Masterpieces hanging on the gallery walls of the National Gallery in London were never really seen. Not the whole story, anyway. The sketches that were done on the plain, white canvases that were the guidelines to greatness, called "underdrawings", were sometimes masterpieces themselves.
Infrared photography has let curators see what was hidden under the paint for the last few decades and taught invaluable lessons on the techniques used by not only the different artists but the chemistry of the paints themselves. Surprises were seen on all levels as a painted, minute detail was seen to have gotten it's start as a messy scribble or in the case of da Vinci's "Mona Lisa", several complete versions of the masterpiece had been rejected and painted over. Though the photographic process was there to see into the past, it was a tedious process to piece the photographed patches together and never quite made a perfect version of the art. Thanks to computers and the art of stitching, those perfect touches can now be put accurately and quickly on the secret creations.

Using infrared cameras, fifteenth- and sixteenth-century paintings were photographed by the National Gallery in London. The photos were then matched together by computer to form an unlined view of what lies beneath some of the world's masterpieces by Raphael, Bruegel, Crivelli, Cranach and even the unknown "Master of 1518", who painted the beautiful "Flight to Egypt", one of my favorites. It shows the Virgin Mary and the Baby Jesus fast asleep in the arms of the Sphinx. :)

Infrared light passes through most paint pigments but carbon-rich charcoal, inks and paints often used for underdrawing, absorb it. This causes those areas to remain dark.

Some underdrawings are very beautiful. Raphael's preparation for "Madonna and Child with the Infant Baptist", painted around 1509 is "a wonderful work in its own right", says curator and conservator David Bomford.

Underdrawings also highlighted how none of the great masters worked alone. Workshops where assistants did the boring jobs like as pigment grinding were sometimes given the honor of painting "safe" parts of the work. Some seem to be almost all the work of expert apprentices who signed their Mentor's name. It was typical for an apprentice to excel in a facet of the process - one might sketch only animals, another might be expert at buildings or perspective. Some paintings were found to have two or three versions of themselves hidden under the final product.

Artists often made preliminary cartoons on paper, then transferred them to a white-primed wooden panel as a guide. This could be done by 'pouncing' - pricking the lines of the cartoon with a needle and then dusting it with powdered charcoal to leave an outline of black dots on the board. Such dots are often still visible in the underdrawing, as in Raphael's Procession to Calvary (around 1504).

The above mentioned book was made as a companion to an exhibition at the National Gallery In London and details the peek into the secret world underneath the pigments of the Masters. It's nice to know even the Masters screwed up once in a while.