Panel painting is a system of painting, which has evolved from Byzantium views to enable very fine craftsmanship to take place (even the laying of gold leaf), and for the painting to be portable.
These objects were used as altar decorations "reredos" or portable devotional objects, in their peak of evolution in the Italian Renaissance. Wooden frame and panels were incorporated from the start (so were the doors if there were to be any) if the painting surface is to be wider than a plank of wood it is but jointed. Usually this surface is covered with a layer of linen cloth. All are painted in many coats of gesso; gypsum or whiting are combined with warm animal glue and painted on in many layers, sanding between each coat. The end result is a perfect smooth painting surface. This extremely absorbent surface is sized to reduce this. The medium used with the pigment is the egg yolk. All these materials are capable of taking wide variations of climatic conditions; a painting incorporating these techniques is a painting that will survive many centuries.