The term medium has several different meanings in art. For starters, it is used to describe the substance that binds the pigment in paint. It is also called a binder, vehicle, or base. In acrylic paints, this is a synthetic substance. In oil paints, it is a natural oil such as poppy or linseed oil. In tempera paint, it is egg yolks. The plural is mediums.
A medium can also be something mixed with paint to change the body of the paint in some way.
For example, a gel medium is used to thicken paint for impasto. The plural is mediums.
To add to the confusion, the term medium is also used to describe the materials an artist uses, or simply the type of paint, such as oil, acrylics, or watercolours, as well the type of support, or surface used for painting. The plural is media. For example, the description “oil on canvas” indicates an oil painting done on a canvas surface; both oil and canvas are the media used. A “mixed-media” piece is one consisting of multiple media – for example, acrylic paint, graphite, and oil stick on a single surface or on a support consisting of canvas and newspaper.
For Oil Paint
Mediums for oil paints are made from mixtures of separate products with which an artist plays to develop his/her own recipe. For example, a traditional medium mixture is one part Damar varnish, one part stand oil, five parts turpentine and a few drops cobalt Mediums may also be bought in pre-made mixtures. Commercially made mediums can vary as much as home-spun recipes. Whether homemade or store-bought, mediums can be placed in a lidded glass jar and reused for many painting sessions
Although acrylics can be thinned with water, they lose quality in the process. Mediums can aid in the flow of acrylics without changing the hue or intensity of the color. In fact, acrylics can be heavily manipulated with the bevy of new mediums now on the market. Gels will thicken acrylics for an impasto effect, retarders slow drying time, and tinting mediums alter hue and/or color intensity.
Generally, watercolours are mixed with water for creating transparencies, but mediums such as Gum of Arabic or even acrylic mediums can increase the brilliance of colours. There are also several different mediums available to increase the fluidity and texture of watercolours.