4.2. Polarized Light Microscopy (PLM)

Polarized light microscopy (PLM) is a common method of pigment identification based on the optical properties of the crystals. For PLM, pigment particles were scraped from the bulk paint with a scalpel, transferred to a glass slide, and covered with a coverslip. The heat-reversing mounting medium, Meltmount™ (refractive index of 1.662 at 25 °C) (Cargille Laboratories, Cedar Grove, NJ, USA), was melted at approximately 70 °C and then injected between the cover slip and the slide. After cooling, the pigment particles dispersed in the mounting media. A Nikon LV100ND microscope with a polarized attachment and a DS-Ri2 microscope camera were used for sample observation and photomicrography, using NIS-Elements software (Nikon Instruments, Mellville, NY, USA).