Towards the understanding of Keim'sche Mineralfarben in architectural paints: Material characterization and phase quantification on selected historical pigment admixtures

‘ Keim’sche Mineralfarben’ are widely recognized silicate-based paints historically employed for painting and decorating architectural surfaces, as well as in the conservation and restoration of historic buildings and monuments, particularly where high-quality architectural finishes are required. Comprehensive knowledge of the composition of historical Keim’sche Mineralfarben pigment admixtures and their evolution—particularly for dating purposes—remains limited within the field of architectural conservation. The study demonstrates the effectiveness of a multi-analytical approach in characterizing the composition and phase quantification of selected yellow-to brownish-toned historical Keimfarben pigment admixtures and suggests a methodology for the characterization of this product on site, offering new perspectives on understanding the chronology of the paint layers, thus planning future restoration interventions. A multi-analytical approach was employed, combining optical microscopy, X-ray fluorescence, in situ Raman and micro-Raman spectroscopy, ATR-FTIR, and X-ray powder diffraction with Rietveld refinement. The results show that the studied Keimfarben pigment admixtures are composed of calcite, fluorite, kaolinite, quartz, barite, rutile, and witherite, as well as different quantities of hematite, goethite, and eskolaite depending on the hue. Certain identified phases have been shown to serve as diagnostic markers, limiting the application of the examined products on architectural surfaces to the period between the 1958 and 1980s. Moreover, quantitative analysis of the components may help elucidate their function within the admixture and will be essential for advancing the investigation into their relevance in the coating’s formation. • Multi-analytical approach reveals Keimfarben phase composition and pigment ratios • Specific compounds and phases date the pigment admixtures to the 1960s–1980s • Rutile, witherite, and fluorite serve as reliable chronological markers for Keim products • Fluorite of natural mineral origin was identified in all pigment admixtures • Raman and XRF techniques show strong potential for in situ paint layer identification