A formal event calls for the elegance of a black dress or well-tailored black suit. The point of clothing made from black fabric is that it doesn’t have the distraction of a pattern or varying color pallet. In composing your outfit, a fine necklace or tie can provide a singular focal point. In creating a piece of furniture out of wood, like clothes, there are elements to the design beyond function. Similar to fabric, wood as a natural material has texture, patterns and color. Yet unlike fabrics, nature has limited the design options. In designing a wooden object, the craftsperson is limited largely to selecting the type of wood and how the board was cut out of the tree to select the texture and patterns. For instance, red oak has pronounced pore structure that can give rise to v-shaped or cathedral-shaped patterns when cut tangentially to the central axis of the tree (called flat-sawn). When cut radially to the tree (called quarter-sawn), these pores become vertical lines – like pin-striping – interspersed with wavy flake patterns created by the medullary rays in the tree. When cut In between these two extremes (called rift-sawn), the vertical lines remain. These three grain patterns are depicted in the image below.
Changing the natural color of the wood can be done during the finishing process. In contrast to say holly that has a uniform white color, very large differences in the size of earlywood versus latewood pores creates texture and varying color of red oak. Like having a color combined with a pattern on fabric, this combination can sometimes be distracting from the overall design. Like a black outfit, coloring the wood black, a process called ebonizing, helps focus the design of a wooden object to it’s overall form and possibly texture. Burning or charring of the wood, called shou sugi ban in Japan, is an alternative that I have tried. Differences in the density of early versus late wood in combination with charring can create it’s own patterns on the wood. One may notice that iron, when exposed to oak such as by inserting a nail, can blacken the wood. Following from this observation, an approach for ebonizing wood is to use an iron acetate solution. This can be made easily at home by reacting vinegar with steel wool. In contrast to a dye or paint that colors the surface of the wood, ebonizing is a reaction that changes the color of the wood fibers and imparts a richness to the color without obscuring the texture of the wood.
As mentioned in an article by Laura Mays, the Program Director of the Fine Woodworking Program at the Krenov School in northern California, in Fine Woodworking titled “A Box Worth Repeating” (FWW May/June 2014 Issue 240, pg 44-49), small boxes are a great vehicle for exploring design ideas. I decided to make a little dovetailed box out of quarter sawn red oak to explore ebonizing using iron acetate solutions. As mentioned in [1], the guidance into how to make an iron acetate solution are quite varied. Typically, though it involves submerging steel wool in distilled vinegar and letting it sit for weeks. While [1] was a bit more scientific in their approach, they still let the reaction proceed at room temperature over a 7-day period.
The reaction between the dilute acetic acid present in vinegar and the iron present in steel wool is irreversible due to the liberation of hydrogen gas as a product. As all reactions require some energy for the reactants to combine, increasing the temperature of the solution can speed up the rate of reaction. Following from Arrhenius, this temperature dependence is exponential so just increasing the temperature a bit can really accelerate the reaction.
In this case, one of the products, hydrogen, is released as a gas and is flammable (recall the demise of the Hindenburg). So letting the reaction proceed in a container where the hydrogen gas can escape and in the absence of an open flame is prudent. Hydrogen is less dense than air (why the Hindenburg was filled with hydrogen) so ventilation will carry it away. The reaction is also exothermic, so the dilute nature of the vinegar helps limit the potential speed of reaction. As the reaction also occurs on the surface of the steel, the high surface area of steel wool also helps increase the reaction rate, but this is a linear dependence on surface area.
To kick-start the reaction, I decided to pre-heat the vinegar in the microwave to 160 Deg F. The steel wool was first washed to remove potential residual oils introduced during manufacture and then added to the pre-warmed distilled vinegar. I used a recycled clear plastic peanut butter container for the reaction vessel. Once the washed steel wool was added to the pre-warmed vinegar, the reaction commenced as indicated by the steady stream hydrogen bubbles. I kept the vessel in a larger warming bath to maintain the temperature at 160 deg F. A bamboo skewer helped keep the steel wool submerged and dislodge hydrogen bubbles that form in the steel wool. After 60 minutes, I let the reaction vessel cool, although the reaction had not gone to completion as indicated by the still present stream of hydrogen bubbles. The resulting solution was clear with the remaining steel wool removed from the solution, as shown below.
As the solution aged over the next weeks, it changed in color from clear to orange tinted (as seen in the picture above) and an orange-brown precipitate formed on the walls and bottom of the container. While I didn’t analyze the chemical composition, I’d surmise that the initial solution contained primarily ferrous (Fe2+) acetate, which is highly soluble in water. In contrast, the aged solution likely contains primarily iron (ferric Fe3+) acetate, which is insoluble in water.
Using masked-off samples of some different woods, I then compared the ebonizing potential of a fresh ferrous acetate solution with an aged ferric acetate solution. As ebonizing results from the reaction of tannins (polyphenols) present in the wood with iron salts, which create iron ions in solution, to produce brown and blue-black amorphous complexes that become integrated with the wood fibers. One might surmise that having the highest concentration of iron ions in solution is likely to give the best result.
As expected, woods with the highest tannin content gave the best results. While initially the color was matte gray, a wash with a 50:50 mixture of tung oil and mineral spirits imparted a richness to the color. Red oak became the darkest, almost black with some variation with the grain. Hickory, which has a much lower tannin content than oak, took on a brown hue. With the lowest tannin content of the three woods, white pine also took on a brown color with gray overtones. White pine also absorbed the most solution, which makes it appear almost darker than the hickory.
Overall, there didn’t seem to be much difference between the fresh and aged iron acetate solutions. The fresh solution might be slightly darker but it’s hard to tell. It is likely that the tannin content of the wood limits the color rather than the iron ion concentration. The advantage of using the fresh solution might be that one can generate this solution in about an hour compared to weeks with the traditional approach. A second advantage was that you didn’t need to filter the fresh solution as the remaining steel wool can be easily removed from the solution. A working aliquot can be decanted from the reaction vessel.
In short, ebonizing wood can be a simple at home experiment with useful applications using common ingredients. It touches on a variety of concepts including chemistry that you can see what is happening (formation of bubbles and a color change), the effects of temperature on reaction, and a discussion of safety considerations.
DK Design Blog 