UV light and ageing

Why wood changes colour in light

Two processes run at the same time. Ultraviolet and short-wave visible light break down the lignin at the surface, the substance that holds the wood fibres together. Light woods become more yellow and browner as a result. At the same time, light and oxygen change the extractives that give the wood its colour. Depending on the species, they darken or bleach.

The change sits in a very thin layer at the surface and is strongest in the first months. After that it slows down, but never stops entirely. On solid wood the changed layer can be sanded off. On veneer the whole visible layer is only a fraction of a millimetre thick, and there is little room for sanding.

How the Reihax woods change

Reihax works with seven woods. Each changes differently in the light:

• Ash: becomes more yellow and warmer. The cool, almost white tone of the freshly sanded wood turns into a light yellowish brown.
• Oak: becomes more golden and somewhat darker, the process that gives oak its familiar honey tone over the years.
• Walnut: usually becomes lighter and warmer. The cool chocolate brown loses depth and moves towards golden brown.
• Mahogany: darkens and becomes a deeper red-brown. Freshly sanded mahogany looks lighter and pinker than a piece that has stood in the light for a few months.
• Wenge: changes comparatively little. It stays dark, and changes are less noticeable on so dark a wood anyway.
• Maple: yellows and over the years becomes ivory rather than white.
• Cherry: darkens most clearly of all seven woods, from a light, pinkish brown to a deep red-brown.

These are directions, not fixed values. How much a piece changes depends on how much light and what kind of light it gets, which finish it has and which log the veneer comes from.

What that means for the contrast

An intarsia is built for a particular contrast. Because the woods involved change in different directions, this contrast shifts over time. How much depends on the pairing:

• Walnut with ash: walnut gets lighter, ash more yellow. The two move towards each other, the contrast decreases most clearly, and the play of warm against cool weakens.
• Wenge with oak: wenge stays dark, oak becomes more golden. The gap in lightness stays large; the pairing becomes warmer overall.
• Oak with walnut: oak becomes more golden and somewhat darker, walnut lighter. These two also move towards each other, and the gap in lightness gets smaller.
• Ash with mahogany: mahogany darkens, ash becomes more yellow. The gap in lightness stays roughly the same; both woods become warmer.

Photo: Reihax

For planning, that means a pairing that lives on the gap in lightness ages more predictably than one that lives on fine differences of colour. How lightness and colour temperature work together is described on the page contrast and colour temperature.

What a finish can do

No clear finish stops ageing by light. It can slow it down or slightly change its direction, no more:

• Oils and hard-wax oil deepen the tones when applied and bring a slightly warm tone of their own, especially on light woods. They give hardly any protection against light.
• Lacquers with UV absorbers slow the change down, especially on light woods. They cannot stop it (lacquering).
• Pigmented finishes protect better, because the pigments keep light out, but they also cover the wood and are hardly an option for intarsia.

Every Reihax piece is finished with hard-wax oil in a satin sheen (hard-wax oil). The woods age under it as described above. What keeps the change even is therefore less the finish than the place where a piece stands or hangs.

Shadow lines

Where something covers the wood, it does not age along with the rest. A box that stands in the same place on a wooden surface for months leaves a lighter outline when it is moved. On a wall piece, the edge covered by the rebate of the frame stays lighter than the rest, which only shows when the piece is taken out of the frame. A pen holder that always stands with the same side to the window also ages faster on that side than on the others.

Shadow lines never quite close up over time, but they fade when the covered area is then exposed to the light. Moving or turning objects now and then keeps them from forming in the first place.

Placing a piece well

• No direct sun: a spot that the sun falls on regularly ages a piece as much in months as a shady one does in years. That applies behind glass too, because window glass lets part of the UV light and the visible light through.
• Even light: a wall piece hangs better on a wall with even, indirect light than next to a window that lights one half more strongly (intarsia wall art).
• Artificial light: LED lights give off hardly any UV light and contribute little to ageing.
• Turn now and then: turn or move pen holders and other objects on a desk from time to time, so that all sides age alike and no shadow lines form.
• Store veneer in the dark: anything not yet used is kept covered, so that the top sheet does not age differently from the ones below (storing veneer).

How wooden objects are otherwise looked after is described under caring for handmade wooden objects.

Patina, repairs and a second piece

Ageing by light is not damage but patina, and many people value precisely the warm tone of aged oak or the deep red-brown of old mahogany. It becomes a problem where new meets old: a replaced field in an older intarsia is at first lighter or darker than its neighbours (fixing mistakes). And a piece made today looks different next to an identical piece that has stood on a bright desk for two years, even though both are built the same way.

Photo: Reihax

Anyone looking for a second piece to go with one they have should know this: the difference is greatest in the first months and becomes smaller once the new piece ages in the same room.

Checklist for even ageing

• No spot with regular direct sun, not even behind glass
• Even, rather indirect light across the whole surface
• Objects turned or moved now and then
• No mats, coasters or objects left permanently on wooden surfaces
• When choosing woods, consider which pairings keep their contrast
• Unused veneer stored covered and in the dark
• New parts next to old ones judged only after some time in the same light