Song of the Angels
Here is a link to another outstanding article by George O'Hanlon for Natural Pigments of California: Painting for Posterity. Have you ever wondered what is the effect of humidity on the medium in which you paint? Should your canvas be stretched on supports? What exactly is oil paint? Old world vs Modern oil paints? Additives? It's all comprehensively covered in this article.
Photo Credit: Songs of the Angels by William Adolphe-Bouguereau, 1881, oil on canvas