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 40 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. Surface and techniques The surface chosen for oil pastels can have a very dramatic effect on the final painting. Paper is a common surface, but this medium can be used on other surfaces including wood, metal, hardboard (often known as "masonite"), MDF, canvas and glass. Many companies make papers specifically for pastels that are suitable for use with oil pastels. Building up layers of color with the oil pastel, called layering, is a very common technique. Other techniques include underpainting and scraping down or sgraffito. Turpentine, or similar liquids such as mineral spirits, are often used as a blending tool to create a wash effect similar to some watercolor paintings. Commercially available oil sketching papers are preferred for such technique. Grades There are a number of types of oil pastels, each of which can be classified as either scholastic, student or professional grade. Scholastic grade is the lowest grade; generally the oil pastels are harder and less vibrant than higher grades. It is generally meant for children or new users of oil pastels, and is fairly inexpensive compared to other grades. The middle grade, student grade, is meant for art students and is softer and more vibrant than scholastic grade. They are usually more expensive. Professional grade is the highest grade of oil pastel, and are also the softest and most vibrant, but can be very expensive. See also Oil stick References Further reading Leslie, Kenneth. Oil Pastel: Materials and Techniques for Today's Artist, Watson-Guptill Publications, 1990. . M.H.Ellis, "Oil Pastel", in Media and Techniques of Works of Art on Paper, New York University Conservation Center of the Institute of Fine Arts, New York, 1999. Elliot, John. Oil Pastel: for the Serious Beginner, Watson-Guptill Publications, 2002. . External links The Oil Pastel Society https://web.archive.org/web/20070716232500/http://palimpsest.stanford.edu/waac/wn/wn21/wn21-1/wn21-106.html for a discussion of the evaporation and efflorescence. Visual arts materials Visual arts