Soap Ground Aquatint
Soap ground aquatint or white ground aquatint is a technique in which the artist paints with a ground made of soap, fat and white pigment directly onto a plate already prepared with an aquatint.
The soap ground is a semi-permeable resist, which means that acid will penetrate it slowly. When exposed to acid, the soap ground will protect the plate depending on how thickly it has been applied, longer where it is thicker and shorter where it is thinner, resulting in a wide range of tonal values in the print.