Amadori found a lead tin yellow in a glassy matrix as well as colourless glass particles in works by the Venetian painter Lorenzo Lotto (1480–1556), both materials could plausibly be connected to the local glass industry.

Here the glass particles contain manganese, added by Italian glass makers from the late thirteenth century onwards to counteract the greenish tinge caused by iron impurities in the sand or pebbles used. In the latter it occurs mixed with various pigments and in different layers in the paint stratigraphy. However, Spring also established extensive use of glass in 15th and 16th-century Italian paintings from Bologna and Ferrara, and in works by Raphael. proposed that the addition of ground glass was ubiquitous in Florentine paintings from 1490 to 1530.

Spring identified powdered colourless glass in 15th and 16th-century German and Netherlandish school paintings in the collection of the National Gallery, London, amongst others in Jan van Eyck’s Arnolfini Portrait and Annunciation, and quantified their chemical composition using SEM–EDX. Dietz studied the extensive use of ground glass in works by Hans Holbein the Elder (c.1460/70-1524), where glass particles are mixed with a large variety of pigments in almost all paint layers, not just red glazes, and are also present in mordant layers for gilding. Although adding dryers to oil paint is indeed quite common, the function of the colourless ground glass is not yet fully understood. Contemporary art technological sources recommend this practice mainly to speed up the drying process. The presence of colourless glass particles in paints in European easel paintings from the 15th to the 17th century, especially in transparent oil-bound red glazes, is a regular occurrence. The results of this study contribute to a better understanding of the artists’ addition of ground glass in paint layers. 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 homogeneous size distribution, obtained by grinding the oil, pigment, and glass together, induces higher elastic and viscous moduli. The nanoindentation results indicate that the addition of glass particles increases the elastic and viscous moduli of the red glaze layers. 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. Moreover, we used a nanoindentation protocol to measure the viscoelastic properties of the dried red glaze film. 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. The dried reconstructions were non-invasively imaged using optical coherence tomography (OCT). The latter gave the desired consistency and colour based on visual inspection. 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. With the use of reconstructions, we examined the influence of the paint preparation process on the morphological and mechanical properties of the paint film. 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.

Painting external coherence