Since the ammonia fume diffused through the finishes so easily, I started to wonder how far it penetrated the wood. I had assumed the fume darkening would be a thing film, so I planed one of the edges to see how far it would go. I planed and planed, and found out that the fume had penetrated 1-2 mm through the face grain, and up to 10 mm on the end grain. You can see this in the planed edge shown above. The fumed finish on oak can easily stand up to extensive sanding. As always, please test this with your materials and techniques before you decide to try it on a real piece.