Once the gesso has dried and any raised detailing applied and smoothed the final stage of building up the substrate is to seal its surface.
This is done using a traditional ceramic based paint called boll. This is made up of a pigment and rabbit skin glue.
Up to three coats of boll may be required. The particulates in the boll fill in the pores in the gesso making its surface semi-permeable and ultra-smooth -once its surface has been burnished with a soft rag.
As well as serving as a sealant, the boll acts as an undercoat to the leaf. Traditionally an ox blood red is used under gold and black or grey under silver leaf.
For a more contemporary look I have experimented using different coloured boll underneath the leaf and then rubbed the leaf back to partly expose the boll to create a striking two-tone effect.
Again, various alternatives to boll have been developed but this traditional paint gives the metal leaf a greater lustre as it bonds with the gesso to form a harder more durable surface, which can be burnished using a hard agate stone.