Silverpoint
Using a metal point of silver or gold on a rough ground is an ancient form of drawing used before the introduction of lead pencils. The ground is made of gum Arabic mixed with ground chicken bones, egg shells or beach shells, with titanium pigment and tint added if required.
When drawing, neither the width nor the tone value of the line responds to changes in pressure, so applying greater force on the stylus would only tear the paper. The Renaissance artists used this method to teach drawing as lines cannot be erased and accurate observation is required. Over time the silver lines oxidise turning a brownish colour, reflecting the process of change.. The technique feels much like carving in stone, slowly finding the form by degrees of cross hatching.
The challenge of working in this medium gave a special meaning to the bird portraits, giving a clear connection to the birds themselves.