The naked eye cannot see the UV reflection, but the UVR image can be very informative revealing the presence of retouches, varnishes and later additions on the artwork surface. It is specially relevant before restoration treatments, as it maps the conservation state. It is recommended to repeat it at the various stages of the intervention, to monitor and document them
Cardinal Baronio, attr. to Caravaggio, Uffizi Gallery. Art-Test analyses, 2009©
UV False colour
Observing an artwork under UV light is a standard diagnostic method used since the discovery of the Wood filter in the twenties. Because different painting materials exhibit characteristic fluorescence colours when exposed to ultraviolet light, UV fluorescence has been used to identify areas of impaintings and retouching and to determine and localize different types of varnish or wax. Currently it is also possible to quantify the fluorescence emission and to recognize some of the paint materials and their age.
The traditional acquisitions method consists in shining UV light onto the art object and to record, generally with a traditional camera and special film, or with a digital camera, the visible radiation, emitted by the materials on the object’ surface.
In order to eliminate the stray visible radiation which is present in the emission spectrum of UV lamps, and which would add to the fluorescence emission, Art-Test adds in front of the lamps a special visible blocking filter. In order to correctly document and analyze fluorescence emission it is necessary to know as precisely as possible the emission spectral curves, the multispectral method developed by Art-Test, greatly helps in this direction.
UV diagnostic and documentation possibilities
Traditionally UV photographs have been the only possible way to document fluorescence emission. However such images do not allow to quantify the emission not to faithfully render the colour, limiting the possibilities of identifying the painting materials, to compare fluorescence emission of different artworks, different times of with different devices.
The method developed by Art-Test allows one to overcome these present limitations. It is now possible, with a calibrated CCD camera to document the amount of UV light used. With multispectral system one can have anaccurate colour rendering, and achieve spectral profiling of the fluorescence emission.
This allows for a better identification of materials and of their age.
Fluorescence emission of organic materials has been widely reported and documented. Traditionally, it was common to refer to fluorescence emission by describing its colour. For example the fluorescence emission of shellac was described as orange.
However, it is clear that it is not possible to be precise and to quantify fluorescence in this way. In order to correctly document and analyze fluorescence emission it is necessary to know as precisely as possible the emission spectral curves, and to consider aging and interaction with underlying materials.