Mystery marks muddle my mind

July 30, 2006 11:46 PM

What caused these stains in such a weird shape? The day's events...

I hiked up scenic Mt. Si on Saturday and, naturally, sweated up a storm. When I got to the top I put on a sweatshirt I had brought with me, which I later took off on the way down. I drove home, took a 2 hour nap (still wearing my clothes), and when I woke up I noticed these odd marks on both sides of the shirt.

Information that might be relevant:

• I was wearing deodorant (a "goes on clear" blue thing by Old Spice)
• I wore a backpack during the hike
• I haven't washed the shirt yet so I don't know if the marks are going to wash out
• There's no difference in odor in the area where the marks are
• The marks are on the outside of the shirt - barely visible at all on the inside

My main curiosity is how the odd shape was formed - why are there lines going in both directions on both sides? Why are they so close to symmetric? I'm also wondering if the marks were caused by something other than deodorant

posted by 0xFCAF to clothing, beauty, & fashion (12 comments total)

You've answered your own question - they're deodorant stains. The shape is just the drying pattern of your sweat - the symmetrical appearance is just chance.
posted by StuMiller at 11:54 PM on July 30, 2006

Is it not salt from your sweat?
posted by blag at 12:38 AM on July 31, 2006

I'd say it's salt as well.
posted by swordfishtrombones at 12:41 AM on July 31, 2006

Absolutely; it's salt. I get the same thing on my shirts.
posted by Steven C. Den Beste at 1:06 AM on July 31, 2006

Sweat plus horizontally symmetrical folding of the shirt before leaving it set for a while?
posted by datacenter refugee at 1:55 AM on July 31, 2006

Definitely salt. Lick it to be sure. It'll wash right out.
posted by friedrice at 2:50 AM on July 31, 2006

It'll also brush out if you have the patience. I get salt marks like this where the strap of my satchel goes all the time, especially on darker-coloured shirts.
posted by macdara at 6:07 AM on July 31, 2006

I can't judge whether it's deodorant or salt, but I do know that I only get these when I've applied deodorant earlier.
posted by goodnewsfortheinsane at 6:17 AM on July 31, 2006

As a glassblower, I speak from experience, that is salt from your sweat. Replace those electrolytes, drink some Gatorade.
posted by sgobbare at 7:49 AM on July 31, 2006

The symmetry is probably due to the backpack - your sweat would probably collect on the edges of said pack or it's straps.

No one has considered the obvious possibility that this may, in fact, be a manifestation of your latent mutant powers, which are attempting to convert an ordinary t-shirt into an official X-Men uniform. I suggest that you contact Professor Xavier immediately.

Or, you know, just wash the shirt.
posted by sluggo at 8:34 AM on July 31, 2006

Those are the fronts of the diffusion patterns of sweat from your front and your back.

The front and back parts of your torso are in direct contact with the fabric of your shirt. Most t-shirts tend to hang loose on the sides on males. A shirt is a tube and sweat is produced on the opposite sides of the tube.

Sweat diffuses through fabric in a circular shape. Imagine a spot of ink spreading through a piece of tissue. If you try this, over time, you'll see a black circle expanding from the original central dot. Now, imagine two spots of ink on a piece of tissue (if you try this at home, try wetting the paper first). You get two circles which form a lenticular shape where they overlap. Now wrap the tissue around your finger and spot the ink on opposite sides. Two lens shapes form on the sides where the two circles overlap. As the french say, viola!

When the sweat dries, the salt lines show you where the outer edge of the sweat circles are. The salt lines are at the edge (the solvent front) of the expanding sweat circles because salt doesn't bind well to the fabric. Stuff that sticks to the fabric, like coloured dyes, will be slower to travel and might not make it as far as the edge of the circle---this is how tie-dies work.

Finally, this is basis for many kinds of chemical analysis, and is called chromatography, the separation of colour. Those big Agilent systems on you see on CSI work the same way (though they don't use t-shirts inside).
posted by bonehead at 8:55 AM on July 31, 2006

I agree with bonehead - I play a lot of sport in hot weather and cotton t-shirts always get roughly circular sweat patterns on the front and back from the dried salt. The stains on your shirt definitely look like the sweat stains from your front and back meeting up at the side.
posted by EndsOfInvention at 10:00 AM on July 31, 2006