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Drying defect and causes
Lumber drying defects and their causes.
Drying mistakes and their degrade and defects.

Drying 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.
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.
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.
Drying defect - "A drying defect is any characteristic or blemish in a wood product that occurs during the drying process and reduces the product's intended value."
Drying degrade - A defect which lowers the lumber's grade.
Wood has many natural defects; knots, bark, mineral streak, pitch pockets, compression & tension wood, spiral or interlocked grain etc.
Defects that reduce grade can occur at many steps in the process: logging, sawmilling, drying, finishing, or material handling.
High temperature reduces wood strength , temporarily, if temps are reduced quickly and if-high temps are persistent - permanently. This temperature effect is made worse with increasing Moisture Content.

Rupture of Wood Tissue
  • Surface Checks
  • End Checks and Splits
  • Collapse - Crushing of wood cells (Usually associated with too high dry bulb temperatures too early in the drying schedule)
  • Honeycomb - Internal crack caused by tensile failure across the grain
  • Boxed-heart splits - Eminating from pith
  • Checked knots
  • Loose knots ( Anyone ever hear of STK grade decking?)
Warp Warp , Cup , Bow , Crook , Twist , Diamonding Uneven Moisture Content

Lumber not dried to target MC
Some of lumber too high MC (inadequate Equalizing)
Lumber too dry!
Water Pockets

Remember consistency, and standardization
Sort based on original MC and species, thickness, etc.


Discoloration

Sapwood vs. Heartwood
Sapwood
  • Chemical
    • - Live parenchyma cells are subject to enzymes and oxidation which darken wood. Remember premiums paid for lighter shades of wood that are preferred by today's market.
    • - Pink, blue, yellowish, brown (Douglas Fir Brown-stain, Eastern White Pine Brown Stain)
    • High initial temperatures (~ 140 deg) spur growth of chemical stains (start near 110 deg)
  • Fungal
    • Blue-stain - fungus feeds on sugars and starches in wood
    • Will not grow in M.C. < 20% and temps > 110 deg f
  • Biological
    • Some tropical woods - aerobic bacteria feeding on chemical extractives
Heartwood
  • Chemical
    • Function of chemical extractives and high drying temperatures
  • Fungal
    • Most mold-type fungi (like blue-stain) can't grow in heartwood.
    • Exception Paecilomyces Varioti - feeds on tannins in oak
Drying Defects and Machinability

  • Planer splits from cupped lumber
  • Broken knots and knot holes
  • Torn grain
  • Raised grain
  • Residual drying stresses - Casehardening could lead to
  • End checks
  • Planer splits
  • Warp

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)
For every 1 unit of lumber damaged during the drying process,
you will have to dry 10-20 units to make up for the loss (break even).
Remember lumber drying is all about maximizing profit by increasing
the value of the lumber together with minimizing the cost of drying. (Cost of drying = Energy cost + Degrade + Depreciation + Maintenance).



Twist,Spring & Bow