Drying defect and causes Home Kiln theory General theory

Drying too fast	Surface checks, End checks, Internal checks (honeycomb), Splits and cracks.
Drying too slowly	Fungal stain (blue stain), Mold and mildew, Decay, Chemical stains (brown stain, sticker stain), Warp (especially cup).
Poor stacking	Warp (especially bow), Uneven drying.
Operational errors	Incorrect final wood MC%, Casehardening not relieved, Resin not set.
Natural defects	Ring failure (bacterially caused), Logging damage (end damage), Checked knots, Loose knots, Heart splits, Crook, twist and cup.
Incorrect wood MC	Raised and fuzzy grain after machining, Chipped and torn grain during machining, Planer splits, Splits after manufacturing, Warp after manufacturing.
Casehardened lumber	Planer splits, Saw pinching, Immediate warp when manufacturing.

Type of defect	Drying too fast	Drying too slow	Poor stacking	Operational errors	Natural wood defects
Surface checking	X			X(1)	X (2)
End checks	X
Internal checks	X			X(3)	X (2)
Split/craks	X				X (4)
Collapse	X				X (5)
Blue stain		X	X
Mold, mildew		X	X
Decay		X	X
Coffee or brown stain		X	X	X	X (2) + (5)
Sticker stain		X	X	X (6)
Pinking				X (7)
Twist					X (6)
Cup		X		X(9)	X (10) + (11)
Bow			X		X (8)
Crook					X (4)
Diamond/oval					X (11)
Warp after drying				X (12)
Uneven drying		X	X		X (12)
Too wet				X (14)
Too dry				X (14)
Casehardened				X (14)
Resin not set				X (14)
Ring failure/wind shake					X (2)
Loosened knots					X (16)
Checked knots	X
Heart split					X (11)
Raised grain					X (17)
Chipped, fuzzy, torn grain				X	X (18)
Planer splits				X (9)

Foot notes and comments to above table.

1: Rewetting a dried surface; too high initial temperature.
2: Bacterially infected wood.
3: Rewetting a dried surface; too high temperature above 30% MC.
4: Tension wood, compression wood, or juvenile wood.
5: Enzymatic oxidation reaction influenced by temperature, humidity and MC.
6: Wet stickers or wide stickers.
7: Excessive temperature.
8: Grain angle.
9: Too dry or casehardened.
10: Lumber from small diameter trees.
11: Difference between radial and tangential shrinkage.
12: Change in MC of lumber after drying; lumber not equal to air's EMC.
13: Mixed species; heart/sap; think/thin; lowland/upland; variable air velocities.
14: Poor samples; incorrect MC measurement; equipmetn problems; poor schedule or procedures.
15: Softwoods only; use 72C/160F or higher
16: Keep temperatures below 90F.
17: Knot was held only by resin in tree.
18: Density differences springwood to summerwood and equipment setup.
19: MC too high or too low; equipment set-up.

Information was originally provided by Virginia Polytechnic Institute and State University,
Blacksburg, Virginia.

Drying defect and causes Home Kiln theory General theory

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