As defined by Wikipedia, egg tempera, is a permanent fast drying painting medium consisting of colored dry pigment mixed with a water-soluble binder medium (usually a glutinous material such as egg yolk). Tempera also refers to the paintings done in this medium. Egg tempera paintings are very long lasting, and examples from the first centuries AD still exist. Egg tempera was a primary method of painting until after 1500 when it was superseded by the invention of oil painting.
Given reprographic capabilities at the time, the preferred method for creating crisp, high-contrast art intended for newspaper reproduction was called "scratchboard".
Scratchboard, is explained by Wikipedia as: Modern scratchboard, as we know it originated in the 19th century in Britain and France. As printing methods developed, scratchboard became a popular medium for reproduction because it replaced wood, metal and linoleum engraving. It allowed for a fine line appearance that could be photographically reduced for reproduction without losing quality. It was most effective and expeditious for use in single-color book and newspaper printing. From the 1930's to 1950's, it was one of the preferred techniques for medical, scientific and product illustration.
Using a sharp, angled blade or scratch tool an outline is made on the surface of the scratchboard. Scratchboard can be purchased in either all black or all white sheets. [Musselman would apply india ink to a white china board.] Shadows and Highlights are created by "scratching" away at the board. Artists using the white scratchboard paint or draw [or inked] black areas onto it and then proceed to scratch into the black portions to create their drawing. Alternatively, the cleared portions of the scratchboard may be left blank for a stark black-and-white image.
As the thumbnail above for one such Bank of America ad does not give the hand or process justice, a close-up of the printed ad shows the mastery of the medium.
This close-up at left of a scratchboard sketch or comp most probably is a self-portrait, circa 1961.
This close-up of a scan from an actual board shows the detail of the highlight and shadows, and shows the underlying strength of composition found in all of Musselman's work.