It's been YEARS since I added to my series of blog posts on dyes and colors (links in the side bar). So, it's a fine time to do it.
White, as a color name in English, goes back through Old English and eventually traces to several older Scandinavian languages that mean 'bright' or 'shiny'. There's a really interesting listing of idioms using the term white, and when and how they came about, here.
The oldest form of white paint or pigment would have been chalk, and related soft, white ores. It's available pretty much world wide, often on the surface, so it's not what you'd call hard to find.
But, oil darkens chalk, and it's not white any more. So when oil painting was developed, sometime around the 1400s, they had to find something else.
The 'something else' turned out to be "white lead", or lead carbonate. Powdered lead, what could possibly go wrong? It was beloved by many artists because it was so opaque - you could paint it over other colors, and it would cover them. It could also be mixed with other colors and make THOSE opaque. It had a couple problems, though. The toxicity was one, but I don't think that's why it was abandoned. Compared to other paints they were using - cadmium yellow, orange, red and green; cobalt blue; arsenic based yellow and oranges - really, lead white was relatively stable in comparison. No, I think lead white fell out of use because it turns BLACK when exposed to sulfur. And guess what the coal fuel of the Industrial Revolution belched into the air in amazing amounts? Yup. Sulfur. You wind up with cave paintings originally painted with lead white, that have sadly gone black on us.
And this brings us to textiles.
For much of history, truly white clothing/fabric didn't exist. They could hunt albino animals, and did. They could color leather with chalk, and they did. Later, after animals and plants were domesticated, they could breed for white in both animal and plant fibers - and did. But as everyone knows, there's no such thing as true, 'bright' white in nature. For that, you need bleach.
Linens were originally washed with lye-based soaps and left out in the sun to whiten. Wool and animal fibers, well, they bred the animals to be as white as possible, and that was about it. In the 1700s, with industrial chemistry gearing up, people began 'bleaching' (read, 'damaging all to hell, but whitening') finished textiles with SULFURIC ACID. That didn't work out so well for the long term (good gourd), and finally, in the 1780s, chlorine bleach was invented - the stuff we know as 'bleach' today. But even with bleach, that's not the end of it.
What we, today, think of as a white textile - let's say a white tee shirt - is the end result of some wild and crazy processes. First the cotton was bred, over thousands of years, to be as white as possible. Then, as part of processing, it is bleached as white as it's possible to get it, without (overly) damaging the fiber. Then, it is in fact dyed. The class of dyes used are called "optical whiteners" and are there to reflect back as much of the visible light spectrum as possible. Have you ever noticed how your white clothes will often glow under black light? That's why - the optical whitener is bouncing back all the light possible.
Oh, and one last thing - white LED lights? They aren't true white, in that they don't produce ALL colored light wavelengths. They're really an optical illusion; they produce yellow and blue wavelengths and trick the eye into 'seeing' white. So don't expect colors to look right under them.
PS: Traditional knitting NEVER uses true white yarn. Because it didn't exist. The closest they got was undyed, 'natural' yarns. And there's no way in hell any house wife worth a damn would send her family out in light clothes, because she didn't have the detergents it takes to get them clean again. There's a reason real fishermen's sweaters were dyed nearly black with indigo.