Sunlight, radiators and the walls you should not use

What light actually does

Two separate things, and only one of them is famous.

Fading. Ultraviolet and visible light break down the molecules that give a pigment its colour. Some pigments are highly stable and some are not, and there is no way to tell by looking which is which in a given canvas. What fades goes first in the most saturated passages, and it does not come back.

Embrittlement. The less discussed one and the more serious. Light and heat together accelerate the ageing of the oil binder, which continues to harden for decades after a painting is finished. A brittle paint film cracks when the canvas moves, and the canvas moves with every change in humidity.

Both are cumulative and neither has a threshold. There is no dose below which nothing happens; there is only more or less of it. Museums manage exactly this by controlling illuminance — oil paintings are typically displayed at around 150 to 200 lux, and works on paper far lower. Direct sunlight through a window on a clear day is tens of thousands of lux. That is the entire argument in one comparison.

A south-facing window, over ten years

The question people actually want answered is whether it matters in a normal house on a normal wall, so here is the honest version.

Indirect daylight is fine. A room with big windows and a painting on a wall the sun never reaches directly is a good room for a painting, and always was. Paintings have hung in rooms like that for four hundred years.

Direct sun for an hour a day is not fine. It is not a catastrophe either, and nothing visible will have happened in year one or year three. Somewhere between year five and year fifteen you will notice that a colour has gone, and by then it went a long time ago. The rate depends on the pigments, the glazing and the season, and none of those are things you can measure at home.

The practical line, and it is a clean one: if a patch of sunlight lands on that wall at any time of year, it is not a wall for a painting. Not for this one and not for a print. Watch the wall for a day in summer and a day in winter, because the sun is in a different place in each and the winter one is lower and reaches further into the room.

Radiators, fireplaces and dry heat

Rising heat off a radiator does two things to a canvas hanging above it. It raises the temperature of the canvas, and it drops the relative humidity in the column of air directly against the back of the painting.

Dry canvas contracts. It contracts every time the heating comes on and relaxes every time it goes off, twice a day for the whole heating season, and the paint film on top of it is less flexible than the canvas underneath. That is the same mechanism that rules out a bathroom, running in the opposite direction: too dry rather than too wet, with the same result.

The chimney breast above a working fireplace or a wood burner is worse, because it adds soot and a much larger temperature swing. It is also, in most Dutch and British houses, the most attractive wall in the room and the one people most want to use. It is still a no while the fire is in use.

An electric fire or a decorative fireplace that has never been lit is a different matter and that wall is fine.

Walking an ordinary house

Row three is the one worth taking seriously, because it is the wall most people have already chosen. The wall opposite the window is the wall you see from the door, it is usually the largest one, and in a west-facing room it takes a rectangle of direct sun for two hours every afternoon from March to September.

If the wall you want is a problem wall

Sometimes there is one good wall in a room and the sun lands on it. Four things help, in descending order of how much.

UV film on the window. The most effective single measure, it is invisible once fitted, and it protects everything else in the room as well — upholstery, floors, book spines. It reduces ultraviolet substantially. It does not stop heat or visible light, so it is a mitigation and not a solution.

A sheer curtain or blind during the hours the sun is on that wall. Unglamorous and extremely effective, because the problem is two hours a day rather than the whole day.

Move it up or across by 40 cm. Sun patches are smaller than people think. Watching where the rectangle actually falls often shows that the wall is fine 50 cm to the left of where you were going to hang it.

Move it in summer. Legitimate, if the room genuinely only has the problem for part of the year. Store it properly while it is down — upright, covered, never flat — the way moving house with an oil painting sets out.

What does not help: hanging it slightly further from the radiator. A radiator wall is a radiator wall.

The check that costs nothing

A year after you hang it, take the canvas off the wall and look at the wall.

If there is a visible rectangle where the paint behind the painting is a different shade from the paint around it, that wall has been getting enough light to change a painted surface in twelve months. Your wall is one coat of emulsion. The painting is oil that will keep hardening for another fifty years, and it is getting the same dose.

That single observation settles more arguments about hanging positions than any amount of theory, and it is the reason the unfaded rectangle keeps coming up elsewhere in this pillar — it is also what gives away a painting taken down before visitors arrive.

Everything else about looking after the canvas — dust, cleaning, what not to do yourself — is a separate subject. The short version is a dry, soft brush and nothing else: no cloth, no spray, no water. And if the answer to all of this is that the room you wanted is the wrong room, where to hang it, room by room has a verdict on each, and the collection will still be here.

Questions people ask

Does sunlight damage an oil painting?

Yes, cumulatively and permanently. Ultraviolet and visible light break down pigments and accelerate the ageing of the oil binder, which makes the paint film brittle. Bright indirect daylight is fine; a patch of direct sun landing on the canvas is not.

Can I hang a painting above a radiator?

No. Rising heat warms the canvas and drops the humidity in the air against the back of the canvas, so it contracts every time the heating comes on and relaxes when it goes off, twice a day all winter. It is the same mechanism as a bathroom, running dry instead of wet.

Is a wall opposite a window safe?

Check it before you commit. That is exactly where an afternoon sun patch lands in a south or west facing room, and it is also the wall most people choose first. Watch it for a day in summer and a day in winter, because the winter sun is lower and reaches further in.

How can I tell if a wall gets too much light?

Take the painting down after a year and look at the wall. If there is a rectangle where the paint is a different shade from the wall around it, that wall changed a painted surface in twelve months, and the canvas is getting the same dose.