MITRA Forum Question Details

​While some oil colors have enough oil to reduce simply with solvent, most will be reduced too much to retain the critical optical function of oil in the glaze. Generally, application of paint in thin "washes" with solvent alone tend to settle in a dull, lackluster layer that is weakly bound and lacks optical clarity. Viscosity is important in glazing, because the consistency must preserve pigment suspension, but also allow manipulation and removal while wet. A small amount of bodied oil (stand oil or sun-thickened linseed) to the medium lends this property, but too much makes the paint sticky and difficult to distribute and wipe away.  A good glazing medium should also have enough tack to prevent beading. Composition of the underpainting is important here too- colors need to be resistant enough to wipe clean, if necessary and to resist staining from subsequent layers. 

One method of preparing glazes from modern commercially-prepared oil paint is to add low-refractive index white pigment—commonly referred to as "extender pigments" or "fillers"—to the paint. While this increases the pigment volume concentration (PVC) of paint, a small amount of oil may be added to maintain the paint near its critical pigment volume concentration (CPVC). Pigments of low-refractive index, i.e., chalk, silica, kaolinite, etc., do not absorb or scatter light as do color pigments or high-refractive index white pigments, and hence appear translucent in oil paint. These were commonly used to extend paint by the old masters and can be used to create glazing tints with color pigments. Extender pigments also alter the rheology of paint to provide interesting behavior in their flow and brushing properties.