LK13320 - The hue of the yellowish
color in ancient paper may be placed in precise relationship with the environmental
conditions to which the artefacts were exposed during their life. This important
outcome is the result of our experimental and theoretical investigation on the
optical degradation of 15th century papers.
Paper degradation
results in a yellowing of the sheets mainly as a consequence of the oxidation
of cellulose fibres. The oxidized products act as chromophores capable of
selectively absorbing light and give rise to the yellow coloration of ancient
paper sheets. The complex chemical and physical properties of cellulose have
prevented a detailed identification of chromophores up to now.
To solve this problem, we measured the optical
properties of both ancient and, as comparison, modern paper samples artificially-aged
in several environmental conditions and interpreted the collected data by means
of ab-initio theoretical calculations based on time-dependent density
functional theory. Through this approach the relative concentrations of
chromophores responsible for the yellowing of the ancient paper were clearly
identified and quantified. A given set of chromophores produced upon aging is
strongly related to the environmental conditions to which the artefacts were
exposed during their life, such as dry, humid or closed storage.
Our work
contributes to a contemporary Preservation Science which attempts to provide
conservators a rationale based methods of artefacts analysis and treatment.
