Why Sunlight Fades Wall Decor on One Side Only

Because fading runs on a dose, and one edge of the piece is on a much bigger dose than the other. Sunlight fades wall decor on one side only when one end sits inside the moving patch of direct sun for a few hours a day while the far end never leaves shade. The Canadian Conservation Institute sets out the arithmetic: total exposure is light intensity in lux multiplied by hours, counted in millions of lux hours, and the rate of light damage is proportional to intensity.

Both halves of that product favour the same edge, which is why the gradient is steeper than it looks like it ought to be. The lit end takes more lux, and it also takes more hours, because the sun crosses the sky at 15 degrees an hour and the patch reaches that end first and leaves it last. Nothing is faulty. It is one object living in two different rooms.

Why does sunlight fade wall decor on one side only?

Because the dose at the two ends can differ by a factor of 20, and a factor of 20 on the clock is the difference between five weeks and two years. Wikipedia's illuminance table puts direct sunlight at 32,000 to 100,000 lux and an overcast day at 1,000 lux, so the lit end is being worked on by a source hundreds of times stronger than the one reaching the shaded end. The shape of the piece changes what that reaches, too: the Houseland box modules have no back panel, so daylight carries on through the opening instead of stopping on a backing board, and what takes the dose is whatever stands inside them.

Daylight factor is the share of the outdoor level that reaches a point indoors, and CIBSE Lighting Guide 10 calls under 2 per cent not adequately lit and over 5 per cent well lit. Set against an indirect daylight level of 10,000 lux outside, that is 500 lux at the good end of a room and 200 lux at the poor end. Put those three numbers through the dose formula and the two halves of one piece stop looking like the same object.

The Conservation Institute's table of blue wool categories gives a just noticeable fade for a medium sensitivity material, category 4, at 3.5 Mlx h. The lit edge crosses that in about 5 weeks. The shaded edge takes about 2 years, and the dim end of the same wall takes nearly 5. A 19 to 1 dose ratio produces a visible step on its own, with nothing wrong with the object and nothing wrong with the finish.

What makes one end of the same piece take more light?

Angle and how much sky that end can see, far more than distance. Lambert's cosine law says the light a surface collects falls with the cosine of the angle between the beam and the surface: at 45 degrees it takes about 0.71 of what it takes square on, and at 75 degrees about 0.26. A piece on the wall the window looks along presents a different angle at every point across its width, and the far end is also seeing less of the opening.

The rule most people reach for does not apply here. Inverse square is a point source law, and its error stays under 1 per cent only once the source is smaller than a fifth of the distance to the subject. A window 48 in (122 cm) wide needs 20 ft of room before that holds, so inside an ordinary room the falloff from the glass is gentler than one over r squared and the angle term is doing most of the work.

Then the hours term stacks on top of it in the same direction. At 15 degrees an hour the patch sweeps steadily along the wall, so the end nearest the glass enters it earliest and leaves it latest, and it collects the deepest part of the sweep in midsummer as well. The seasonal half of that is why the same finish reads differently in summer light before any pigment has changed at all.

Does the window glass stop the fading?

It stops one part of it, which is why a film alone never gets you to zero. Ordinary soda lime window glass passes about 90 per cent of the light above 350 nm and blocks over 90 per cent below 300 nm, so the short ultraviolet is largely gone by the time light lands on the wall and the long ultraviolet is not. Visible light does its own share of the damage, which is exactly why the conservation dose above is counted in lux and not in ultraviolet alone.

The Conservation Institute's own ceiling is to keep ultraviolet below 75 microwatts per lumen, and it pairs that with three lux tiers that are worth borrowing at home: 50 lux for textiles, works on paper, watercolours and photographs; 150 lux for oil and acrylic paint surfaces, polychrome and furniture; 300 lux for stone and metal. A spot next to the glass reading 500 lux at midday is already ten times over the paper tier. For scale on what film can do, published measurements of car glazing put transmission at 3 to 4 per cent of ambient ultraviolet, mostly above 380 nm.

How do you stop a piece fading unevenly?

Cut the peak hours first, because they carry almost all of the dose.

1. Find the line before you move anything. Take the piece off the wall and hold the face against the back, or against an edge that has never seen daylight. Every object carries its own control sample, and a step you cannot see against that reference is not yet a fade.
2. Close the covering for the 3 hours the patch actually crosses. Dose is lux times hours, so removing 3 hours a day of 32,000 lux removes roughly 35 Mlx h a year, which is more than the whole rest of that year at 500 lux ever delivers.
3. Halve the intensity and you double the clock. Damage rate is proportional to intensity, so a sheer that cuts the level by half moves a 5 week threshold out to 10 weeks. Real, and small next to moving the piece.
4. Rotate symmetrical pieces every 6 months so both ends share the budget. That works on a plain rectangle and fails on anything with a top and a bottom.
5. Hang on the wall the window looks along, not the one it faces. At 75 degrees to the glass a surface takes about a quarter of what it takes square on, and that quarter applies for the whole day rather than for one hour of it.
6. Do not plan on reversing it. Light damage is cumulative and the dose only ever counts up, so the honest options are shading, moving and replacing.

The step that changes the answer most is the cheapest one: a piece 24 in (61 cm) further along the wall, out of the patch, is on the 1.8 Mlx h budget instead of the 35 Mlx h one. On a wall of modular boxes you can rearrange, that is a Saturday morning rather than a purchase.

From the shop

6-Piece Rounded Pine Box Wall Shelves

The set is 6 rounded open boxes in mixed sizes, natural pine with a clear matte lacquer, each 3.5 in (9 cm) deep on a 0.6 in (1.5 cm) board. Load rating is up to 11 lb (5 kg) per piece and every module hangs on a keyhole hanger, so a box can be moved along the wall without new hardware.

View the 6-piece rounded pine box set — dimensions & current price

Which pieces show a one sided fade first?

Dyed organic material shows it first and stone shows it last, with a spread of more than 500 to 1 between the two ends. The Conservation Institute's blue wool categories put a just noticeable fade at 0.22 Mlx h for the most sensitive category, 3.5 Mlx h at category 4, 50 Mlx h at category 7 and 120 Mlx h at category 8. Category 1 crosses the line in about 2 days of the sun patch above; category 8 takes roughly 3 years of it.

Timber is the awkward middle case, because it does not only lose colour. Some species darken under light and some bleach, and a clear finish over the top ambers on its own schedule, which is the same drift that makes a wood fitting change tone over years of use. On a sunlit wall that shows up as a warm edge and a cooler one rather than as a pale patch.

We do not test any of that, and the listing does not claim it. What the product page does say is still the useful part: the Houseland box set is natural pine under a clear matte lacquer, and the listing states plainly that the timber varies in grain, knots and tone from set to set. Two boxes hung 24 in (61 cm) apart can therefore start slightly apart in tone, which is worth knowing before you read a difference as sun damage.

What I'd do

Spend one sunny day watching the wall instead of guessing. Note the clock time the patch reaches the piece and the time it leaves, because those two numbers are the whole hours term, and 3 hours against 30 minutes changes the answer by six times on its own.

Then hang the things that carry colour on the wall the window looks along, and put the things that do not care in the patch. Pottery, metal and stone sit in the 300 lux tier; paper, textile and photographs sit at 50 lux and belong out of the sweep entirely. Boxes and ledges from the wall shelves range are useful here for a flat reason: what fades is usually what stands on the shelf, not the shelf, so the shelf can take the sunny end while the framed photograph moves 24 in (61 cm) along.

Last, write the date on the back. A pencil note behind the piece turns an argument about whether it has faded into a comparison against an edge that has been in the dark since the day it went up.

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