Introduction to Wood Acidity
“A Cautionary Saga,” the article about keelbolt replacement by Michael Sauter in WB No. 297, prompted Tom Jackson, WoodenBoat’s senior editor, to suggest to me that a column on the topic of wood pH might be useful. He commented that although oak is well-known as an acidic wood, he was surprised to learn from Sauter’s article that Douglas-fir wood also has a low pH.
As I dug into the topic of wood acidity, I became ever more frustrated and soon learned that I wasn’t the only one. In recent years, commercial growers of container plants sold at nurseries have been switching from organic peat, bark, and coconut coir—planting substrates with known pH values that affect plant nutrient uptake—to more variable wood-chip-based materials. One website complained that “the natural acidity of wood can be attributed to several factors, including tree species, age of tree, location of wood within the tree, site location, soil type, and season of harvest.” I would add the variable of how wood pH is measured, as there is no standard.
With a range of 0 to 14, the pH scale is a measure of the acidity or alkalinity of a substance, with 7 representing the neutral zone. The scale is logarithmic, so a change from 5.0 to 4.0, for example, indicates a tenfold increase in acidity. The main components of wood are cellulose, lignin, and hemicellulose. Of these, hemicellulose is the most reactive. Acetyl esters on hemicellulose molecules can be hydrolyzed to produce acetic acid, which has a pH of about 2.4. Hemicellulose is found in concentrations of 4 to 6 percent in hardwoods and 1 to 2 percent in softwoods. This suggests that hardwoods should be more acidic, but the pH balance in wood is complex and can lead to some highly acidic softwoods.
Range of Acidity
Some hardwoods contain high-enough levels of hemicellulose that extraction for acetic acid production from wood chips destined for paper production is a commercially viable option. A recent chromatographic analysis of the chemical compounds in teak (Tectona grandis), revealed acetic acid in concentrations of 10 percent in sapwood and 6 percent in heartwood—prompting the authors to suggest that “the high acetic-acid content enhances the use of teak wood to [the] production of artificial essences for perfumery, paints, and dyes” (www.nature.com/articles/s41598-022-22800-1). Imagining valuable teak wood ending up in some boudoir perfume gives me the willies! Hopefully, the authors were only thinking of sending teak sawmill waste to the chemical factory.
In 1999, the Canadian Conservation Institute published a list of pH values of 88 woods that they had compiled from several sources. In the accompanying table, I have condensed that list to two dozen woods that might be used by boatbuilders. The pH values range from 8.0 in a tropical rosewood to 2.5 in western red cedar. Most woods are mildly acidic (pH 4 to 5, which is similar to tomato juice or black coffee). But oaks, Douglas-fir, at least one Australian eucalypt, and western red cedar are as acidic as orange juice or even vinegar.
Although the determination of wood acidity is not standardized, it is most often measured by mixing wood sawdust or other wood particulates with cold water and testing the pH after several minutes or a few hours of soaking. An alternate method substitutes hot water. Recently, piercing electrode meters, which are not unlike moisture meters, have been sometimes used. As a consequence, reported wood acidities can vary, depending on testing lab protocol. Additionally, fresh, green wood can have an acidity different from aged wood. Finally, acetic acid is the most common wood acid, and the most volatile, but formic, oxalic, and tannic acids are among the other acids that are present in amounts varying from very low to quite high. The pH values in the table reflect the totality of acids present in the wood at the time of measurement.
Wood acidity can be temperature-, time-, and moisture-dependent. Kiln-drying has been reported to enhance wood acidity over air-drying, and long-term monitoring has revealed increased acidity with extended wood storage times. But moisture content likely has the greatest influence by allowing hemicellulose to be hydrolyzed to release acetic acid and, in addition, by creating an environment for tannins to become reactive acids.
Corrosion and Rot
Many metals are corroded by wood acids. Lead can be rapidly destroyed by acetic acid, while iron and steel are particular targets of tannic acid. Zinc and bronze are only mildly corroded by wood acids. Over time, acetic acid is liberated from wood by volatilization or by leaching if in water contact. Tannins, being larger phenolic compounds, are more resistant to leaching and, thus, can be reactive with iron in wet wood for long periods.
After a boat failure involving iron fastenings, we tend to focus on the single factor of corrosion of the iron screws or bolts, but that is often only part of the problem. As I noted in my column in WB No. 280, the reaction between tannic acid and iron produces ferric tannate complexes that are sparingly soluble. In the presence of these ferric ions, many brown rots are catalyzed to develop accelerated growth rates. So, as the iron is rusting, brown rot fungi are flourishing in a ferric ion bath. The resulting hull damage is then a combination of corroded fastenings and wood rot around those fastenings.
Color and Location
It’s been suggested that dark-colored wood is an indicator of high tannin concentration. If we look at woods in the table with pH values between 4.0 and 5.0, we find that dark-colored cherry and teak have lower concentrations of tannin than more-acidic, lighter-colored oaks or Douglas-fir. I would be leery of using color as a criterion for tannin content. For example, Rowan trees (Sorbus spp, also known as mountain ash) have whitish sapwood and medium-yellow or pale-tan heartwood, yet they contain some of the highest concentrations of tannin compounds of any wood.
Are tannins evenly distributed in a tree? Because oak woods are prized for barrels used for aging wine and spirits, researchers have measured the relative concentrations of tannins in living trees. They have determined that heartwood contains much more tannin than sapwood, and the highest concentration is at the base of the tree in the most recently formed, or outer, heartwood. Slow-growing trees, which have narrow annular rings, have higher tannin concentrations than rapid-growth trees. Fortunately, wide rings in a ring-porous wood such as oak indicates the strongest and preferred wood for boat frames.
When researching this subject, like Tom Jackson I was surprised that iron-fastening problems are well-known for oaks but not for Douglas-fir or western red cedar. Perhaps it is because oak is prized for frames and keels, where the wood is often water-saturated for long periods, while Douglas-fir and cedar are generally planking woods that may be only intermittently wet. Another possibility is offered by the large pH range measured for Douglas-fir: 3.1 to 6.1, from four different sources. This suggests that beyond the measurement variables already noted, different soil chemistry conditions, growth rates, or both may be causes of this variation. Douglas-fir grows over a very large geographic range, and this might result in varying concentrations of tannins in the wood. I would be interested in hearing from any reader who has observed iron-fastening corrosion in Douglas-fir or western red cedar, particularly if the source of the wood is known.
Dr. Richard Jagels is an emeritus professor of forest biology at the University of Maine, Orono. Please send correspondence to Dr. Jagels by mail to the care of WoodenBoat, or via e-mail to Senior Editor Tom Jackson, tom@woodenboat.com.