And how is it different from an oil or watercolour?
Pastel is a painting in chalk with a soft and detailed finish. The technique allows for a sense of movement and animation. 19th Century artist Edgar Degas and impressionist Mary Cassatt produced some of the finest pastel paintings.
I use a combination of Rowney soft pastels, which have bright colours and can be readily smudged and blended for background work, and Stabilo pastel pencils, for the finer detail, working on Canson fine art paper. Pastels will last well when framed behind glass in a mount.
Oils are oil based and therefore slow drying. They have a feeling of tradition and permanence. Oil paintings are built up from a series of "layers", which need to be allowed to dry before a new layer can be applied. I don't undertake portraits in oil. However, I have including one of my semi-abstract works in oils here - this will hopefully give you an idea of this medium. The work is entitled "Dwell" and was undertaken for an album cover in 2017 for Prayerscapes.
"Tintern Abbey: The Crossing and Chancel, Looking towards the East Window."
J.M.W Turner (1794, graphite and watercolour on paper. Photo in the Public Domain.)