90 PAINT TECHNOLOGY AND TESTS 



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. 



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 



Microscopic View of Barytes under 

 Polarized Light 



throughout, except in such places where crystalline substances 

 are present, when the crystal is shown up with beautiful dis- 

 tinctness. Photomicrographs of various single pigments and 

 pigment combinations are shown under Chapter III. 



Effect of Pigments on Oil. Certain pigments have the prop- 

 erty 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 



