Wood and wooden objects

Wood exchanges moisture with the air until it reaches equilibrium, and it changes dimension as it does. That much is familiar. The consequence that matters is that it changes dimension very unevenly: substantially across the grain, considerably less radially than tangentially, and almost not at all along the grain.

Nearly every characteristic failure in wooden objects follows from that asymmetry.

Why objects fail

Panels crack. A board restrained at its edges by a frame cannot expand across the grain when humidity rises, so it is compressed. When humidity falls again, it shrinks from a compressed state and splits.

Panel paintings cup. Ground and paint layers restrain the front face while the back responds freely. The panel curves towards the restrained side, and the movement is repeated with every seasonal cycle.

Veneer lifts. Veneer is thin and responds quickly; the substrate is thick and responds slowly. Between them sits a glue line, usually an animal glue that is itself moisture-sensitive. Repeated differential movement fatigues the bond.

Marquetry falls out. Pieces of different species and grain directions in one plane, each moving differently, all restrained by their neighbours.

Joints open. Mortice and tenon joints work loose as the members move relative to one another and as the glue is repeatedly stressed.

Note what all of these have in common: they are caused by change rather than by any particular level. This is why the target range discussion emphasises stability over the specific number.

Rate matters more than range

Wood responds slowly, and slowness is protective. A slow seasonal drift lets moisture content equalise through the thickness, so stress stays low. A rapid change wets or dries the surface layer while the core lags, and the resulting gradient is where cracking and cupping originate.

The practical consequences are worth stating plainly. A weekly cycle of ten points is worse than an annual drift of twenty-five. A gallery whose heating comes on hard each morning is doing more damage than a cold, damp store that never changes. And an object surrounded by a buffered enclosure experiences a flattened version of whatever the room does, which is why case and box microclimates are so effective for furniture.

Historic equilibrium

An object that has stood in an unheated house at 60 percent for two hundred years is in equilibrium with that condition. Its joints, its finish and its cracks have all accommodated it.

Moving that object to a textbook 50 percent is an intervention. It will shrink, joints will move, and old repairs will be stressed. Where the existing condition is not actively harmful, matching it is usually safer than correcting it. The exception is genuinely damp conditions, above roughly 65 to 70 percent sustained, where mould and wood-boring insects become a real risk.

Other agents

Light affects wooden objects mainly at the surface. Finishes yellow, some woods darken and others bleach, and dyed or stained surfaces fade. The wood itself is not highly responsive, but a fine finish is.

Insects. Wood borers generally need a timber moisture content that dry indoor conditions do not sustain, so a live infestation is usually also a damp finding.

Handling. Furniture invites the wrong lift. Carrying a chair by its back, a table by its top, or a case by its cornice puts load into joints designed for compression rather than tension. Handling covers this.

What to actually do

1. Log the space for a year, because the swing is what matters and only a record shows it.
2. Reduce the rate of change before chasing an absolute value. Slower heating, better draught sealing, buffered enclosures.
3. Match historic conditions where they are not harmful, rather than correcting to a specification.
4. Keep objects away from external walls, radiators and direct sun, all of which create local gradients across a single object.
5. Where one object is at risk in an otherwise acceptable room, buffer that object rather than the building.