First we’ll scare you: nitric acid is used in explosives and rocket propellants—although in much more concentrated forms. This explains why nitric acid is also known as “Aqua fortis” which in Latin means strong water. For the purpose of coloring wood, however, a 35 percent concentration is fine. Nonetheless, you still want to handle this odorless, colorless liquid with great care because it is unstable.
Besides being explosive or flammable in the right concentrations or if handled negligently, nitric acid can also be corrosive. For this reason it is used to etch copper or brass.
Now that we’ve raised your eyebrows, let us attempt to lower them. The main chemical produced from nitric acid is ammonium nitrate, which is used in fertilizers—and that is what its primary use is.
So why did we include nitric acid in this book if it comes with so many warnings? Some chemicals amplify the colors within certain species to spectacular effect (look at lye on cherry!) while these same chemicals have merely pleasing effects on other species (look at lye on alder!). Nitric acid—while tricky to work with—hands-down produces the most consistently warm enhancements of tones across all species. An additional benefit of nitric acid is that it does not rely on tannins in the wood for its magic to occur. Look at the sample of maple (a species containing little to no tannins) for proof. Nitric acid is an equal-opportunity coloring agent!
You can buy nitric acid at a chemical supply house. It is sold as a liquid and should always be stored in a dark glass bottle in a very cool place where it won’t be jostled or disturbed—and should most definitely be placed out of the direct sun. Under these conditions it has a shelf life of roughly three years before it begins to lose its potency.
If nitric acid splashes onto your skin while you are mixing it, you should “irrigate” your skin for 15 minutes, that is, run cool water over the area where the spill occurred.
1½ oz. of nitric acid
8 oz. cold distilled water
Slowly pour the nitric acid into the cold distilled water while stirring.
Unlike the application of other chemicals, nitric acid is applied to the edges and end grain first—using the side of the brush. If you are concerned about the end grain becoming too dark, pre-treat it with distilled water immediately before applying the nitric acid. Then brush the nitric acid solution onto the surface of the wood.
It is not necessary to wipe off the solution with a rag as you do when applying other chemicals, because nitric acid “evens out” on its own and doesn’t leave lap marks.
Nitric acid tends to dry more quickly than other chemicals. After you are sure that the nitric acid solution is completely air dry, it is time to use the heat gun. The heat gun develops the color change. It would take days or weeks for the color to develop on its own if you did not use a heat gun.
Slowly wave the heat gun over the wood, holding it about 2 inches above the surface and constantly moving it along the wood. If you hover over one spot for too long, you’ll burn the wood. You will see the rich color develop as you wave the heat gun over it, and you’ll know you’re finished when the color doesn’t darken any further.
You have to use a legitimate heat gun because a hair dryer simply doesn’t produce enough heat.
1 tbs of baking soda
3 oz. of hot distilled water
3 oz. of cold distilled water
A clean brush
Slowly add one tablespoon of baking soda into 3 ounces of hot distilled water and stir until dissolved. Then add 3 ounces of cold distilled water and mix thoroughly. Strain into a clean container. Using a clean brush, apply the solution to the wood.
Before You Begin
Alder
Cherry
Mahogany
Maple
Oak
Walnut
