I normally use for my portraits is Acrylic,
which was developed in the 1920/30s in South America,
to withstand climatic conditions.
dries quickly, within a few hours & no further
chemical reactions occur, therefore the finish is
permanent, flexible & not prone to yellowing.
The versitility of the medium allows it to be diluted to
give the appearance of water colour, alternatively it can
be used straight from the tube to achieve the creamy
consistency of oil paint. The finished portrait
cleaned, simply by wiping over with a damp cloth.
Portraits can also be completed in 'Pastels',