Oil pastels are quite a new medium when compared with most of the others that have been used for centuries. Picasso said in 1947 that he wanted a coloured pastel that he could use on almost any ground without having to prepare or prime it. Two years later the Sennelier company came up with the first professional oil pastel. The very wide range of greys were chosen by Picasso.
I use oil pastels when I think that I am painting in too tight a style. They are not my natural medium so I think that struggling with them does me good. The first time that I used them was so disappointing. My final work looked no better than work that I did at school using school crayons.
Completed oil pastel paintings never dry, so they can always be worked on. Because of this when completed they need to be framed behind glass. As mentioned earlier, these pastels can be used on almost any surface. Paper made specifically for the pastels is available, but I don't like the texture of these papers. I enjoy working on mountcard.
I always split any new pastels in two. One half I use for traditional drawing, the other half I use for putting down larger flat washes - good for building up the colour in layers, or for scrumbling colour over blended layers.
The technique that I use more than any other when painting with oil pastels is the blending technique. Pastels made by different firms have different grades of hardness/softness. I find that Sennelier oil pastels are softer than most. Using these pastels I am able to create a smooth and gradual transition between colours or tones by rubbing with my fingers. If I am using harder grade pastels I occasionally use a rag to blend the pigment. Some oil pastelists use sand paper as their ground. I don't, but if I did I would rub less with my fingers and more with a rag.
My favourite way of blending the oil pastels is with the use of solvents. There are a number of odourless solvents on the market, but I prefer turps as I find that it works more aggressively on the pigment giving me a smoother gradation. Another reason that I like turps won't go down well with the safety brigade, but I like my studio to smell like an artists workshop so I almost always use turps.
My way of working is to build up layers of pigment using the flat side of the pastel, and then blend in using turps and a brush. I work layer over layer, blending when needed. I finish by adding detail with the point of the pastels. I prefer to compromise my pigment between fairly runny and fairly stiff so that I can manipulate it better. I don't use masking techniques very often, but on the odd occasion that I do I find it easier using turps and brush than using fingers.
I recommend that if you haven't used oil pastels before that you go out and purchase some. They are not too expensive and I am sure that you will enjoy using them.