The Magic of Materials: Buried Bog Oak
Sourcing materials has become a relentless pursuit for founder Mark Boddington, whose adventures include visits to the USA to inspect gypsum and extraordinary quilted (heavily patterned) maple, as well as trips to Spain to examine large end–grain European oak veneers. He is also constantly on the lookout for materials with rich histories, such as 3,000-year-old bog oak, excavated from the ground, or leather salvaged from a ship that sank in 1786.
Among these remarkable finds, bog oak stands out as an extraordinary semi-fossilised timber. Incredibly rare, it is only found buried underground, such as in the Fens of east England, where it has been preserved for 3,000 to 7,000 years in peat bogs. Excavating and drying bog oak is no easy task, but artisans prize it for its beauty and exceptional durability. The finest bog oak, often compared to tropical hardwoods, boasts a distinctive charcoal-black hue.
English bog oak is a beautiful timber that is greatly prized for its deep charcoal-black colour and exceptional hardness. Bog oak has an unusual and fascinating provenance, as it comes from trees that have been buried underground for thousands of years. Such ancient trees are difficult to excavate, and sites of quality bog oak are very rare. Once exposed, the trees become extremely unstable, so great skill is also required to dry them out. In fact, the process shares some of the challenges faced by conservationists salvaging the massive oak hull of Henry VIII’s ship the Mary Rose after 437 years at the bottom of the Solent.
Bog oak is specifically excavated from bogs and other wetlands such as the Fens, a vast low-lying marshy region in the east of England. Today’s somewhat flat and featureless Fens looked totally different 10,000 years ago, just after the last ice age. The glaciers had retreated, the sea levels had dropped and the Fen basin was a very dry area that had become colonised by trees – as had many other large areas in Britain and Europe. English oaks were prevalent in these dense deciduous forests, and grew to a great size and age.
Then, ~7,000 years ago, rising sea levels and adverse weather conditions caused the Fens to flood, and their familiar boggy habitat was first formed. Bogs are largely made up of peat, a compact brownish deposit that is formed over thousands of years by the growth of mosses (especially Sphagnum) and other plants. Because of the low-oxygen (anaerobic) and acidic conditions in these waterlogged bogs, the moss is only partially decomposed when it dies and therefore accumulates, forming peat.
Following the flooding, the trees in the forests would have rotted and eventually died. Some of the massive oak trees would have fallen directly into the silt that built up on the forest floor. Although their branches and leafy canopies decayed quickly, their huge submerged trunks were ultimately preserved in the acidic peat bogs. However, because the forests were so dense, most of the trees would have fallen haphazardly on top of one another, and therefore suffered different degrees of decay due to exposure to the elements, insects, fungi and microorganisms, before eventually becoming buried when the flood waters rose and bogloving vegetation grew over them.
Fascinating Fen Facts
Today, the Fens are one of the richest crop-growing areas in England – and one of the driest areas too. But this is only because about 625 square miles of low-lying land have been reclaimed from the sea, creating one of Britain’s largest m an-made landscapes.
Draining of the Fens, via a network of canals, began in earnest during the 17th century, led by the Dutch engineer Cornelius Vermuyden. But seasonal flooding continued until steam-powered pumps were developed in the 19th century and more water-control projects were introduced in the 1960s.
Drainage is still important in the Fens to this day. For example, Burnt Fen, one such area of low-lying prime agricultural land, is surrounded on three sides by rivers and dependent on artificial drainage through a network of dykes, ditches and drains to protect it from flooding. And it was close to Burnt Fen that several bog oak trees were recently excavated.
Only a few hills exist within the Fens, which have historically been called islands, as they remained dry when the surrounding areas flooded. The Isle of Ely, on which the small city of Ely was built, is the largest island; at 85 feet (26 metres) above sea level, it is the highest land in the Fens. The name Ely means ‘island of eels’ and is a reminder of the importance of eel fishing as both a source of food and income many centuries ago.
Known locally as The Ship of the Fens, Ely’s majestic cathedral has long dominated the flat fenland landscape. At 537 feet (164 metres), it is the fourth longest cathedral in England and is visible for miles around. It was built by the Normans (between 1080 and 1189) when Ely was just a small settlement in a relatively remote place.
It features a distinctive central tower that is a triumph of medieval engineering, which required exacting carpentry skills on a vast scale, and continues to be a delight to the eye. It was built in the wake of the disastrous collapse of an existing Norman tower in 1322, just after the monks had finished matins – the noise was apparently so great that they thought there had been an earthquake.
Built in a Gothic style, this unique octagonal lantern tower took 18 years to build. So-called because it lets light in through tall windows at each side, the upper timber lantern was designed by William Hurley, King Edward III’s master carpenter (who was in charge of timber works for all royal buildings, including the Tower of London, Windsor Castle and the Palace of Westminster).
