recently, digital image processing has been used – particularly in advertising – to recreate the appearance and effects of hand-colouring. Colourization is now available to the amateur photographer using image manipulation software such as Adobe Photoshop or Gimp. Materials and techniques Dyes Basic dyes are used in the hand-colouring of photographs. Dyes are soluble colour substance, either natural or synthetic, in an aqueous solution, as opposed to pigments which are generally insoluble colour substance in an aqueous suspension. Aniline dyes, the first synthetically produced dyes originally used for the dyeing of textiles, were first used to dye albumen prints and glass transparency photographs in Germany in the 1860s. When hand-colouring with dyes, a weak solution of dye in water is preferred, and colours are often built up with repeated washes rather than being applied all at once. The approach is to stain or dye the print rather than to paint it, as too much paint will obscure photographic details. Blotting paper is used to control the amount of dye on the surface by absorbing any excess. Watercolours Watercolour paint has the virtue of being more permanent than dyes, but is less transparent and so more likely to obscure details. Hand-colouring with watercolours requires the use of a medium to prevent the colours from drying with a dull and lifeless finish. Before the paint can be applied, the surface of the print must be primed so that the colours are not repelled. This often includes prepping the print with a thin coating of shellac, then adding grit before colouring. Watercolour paint used in photographic hand-colouring consists of four ingredients: pigments (natural or synthetic), a binder (traditionally arabic gum), additives to improve plasticity (such as glycerine), and a solvent to dilute the paint (i.e. water) that evaporates when the paint dries. The paint is typically applied to prints using a soft brush. Watercolours often "leave a darker edge of colour at the boundaries of the painted area." Since different pigments have varying degrees of transparency, the choice of colours must be considered carefully. More transparent pigments are preferred, since they ensure greater visibility of the photographic image. Oils Oil paint contains particles of pigment applied using a drying oil, such as linseed oil. The conventions and techniques of using oils demands a knowledge of drawing and painting, so it is often used in professional practice. It is necessary to size the print