^(a) DFA, Dipartimento di Fisica e Astronomia “Ettore Majorana”, Università degli Studi di Catania, Via Santa Sofia 64, 95123 Catania, Italy

^(b) INFN CHNet-CT, Catania node of the Cultural Heritage Network, Istituto Nazionale di Fisica Nucleare, Via Santa Sofia 64, 95123 Catania, Italy

Email:anna.gueli@unict.it

Keywords: colour specification, accuracy, protocols

Colour is an essential characteristic of artworks and all diagnostic projects must not disregard the characterisation of the related optical quantities and the evaluation of its stability over time. In this context, an ideal conservation programme includes colour measurements at established time intervals, while in restoration projects they should normally be performed before and after each intervention.

The "measurement" of colour is essentially carried out by detecting the Spectral Reflectance Factor (SRF%) and calculating the chromaticity coordinates obtained considering the light source emission and the response of the human eye to the light reflected from the object (Oleari and Simone, 2015). The CIELAB is the most widely used colour space and the association between optical behaviour and colour coordinates is defined as colour specification. While the SRF% behaviour is characteristic of the material and can contribute to the identification of a pigment, the colour coordinates depend on the experimental and data processing procedures. The assessment of colour variations is possible thanks to the calculation of the DE parameter, the value of which depends on every step, and each experimental parameter plays a crucial role in the assessment of colour differences. Particular attention should be paid to the lightness scale adjustment (Gueli et al., Instruments 3(3), 2019).

The influence of each experimental parameter will be illustrated by the results of several methodological studies conducted using contact spectrophotometric and remote spectroradiometric methodologies. The limits and strengths of both approaches will be outlined as well as the impact of some characteristics of paint layers such as pigment particle size (Gueli et al., CRA 42(2), 2017) and binder (Gueli et al., CRA 42(6), 2017) on colour measurements.

The operative procedure and best practices developed at the PHysics for Dating Diagnostics Dosimetry Research and Applications (PH3DRA) laboratories that ensure the high precision required for colorimetry applications in the Cultural Heritage field will then be described.

References

Oleari C., Simone G., Standard colorimetry: Definitions, Algorithms and Software, (2015), Wiley & Sons Inc, 493 pg.

Gueli A. M., Bonfiglio G., Pasquale S., Troja S. O., Effect of particle size on pigments colour, Color Research & Application, (2017), 42(2), 236-243.

Gueli A. M., Pasquale S., Troja S. O., Influence of vehicle on historical pigments colour, Color Research & Application, (2017), 42(6), 823-835.

Gueli A.M., Pasquale S., Politi G., Stella G., The role of scale adjustment in color change evaluation, Instruments, (2019), 3(3), 42 (10 pages)