What green wood is
The Wood Handbook says green wood is often defined as freshly sawn wood in which the cell walls are completely saturated with water, with more water possibly in the cell cavities. Its moisture content — the weight of water as a share of the dry wood's weight — can run from about 30% to more than 200%. It differs by species, between heartwood and sapwood, and even within boards from the same tree. The craft uses that wood before it dries: splitting it, hewing it with an axe and shaping it, then letting the finished or roughed-out parts dry.
| Species | Heartwood | Sapwood |
|---|---|---|
| White ash | 46% | 44% |
| Black cherry | 58% | — |
| Sugar maple | 65% | 72% |
| White oak | 64% | 78% |
| Black walnut | 90% | 73% |
Averages from the Handbook; a dash means it gives no figure. A moisture meter measures a real piece, within its range; the weight of green wood is estimated in the wood weight calculator.
Fiber saturation: when shrinking starts
Water sits in wood two ways: free water in the cell cavities and bound water held within the cell walls. The fiber saturation point is the moisture content at which the walls are still full but the cavities are empty. Above it, the Handbook says, wood is dimensionally stable; below it, wood shrinks as the walls give up bound water. The Handbook uses about 30% for the fiber saturation point in its calculations, but it also says the point varies with the way it is measured and that its exact definition is still debated, so treat 30% as approximate, not a line every piece crosses at once. Strength changes too: the Handbook lists separate values for green wood and for wood at 12%, and gives the moisture content at which properties begin to change on drying — 24% for white ash and 27% for yellow birch, for example.
Shrinking, unevenly
| Species | Radial | Tangential | Volumetric |
|---|---|---|---|
| White ash | 4.9% | 7.8% | 13.3% |
| Black cherry | 3.7% | 7.1% | 11.5% |
| American beech | 5.5% | 11.9% | 17.2% |
| White oak | 5.6% | 10.5% | 16.3% |
| Black walnut | 5.5% | 7.8% | 12.8% |
As a percentage of the green dimension. The Handbook says tangential shrinkage is about twice radial; in these five species the ratio runs from about 1.4 to 2.2. Movement at a given moisture change is worked in the wood movement calculator.
Two things follow for green work. A part shaped green will get smaller as it dries, more along the growth rings than across them, so a round section can dry oval and a square one diamond-shaped — two of the six types of warp the Handbook names. And it will not dry evenly: the Handbook explains that the surface dries first, so it can start to shrink while the core is still above fiber saturation, which puts the surface in tension.
Checks, and coating the ends
Those stresses are what crack drying wood. Surface checks open early in drying when the drier shell is pulled in tension; end checks open at the ends of boards and logs because moisture moves quickly along the grain and the ends dry fastest. The Handbook calls end coatings on logs or freshly sawn lumber an effective preventive measure, and says elsewhere that end coating may alleviate end checking — it reduces the fast drying at the ends; it does not promise a piece will not crack. How long drying takes, the Handbook says, varies widely with species, thickness, location and season, so no rule of thumb is given here.
Riving: splitting along the fibres
Splitting opens wood along its fibres; the Handbook's own best test for spiral grain is to split a sample in the radial direction and look. A riven blank therefore follows the grain, while a sawn one can cross it, and the Handbook's table shows why that matters: at a slope of grain of 1 in 10, clear wood keeps 81% of its straight-grained modulus of rupture and 62% of its impact bending strength. A split still follows whatever the tree did — knots carry their own local slope of grain, and spiral grain takes the split around with it — so a riven part is only as straight as the wood it came from.
Gränsfors Bruk's froe shows how it is done: the wood is placed under the edge, the tool is steadied by its upright handle, and the blade is struck with a mallet; the maker sells it for splitting wood very finely, as for roof shingles. A froe is struck with a mallet, not with the poll of an axe, which Gränsfors says is not made for hammering; mallets are covered in woodworking mallets.
