oil pastels

Oil pastels can be used directly in dry form; when done lightly, the resulting effects are similar to pastel chalks. Heavy build-ups can create an almost impasto effect. Once applied to a surface, the oil pastel pigment can be manipulated with a brush moistened in white spirit, turpentine, linseed oil, or another type of vegetable oil or solvent. Alternatively, the drawing surface can be oiled before drawing or the pastel itself can be dipped in oil. It should be noted that some of these solvents pose serious health concerns.
Oil pastels are considered a fast medium because they are easy to paint with and convenient to carry; for this reason they are often used for sketching, but can also be used for sustained works. Because oil pastels never dry out completely, they need to be protected somehow, often by applying a special fixative to the painting or placing the painting in a sleeve and then inside a frame. There are some known durability problems: firstly, as the oil doesn't dry it keeps permeating the paper. This process degrades both the paper and the colour layer as it reduces the flexibility of the latter. A second problem is that the stearic acid makes the paper brittle. Lastly both the stearic acid and the wax will be prone to efflorescence or "wax bloom", the building-up of fatty acids and wax on the surface into an opaque white layer. This is easily made transparent again by gentle polishing with a woolen cloth; but the three effects together result in a colour layer consisting mainly of brittle stearic acid on top of brittle paper, a combination that will crumble easily. A long term concern is simple evaporation: palmitic acid is often present and half of it will have evaporated within forty years; within 140 years half of the stearic acid will have disappeared. Impregnation of the entire art work by beeswax has been evaluated as a conservation measure.