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Kiln theory and knowledge
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Drying defect and causes
Lumber drying defects and their causes.
Drying mistakes and their degrade and defects.
| 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.
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| 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 |
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X(1) |
X (2) |
| End checks |
X |
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| Internal checks |
X |
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X(3) |
X (2) |
| Split/craks |
X |
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X (4) |
| Collapse |
X |
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X (5) |
| Blue stain |
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X |
X |
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| Mold, mildew |
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X |
X |
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| Decay |
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X |
X |
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| Coffee or brown stain |
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X |
X |
X |
X (2) + (5) |
| Sticker stain |
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X |
X |
X (6) |
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| Pinking |
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X (7) |
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| Twist |
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X (6) |
| Cup |
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X |
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X(9) |
X (10) + (11) |
| Bow |
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X |
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X (8) |
| Crook |
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X (4) |
| Diamond/oval |
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X (11) |
| Warp after drying |
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X (12) |
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| Uneven drying |
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X |
X |
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X (12) |
| Too wet |
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X (14) |
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| Too dry |
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X (14) |
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| Casehardened |
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X (14) |
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| Resin not set |
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X (14) |
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| Ring failure/wind shake |
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X (2) |
| Loosened knots |
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X (16) |
| Checked knots |
X |
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| Heart split |
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X (11) |
| Raised grain |
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X (17) |
| Chipped, fuzzy, torn grain |
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X |
X (18) |
| Planer splits |
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X (9) |
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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.
About information was originally provided by:
Virginia Polytechnic Institute and State University,
Blacksburg, Virginia.
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