Wikipedia:Reference desk/Archives/Science/2021 September 14

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September 14

Red colored fabrics

Goodmorning! Why do red colored fabrics (e.g. a t-shirt) tend to fade more over time when exposed to light than those of other colors? Thank you--93.43.187.226 (talk) 05:42, 14 September 2021 (UTC)

Who says they do? ←Baseball Bugs ^{What's up, Doc?} carrots→ 05:47, 14 September 2021 (UTC)

There is a book called Red: The Art and Science of a Colour that discusses this issue. If you go to Google Scholar and search for "Fading of red dye" or "paint" or "objects" or "fabrics", you will get many hits in the academic literature. One I found began, "The irreversible fading of organic red colorants in art is well documented and greatly affects the perception of masterpieces from antiquity to the present day." So, there's that, Bugs. Cullen³²⁸ Let's discuss it 06:05, 14 September 2021 (UTC)

Thanks for the citation! ←Baseball Bugs ^{What's up, Doc?} carrots→ 06:09, 14 September 2021 (UTC)

I was looking around WP for a discussion of the lightfastness of red pigments and didn’t’t find much. However, both red pigments and more particularly, red dyes (which would be the choice for fabrics) are notoriously fade-prone. Looking around elsewhere, this is commonly attributed to preferential absorption of UV, since red light is reflected for it to appear red. However, I think there’s more to it than just that. Acroterion (talk) 06:12, 14 September 2021 (UTC)

In connection with the fading of outdoor printed material (such as advertising posters) which, as commonplace observation reveals to anyone who actually looks, lose reds fastest and blues least fast, I once read an explanation that disruption of redder pigments inherently require less light energy than that of bluer ones, not unconnected with the fact that higher frequency (= shorter wavelength) blue photons are more energetic than lower frequency (= longer wavelength) red photons. However, I can't remember all the details or find the original source. Is there an optical physicist in the house? {The poster foremrly known as 87.81.230.195} 90.200.67.3 (talk) 07:42, 15 September 2021 (UTC)

Some time ago I read an explanation. I don't remember where I read it and can't guarantee it's true, but it made sense. Red pigments reflect red light and absorb green and blue, blue pigments reflect blue light and absorb red and green. Blue light, due to its higher energy per photon, does more damage when absorbed than red or green light, so the red pigments, absorbing more of the higher energetic radiation, are damaged faster. PiusImpavidus (talk) 08:32, 16 September 2021 (UTC)

How sweat comes out of human skin at the same time water can't go inside the skin?

I didn't understand the mechanism of human skin towards fluids. Rizosome (talk) 08:01, 14 September 2021 (UTC)

Who says skin does not absorb water? ←Baseball Bugs ^{What's up, Doc?} carrots→ 08:27, 14 September 2021 (UTC)

One Dr Hoffman says: "The epidermis, the outermost layer of skin, provides a waterproof barrier" [1] Alansplodge (talk) 20:03, 14 September 2021 (UTC)

In any case, read Sweat gland for some answers on the first part of your question. ←Baseball Bugs ^{What's up, Doc?} carrots→ 14:29, 14 September 2021 (UTC)

Does it depend on whether or not you have anhidrosis? Martinevans123 (talk) 14:32, 14 September 2021 (UTC)

Courtesy link: Anhidrosis. hydnjo (talk) 16:47, 14 September 2021 (UTC)

Alansplodge "The epidermis, the outermost layer of skin, provides a waterproof barrier" is enough for me. Rizosome (talk) 00:09, 15 September 2021 (UTC)

That's why your fingertips don't wrinkle when they've been in water for a long time. ←Baseball Bugs ^{What's up, Doc?} carrots→ 01:23, 15 September 2021 (UTC)

Obviously you know that they do, and are presumably being sarcastic. However Alansplodge is correct – the epidermis does indeed prevent water from penetrating deeper into the dermis. However, the very outermost layer of the epidermis is made up of dead, dry cells made mostly of keratin; when these get wet they absorb water (which does not penetrate beyond them) and swell up, but since their attachment to the underlying dermis prevents them from expanding like a balloon, their increased volume instead forces the skin into wrinkles.

With reference to the implied question in the title: sweat does not just ooze out of the skin randomly, it is formed in sweat glands and expelled out by internal pressure via sweat pores. Due at least in part to surface tension, this is a one-way process – water cannot enter the body via the pores. {The poster formerly known as 87.81.230.195} 90.200.67.3 (talk) 07:33, 15 September 2021 (UTC)

Your fingertips do not "swell up" when in water - if they did, your fingertips would be less wrinkled, not more wrinkled. And in fact "prune hands" has nothing to do with water being absorbed or released at all (cite). Matt Deres (talk) 17:43, 15 September 2021 (UTC)

I didn't say that the fingertips swell up, I said that the outermost keratinous cells swell up but do not expand outwards because of their attachment to the deeper epidermal (and indirectly dermal) cells, and since they can't cause the skin overall to expand, they must accommodate their larger size within the same lateral span, which forces the skin into wrinkles, thus: ---------- —> /\/\/\/\/\.

This is what your reference says scientists used to think, and I concede that you have found a more up-to-date reference. I'm reminded that the "half-life" of "facts" on the well-researched programme QI is said to be about 18 months (or something similar). {The poster formerly known as 87.81.230.195} 90.200.67.3 (talk) 22:50, 15 September 2021 (UTC)

18 months ago it was about 18 months. Now it is more like 9 months. Soon we'll have the 24/7 science cycle.  --Lambiam 08:21, 16 September 2021 (UTC)

That's fair. Apologies for misrepresenting your post. Matt Deres (talk) 13:53, 17 September 2021 (UTC)