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Lesson 6 of 7
Drying stress, defects and conditioning
Most kiln defects have one root cause: stress from uneven water loss combined with anisotropic shrinkage (Yin and Liu, 2021 review; Wood Handbook ch. 13, North America).
How stress develops
Early in drying the shell dries below FSP and tries to shrink while the wet core resists, so the shell is in tension and may check if tension exceeds its strength. Later the stress reverses: the shell is in compression and the core in tension, which can cause internal checks (honeycomb). Residual stress left at the end is case hardening. Schedules must keep stress below wood strength at each temperature and moisture level.
Defects therefore point to a stage: surface checks to the early stage, case hardening to the middle, internal checks and honeycomb to the late reversal. (This diagnostic mapping is the portal's reading of the review.)
Defects, causes and remedies (FPL manual ch. 8, North America)
| Defect | Main cause | Main remedy |
|---|---|---|
| Surface check | surface dries too fast, humidity too low | higher humidity early |
| End check | ends dry first | high humidity early, end-coat green ends |
| Honeycomb | internal tension failure, usually in rays; high temperature while core is above FSP | no temperature rise until the core is below FSP |
| Collapse | cell flattening, linked to excessively high early dry bulb | low initial temperature, air drying first, reconditioning |
| Case hardening | unbalanced residual stress | conditioning |
| Brown stain (softwood) | oxidative reactions of sapwood chemicals | low dry bulb, large depression, steaming |
Uneven final MC: presort on the green chain, or sort after drying and redry wet boards, which can add 25% or more to drying cost.
Tests for stress and gradient
Shell-and-core test. A 1 inch moisture section is split into shell and core, and each part is oven-dried. Best made at an average of about 20 to 25%. For oak, delay raising temperature in the last steps until the core is at 25% or lower.
Prong test. For lumber under 1.5 inches thick, cut three prongs and remove the middle one; thicker lumber gets six prongs with the second and fifth removed. Outer prongs pinching inward indicate case hardening (core in tension, shell in compression). Residual stress shows at once; moisture-gradient movement takes hours. The review notes that prong and slicing tests describe stress only at one moment, not during the run.
Equalising and conditioning (FPL manual ch. 7)
- Equalising starts when the driest kiln sample is 2 percentage points below the desired final average. The equalising EMC is set at that value and runs until the wettest sample reaches the desired average.
- Conditioning relieves compressive shell stress with high temperature and humidity: EMC 4 points above the desired final MC for hardwoods and 3 points for softwoods, at 160 to 180 degrees F for 16 to 24 hours for 4/4 hardwood, up to 48 hours for 8/4. It is generally applied to final MCs of about 5 to 11% and is not really needed for softwood dimension lumber dried to 15% average (maximum 19%).
Worked example. A hardwood charge is to finish at an 8% average. Equalise when the driest sample reaches 8 - 2 = 6%, with the equalising EMC set at 6%, until the wettest sample is at 8%. Then condition at EMC 8 + 4 = 12% for 16 to 24 hours (4/4 stock). If the same logic were applied to a softwood target of 12%, conditioning EMC would be 12 + 3 = 15%.
Checklist for a defect investigation
- Which stage of drying does the defect point to?
- Run a prong test and a shell-and-core test on sample boards.
- Check the log: was temperature raised before the core was below FSP?
- Check sorting: species, thickness, sapwood share.
Key points
- Shell tension early, reversal later, core tension and honeycomb at the end if the schedule is too hard.
- Never raise temperature mid-drying while the core is above FSP.
- Prongs pinching inward means case hardening; the cure is conditioning.
- Equalise from 2 points below target; condition at target + 4 (hardwood) or + 3 (softwood).
- Sorting before drying avoids costly redrying.