White cotton yarn in bleached, using a variety of chemicals, ALL of which leave toxic residues of varying degrees of toxicity. Natural cotton is actually sort of REALLY pale yellow-tan.
The ONLY plant fiber that is naturally snow-white without chemical bleaching is hemp, but that is illegal to grow in the US.
It's not easy to find as a raw fiber, or in yarn or thread form, but it IS gaining popularity. Most of the hemp yarn or thread you find in the US is from China, but some of the high-grade stuff comes from Canada or Romania.
If you could find natural, unbleached, 100% organic undyed hemp yarn, that would probably be MUCH better than cotton--it has a higher flash point, is about 20 times as strong and abrasion-resistant, and it stands up to getting wet MUCH better than cotton or linen.
Oh, yeah and one more thing about cotton--almost ALL cotton grown in the US is Genetically Modified. Non-GMO cotton is nearly extinct in the US these days, and it's isappearing fast in most of th erest of the cotton-growing world too, thanks to Monsanto's agenda to patent the entire genome of the planet, and poison us all with their evil, toxic franken-plants...
I may try this myself. Hemp wicks might just be the "holy grail" for vaping! We won't know until someone tries it..