Pastels are......

A  strong   colourful way of painting. The medium itself is
dry and because of its powdery surface the light reflects
well. So, with pastels, they give out very intense colouring,
more so than any other medium.
Therefore it will be longer lasting than oil paint, simply
because it has no varnish or oil contained in it to yellow
and crack over the years.

Amazingly enough, pastels used in the 18th Century, are
still lovely, bright and fresh today. You can frame pastels
under glass as long as there is a mount mat between the
surface and the glass, as you do not want the glass to
come into contact with your picture.
In actual fact, strangely enough, the pigments in pastel
paints are just the same as the ones used in oil and
watercolours. The main difference is how they are
produced. For example, watercolours are made by mixing
gum with pigment and pastels have chalk and water mixed
with the pigment so that it forms a paste.