The masks date from 1994 to 1999. They are a strong departure from his previous work.

This text from Pat Utermohlen's 2006 essay helps clarify their context.

In these pictures we see with heart-breaking intensity William’s efforts to explain his altered self, his fears and his sadness. The great talent remains, but the method changes. He sometimes uses water-colour and paints a series of masks, perhaps because he could more quickly express his fear. In both the oils and water-colours these marvellous self portraits express his desperate attempt to understand his condition. There is a new freedom of expression, the paint is applied more thickly, art-historically speaking the artist seems less linear and classical, more expressionist, and I see ghosts of his German heritage.