Reactive Oils and Finishes

“The woods are lovely, dark, and deep. But I have promises to keep and miles to go before I sleep.” – Robert Frost 

Reactive Oils and Finishes

Hidden within the cellular structure of many hardwood species are chemicals that will oxidize and reveal attractive reactions. These unique reactions were developed over 300 years ago, before the advent of pigment for stains or paints. Some of these custom colors can take from seven to nine steps to produce, which replicate the true chemical weathering process that wood experiences in nature. Each step of the process creates a physical change inside the wood on a molecular level. The chemical reactions which take place, such as the oxidation of metallic minerals embedded within the cellular structure of the tree, actually change the physical properties of the wood. In fact, it actually becomes harder and denser, which adds to the durability of the material. Since there is no stain or paint used, you won’t see that artificial, manufactured, uniform look that most wood floors have today. This wood is also finished using a natural oil, which penetrates into the wood instead of sitting on the surface. This accentuates the natural grain and texture of the wood, and as the oil cures naturally within the wood’s pores, it hardens to create a strong surface but allows for easy repairs. Careful experimentation will expose a myriad of colors to the inquisitive and resourceful finisher.

When I think of reactive finishes, I am reminded of our own skins natural tanning process. The amount of pigmentation in each individual’s skin determines whether they will tan quickly or it will take longer timed exposure to develop the degree of tanning desired.

How it Works

Woods like Oak, Cherry (heartwood), Mahogany, and Walnut contain significant amounts of tannins or phenolics, which in their electron-rich reduced state tend to be colorless to a very slight yellow. However, rearrangements in electronic structure that occur with oxidation (loss of electrons) cause the compounds to become a more highly colored brown to reddish brown. Some tannins are photosensitive and will oxidize in light with the atmospheric oxygen acting as an oxidizing agent. The reaction tends to be fairly slow, and is the process which Cherry and Mahogany darken over time.

Placing the wood in direct sunlight will speed up this process. Sunlight on Cherry causes the color to change much faster as the light acts as a catalyst to speed the oxidizing reaction. A week in the bright sun will darken your wood, but may not be practical for a large project.

Chemicals can also accelerate the oxidizing reaction particularly those that are oxidizing agents. Potassium Dichromate has often been used historically in woodworking. Granules mixed with water will oxidize the wood and change color in a few minutes depending on the concentration of the chemical. This produces the color the wood would ultimately achieve after years of atmospheric oxidation.

The appearance of the brown color is the same process that occurs on a fresh slice of apple. An application of acidic lemon juice will retard this browning. Conversely placing wood tannins in a basic pH environment will accelerate their oxidation. A basic solution of Sodium Hydroxide (household lye – Part A in Wood Bleach) can also accomplish this color change. Fuming in an ammonia gas can also be used and is great for getting in every nook and cranny to create an even color change and natural tone.

White Oak Finished Flooring

(Above: White Oak stained with 3 concentrations of Potassium Dichromate)

White Oak Stained Flooring

(Above: White Oak stained with 3 concentrations of Sodium Hydroxide)

Fumed White Oak

(Above: White Oak Ammonia fumed 53 hours)

Likewise the amount of chemicals in the make-up of certain species of wood will determine the degree of coloration or reaction to the chemical applied to the wood. The natural color of wood species is dependent on chemical codes encapsulated in the seed that then determine the predominant color or colors of the species.

Even within a species there will be a distinction between sapwood being lighter in color and the darker heartwood. The natural progression of growth in the tree will produce extractive chemicals that alter the chemical makeup of the wood cells. These extractives produced in the heartwood protect the tree from fungal growth, decay, and decomposition. They also will reveal the color difference between the sapwood and heartwood. For example, the desirable lighter sapwood of Maple versus the darker heartwood of the Black Walnut. There are even anatomical features such as the tyloses in the White Oak cells that help prevent fungus and bacteria from intracellular moisture movement that could decay and destroy the tree.

The skill of the finisher using reactive chemicals depends on knowledge of the chemical make-up of the boards that he is applying finish and stain to. It becomes more of an art than an exact science. Even if the application of the oil or stain was applied perfectly following the known procedure there still will be variations.

The client that is a candidate for reactive stains and finishes can expect color variation from board to board even if it has been harvested from the same region. This is the beauty of reactive oils and finishes, they reveal a myriad of natural colors that cannot be achieved by pigmented stains and dyes. Pigment stains and dyes will produce a more even color to the woods if that is the desired look.

The way many of these reactive stains work is that they react with the tannins in the wood. Woods with higher tannin content will have a stronger reaction than those that don’t.

Some of the reactive chemicals are mixed with lime and other special pigments, along with wire brushing, help with the ceruse or cheeseboard effect that give the boards an aged or weathered look.

HH-302 Showroom

When it comes to selecting wood for your next project, realize that the artistry of finishing is knowing the substrate that you are dealing with and the natural chemistry will bring out its beauty.

“Nature will bear the closest inspection. She invites us to lay our eye level with her smallest leaf, and take an insect view of its plain.” – Henry David Thoreau