Try a Traditional Ebonizing Recipe for Black Oak
Years ago, I knew of a vinegar solution that blackens oak. I remember the vinegar part, but what else goes with it?
—Scott Swager, Jolon, Calif.
Add a little iron and now you're cookin', Scott. Iron mixed with white vinegar produces a centuries-old finish, called "iron buff," that can be brushed onto oak, and most other woods, to darken their color.
To make iron buff, rinse a pad of steel wool with lacquer thinner to remove the protective oil. Allow the pad to dry. Then tear the pad into pieces and soak them in a pint of vinegar until the acid begins to dissolve the steel. Soak the pad overnight for a light color change or for a week to achieve darker colors. When a dab of iron buff turns a test board the desired color, strain the solution through a tightly-woven cloth, such as an old t-shirt, to remove the largest pieces of undissolved iron. Then strain it again through a coffee filter to remove the remaining particles.
The vinegar itself doesn't change to a dark color that dyes the wood. Instead, the iron in the vinegar reacts with chemicals in the wood, causing them to change color. Because it's a chemical process instead of a wiped-on stain or dye, you get blotch-free color. Each batch will vary, though, so mix enough to stain an entire project. Apply an optional oil-based stain to deepen the gray or black color for a darker tone. Then apply a film finish.