The old adage, “wood that remains completely submerged in water will not decay,” is unfortunately incorrect. Recently I read an article about a Revolutionary War-era shipwreck found submerged in the Nanticoke River near Baltimore. Divers were cleaning up debris in the river channel under the US Route 50 bridge after barge had apparently hit a system of fender piles beneath the bridge when they unexpectedly pulled up some old timbers that were not part of the barge debris. The timbers appear to be from a shipwreck that may extend back to the Revolutionary War era, which would make the timbers over 200 years old. According to the Baltimore Sun article, it "was a merchant ship that likely greed grain to or from the Eastern Shorte port of Vienna in the late 1700s."
It should come as no surprise that this story attracted my attention because of the wood science involved. Upon reading this story I found it very likely that the submerged timbers were quite deteriorated, and that soft rot wood decay would be the culprit. I contacted Nichole Doub, one of the archaeologists from the Maryland Archaeological Conservation Laboratory (MAC Lab) working on preserving the wood, to discuss the process by which they preserve the wood, and to share some knowledge about how wood deteriorates in fresh water.
Soft rot wood decay is a common issue I encounter when working with diving, marine and geo-technical engineers. It's a slow-acting wood decay that is actually capable of causing significant damage and strength loss in wood, even though it that has been submerged in water for very long periods of time, especially untreated wood. Soft rot is a unique type of wood decay because it can survive in oxygen deprived environments, whereas its more aggressive counterparts require oxygen for growth. For now, I’ll simply refer to the aggressive types of wood decay as 'common wood decay,' or, as they are often referred to in construction: "dry rot." Common wood decay, which is what most of us are used to seeing, decays wood rapidly in optimal conditions. For example, stick an untreated 2x4 in the ground in the deep south and it's pretty much gone within 1-2 years. Do the same in the northeast and the untreated 2x4 will be gone in 5-6 years. Ground contact conditions are perfect for common wood decay because there is ambient oxygen, favorable temperature, and favorable moisture. Soft rot, on the other hand, is very slow acting and occurs in oxygen deprived environments such as fresh water and below grade conditions. Significant damage from soft rot can take decades, or hundreds of years, depending on how wanting the oxygen is in that environment.
Soft rot is not easily detectable upon visual examination. While the outer surfaces of timbers will be soft when inspected they retain their shape because they are saturated, and as a result they can appear to be in relatively good condition. However, upon drying, some interesting things can happen to soft rot infected wood. While talking with Ms. Doub of the MAC Lab, she explained to me how they use bulking agents to essentially help "bulk" up the damaged wood cells so that the timbers won't be prone to warping, cracking and splitting when they dry. She also explained that the timbers from this ship wreck were immediately stored in water once they were removed from the river so that they did not start drying prematurely (see image above).
In my experience working with pilings or timbers, I incorporate the same concepts as the MAC Lab. It is crucial that I keep samples saturated prior to testing. Any type of drying could lead to drying defects. Plus, it's critical that the samples maintain the same moisture content throughout the testing process. While I was talking to Ms. Doub, I told her about the two wood samples in the image above which were originally 1"x1" square cross sections, but had collapsed into irregular shapes once they dried. This is exactly the type of phenomenon that soft rot can cause and what the archaeologists are tying to avoid with the timbers from the shipwreck during the preservation process.
Above is a microscopic image I recently captured from a marine piling. In it there are several spiral-like, diagonal, formations. Those are soft rot cavities. The shape of the cavities are developed because the decay is consuming the cellulose in the wood cell walls. Cellulose chains are aligned in a helical formation down the length of the wood cells. The result of soft rot wood decay is the formation of these diagonal cavities. The image is proof positive that soft rot is present and causing cellular damage. There is absolutely decay present in the wood cells, but the timber deceivingly keeps its shape when wet, and as a result also has elevated moisture content levels as well.
In circumstances like the Nanticoke river shipwreck, when the damaged wood dries, and the moisture migrates out of the wood cells, the timbers are then prone to collapse, splitting, and warping. This type of damaged wood, therefore, makes archaeological preservation quite a challenge.
I want to extend my sincere gratitude to Nicole Doub for sharing her expertise and experience as well as permission to use photos from this project. Best of luck in your work, Ms. Doub!