Oil pastels' non-drying binder can cause problems.
Artists and art students use oil pastels, also known as "wax oil crayons," for both painting and drawing. Oil pastels are usually cheaper than chalk pastels (which are generally called just "pastels"), so art students and children often use oil pastels. Professional artists use oil pastels more often in oil paintings, as oil pastels are made from similar elements and can add a more drawn element to paintings. But oil pastels' chemical composition has several drawbacks.
Spreadability
Unlike more powdery chalk pastels, oil pastels are pigment combined with a sticky and non-drying binder. Oil pastels thus do not spread as easily as chalk pastels and cannot achieve the same effects.
The non-drying binder also makes it harder to apply fixative to a finished drawing. Without fixative, unintended rubbing or smearing can occur.
Fixing Mistakes
Because oil pastels do not spread easily, they are not easy to erase. Artists using oil pastels must plan drawings and paintings carefully, because a stray mark will remain. It's possible to cover a mark with more oil pastel--however, a surface covered with thick marks is not always the desired effect.
Durability
Because oil pastels don't dry, the oil of the pastels will penetrate paper over time and create a grease stain. This degrades the paper and makes it difficult to preserve a drawing made with oil pastels. The stearic acid and wax in oil pastels can also result in "wax bloom," where the fatty acids in the oil pastels create a milky white layer. Though gentle rubbing with turpentine or mineral spirits can remove this layer, this is a potentially corrosive technique to use on older material.
