Bog oak

Bog oak is oak that has lain buried in waterlogged ground for centuries or thousands of years. It is not a separate species: The Wood Database calls it a term for oak preserved in peat bog, with English oak the species most commonly salvaged¹. The wood is dark brown to almost black, supply is very limited, and, as far as we could verify, its hardness has never been published.

How does bog oak form?

Preservation comes first. The Wood Database explains that the very low oxygen levels in a bog protect the wood from normal decay, and that acidic peat and iron salts react with the tannins in the wood and darken it. It describes the wood as being in the early stages of fossilisation¹. That is why the material is called subfossil: it is still wood, not stone.

Peat is only one setting. In southern Poland, researchers describe "black oaks" in the alluvial deposits of the Vistula and Odra. Rivers shifted their beds, felled trees on their banks and buried them in sediment, and the black colour came from a reaction between wood tannin and iron compounds in the groundwater⁴. In the Fens of eastern England, the Great Fen Project says trunks lay undisturbed in acidic bog conditions until drainage shrank the peat and brought them closer to the surface⁷. Woodland Heritage offers a theory that rising water levels swamped the trees, and that tannins in the oak and the surrounding soil produced the black colour⁶.

Why does bog oak turn black?

The authors of a study of a subfossil trunk from Chiarano, Italy, explain it directly: oak that lies in water for a long period may turn black because its tannins react with iron ions in the water and sediment, typically in an acid, poorly oxygenated environment³. The colour is not uniform. In that trunk it ran from black on the outside to natural light brown in the centre³. Colour therefore depends on the burial conditions and on which part of the log a board is cut from.

The same paper notes that this kind of oak was used in the past as an economical substitute for ebony in marquetry, and that it is still sought after for furniture³.

How old is bog oak and how is it dated?

Ages vary enormously. Woodland Heritage gives roughly 2,000 to 5,000 years for English bog oak, with Irish examples sometimes over 7,000 years⁶. The Chiarano trunk is far younger: radiocarbon dating gave 795 ± 35 years BP, a calibrated range from the second half of the 12th to the second half of the 13th century AD³. Subfossil oak is therefore defined by how it was preserved, not by a minimum age.

Oak is well suited to tree-ring dating because it is ring-porous and does not form false rings³. A Polish team built a chronology covering 1795 to 612 BC from subfossil trunks of the Vistula and Odra basins, plus floating chronologies fixed by radiocarbon for older periods, the earliest at about 6650 to 6150 BC⁴. In England, a bog oak recovered from a field drain near West Deeping had a ring series spanning 489 BC to 126 BC⁹. The Chiarano trunk, by contrast, could not be dated by rings alone, which is why radiocarbon was used³.

Where does it come from, and are there rules on taking it?

Most finds are accidental. Fenland farmers cleared bog oak from arable fields and piled it into clearance cairns, and some of it was burned as fuel while better pieces went to furniture and carving⁷. The Chiarano trunk surfaced during canal maintenance, and a Bosnian log used in the Croatian study was found during gravel extraction at about six metres depth³⁵. The Wood Database says the wood occurs most often in the United Kingdom¹. Woodland Heritage advises owners wanting to find furniture makers to contact the Worshipful Company of Furniture Makers⁶.

None of the sources we opened sets out a general legal rule on ownership, export or sale of bog oak, so we state none. One point is worth keeping in mind: the Chiarano trunk showed signs of felling and debranching, so it was not simply a fallen tree, and it was found in an area rich in Roman and Venetian remains³. Ask a supplier where a batch was recovered and under what permissions.

How does bog oak behave when dried and worked?

The one laboratory dataset we found is from a Bosnian log studied by a Croatian forestry team. It was recovered by gravel extraction and then dried slowly under controlled conditions. Compared with recent pedunculate oak, it had a higher absolutely dry density (0.704 g/cm³, 15.8 % higher) and slightly higher bending strength (101.3 MPa, 10.8 % higher), while compressive strength parallel to the grain was not significantly different (52.3 MPa)⁵. Shrinkage was much larger: total radial 10.5 %, tangential 18.6 % and volumetric 27.6 %, each roughly double the figure for recent oak⁵.

For scale, The Wood Database lists English oak at 4.7 % radial, 8.4 % tangential and 13.0 % volumetric shrinkage². Those figures come from a different dataset, so treat the comparison as indicative. Properties also vary by site: the Chiarano trunk had a basic density of 0.5217 g/cm³ against 0.57 g/cm³ for healthy oak, and the authors suspected mineral matter had entered the wood during its time in river sediment³.

Practical drying is slow. The Carpenters' Company project sawed a 13 m, 5,000-year-old Fenland trunk on site and dried the planks in a purpose-built kiln over six months, a process it says reduces weight and some of the girth⁸. Our reading of the data: a wood that shrinks this much needs patient drying and careful acclimatisation, so read our guide to moisture and acclimatisation before specifying it.

How hard is bog oak, and what does that mean for floors?

The Wood Database entry for bog oak lists neither a Janka rating nor a density¹, and the Croatian study reports no hardness⁵. We have not invented a figure. The Janka row above is for ordinary English oak (Quercus robur): 1,120 lbf (4,980 N)². It describes the wood before subfossilisation and should not be read as a measured value for bog oak. Our guide to Janka hardness explains why the scale is only a rough guide anyway, and the European oak page covers the living species in more detail.

The Croatian authors say bog oak is valued for its durability, appearance, acoustic properties and age, and is used for veneer, furniture, musical instruments and sculpture⁵. For floors, one manufacturer's published data shows how it is sold. Havwoods lists a bog oak range of multi-layered engineered boards 20 mm thick with a 5 mm wear layer, in herringbone, chevron, plank and Versailles formats; the page describes the colour as deep and ebony and does not discuss stability or drying¹¹. That is the maker's description, not independent test data. See engineered wood flooring for how a thin top layer works and herringbone for the pattern. Boards cut from old timbers are also sometimes confused with reclaimed wood, but reclaimed wood is timber salvaged from earlier use, whereas bog oak is a natural deposit.

Is bog oak the same as fumed or smoked oak?

No, though the finished colours can look similar. Bog oak darkens over centuries through iron and tannin chemistry in the ground³. Fumed oak is darkened on purpose: in a peer-reviewed study, oak heartwood exposed to ammonia gas for 16 days reacted intensively in a very short time, and oak had both the highest tannin content of the four species tested and the biggest drop in tannin after fuming, which suggests the ammonia reacted with it¹⁰. Both processes depend on oak's tannins, but one is a natural burial and the other is a controlled treatment.

We did not find a trusted source defining "smoked" oak, so we make no claim about it here. When a floor is described as bog oak, fumed oak or smoked oak, ask the seller which process was used and whether the colour runs through the board or sits at the surface. Our wood floor finishes guide covers what coatings do to colour. The sources we opened do not say how deep fuming colour penetrates, so we make no claim about that either.

What should you check before buying?

• Origin and recovery. Ask where the logs were recovered, as above.
• Drying. Ask how the timber was dried. The Carpenters' Company's six months in a kiln for one trunk gives a sense of the timescale⁸.
• Build. Check thickness and wear layer, as with the Havwoods figures above¹¹.
• Colour variation. The Chiarano trunk shows that colour can change from outside to centre³, so ask to see several boards, not one sample.

This page compiles published data from the sources listed above. Read our editorial policy. Last updated 3 Nov 2026.