Leaving a stain in the sun to fade it

Does it work?

Plausible mechanism, untested in this application

The photochemistry is settled and the effect on materials is documented from the damage side by more than one source in this register. What is missing is any read source testing the technique as a stain treatment, which is why this sits at plausible mechanism rather than supported. The distinction is exactly what that level is for: the mechanism is coherent and consistent with known behaviour, and the specific application has not been tested in anything CleaningHQ has read.

This is a verdict on whether the technique is effective. Whether it is safe on a particular surface is a separate question, answered below.

Where not to use it

Why it works

Photodegradation. Ultraviolet and short-wavelength visible light carry enough energy to break the conjugated double-bond systems that make an organic molecule absorb visible light. Break enough of them and the molecule stops being coloured. Carotenoids, which have long conjugated chains, are among the most light-sensitive food pigments, which is why they respond disproportionately well. The same photochemistry is why upholstery and curtains fade near a window, and why it is a hazard as well as a technique. Moisture assists the process, which is the mechanistic reason the traditional instruction is to put the item out damp rather than dry.

Background

This is a rare case where the household remedy is aimed at exactly the right mechanism and the commercial products are not. Most stain treatments try to dissolve, emulsify or lift a pigment out of a fibre. Sunlight does something different: ultraviolet and visible light break the chemical bonds that make the molecule coloured, so the pigment is destroyed in place rather than removed. That is why it works on the stains that resist everything else. Carotenoids — lycopene in tomato, curcumin in turmeric and mustard — are fat-soluble, so water and detergent barely touch them, and they are strongly light-sensitive. A yellow curry mark on a white cotton shirt that has survived three washes will often be gone after a day on a line. Three conditions matter. The item should be damp rather than dry, because water helps. It needs direct light rather than daylight through glass, since window glass absorbs much of the useful ultraviolet. And it is slow, measured in hours to days rather than minutes. The obvious limitation is the obvious one: light does not distinguish the stain from the garment, so this belongs on whites and on colourfast items only.

Conditions

• Whites and colourfast items only
• Direct sunlight rather than daylight through glass
• The article damp rather than dry
• Hours to days rather than minutes

What it does not do

• It works on pigment and does nothing at all for a greasy, protein or particulate deposit.
• It cannot distinguish the stain from the fabric's own dye, so it fades coloured items along with the mark.
• Prolonged exposure weakens the fibre itself as well as the colour, so it is a treatment rather than a drying habit.

What to use instead

• Sodium percarbonate (oxygen bleach) — An oxidising bleach that releases hydrogen peroxide in water. Gentler than chlorine bleach on fabrics and dyes, and it does not produce chlorine gas.
• Laundry detergent — A built, multi-component formulation — not simply soap for clothes. The biological and non-biological split is about enzymes, and it is the one that changes what you can wash.
• Solvent treatment — The branch that starts before water rather than after it — and it carries a warning the aqueous branch does not.