Absorption Rates | Oils

Oil absorption rate is explained here in order to help you understand why the quantity of oil varies in different colours. The oil absorption rate describes the ratio of oil to pigment that is required to create a viable paint. A ‘fat paint’ has a higher ratio of oil to pigment and ‘lean paint’ has a lower ratio. Oil paint is made from pigment and a binder; the binder dictates the drying time of an oil paint, and some oils are slower to dry than others. The answer to the question of what makes for a high oil absorption rate lies in the shape and size of the pigment particles. Oil must completely cover the surface of pigment particles in order to bind the pigment, and the smaller the particle, the more oil is required. That leads to transparent colours such as Alizarin Crimson. Also, the more irregular the surface of a particle is, the more oil is required to cover the jagged surface. This explains why fat paint dries slower than lean and why the oil absorption rate varies from colour to colour.

Video Transcript

0:08    Hi there, I'd like to explain how the oil absorption rate of pigment works. The oil absorption rate refers to the ratio of oil to pigment that is required during the colours manufacture in order to create a viable paint. Oil paint is comprised of pigment and a binder - a drying oil such as linseed oil. The oil absorption rate varies from pigment to pigment and has a bearing on how fat the colour is and in most cases, how fast it will dry.

0:40    A fat oil colour has a high oil absorption rate, whereas a lean one has a low rate. So, what makes one pigment absorb more oil than another? These over-sized models show pigment particle size and show one reason why oil absorption rates can vary. Within a volume of colour, the number of particles may vary according to their size. During manufacture, pigment particles need to be completely coated in oil in order to bind them to neighbouring particles and create paint.

1:16    The pigment particle, which might represent, say a cadmium red, has a large surface area to coat. But let's compare it to this agglomeration of small particles, which you'd find in say Alizarin Crimson. Although the cluster of small particles appear to be the same volume, they have a greater combined surface area to be coated. So this pigment would require more binder to create a paint.

1:50    Particle shapes of pigment also range from smooth or regular, to irregular and rough. This particle is smooth and fairly regular, but here we have an irregular particle, which although it has a similar size, has a slightly greater surface area to coat and so will again need a greater quantity of binder to create paint. I hope this has helped you to understand oil absorption rates.