The Chemistry of Art > 2. The Structure of the Atom and Colours > The Use of Infra-Red And Ultraviolet Light >
The use of infra-red and ultraviolet light in the analysis and identification of pigments and their chemical composition
Infra-red Light:
- Extends from wavelengths of 700 nm to 1 mm
- Near IR radiation lies closest to ‘red’ part of visible region and far IR radiation lies closest to microwave region
- In IR region, many organic polymers are excited by and are able to absorb energy of varying wavelengths, making IR light as tool to analyze molecular structure of organic paint films.
Uses
- Good for detecting painting under-drawing wherein artist has used graphite pencil, charcoal or black ink since carbon absorbs IR radiation and does not reflect it back
- Detects presence of copper-containing green pigments since these pigments also absorb IR radiation
- IR radiation penetrates pigments and reflects from white ‘ground’
Example: Titanium Oxide, Cadmium Sulphide, mercury sulphide vermillion
Ultraviolet (UV) light:
- Absorbance plotted as function of radiation wavelength
- Unit of wavelength: nanometer (nm)
- Contains enough energy to break the covalent bonds of most organic molecules
Uses
- UV light (in tandem with water and oxygen) are used to induce slow degradation of several organic polymers which include polymers used to bind paint film
- Useful for detection of chemical changes in sketches (Carbon) and paint over attempts
- Helpful in the identification of pigments containing Cu and ZnO (fluoresces in UV light)