Paintings are not always what they seem to be on the surface. Technical investigation, particularly of pigments, has revealed not only surprising differences between the present and the original appearance of works – a violet colour transformed to pale orange, for example - but also the use of unexpected ingredients for certain effects. Recognising the changes to colouring as well as identifying materials can lead to re-evaluation of both the meaning and sometimes the date and attribution of images. This talk uncovers the practices of artists as different as Hilliard and Reynolds; highlighting those of Elizabethan portraitists in the lifetime of Shakespeare. It also asks what this new information means.