Mixing a pigment with linseed oil and making a paste is a preliminary test applied to pigments, known as oil absorption, that has been in use in a systematic manner for almost a century and perhaps even longer in a less defined manner. This proved to be a practical means to qualitatively characterize the approximate color and texture of pigments quickly with materials and tools readily at hand.
For a long time, the paint industry formulated on a weight basis. In the early part of the twentieth century researchers concluded from a statistical study of house-paint test results that the pigment/binder system should contain at least 28% pigment by volume. Not long after this, reseachers related optimum performance of exterior paints to their critical oil contents, and simple graphical methods for determining this parameter were devised.
From such studies, the relationship known as the pigment volume concentration (PVC) came into use. Studies such as these led to the critical pigment volume concentration (CPVC) concept, and its importance to paint formulation was established.
At the time, drying vegetable oil, alkyds and later acrylic latex were the predmoninate binders used in these studies. These binders envelop pigment particles in a solid film.
High PVC paints, which include most tempered paints, such as egg tempera, casein, distemper (animal collagen), etc., are unlike paints in which the binder envelops pigment particles, such as oil and acrylic paints. High PVC paints rely on the adhesiveness of the polymers of proteins or polysaccharides to bind pigment particles, but do not cover all surfaces of the particles, and for this reason are naturally high PVC films, because voids are created between pigment particles as the water in these systems evaporate.
These porous films are not only suscpetible to moisture and solvent vapors, but readily absorb liquids. These binder systems are less successful as exterior paint, and usually were limited to interior paint applications. This, however, did not limit their use in fine art painting, since pictures are typically kept indoors.
The CPVC for high PVC systems (tempera paint) will be diferent from that of oil and alkyd paints. In fact, they share similar characteristics to acrylic paints where the CPVC is typically lower than for oil paint. The problem is determining the CPVC for tempera paint based on the pigment or pigment blend and binder. And secondly how to make the caulation practical for artists.