Short answer
Reclaimed timber that was outdoors turns silver-grey because sunlight breaks down lignin at the surface and rain washes the coloured fragments away; the black rings around nail holes are iron from the fastener reacting with tannins in damp wood. Both are surface effects, so how the wood looks depends on how deep it is planed or sanded and whether the species is rich in tannin.
Lignin binds wood cells together and makes up roughly a fifth to a third of wood's mass. The USDA Forest Products Laboratory's Wood Handbook explains that ultraviolet and some visible light break lignin down within hours at the surface; rain then washes out the fragments along with coloured extractives, leaving loose cellulose fibres that look silvery grey. Dark fungal growth on weathered boards adds to the grey or makes it blacker. The layer is shallow: the Handbook puts erosion at around six millimetres a century. Anything more than a light sanding exposes wood that is close to its original colour, which is why re-planed reclaimed boards can look surprisingly new.
Blue-black rings and streaks around old nails, bolts and plates are iron stain. Iron from the fastener reacts with tannins in the wood when the timber is wet — the Handbook gives a wood moisture content above about 16% — forming a dark compound chemically related to historic iron-gall ink. Tannin-rich species such as oak, western red cedar and redwood show it strongly; low-tannin woods such as pine show a brown rust mark instead. Because the reaction needs moisture, it is typically an outdoor or damp-building history, and it can go deep along the grain around the hole.
Iron–tannin stain can be lightened with oxalic acid, which the Handbook recommends for the blue-black discolouration; it reverses the stain chemically rather than bleaching the wood. Weathered grey can be kept by brushing and sealing with a clear matt finish, but any clear film finish will darken and warm it, because wetting the loose surface fibres reduces scattering. Matching new timber to reclaimed grey with a stain rarely convinces for long: the reclaimed surface is a texture as much as a colour, and new wood exposed to light will change on its own. Reclaimed-wood projects usually look most coherent when the grey, the nail marks and the colour variation between boards are all accepted as the design.
Joists, floorboards and furniture timber reclaimed from inside buildings did not weather to grey. Instead they usually darkened and warmed with age, as light and oxygen changed extractives and any old finish yellowed; they may carry paint, soot or wax. These boards look amber to brown rather than silver, and planing reveals a paler interior. Mixing interior-reclaimed and exterior-reclaimed boards in one floor therefore gives two quite different colour families, so it is worth asking where the timber came from, not only which species it is.
Why: A clear film finish wets the loose weathered fibres and reduces scattering, darkening and warming them.
Fix: Test on an offcut first; use a finish marketed for weathered wood or leave the surface unfinished indoors.
Why: Iron–tannin reaction where the wood is damp.
Fix: Use stainless or coated fasteners in tannin-rich species exposed to moisture.
Why: The grey is only a shallow surface layer.
Fix: Brush rather than plane if the patina is the point, or accept the fresh colour.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactStrong evidence
Weathered wood has a silvery grey appearance because UV and visible light degrade lignin at the surface and weathering removes coloured components.
Source: Wood Handbook (FPL-GTR-282), Chapter 16: Finishing Wood
FactStrong evidence
Iron stain is a black discolouration formed when iron, most commonly from fasteners, reacts with tannins in wood whose moisture content is above about 16%.
Source: Wood Handbook (FPL-GTR-282), Chapter 16: Finishing Wood
FactStrong evidence
Oxalic acid solution removes the blue-black iron–tannin discolouration from wood.
Caveat: Oxalic acid is hazardous; follow the product's safety instructions.
Source: Wood Handbook (FPL-GTR-282), Chapter 16: Finishing Wood
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.