What’s in a Soundboard Crack Repair: Subsurface Scattering & Color Matching

Many people wonder about cracks in their soundboards. Mostly they don’t cause problems and more often than not, the repairs are cosmetic. This last month we repaired some soundboard cracks that had formed in a newer piano: the cracks were filled with epoxy, which is a very stable and common repair for cracks. But afterwards, there were some very noticeable color differences between the repair and the regular tone of the soundboard, so we went to work on painting the cracks to look like the surrounding wood. How hard could color matching be? Turns out, it gets complicated.
There is a similar reason that painting skin tone is such a difficult thing for painters, and it actually has less to do with color than it does with opacity vs. transparency. Our skin is not completely opaque: there are many subtle color shifts that happen because of a phenomena called “Subsurface Scattering.” It’s how light diffuses as it penetrates beyond the surface of the skin (which it does). It causes thinner areas of skin to appear more red, and thicker areas of skin more yellow, cream, or brown. It causes your lips to look naturally pinkish red. It also causes your veins to appear slightly blue. These colors shift based on the intensity and direction of light: from one angle it might look one color, and from another you might see a slightly different color. Painters solved this problem of color matching by using a series of glazes over what is called a “Grisaille” (pronounced gris as the gris in gristle, and eye). It’s a purely black and white painting that is left to dry, which is then covered in very thin films of paint mixed with a medium. You might imagine it like stacking sheets of stained glass on top of each other to make colors. Van Eyck and others around his era began using this method with oil paints, which resulted in very beautiful skin tones and quite vibrant colors.
So how does this relate to wood and color matching? Well, we matched the color, and as soon as we sprayed the wood with a finish, the color would be drastically different. And it happens for the same reason. When sanded, light interacts differently and more opaquely with the wood than it does coated with a clear material that is transparent and allows light to penetrate deeper into the wood. To add to the frustration, from one angle, the color matched while from the other angle it did not match at all! How did we go about figuring out how to match it? We had to first paint a completely opaque layer of paint 3-4 inches wide to make it look like an entire piece of wood was the same color. After a base color was made, layers of glazes were added to match the yellow of the soundboard when coated with clear lacquer, and striations were added to match the grain of the wood.
It wasn’t perfect, but it turned out decently well. Can you find the cracks?

— Piano Technician Bryan Hutchison