An old canoe or a small wooden boat hanging from the rafters of a barn or garage seems to trigger in me an involuntary nerve response—call it “restoration project syndrome”— for which I have yet to find a cure. A note from reader Peter Fenwood of Rockland, Maine, seems to show he is infected with the same bug:
I am fortunate to have in my care a 15′ wood-and-canvas Old Town square-sterned runabout that dates from the 1940s, when my grandfather purchased it for use on Cold Stream Pond in Enfield, where he had built a camp in the ’30s. The boat underwent a restoration about 40 years ago during which the interior was stripped, and rather than varnishing the interior it was treated with boiled linseed oil and turpentine. This has darkened over time, as it is wont to do, making the interior almost black throughout. My brother would like to bring it back to its original golden tones, but we are at an impasse as to how to do this, if it is indeed possible. Do you know how this can be done?
Some of the darkening Peter sees in this classic Old Town boat may be accumulated dust and other airborne debris, but I am quite certain most of it is caused by embedded spores produced by a black-mold fungus.
Molds and wood-staining fungi belong to a group called ascomycetes. Unlike basidiomycete fungi, ascomycetes don’t cause wood decay. Staining fungi can penetrate deep into sapwood and their colored hyphae (fungal strands) are the cause of blue or other colors of staining in sapwood. Molds, on the other hand, inhabit the surface of wood with little or no hyphal penetration. They have colorless hyphae but produce colored spores, called conidia, on the surface. Black molds are most pernicious because the conidia they produce are embedded in a slime droplet that dries to a protective covering, hindering mold removal.
My last foray into the realm of mold and mildew was in 1991 (see WB No. 100). In the intervening years, a controversial phenomenon called “sick building syndrome” has captured the interest of the news media and prompted, in some cases, the closure of schools or abandonment of residences. The culprit, Stachybotrys chartarum, a species of mold fungus that produces black spores, may cause illnesses in humans and other animals.
First described in 1837 after being found on wallpaper collected from a house in Prague, this black mold became more widely known in the 1930s when outbreaks of a new disease of horses in Ukraine and other parts of Eastern Europe was traced to S. chartarum growing on straw and other grain fodder. Symptoms in horses included irritations of the mouth, throat, and nose; shock; dermal necrosis; a decrease in leucocytes; hemorrhage; and, in some cases, death.
Not all black molds are S. chartarum, but the prudent action when attempting to remove any mold is to wear protective gloves and an N95 or other high-filtration face mask.
Mold growth requires high humidity—generally above 80 percent—but some molds, once established, can go dormant during dry periods and then grow when moisture in the form of high humidity and surface wetness returns. This cycling between growth and stagnation can continue for years.
Black mold, particularly S. chartarum, is characterized as being “cellulolytic,” that is, it can hydrolyze cellulose. Hence its affinity for cotton, wood, or paper. Hydrolysis of cellulose yields sugars that nurture the mold fungus and lead to rapid growth. But in order for this to take place, the relative humidity of the air must be very high and the wood, paper, or canvas must be quite damp.
A canoe stored in a protected space such as a barn or garage may, at times, be surrounded by air at relative humidity above 80 percent, but the canvas and wood are not likely to have a moisture content high enough to allow for hydrolysis of cellulose; yet black mold often develops in these situations. How is that possible?
The culprit is linseed oil, either in the form of boiled linseed oil or some old linseed oil–based varnishes.
Derived from flax seed, linseed oil is composed of fatty acids, primarily linoleic and oleic acids. Here is a food source for mold that doesn’t require cellulose hydrolysis. Linseed oil undergoes oxidative polymerization but it stays slightly tacky, trapping additional mold spores and other airborne debris, some of which may serve as additional food sources for mold growth. Over extended periods of storage, the oiled wood becomes darkened with black mold growth.
While black mold development on a sheltered canoe may be quite slow, the process can be hastened if linseed-oiled wood is directly exposed to rain. I learned this lesson recently when I installed a natural-crook spruce hand support on steps leading to my workshop. In less than six months, as the accompanying photo on page 90 shows, the post had turned black. This provided me with an experimental surface to test mold-removal chemicals.
A search on the Internet turned up a number of chemicals that have been used to remove, or at least bleach, black mold. Commercial cleaning solutions often contain proprietary chemicals not revealed on the label, although the active ingredient is often weak chlorine bleach. I did not test any of these.
Several common household chemicals have been touted as mold removers, including hydrogen peroxide, vodka, white vinegar, baking soda, tea-tree oil, grapefruit-seed extract, diluted chlorine bleach, and borax. In a previous column (WoodenBoat No. 100) I recommended using oxalic acid to remove mold. But more recent research has shown that although oxalic acid is effective in removing tannin and iron stains in wood, it is not any longer recommended for mold removal.
Rollin Thurlow, a canoe builder from Atkinson, Maine, recently told me, “I use a mix of bleach and water and deal with any bleaching of exposed wood with a stain later on.”
With the exceptions of vodka, tea-tree oil, and grapefruit-seed extract, none of which I have lying about in my pantry, I tested each of the household chemicals listed above. This was a very crude test in which I applied the chemicals with a paintbrush on a fall day that reached 60°F. After about 10 minutes, I scrubbed the surface with a soft-bristle scrub brush wetted with the solution. After leaving it to dry for about a week, I photographed the results.
The hydrogen peroxide, baking soda, and borax treatments removed very little of the black mold. A 50 percent solution of white vinegar and a 10 percent solution of Clorox bleach, both mixed with water, effectively removed much of the black color, but the bleach-treated wood had a duller, slightly gray color and the bleaching solution whitened and roughened the ends of the wood fibers. I should note here that the spruce post had been smoothed with just a drawknife, creating a faceted surface ideal for mold growth and one more stubbornly resistant to mold removal than would have been the case for most finished boats.
(Left) A 10 percent solution of Clorox bleach in water worked to remove mold, but caused some deterioration of wood fiber ends. (Right) A 50 percent solution of white vinegar in water proved to be the author’s favored way to remove mold.
In my view, the vinegar solution was the winner. It is inexpensive and reasonably safe to use. Some have recommended applying the solution warm and letting it soak for an hour. This would likely improve penetration, killing more of the embedded fungal hyphae. Also, as others have suggested, a follow-up solution of borax will help inhibit additional fungal growth and neutralize the acidic vinegar, creating an ideal surface for varnish or paint, preferably not with a base of linseed oil.
The results from my crude experiment seem to corroborate the experience of others. Vinegar is reported to kill 80 percent of mold fungi and penetrates deeper than household bleach. Because dried slime covers black molds, removal is more difficult than for other surface molds, which are sometimes called mildew, that occur on many food items and occasionally on various surfaces such as those found in warm, humid boat interiors. Black molds are tolerant of a wide range of temperatures, and they can even be found in Arctic regions of the world.
One final caution: Some black molds, especially S. chartarum, have been reported to cause several ailments in humans. By entering breaks in the skin, they may form large warts or other skin lesions, including swollen or reddened areas that may constantly leak fluid. Inhaled spores may lead to pneumonia in immunocompromised people, and they have been reported to cause sinusitis that is often resistant to antibiotic and decongestant therapy. As I mentioned already, adequate face and hand protection is essential, and working outside is the preferred choice. If you can’t avoid working indoors, avoid fans, which will only cause loosened mold spores to become airborne.
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 email to Senior Editor Tom Jackson, tom@woodenboat.com.