Timber Drying and Moisture Control for Dining Chairs

Kiln schedule, the 8-12 percent band, case-hardening and why most joint failures in dining chairs are actually moisture failures.

Why the kiln is the most important room in the factory

Most seating failures blamed on joinery are actually moisture failures. Timber that leaves a kiln too wet shrinks after it is made into a frame, and the glue line opens. The joint was fine; the water content was not.

We dry to 8 to 12 percent and meter every batch. The reading is recorded against the batch number and printed on the packing list, so it can be checked at destination.

The schedule

A kiln charge runs 15 to 25 days depending on species and section thickness, stepping temperature up and humidity down in stages. Rushing it by raising temperature early produces case-hardening: the outside of the board dries and locks while the core is still wet, and the board moves later, inside a finished chair.

Kiln schedule

Defects we reject for

End checking, collapse, honeycombing, sticker stain and fungal discolouration. Any of these disqualify a board from a frame component. They are cheap to reject at the kiln and expensive to discover after assembly. Solid Wood Dining Chairs use the same graded stock as every other model we build.

Equilibrium, and what it means for your market

Timber settles at the moisture content the surrounding air dictates. A frame that arrives at 10 percent into a heated Nordic restaurant at 30 percent relative humidity will drop towards 6 percent and shrink as it does. That movement is normal and it is why we hold the band, seal end grain, and specify a joint that tolerates it.

Species behave differently in the kiln

Ash — around 70% of what we run — dries steadily and holds its colour if the schedule is respected. Rubberwood, the other 30%, dries quickly and evenly and is the most forgiving charge in the yard. Oak is slow and prone to checking if pushed, and beech dries fast but collapses if the temperature goes up too early; both are custom runs for us. The schedule is written per species, not per thickness, and the two get confused constantly.

Thickness matters as much: a 60 mm blank takes far longer than a 25 mm one, and drying them in the same charge guarantees one of them comes out wrong.

Stacking, stickers and air flow

Boards are separated by stickers of uniform thickness, aligned vertically, with the stack built so air moves evenly through it. Poor stacking produces uneven drying no matter how good the kiln is: stagnant pockets stay wet and the charge comes out with a spread of moisture contents rather than one.

Sticker stain — the discolouration where the sticker sat — is a stacking defect, not a drying one, and it is why stickers are dry, uniform, and placed on a marked pitch.

What happens after the kiln

Dried stock is held in a covered, conditioned store until it is machined. Leave it in an open yard through a Foshan wet season and it will take water back up within days, which undoes the schedule. This is the step most factories skip and the reason a moisture reading taken at packing matters more than one taken at the kiln.

We meter at both. If the two disagree, the charge goes back before it reaches the machining line.

Reading a moisture meter properly

Pin meters measure between two pins driven into the timber and are the reliable method for a frame component; pinless meters scan a surface and are useful for a quick survey but read the wettest thing in range. Take three readings along a blank and average them, and correct for timber temperature if the sample has just come out of a hot kiln.

A single reading is not a measurement. Most disputes about moisture content are disputes about where the probe was put.

What to ask your supplier

Three questions: what moisture content do you ship at; do you meter per batch; and can I have the reading on the packing list. A factory with a kiln answers all three immediately. A workshops buying in dried stock usually cannot answer the second.