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Title: Optical and material properties of varnishes for paintings
Author: Lawman, S. J.
Awarding Body: Nottingham Trent University
Current Institution: Nottingham Trent University
Date of Award: 2011
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Clear varnish coatings are applied to the surface of historic paintings to provide a protective barrier layer and/or change their appearance. The natural varnishes used by the original artist would have given the painting its original finished appearance. Over a period of years these varnish layers degrade with age, which changes the appearance. Then it becomes necessary to remove and replace the coating. This removal and replacement is undesirable as it can lead to damage of the painting. Synthetic alternatives can provide a much longer life coating but may not give the same appearance. This thesis details research into the soft matter physics and fluid mechanisms that determine the relationships between the material properties of varnishes and their effect on the appearance of paintings. The mobility (self diffusion coefficient) of a high molecular weight varnish resin (AYAT) was measured with Nuclear Magnetic Resonance (NMR) to be up to three orders of magnitude lower than the solvent (Toluene) and a low molecular weight resin (Regalrez 1094). The similarity of the low and high molecular weight resins NMR relaxation times, T1 and T2, showed the important difference was the size of the molecules. A unilateral NMR imaging system (using a CPMG sequence with a TE time of 100μs) was able to resolve a signal from the varnish coatings until the solvent content was approximately 10%. The use of a NMR system, capable of shorter TE times, could allow the non-invasive measurement of solvent content of varnish coatings until completely solvent free. To measure the formation of substrate induced roughness of μm and nm amplitudes, a new method was developed using Optical Coherence Tomography (OCT) to simultaneously measure the air-varnish and varnish-substrate profiles. The potential accuracy of this technique was shown to be the same as any other White Light Interferometry method (<10nm for smooth surfaces but decreases with surface roughness). The experimental accuracy achieved for smooth surfaces, without significant vibration suppression, was 50nm. The high sensitivity of OCT allowed the otherwise unachievable measurement of extremely faint varnish-ground glass interfaces. The development of varnish surface profiles and power spectral densities inherited from a substrate (crucial to varnish appearance) were accurately modelled using the lubrication approximation of the Navier-Stokes equation. Though for better levelling varnish coatings, a different process dominated low spatial frequencies. Viscosity, as a function of concentration, was shown to be the crucial variable determining surface roughness. The viscosity functions of high molecular weight polymer resins (AYAT and Paraloid B72) were shown to be considerably different to low molecular weight resins (Dammar, Laropal A81, MS2A, Regalrez 1094 and Regalrez 1126). The differences in the viscosity between low molecular weight resins (including natural resins) are less substantial and other factors, such as choice of solvent, are of increased importance. In conservator applied coatings, the amount of varnish applied was shown to be a key variable along with choice of resin in the final surface roughness. OCT measurements of these provided strong evidence that some coatings completely wetted substrate pores, while others showed different fractions of wetting. This showed dependencies on the resin and variations in the conservators’ application. The assumption that the surface state is the only dominant factor in the appearance of coatings is not necessarily true. The thesis concludes that, to understand the difference in appearance between different coatings, the material properties of the varnish solution should be directly measured. With these the surface roughness, hence appearance, of a coating can be accurately predicted. This will allow the prediction of how a varnish solution can be manipulated, by a conservator, to get precisely the desired surface state.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available