Jacopo di Cione: Gold

'The Crucifixion' is a small altarpiece, painted in Florence around 1369-70 by Jacopo di Cione, with the possible assistance of another painter. It shows Christ and two thieves on the cross, surrounded by depictions of saints and three-dimensional architectural flourishes.

Perhaps the most striking aspect of this picture is its large-scale use of gold. 

Water gilding is used to make large, glossy expanses of gold. First a surface must be made flat through painstaking layering of glue size (liquid animal glue) and gesso (a form of calcium sulphate), followed by a thick layer of red bole (a slippery clay rich in iron oxide).

A thin layer of water is brushed onto to this layer and then the gold leaf gently placed on top. The gold surface is then burnished until smooth. Gold leaf is so thin that when it is applied on top of the bole the red colour shines through, giving the gold a warm glow.

The final product looks like a solid block of gold. However, if we look closely at the small scratches in the gilded background of 'The Crucifixion' we can just see hints of the red bole layered beneath.

The flat expanse of water gilding can also be textured using tools to create incised lines, rings and punch marks. This breaks up the uniformity of the gold and makes it shimmer, creating beautiful ornamental effects. The haloes in 'The Crucifixion' were created in this way.

Three dimensional gilded patterns can be made using pastiglia (applying thick gesso to make a raised design before applying the bole, water and gold). Pastiglia can be seen in the floral decoration throughout the outer frame and architectural elements.

Mordant gilding is used to create gold details. A sticky adhesive, or ‘mordant’ (usually made of pigmented oil), is added onto the painted surface. The gold leaf is applied and sticks to the mordant. In 'The Crucifixion', this method is used to create embellishments on fabric.

Gold leaf can be ground into powder and mixed with a binder (i.e. egg tempera or oil) just like paint. This makes 'shell gold' which is used to create very fine details. The intricate decoration on Christ’s loincloth is likely to have been painted in shell gold.

It is not just gold that can be treated in this way. Other metals, such as silver, can also be beaten into sheets and ground up in a binder to create a paint. Many details in 'The Crucifixion' were made with shell silver, such as the armour and the tips of the raised lances. 

These fine silver details would have originally shone bright, contrasting with the warm glow created by the large expanse of gold. Unfortunately, silver is much more reactive than gold and tarnishes over time, as such, the areas in which shell silver was used now look dark brown.