The lantern is 30-feet (9.1-metres) wide and made from timber and covered in lead. It is framed by eight vertical oak posts, each measuring 63 feet and weighing ten tonnes. Just imagine the difficulty of raising these posts more than 100 feet in the air to position them on top of the eight massive stone columns that form the octagonal base. The posts are braced by an ingenious arrangement of huge beams and baulks (roughly squared timber beams) that hold up the lantern and its roof.
All of the timber was sourced from eight great oak trees, and it is said that the lantern tower could not be built in the same way today as no trees exist that would be large enough to construct the complex timber structure.
So it was not until medieval times, when the Fens began to be drained, the peat started to dry out and farmers ploughed up their fields, that these gigantic semi-fossilised black oak trees came to light, literally, after being entombed for hundreds or even thousands of years. Unfortunately, their uniqueness and true worth were not entirely appreciated at the time. Instead of being seen as ‘buried treasure’, the ancient oaks – which could be up to 100-feet (30-metres) long – were generally considered an inconvenience. Consequently, many thousands of tonnes of logs were hauled out of the peat bogs, left to dry and then burnt (often providing fuel for cooking or heating).
Awareness about the origins of these ‘black oaks’ has thankfully been raised since then. Thus, on the rare occasions that bog oaks are discovered by fenland landowners or during drainage works, experts are usually brought in to carefully excavate them.
Astonishingly, some of the bog oak specimens that have been unearthed from the fenland basin in East Anglia have been estimated to be ~3,500–7,000 years old, using radiocarbon dating. Very simply, this technique estimates the age of dead organisms such as trees by measuring how much carbon-14 (a radioactive isotope of carbon) they contain. To put this into historical context, the trees date back to 5000–1500 BC, which encompasses the Bronze Age. What’s more, given their gigantic size, many of the trees were probably hundreds of years old when they fell.
But unearthing them is just the start of the challenging recovery process, which should ideally be done during winter. On exposure to air after excavation, the saturated wood is quite fragile and therefore easily damaged. The degree of hardness and condition of a log can be judged by hitting it with a small sharp axe and then listening for a particular sound. All too frequently much of the raw wood will be unfit for further processing. Oaks that are found about four or five feet below sea level tend to yield the best timber. Sometimes the trunks, especially if they are too large and unstable to move, will be cut up in the field. Depending on their size and condition, the logs may be sawn into quarters and/or sliced into smaller sections or planks. The saturated logs or planks are then wrapped in waterproof material and transported to a specialist workshop for immediate drying before they rot.
The bog oak is first cut into thin boards, and then stacked up with wooden stickers in between them to allow the circulation of air. The boards are then slowly air dried under cover or in a timber shed, which allows moisture to evaporate at an even rate, and importantly minimises warping and cracking. As a rule of thumb, each inch thickness of timber takes approximately a year to dry down to the desired moisture content of 30%. Thus, one-inch boards take one-year, two-inch boards take two years, and so on.
Once partially dried, the boards are usually transferred into a specialist dehumidifying kiln and dried at ~30°C (86°F) for several more months, until the moisture content is below 12%. The challenge is to remove huge volumes of water, yet maintain the timber’s integrity – so regular monitoring is essential. Typically, a staggering three to four gallons of water are removed per cubic foot during the drying process, which unsurprisingly results in a significant degree of shrinkage (more than half the original volume).
In 2012, a massive black oak was found buried in a field in Norfolk. Remarkably, the tree trunk was perfectly preserved along its 44-foot (13.4-metre) length and estimated to be ~4,800 years old. What’s more, as there was no tapering at either end, it was deemed to be a middle section (possibly just a quarter) from the original giant living tree. Its successful recovery involved a huge team effort, masterminded by bog oak expert Hamish Low.
After being unearthed using heavy machinery, the enormous tree was sawn into ten full-length planks on site, by expert millers using a 52-foot sawmill that had been shipped in from Canada. Because of the trunk’s exceptional size and condition – and contrary to normal practice – the 44-foot planks were not cut into smaller lengths and then into thinner boards (which are much easier to dry). The ten waterlogged planks were therefore so long, heavy and fragile that it took 18 people to lift them into the huge custom-built kiln – but only seven people to lift them out several months later when they were dry!
Even when dry, bog oak is challenging to work with – but the effort is always well rewarded. It looks and feels beautiful, and is often likened to some of the world’s most expensive tropical hardwoods. Unlike most wood, bog oak is so dense that it nearly sinks. It has an almost stone-like hardness, due to mineral deposition during preservation, and can only be carved using special tools.
Because no two trees were preserved under identical ecological conditions, bog oak exhibits a remarkable range of colours and grain patterns—from rich tan brown to dark brown and even charcoal black. Older trees often display black outer rings that transition to deep brown at their core, giving the timber an extraordinary, striped appearance. Recognising it’s rarity and unique character, Silverlining recently crafted a statement desk for the CEO of a hedge fund, using bog oak that dates back more than 3,200 years. This rare wood, combined with fine leather, was also used to create bespoke panelling for a private office in London. With few massive trees believed to remain buried beneath the peat, English bog oak is an increasingly scarce resource—one that must be carefully preserved for only the most exceptional projects.