Recently some mold grew on a favorite white sweatshirt of mine. I hung it in the sun and two days later that mold was gone. This is a great stain removing tip, but how does that happen? I’ve also had success with getting dye run out of a white shirt by hanging it in the sun, so it’s not just the sun killing the mold.
Ultraviolet light, of which there is plenty in sunshine, is good at breaking up large molecules, particularly dye molecules, which are used as dyes because they’re good at absorbing light. Once the dye molecules are broken up, they’re don’t act as coloring agents any more.
UV also tends to break up the large molecules in your skin, leading to sunburn, or if the DNA gets broken in the right places, skin cancer. That’s why your skin makes UV-blocking pigments when it notices a lot of UV exposure (you tan in other words).
Things like mold, that can’t tan and don’t have many other mechanisms to protect themselves from having all their molecules busted up, tend to just die with lots of UV exposure.
It’s the UV-light.
To absorb visible light, molecules need to be relatively long – besides some other molecular properties. The UV is braking apart the long molecules and they become invisible.
The stain is still there. Go into a disco and look at your shirt in the stroboscopic “black” light, actually again UV-light. You will easily detect a dark spot where the stain was formerly.
The sun bleach can be assisted by additional amounts of oxygen, also breaking the bonds in the long molecules.
Lay your shirt down on the lawn in the bright sun. Just above the green grass will be a little bit higher oxygen concentration.
This “sun and lawn bleach” is a well known “grandma recipe”.
My ma used it regularly in the age before the strong chemical detergents.
MummyCave