Underdrawing and composition
Although it is no longer on panel, the painting would originally have consisted of a single piece of wood, probably poplar, in a simply moulded frame. Several layers of gesso, a mixture of calcium sulphate and animal glue, would have been painted over the panel surface and the frame to create an even white surface which would be smoothed and polished before painting began. The original frame no longer exists and the painted black border, seen in these images but not when the painting is in its frame, is not original.
The composition would then have been drawn onto the panel. Most of the elements of the composition were incised into the gesso, including perspective lines of the architectural setting, the apse vaulting and contours of the figures. Incisions allowed the artist to see the composition even when gold or paint layers covered the outlines. The perspective lines converge at the centre of the composition on the cross on the altar, rather than on San Bernardino who occupies a far less prominent position than one would expect.
Follow the links in the gallery below to find out more about the techniques used in this painting.