The whitening of oil paints films containing bone black

Whitish spots were observed in the dark paint of 17th century oil paintings from the Oranjezaal (Huis ten Bosch, The Hague). As a result, the areas originally intended as black and dark brown - hair, eye pupils, shadows - have dramatically changed in appearance. Through chemical analysis, this colour change is attributed to the degradation of bone black: the organic part responsible for the black colour is decomposed. Other examples were investigated including a painted ceiling from the Johan de Witt House and two paintings by Rembrandt. Here, the whitening was caused by lead soap crystals or by degraded lakes in glazing surface layers.