Gypsum is tested for fineness in the usual way by rubbing up with turpentine and alone does not work well under the brush. It is readily colored by the oil it is ground or mixed in, no matter how colorless and transparent the pigment itself may be. When used with paint it is always considered as an extender or adulterant because of its comparatively low price. When used as extender for white lead or any other white paint it does not improve the hiding power of the paint, rather the reverse, but it will not discolor white, and in that connection it is useful. A further use for it is as an extender for solid colors from whose brightness it does not deduct, but rather improves the tone. Take, for instance, a color like chemically pure Indian red or red oxide of iron. If ground in oil and thinned for use the color will look dull when painted alongside of the same red that has been extended or stretched with 50 or 60 per cent of gypsum. Ultramarine blue extended with gypsum will look far more brilliant than the straight color, though of course it will not cover up so well. The one great drawback to an extended use of gypsum is, that when used in large percentages, especially with oxide of iron pigments, it is so very prone to settle and cake hard in bottom of containers, even when in stout paste form. When it can be used in connection with a portion of whiting or asbestine, or even with china clay, it will not be so bad on the settling, but it is at its worst when barytes or silex are present.