The polarizing microscope has been used to aid the conservator in treating a painting through increasing his knowledge of the materials and structures with which he is dealing. With polarized light microscopy he can see and identify crystalline materials, which include most pigments, and distinguish them from media and surface coatings, using crossed and slightly uncrossed polars with varying combinations of transmitted and oblique top lighting. An area of a painting has been quickly and completely investigated using two fragments, as small as 25 mum wide, one a sectional fragment mounted and rolled in Aroclor 1260 and the other a crushing mounted in Aroclor 5442. These mounting techniques have eliminated the need for time-consuming solid mount embedding techniques. The Aroclors' 1. 66 refractive index enabled more extensive identification of pigments than would commonly used mounting media in the 1.53 range. These mounting techniques and polarized light microscopy have helped the conservator solve problems such as distinguishing very old overpaint from original paint, comparing materials and techniques used in different areas, matching inpainting colors more readily, and monitoring removal of overpaint and surface films by mechanical means. Compared with more elaborate equipment used for identification of materials, such as spectroscopy, polarized light microscopy produced more specific information, used smaller samples, and was readily accessible within the conservation laboratory. (See also 8-1829 for additional information.)