This remarkable staining effect known as ‘tree calf’ binding occurs when copperas (iron (II) sulphate) and pearl ash (potassium carbonate) interact to produce a tree-like pattern on the boards. The recipe and technique is believed to have originated in Germany. It involves bending and angling the boards so that when the copperas and pearl ash solutions merge they form rivulets which bleed down the centre of the book. The effect mimics the branches and trunk of a tree. This is an excellent example, and was used to bind George III’s personal copy of Metamorphoses held in the King’s Library.