This study investigates the use of white linen as a support in early canvas painting. Documentary sources and the results of technical examination of 15th and 16th century canvas paintings contribute to the hypothesis that bleached canvas was frequently used and that this white support had an impact on the appearance of these works. To study the effects of bleaching treatments on canvas, 16th century St. Gallen bleaching practices were simulated. The combination of alkaline and mechanical treatment with sun-bleaching changed the optical and mechanical properties of the canvas as well as the morphological and chemical properties of the fibres. These changes are studied by SEM/EDX, FTIR and GC-MS and results are described.