Reading a Kiln Schedule

What a schedule is

A kiln schedule is a list of dry-bulb temperatures and humidities, each held until a condition is met. The Wood Handbook calls it “a carefully developed compromise” between drying as fast as possible and avoiding the conditions that cause defects ¹. Most schedules raise temperature and lower humidity as the wood dries, because drier wood is stronger and can take harsher conditions, and because drying slows as wood gets drier, so the kiln has to push harder to keep it moving.

There are two kinds:

• Moisture-content schedules change conditions when the lumber reaches given moisture contents. Most hardwood is dried this way, using kiln samples that are weighed during drying.
• Time schedules change conditions after set hours. Most softwood, and some hardwood dried as structural lumber, uses these.

The walnut schedule

Here’s the handbook’s moisture-content schedule for 1-inch (4/4) black walnut dried to 7 percent ¹:

Reading it line by line

Wet and weak: gentle and humid. While the lumber is above 50 percent, the kiln holds 120 °F and 80 percent RH, an EMC of 14.4 percent. That’s far below the wood’s moisture, so it dries, but the high humidity keeps the surface from drying much faster than the core can follow. This is Stage I, when surface checks and end checks form, and the schedule’s whole job is to avoid them ².

Losing free water: humidity steps down. From 50 down to 30 percent the temperature stays at 120 °F but the humidity drops in steps to 40 percent. The surface is already below fiber saturation; the core is still giving up free water. Stepping the EMC down keeps a steady driving force as drying naturally slows.

Below fiber saturation: heat up. Once the lumber averages under 30 percent, the core is drying below fiber saturation too, the wood is stronger, and the surface-checking danger has passed. Now temperature climbs, 130, 140, 150, and finally 180 °F, and the EMC sits very low, around 3 to 4 percent. The danger in this stage is different: excessive heat while the core is still wet causes honeycomb, and very high early temperatures darken or collapse wood. That’s why the big temperature jumps wait until 30 percent and below.

Equalize. When the driest samples reach a little below the target, the kiln equalizes: it sets an EMC that stops the driest boards from going lower while the wettest ones catch up. The result is a charge with a narrow moisture range instead of some boards at 5 and some at 10.

Condition. Finally the kiln conditions: a short spell at very high humidity, here 79 percent RH and an EMC of 11 percent at 180 °F. The dry, set shell regains a little moisture and swells slightly, relieving the casehardening stress locked in during drying. Conditioning is short, measured in hours, and its result is checked with prong tests ².

A time schedule for comparison

The handbook’s sugar maple 2×6 schedule, dried to 15 percent in 5 days, uses time instead: 160 °F and 84 percent RH for the first 24 hours, stepping to 52 percent RH by 60 to 72 hours, then up to 180 °F and 27 percent RH until dry. And Southern Pine 2× lumber is dried at a single fixed condition, 240 °F and 29 percent RH, until done ¹. Construction lumber can be dried hard; furniture lumber can’t.

Why hardwoods differ by species

Every species has its own schedule because each has different strength, permeability, and stain risk. Oak is refractory and starts very gently and slowly, with weeks in the early stages. Hard maple starts warmer and faster to keep it white. Walnut and cherry tolerate moderate schedules. The Dry Kiln Operator’s Manual and the FPL hardwood manual tabulate schedules by species and thickness ³.

What to take from this

A hardwood kiln schedule steps through moisture contents: humid and modest temperature while the wood is wet and weak, falling humidity as free water leaves, rising temperature once the wood is below 30 percent and strong, then equalizing to even out the charge and conditioning at high humidity to relieve casehardening.