The use of ground glass in red glazes: structural 3D imaging and mechanical behaviour using optical coherence tomography and nanoindentation

Abstract

In this study we investigate the impact of the addition of colourless glass particles to red glazes, as seen in many 15th-17th-century easel paintings. With the use of reconstructions, we examined the influence of the paint preparation process on the morphological and mechanical properties of the paint film. Three sets of reconstructions were made, a control without ground glass, reconstructions with coarse or fine ground glass mixed in, and reconstructions where fine ground glass was ground jointly with the pigment oil mixture. The latter gave the desired consistency and colour based on visual inspection. The dried reconstructions were non-invasively imaged using optical coherence tomography (OCT). A data-analysis pipeline was developed for both the segmentation of the OCT images and the measurement of the size and spatial distributions of the glass particles within the glaze layer. Moreover, we used a nanoindentation protocol to measure the viscoelastic properties of the dried red glaze film. The OCT results show an expected decrease in median particle size with longer grinding-time, for which the additional grinding with pigment/oil resulted in a more narrow size distribution and a homogenous spatial distribution of the glass particles. The nanoindentation results indicate that the addition of glass particles increases the elastic and viscous moduli of the red glaze layers. The homogeneous size distribution, obtained by grinding the oil, pigment, and glass together, induces higher elastic and viscous moduli. Our imaging and analyses approach, combining OCT and nanoindentation, provides a non-invasive and quantitative investigation of glass particles in (semi-) transparent paint layers, and their effect on the mechanical properties of the glaze. The results of this study contribute to a better understanding of the artists’ addition of ground glass in paint layers.

A holistic view on the role of egg yolk in Old Masters’ oil paints

“…The choice was driven by the need of ensuring a constant pigment composition, which was necessary for the preparation of large batches of paints, replicated measurements, and several sets of systematic experiments that lasted about four years. It is well known that pigment shape and size distribution, exact chemical composition, and pre-treatment may all affect paint rheology, drying, and viscoelastic properties of dry paint layers 24 , 25 . As the focus of this work is on the effect of adding egg yolk to an oil paint, we selected two chemically very different, but readily available pigments to perform the comprehensive series of experiments and varied paint preparation only to achieve different repartition of egg yolk.…”

Colourless Powdered Glass as a Drier for Red Lakes: On the Influence of Composition and Particle Size

“…The accelerated drying rates may also be the consequences of the release of manganese ions in the drying paint system, activated during the grinding process used to produce finer particles. The influence of the grinding process of glass on the mechanical behaviour of lakes after natural drying was also investigated in [20], and the results indicate that the finer glass particles induce the higher effect on the properties. All these results illustrate the differences related to the extremely finely ground particles, described in the historic sources.…”

“…The surface of the glass particles detected varies from 2 to 20 % of the total layer area and they present an homogeneous distribution within the considered layer. The morphology and distribution of these particles in historic painting layers could be studied following the non-invasive imaging approach proposed in [20]. The influence of grinding processes on the morphology, size and size distribution and on physical behaviour of glass mixed in paint was evaluated by means of OCT and nanoindentation on dried red lake layers.…”

“…To evaluate the influence of the shape of the glass, we considered two ranges of particles sizes and studied their siccative effect. As mentioned earlier, this study is part of a larger experimental research on the various effects of the addition of crushed glass in paint layers [21,20].…”

Possible modifications of parchment during ion beam analysis

“…By repeating of such images in parallel, adjacent planes volume data is collected. The technique finds many applications in heritage science [35][36][37] especially for imaging sub-surface structure of paintings [38] but also historic glass [39] and other materials. Since the technique is fast and may be used on-site it was also used for monitoring of conservation treatments [40].…”