Some of the processes required pigment to be moved and removed with papers, cards and plastics. Several different types were used. They developed their own mark making vocabulary.
Light refraction affects what is seen. The images (above and below), are photographs of the same item, but taken with the light source at slightly different angles.
Gesso treated paper created delicate cloud forms (presented here on slate).
The gesso surface of this sample has been highly polished.
As has been mentioned elsewhere, photographing these works has been difficult and will be an area requiring development, or specialist photography.