The tools, and where each is covered
| Stage | Tool | What its maker or source says | Covered in |
|---|---|---|---|
| Splitting | Froe and mallet | Wood under the edge, blade struck with a mallet (Gränsfors) | This page; mallets |
| Hewing and roughing | Carving, carpenter's or broad axe; adze | Cut with the grain on a clean block (Gränsfors) | Axes and adzes |
| Debarking and shaping | Drawknife | Arched blade used with both hands, mainly for debarking and shaping (Gränsfors) | Spokeshaves and drawknives |
| Carving | Knives and gouges | — | Carving tools and green carving projects |
| Turning | Lathe | — | Woodturning |
Carving and turning green wood are covered by their own pages; this page explains the wood, not those crafts.
Working tips the sources support
- Rive before you shape, so the blank follows the fibres; expect knots and twisted grain to steer the split.
- Shape oversize where a part must end up a set size, since it will shrink, tangentially more than radially.
- Coat the ends of logs and blanks you are not working yet, to slow the fast drying there.
- Measure moisture with a moisture meter before final fitting or joinery, rather than guessing from time.
- Choose stock from the lumber guide and read its grain with wood grain; no single species is best for every green job.
Safety— Splitting and hewing safely
Strike a froe with a mallet, keep your hands and legs out of the line of any edge, and work on a stable block or support. Axe technique and the limits on using an axe's poll are in the axe guide; protective equipment in woodworking safety equipment; general rules in woodworking safety rules.
Before working a green blank
- Ends coated if it will wait
- Split along the fibres; knots noted
- Allowance left for shrinkage
- Edges sharp; froe struck with a mallet
- Moisture checked before final fitting
Sources
Every moisture, shrinkage and strength figure is the Wood Handbook's (FPL–GTR–282), given as it states it, by species where it gives species. The fiber saturation point is given as approximate because the Handbook says its definition is debated and its value varies. No drying time, ideal species or universal rule for checking is given; the Handbook gives none. Tool use is from the tools' maker. Nothing was tested.
- 1.
Chapter 4: Moisture Relations and Physical Properties of Wood. USDA Forest Service, Forest Products Laboratory — Wood Handbook, FPL-GTR-282, 2021.
Used for: Green wood often defined as freshly sawn wood with saturated cell walls and possibly water in the cell cavities, at about 30% to more than 200% moisture content; green moisture content by species, heartwood and sapwood (Table 4–1), varying even within a board; free and bound water and the fiber saturation point, whose operational definition is still debated, taken as about 30% in the Handbook's calculations; wood dimensionally stable above fiber saturation and shrinking below it; tangential shrinkage about twice radial; shrinkage by species from green to ovendry (Table 4–3); surfaces shrinking first while the interior is still wet.
- 2.
Chapter 5: Mechanical Properties of Wood. USDA Forest Service, Forest Products Laboratory — Wood Handbook, FPL-GTR-282, 2021.
Used for: Separate property values for green wood and at 12% moisture content; the intersection moisture content at which mechanical properties begin to change as wood dries from green, 24% for white ash and 27% for yellow birch; slope of grain and its effect on strength — at 1 in 10, 81% of straight-grained modulus of rupture and 62% of impact bending; knots with their attendant local slope of grain; splitting a sample radially as the best test for spiral grain.
- 3.
Chapter 13: Drying and Control of Moisture Content and Dimensional Changes. USDA Forest Service, Forest Products Laboratory — Wood Handbook, FPL-GTR-282, 2021.
Used for: Bow, crook, twist, oval, diamond and cup as the six types of warp; air-drying time that varies widely with species, thickness, location and season; surface and end checks from drying stresses; end checks because moisture moves quickly along the grain so ends dry first; end coatings on logs or freshly sawn lumber as an effective preventive measure that may alleviate end checking.
- 4.
Gränsfors Froe. Gränsfors Bruk.
Used for: A froe for splitting wood very finely, such as roofing shingles: the wood placed under the edge, the tool steadied by its vertical handle and the blade struck with a mallet.
- 5.
Gränsfors Swedish Drawknife. Gränsfors Bruk.
Used for: A drawknife forged into an arch and used with both hands, mainly for debarking and shaping logs, and perhaps also dowels.
- 6.
Woodworking & Sloyd. Gränsfors Bruk.
Used for: Shaping with an axe on a clean chopping block, holding the work away from the body, cutting with the grain.
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