Pastels in 1930. However, none of these were comparable to the professional quality oil pastels produced today. These early products were intended to introduce western art education to Japanese children, and not as a fine arts medium, although Sakura managed to persuade some avant-garde artists to acquaint themselves with the technique, among them Pablo Picasso. In 1947, Picasso, who for many years had been unable to procure oil pastels because of the war conditions, convinced Henri Sennelier, a French manufacturer who specialized in high quality art products, to develop a fine arts version. In 1949, Sennelier produced the first oil pastels intended for professionals and experienced artists. These were superior in wax viscosity, texture and pigment quality and capable of producing more consistent and attractive work. Picasso requested that these be produced in 48 different colours. The Japanese Holbein brand of oil pastels appeared in the mid-1980s with both student and professional grades; the latter with a range of 225 colours. Use Oil pastels can be used directly in dry form; when done lightly, the resulting effects are similar to oil paints. 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. 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