Virgin and Child Adored by Saints Mary Magdalene and Nicolas of Bari; Christ Crucified with the Virgin and Saint John the Evangelist (1400s) by Sano di Pietro - Public Domain Catholic Painting
The artist covered the sides and reverse of this panel with a thin layer of gesso (a type of plaster), allowing the texture of the rough wood to remain visible. He then painted it to resemble porphyry, a beautiful and valuable stone.