Don’t be intimidated by working with chemicals! Yes, some of them are carcinogenic (potassium dichromate) and others are caustic (lye, aka sodium hydroxide) or corrosive (nitric acid). And let’s be honest here; the word “chemicals” these days makes people wary in light of the trend toward more natural products.
Handled properly, though, chemicals will bring a startling clarity to your wood by imparting an elegant patina, making the wood look richer, warmer and with a greater depth of character. This is due to the way chemicals—unlike stains—highlight(rather than hide) both the subtleties of grain patterns and the range of natural colors within the wood.
Before synthetic dyes were invented in 1856, furniture makers not only colored their wood with natural dyes but also with chemicals. Potassium dichromate, for example, was favored by Napoleon for its dramatic effect upon Cuban mahogany furniture created during the French Empire period in the beginning of the 1800s. Charles and Henry Greene were champions of applying ferrous sulfate followed by potassium dichromate to cedar and mahogany. Ammonium hydroxide, widely used for fuming oak, was favored by Gustav Stickley, and he was also a fan of iron acetate. When you think of these periods and/or designers, a particular color of wood comes to mind along with their designs—and that color was produced by chemicals.
You won’t be working with a huge volume of chemicals. A little bit goes a long way in these recipes. The amounts of various chemicals presented here vary from as little as ⅛ teaspoon up to 1½ ounces. You can always adjust the amount of the chemical to darken or lighten the color.
The mound on top is potassium dichromate, below left is ferrous sulfate and below right is sodium hydroxide (lye).
Over the course of the last century, many woodworkers abandoned chemicals in favor of the less toxic synthetic dyes. Coloring wood with chemicals is now a specialized art.
As they say, “Better living through modern chemistry!” We want to celebrate everything that the wood is and can be. We don’t want to mask the grain pattern—which is what stains do. Chemicals accentuate it!
Tannic Acid Content in Wood
Species containing tannic acid include:
Birch
Cedar
Cherry
Douglas Fir
Lacewood
Mahogany
Oak
Redwood
Walnut
Zebrawood
Species containing little to no tannic acid include:
Alder
Ash
Beech
Maple
Pine
Poplar
Any kind of sapwood
This means that cherry and walnut will respond heartily to chemicals because they contain tannic acid but the sapwood in these species will not have the same chemical reaction.
There are three factors influencing the color imparted by chemicals: the background color of the wood, the strength of the chemical solution, and the presence of tannic acid in the species.
Make sure you’ve raised the grain and dewhiskered the nibs before applying a tannic acid solution to your wood!
A starting formula for tannic acid to apply to finish-sanded wood is:
1 tsp of fluffy tannic acid added to
8 ounces of HOT distilled water.
Strain before applying.
Apply the tannic acid solution while it’s hot because this enables it to penetrate more deeply into the wood fibers, which in turn will promote a more effective reaction when a chemical is subsequently applied.
Be sure the wood is completely dry before applying whatever chemical you have chosen to color your wood. And you do not have to lightly sand the wood after applying the tannic acid, even if the grain is slightly raised. Wait until after you’ve applied the chemical and the first coat of finish before de-whiskering the wood.
Always wear gloves and a mask when working with tannic acid, and be sure to store in a sealed container in a dry, dark place.
Basically, chemicals impart their color by reacting to tannins (popularly referred to as “tannic acid”)within wood. This chemical reaction occurs immediately when the solution is applied to the wood. The higher the level of tannic acid that is present in a board, the more dramatic of a color change. Similarly, the stronger the chemical solution, the more dramatic the color change will be. Even within a single board there can be varying degrees of tannin.
A caveat: the exception to this explanation of how chemicals work is nitric acid. It works on any species—including those that do not contain tannins, such as maple and pine.
However, not all wood species contain tannins. Consequently, when a wood is void of tannins, there is no chemical reaction but the chemical may deposit its own color.
The solution to getting chemicals to work on woods that possess little to no tannic acid is to add tannic acid to the wood first. Fortunately, this is a simple fix! You can easily purchase various types of tannic acid powder that dissolve into hot distilled water and can be brushed onto woods like maple and alder prior to adding chemicals to it.
Tannic acid can be bought online as a dry powder in several different forms:
Fluffy tannic acid does not impart any of its own color to wood, so you might feel as if you are simply brushing water onto the wood. There is no set formula for how much tannic acid to add to wood that doesn’t have it, but bear in mind that a stronger solution of tannic acid will produce a greater reaction when chemicals are applied.
Chemicals will accent different tones within different species. This is why it is so important to test your recipes on scrap wood from your project. For example, potassium dichromate produces a hearty reddish cast on cherry, whereas it results in more golden hues on oak and meatier brown shades on walnut.
Maple has no tannic acid so this is the effect you’d achieve by just applying potassium dichromate. The chemical merely deposits its own color. (Surprisingly, however, potassium dichromate looks quite orange in the mixing bowl!) Note how the dichromate makes the maple look weathered as opposed to aged.
A sample of maple with a solution of tannic acid applied first, followed by potassium dichromate. You can see how the reaction of the potassium dichromate to the tannic acid has significantly altered the color of the maple, producing a blend of harmonious coloration. Why would you want to do this to maple? Perhaps you want maple’s durability but not its color.
Ferrous sulfate is a tricky chemical on maple because it can produce a vast range of color effects with or without first applying a wash of tannic acid. On some maple, ferrous results in a grainy gray to dark blue cast while on other maple, you will see a spectrum of light blues and greens and browns. This variation in effect can occur on both hard and soft maple so it is imperative that you test the chemical and the particular concentration of it on samples of the same wood that you intend to color.
This soft maple sample with no tannic acid applied has one effect; the figured maple tabletop featured on pages ii and 31 has a totally different effect, and it too had no tannic applied in advance.
It is fairly obvious with this sample that the application of tannic acid did not improve the effect of ferrous sulfate on the maple. Tannic acid will improve the final flavor of maple with some chemicals but with others, the effect is not worth the effort of the 2-step process. As we have said repeatedly, you must make a sample because when it works, the results can indeed be spectacular. It all depends on the type of maple you have.
Chemicals are poisonous if ingested. Do not let chemicals come in contact with bare skin.
Cautionary Notes:
Most chemicals, once mixed into a solution, have a short shelf life of up to approximately two weeks before they lose their potency. It’s always better to simply make up a new solution than to store a used one—unless, of course, you plan to use it within a day or two.
Potassium Permanganate
Originally we were going to include samples of wood that Brian colored with potassium permanganate, but we decided against it for two reasons:
