Polar Micro-Examinations and Photomicrographs. By Polar Micro-Examination is meant the examination of pigments under polarized light. A polarizing apparatus, though not an essential in the hands of the paint chemist, is nevertheless much to be desired, for by its help deductions may be drawn as to the contents of a paint, which by other means might not be possible. The polarizing apparatus as marketed by most manufacturers of the microscope is attached in the following manner:
The diaphragm immediately under the sub-stage container is swung out and opened to its widest limit, allowing the insertion of the polarizer. This polarizer carries one of the pair of Nicols prisms and is countersunk to allow of the introduction of gypsum or selenite plates. The analyzer fits over the eyepiece on the tube.
The use of polarized light upon paint is valuable on account of its action upon crystalline substances. The re-enforcing pigments, such as Asbestine, China Clay, Gypsum, Silex, Barytes, etc., are crystalline and consequently act upon the polarized light. In most cases these pigments are used in ready-mixed paints in small amounts, varying between 5 and 25%. When a slide containing a small amount—for example, less than 3%—of these crystalline pigments is examined under the microscope by ordinary transmitted light, they will often escape observation, owing to the small amount in which they are present. However, in the case of polarized light, this could hardly happen.
Microscopic View of Barytes under Polarized Light
A slide of paint containing these re-enforcing pigments is prepared in the usual manner. On examining this under the microscope and using the polarizing apparatus, the crystalline pigments are at once detected by revolving the analyzer. At one position of the analyzer, one sees an ordinary field, as with transmitted light, but if one revolves the analyzer, the field gradually becomes darker until total darkness is obtained throughout, except in such places where crystalline substances are present, when the crystal is shown up with beautiful distinctness. Photomicrographs of various single pigments and pigment combinations are shown under [Chapter III].
Effect of Pigments on Oil. Certain pigments have the property of acting upon the linseed oil in which they are ground, forming metallic linoleates which accelerate the drying of oil. This is especially true of lead and zinc pigments. The inert crystalline pigments, when ground in linseed oil and painted out, distribute the oil so as to allow a great surface to be exposed to the air. Thus by physical action, and possibly catalytic or contact action, these inert pigments stimulate the drying of oil paints in which they are ground. Lead and zinc paints, of course, have the greatest drying values on account of the added effect of the linoleates formed, as outlined above. The writer has made a series of tests in which the action of various pigments upon linseed oil is shown. The tests were made in the following manner:
Five grams of each of a series of commonly used paint pigments, including those of inert crystalline nature as well as the more valuable amorphous pigments which are considered more or less chemically active, were ground separately in an agate mortar, with 5 grams of raw linseed oil. The ground paste in each case was placed in a marked glass beaker, and allowed to stand in a dustless section of the laboratory for one month. The oil-pigment paste from each beaker was then separately extracted with benzine to remove the linseed oil from the pigment. The benzine solutions of oil were then heated to remove the benzine and the residue of oil burned to ash in crucibles. The ash from each test was weighed, and if it ran above the percentage of ash determined on a blank sample of linseed oil (namely, .003%), the ash was analyzed qualitatively for metallic constituents. The following table of results shows the percentage increase in ash, as well as the constituents of ash on the various samples tested:
Table of Results
| Pigment in Oil | Per cent. of Ash in Oil Extracted from Oil-Pigment Paste | Analysis of Ash | |
| Raw linseed oil without pigment | 0.003 | — | |
| Barytes | 0.003 | — | |
| Blanc fixe | 0.003 | — | |
| Silica | 0.003 | — | |
| Asbestine | 0.005 | — | |
| China clay | 0.007 | — | |
| Whiting | 0.008 | — | |
| Chrome yellow | 0.025 | Lead oxide (PbO) | |
| Lithopone | 0.031 | Zinc oxide (ZnO) | |
| Prussian blue | 0.032 | Iron oxide (Fe2O3) | |
| Sublimed white lead | 0.033 | Lead oxide (PbO) | |
| Zinc oxide | 0.105 | Zinc oxide (ZnO) | |
| Corroded white lead | 0.116 | Lead oxide (PbO) | |
| Red lead | 0.211 | 2 | Lead oxide (PbO) |