The paint used for icons is known as 'egg tempera' - the base being the yolk of an egg (mixed and thinned with water and vinegar, for preservation).
Icons are painted or 'written' (as they are considered to be 'a visual gospel') on wooden boards that are covered with a thin material canvas. Onto this is painted a chalk (or 'gesso') surface. This chalk gesso imitates the limestone (chalk) walls on which Ancient Egyptians painted - a surface ideal for preserving this kind of paint.
The painting itself is made by the darkest colours being laid first, and successive layers becoming lighter and brighter each time. This process gives an extremely rich gradation of colours and a beautiful shine to the faces in the icon.
As a guide, it may take around 20 hours to complete an icon as small as an A4 sheet of paper - from start to finish. For an icon with dimensions as large as 4ft x 8ft it may take around 250-300 hours to complete.