Ground Preparation for Altarpieces
What is an altarpiece ?
Altarpiece is a work of art adorns the space above and behind the altar in a Christian church.
The ground preparation of an altarpiece starts with the selection of the wood.
The poplar tree is commonly used for altarpieces, carpenters chooses the middle part of the wood because it is the most straight among the other parts.
They held together the wood panels by using nails or joints. There are different types of joints present such as butterfly, biscuit, etc. Later panel is coated with animal glue to make panels stick together better. After they soak linen cloth into animal glue and lay on the panel to stabilize the imperfections.
Gesso application starts: Gesso is made of calcium carbonate, water and glue.
First, Gesso Grosso (thick layer) application, as Cennino Cennini recommends artist apply Gesso gross more than one layer which is another characteristic differentiated Gesso grosso from Gesso sottile. On top of Gesso grosso the thin layer of gesso sottile applied only one layer. After it dries, dust with charcoal to smooth the surface.
Underdrawing: Sketch the design with charcoal or “spolvero” technique. he use of brush or pen the underdrawing becomes permanent.
Than artists outlines the figure with a help of stylus to plan areas to be gilded and to be painted. The gilding process comes first because it covers the whole background of the altarpiece.
Gilding process: the application of a mix called “Bol” made of glue and reddish-brown clay on areas to be gilded. Later, water is applied to reactivate the glue. Than the gold leafs are applied with a thin brush on moisturized surfaces since gold leaf only stick to area that are wet. After gold leaf application is finished, they burnish with a dog tooth or wolf tooth to last longer.
The gilding requires a gessoed surface to reflect the gold leaf's shine. When applied on a plain surface it appears rather green.
Decoration of altarpiece: Tooling and Punching methods. Tooling is the type of decoration made with the help of compass and a thin stylus, for example halos around the divine figures. Punching is the hold of hammer with a stylus perpendicular to the surface and creating ornamental elements.
For final step, the painting with tempera.
Mordant Gilding: it is another type of gilding that consists oil inside. It is made of oil, varnish, lead white, and verdigris. Mordant gilding is commonly used for decorative purposes; applied with a thin, small brush on small areas.
