conservation treatment and are especially useful during such treatments as cleaning and/or varnish removal.
It is important to remember that these techniques are only tools ; their success in helping assess the true nature and condition of an object relies on the examiner’s ability to interpret the results of the examination accurately.
To be able to do this it is necessary to understand how they 'work', what information they may be able to provide and how they may supplement and complement each other. It is often only after a combination of these examinations have been carried out that an accurate understanding of an object may be achieved, the information gained from one method alone may be confusing or even misleading.
For a start i would like to share some information pertaining the technical examination by using the ultraviolet (UV) methods as general guidelines. The UV equipment now is available at BSVN and I think most of my colleague quite familiar with this technique.
• In this form of examination an ultraviolet light source is used to examine the object, different materials appear differently in ultra violet light and additions and alterations can often be observed. Ultra violet light only affects materials on the surface of an object the light does not penetrate beneath this
layer. The UV light excites a form of light emission from some materials which is known as fluorescence, this can be seen with the naked eye and can also be photographically recorded. Photographing UV fluorescence does not require specialized film but a filter is needed on the camera to filter out the visible UV light and record only the fluorescence, such images are known as ultra violet fluorescence (UVF) images. Some materials such as iron gall ink absorb UV light very strongly and thus appear very dark in UV examination.
• Ultraviolet light is damaging to many objects and to eyes so U.V. filter glasses should always be worn when using this form of examination and it should also be used as briefly as possible. As with all forms of technical examination the results must be carefully interpreted.
• Figs 1 are case studies illustrating the use of ultra violet examination in assessing the nature and condition of ‘Menora’ 1954, Oil on linen by Hoessein Enas, Permanent Collection of The National Art Gallery Malaysia.
Ultraviolet fluorescent examination
• Varnishes, especially aged natural resin varnishes often fluoresce a milky green; if the varnish is very old or physically thick, this fluorescence can mask anything laying beneath it i.e. the paint layer. In a varnished painting areas that appear black or simply dark by comparison to the fluorescing varnish, may be either an area without varnish or an area covered with so much dirt that the U.V. light cannot reach the varnish to excite it to fluoresce. It could also be an area of paint lying above the varnish layer which would also prevent the U.V. light from reaching the varnish layer. If paint is laying above the varnish layer the question must be asked is it an artist’s alteration added after varnishing, or is it an area of nonoriginal paint added by a different hand?
• Even in an unvarnished painting ultraviolet examination may still be useful in identifying areas of nonoriginal paint. Usually the later addition will comprise of different pigments and media and it will also be younger all these things contribute to such areas appearing different in U.V. light; they will have a different response to the light, sometimes appearing lighter than the original perhaps because they have a slight fluorescence, and more often darker because they have less fluorescence.
• Some other materials that have a typical fluorescence include shellac varnish, which fluoresces salmon pink, rose madder pigment which fluoresces a bright pink and zinc white which fluoresces yellow. Iron gall ink absorbs UV strongly and inscriptions made with this can become more legible in UV light.
• As most paint layers are composed of more than one pigment and sometimes more than one medium really clear identification of materials is often not possible which is why experience is needed to develop diagnostic skills.
The UV, Visible and IR Spectrum
Ultraviolet Techniques
Fluorescence
— Fluorescence wavelength is different to source wavelength
— Fluorescence may be in visible or infrared
— Fluorescence may be photographed
Ultraviolet Reflectography
— Record image of reflected UV
— Use filter to avoid fluorescence
— Avoid direct light from source
(i) Natural resin varnishes fluoresce yellow-green to blue-green, whereas some newer varnishes fluorescence with a strong green colour.
(ii) If the varnish is sufficiently thick its fluorescence will mask any reaction to UV of the paint beneath it. When areas have been restored by the application of a new paint layer nearer the surface, the fluorescence in those areas will be much less intense.
(iii) Repainting behaves differently from the original paint. Recent restorations may be distinguished by their very dark or purplish tone with no apparent fluorescence at all. A forged signature may betray itself by its lack of fluorescence
(iv) Certain pigments fluoresce with a characteristic colour which may be used to aid their identification. Zinc white, for example, fluoresces a lemon yellow colour whereas madder lake fluoresces orange.
(v) Ultraviolet light may be used to render visible text and inscriptions which have become illegible due to discoloration, staining or fading.
(vi) Glues, gums, and stains may become visible under UV light.
(vii) Old varnish may have a milky appearance and cover other effects.
(viii) Checking with UV whilst removing old varnish can indicate to the restorer whether there is any varnish left to be removed.
Oil Samples on coloured ground using different light waves
1)Visible Light 2)UV Fluorescence Examination 3)IR
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