On the preparation of model ultramarine blue oil paint layers: factors affecting the wet paint flow and curing

In heritage science, preparing mock-ups is often a fundamental yet challenging aspect of experimental protocols. This work arises from the need to understand the critical factors that must be considered when preparing model oil paint layers. Specifically, the focus is on understanding the rheology and the curing process of oil paints, and the work was carried out selecting ultramarine blue as pigment and (semi)drying oils as binder. The factors investigated here are the types of pigment and oil, paint thickness, pigment storage conditions, and paint solids content. In this work, 10 different ultramarine blue pigments were selected, differing in terms of their origin (natural and synthetic), particle size distribution, presence of coating or additives, and overall composition. Cold pressed walnut, safflower and linseed oils were used, as well as linseed oils subjected to different pre-treatments, i.e., stand oil and alkali-refined oil. The rheological behavior of fresh model paints was investigated, focusing on the effect of parameters such as pigment particle size distribution and water content. Fresh model paints were also analyzed with ThermoGravimetric Analysis (TGA) under air flow at a constant temperature (80 °C), and some selected paints with microUV—Pyrolysis—Gas Chromatography—Mass Spectrometry (microUV-Py-GC-MS). This enabled us to compare the induction period of the curing reactions, as well as the balance between oxidative degradation and polymerization when the paint formulation was systematically varied. Results show how, in the case of ultramarine blue, the natural pigments behave differently from the synthetic ones, both from a rheological and from a chemical point of view. The study demonstrates that all the parameters investigated strongly affected both the rheology and the chemical curing of the oil paints, which must thus be taken into consideration when carrying out systematic studies.