Differentiation between Iron-II-Sulphate (Copperas) Staining and Natural Ochre Pigmented Lime Finishes on Lime Plaster

ABSTRACT

Warm yellow facades, popular for centuries and throughout the world, were typically painted with ochre but could be copperas-based. Copperas is iron sulphate mixed with lime, and oxidation produces a golden colour. Usually, the methods are distinguished by appearance. Knowing the original painting method is critical for historical context and conservation, so the research program evaluated characterization methods to supplement visual assessment. Mock-up lime plaster boards were painted with copperas and yellow ochre paints. XRF, optical microscopy, SEM, EDS and XRD were conducted on surface samples, cross sections, and particles. Results show plaster staining for copperas solutions, optical differences of paint particles in crossed polars, and differences in surface morphology of finishes and pigment grains. The experimental program revealed how strongly the preparation of materials and application technique impacts the appearance and properties of the resulting finishes.

KEYWORDS:

• Copperas
• ferrous sulfur
• historical painting technique
• optical analysis
• plaster staining
• vitriol

Acknowledgments

The authors would like to express their gratitude to Mr Frankie Chan, Senior Technical Manager and Dr. H. M. Karen Wong, Technical Officer at the Electron Microscopy Unit of Queen Mary Hospital, Hong Kong for their assistance with the SEM. The authors would also like to express their gratitude to Dr. Joe Michalski and Jiacheng Liu at the Planetary Spectroscopy and Mineralogy Laboratory at the University of Hong Kong for their help with carrying out and evaluating the XRD analyses.

Disclosure statement

No potential conflict of interest was reported by the authors.