Investigating surface treatments and coatings, their history, application and detection on selected pigments: Lead White, Zinc White and Titanium White

The present work looks at pigment surface treatments and is divided in two parts. Part I explores the relationships between pigment manufacturers, raw materials suppliers, and the artist’s oil paint manufacturer, and describes surface treatments and coatings. This is followed by a summary of the history and characterization of surface treatments and introduces the role of pigments in the formation of certain paint defects in relation to these surface treatments. Finally, it presents a tentative identification of the surface treatments of the white pigments selected for study. The research in Part I involved collecting information through research into the patent and technical literature in conjunction with interviewing a global manufacturer and supplier of raw materials/pigments and an artist’s oil paint manufacturer. This exploration revealed that pigment surface treatments can be very complex consisting of either an inorganic or organic component or both, applied in very small amounts (up to 10% wt of the pigment). This work revealed that surface treatments may find their origins very early on, as in the 14th century Montpellier Manuscript, and that treatments re-emerge in response to industrial uses and needs not specifically linked to artist’s oil paint manufacturers. Materials such as waxes and stearates, known additives for oil paints, but also alkyds, developed as organic binders, were found to have been used as surface treatments for pigments. Part II focuses on the detection and identification of surface treatments on the selected pigments, Lead White, Zinc White and Titanium White, and describes the multi-analytical approach used to investigate their surface treatments with electron microscope techniques (SEM-EDX, FEG-SEM and (S)TEM-EDX) and hyphenated mass spectrometry techniques (EGA-MS, Py-GC//MS and Py-THMGC/ MS, and DTMS). This study highlights the importance of gathering information on the pigment’s provenance and material characterization and reflects on the implications of terminology and commercial secrecy for surface treatment detection and identification. Regarding the detection and identification of inorganic treatments, both FEG-SEM and (S)TEM-EDX proved to be effective for this purpose. These techniques also provided evidence which shed light on the method of manufacture of the pigments. Although the analysis of the organic fraction present in the pigment samples proved to be challenging, pyrolysis methods did enable the identification of anticipated materials (as was the case of the polyols in Titanium White pigments) as well as the discovery of unexpected compounds such as anti-oxidants