
Soft pastels are
pure pigments that have been mixed to produce the required colour, this
is then mixed with a binder, called gum tragacanth and a small amounts
of china clay and or kaolin, and formed into sticks.
This is the purist
colour as an artist can obtain - not even oil paints can match the brilliant
intensity and purity of colours.
The amount of
binder used will determine the 'hardness' of the sticks themselves.
Thus pastel pencils and conté sticks have considerably more binder
than the soft versions made that are normally quite big and chunky in
comparison.
Pastels should
not be confused with chalks - these are limestone - tinted with dye
to produce different colours with only the most minute amount of pigment
present. 
I personally find
that the amount you pay for the soft pastel sticks determines their
quality and the amount of pigment present. The cheaper the pastels generally,
the harder they are thus more binder has been added, often you can see
that the sticks look dull and uninteresting. However if you open a box
of, say, Unison or Senellier pastels you are immediately struck by the
purity and excitement of living colour