The uses of picture postcards

In her review of, “Van Gogh: Irises and Roses”, the Metropolitan Museum’s exhibit of four still lives painted by Vincent van Gogh in May 1890 using an unstable red lake pigment (“A Study in Scarlet: Evanescence in Bloom”, The New York Times, May 22, 2015), Roberta Smith mentions that a slide show which accompanies the exhibit includes postcards of the paintings, produced over several decades which clearly show the fading of the red pigment. Until now, I had not given much consideration to the usefulness of postcards and other inexpensive reproductions as conservation research tools, but from now on I shall.