A couple of months ago I bought some Rayon/Viscose, and ever since then I have thought about making a thorough scientific comparison to finally once and for all determine which is the wicking material to rule them all 
It however looks like that is never going to happen 
so instead I give you a completely unscientific, anecdotal and subjective comparison between Rayon/Viscose and Cotton based on some more or less arbitrary parameters 
Look and Feel
We all know what cotton looks and feels like and even though rayon looks very similar perhaps a bit whiter, it feels very different. If you run your fingers through it it feels much smoother, almost silk like. More importantly it feels like the fibres are generally longer and more uniformly aligned in the length direction of the wick. It simply feels like it ought to be a better wicking material.
Perhaps not the most important parameter, but I will never the less give rayon a clear 1-0 victory on this.
Ease of use
When I was still using cotton I used a variation of the scottish roll, where I would roll it very tightly, thin out the tails heavily and also cut of quite a lot of the tails at an angle. This allowed me to pack in quite a bit of cotton in the coil, while not overstuffing the wicking channels.
With rayon I pretty much do the same thing, except there is one major problem. It seems to be pretty much impossible to squeeze rayon into a slimmer shape, as soon as you release the pressure it just pops right back into its default thickness. This also means it is near impossible to pack enough rayon into the coils without deforming the coils! It might be a bit easier it you use thick wire, but I like to use very thin wire (2x30+40) and my coils always get somewhat deformed. This is made even worse by the fact that rayon doesn’t expand the same way cotton does when it gets wet, making it even more important that you stuff the coils as much as possible.
This is a very clear 2-0 win for cotton.
Break in period
During the break in of rayon I get a very sickening off taste, it is not a strong taste in fact it is quite faint, but it is also quite vomit inducing 
. If you have ever been near a paper mill or other cellulose processing plants you know what I’m talking about. It however doesn’t last long, it only takes about 1-2 ml of juice to wash it away.
With cotton I also get a bit of off taste, it might even be a bit stronger and last a bit longer, but it is a much more pleasant taste, almost vanilla like 
This is a clear 1-0 win for cotton.
Wicking capabilities
This one is a though one, theoretically rayon ought to have better capillary capabilities, which means that it should be able to wick faster, and perhaps it also does. The problem however is that I have never had any problems with cotton, so I simply haven’t personally been able to notice any difference, it doesn’t mean that there isn’t any difference or that others mightn’t notice a difference, just that I can’t.
I have to declare this a 1-1 draw
Flavour
Again a though one. In order to determine this I would really need to do a blind test with two identical setups, or at least try the same juice in one atty with cotton and another with rayon. The problem however is that I don’t do anything half and have already rewicked all of my atties with rayon, and I’m too lazy to rewick one of them with cotton just to perform this test. (And yes I do realise the build in irony in that sentence 
)
Based on my memory of how it used to taste with cotton, I will however say that I think I get a slightly cleaner flavour from rayon, it might all be in my head, but it is never the less the impression I get.
I will declare this a 2-1 win for rayon.
Longevity
With cotton I usually had to rewick at least once a week, with rayon I have however just gone through a 3 week period with the same wicks, before I finally rewicked last night. Mind you, this was purely done as a scientific test to see how long I could stretch it, not because I was too lazy to rewick and it by happy happenstance turned out to be useful for this post … Ahhh … who am I kidding, I’m lazy, I know it, but in this case laziness paid off.
During this 3 week period I didn’t notice any drop in flavour or any off taste at all, which I most certainly would have with cotton. Furthermore when dissecting the rayon it looked much cleaner than cotton would have been after just one week of use. There was some brown blotches of caramelized liquid, especially one very large on top of one of the wicks, but it was only skin deep, and the rest looked crispy white, in one of my rta’s the wicks actually looked so good that I was tempted to just pop the cap back on and give it another 3 weeks
This is a very clear 2-0 win for rayon.
And the winner is …
Rayon/Viscose 6 - 5 Cotton
A close fight but Rayon/Viscose manages to pull ahead at the last minute and win by the smallest possible margin.
The decisive difference for me is the longevity, if it wasn’t for this then I don’t think it would be worth the extra trouble.
So would I recommend this to others … ehh … perhaps, it depends.
If you are perfectly happy with the wicking material you use now, then I don’t really see any reason to rush out and buy something else.
If you are new to rebuildables, cotton is most definitely easier to wick with.
If however you are searching for a better wicking material, or you simply like to experiment and make up your own mind, then sure, give it a try.
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* Cotton I have used was Muji and Koh-Gen-Do, and the Rayon/Viscose Graham’s Cellucotton
but yeah that is also a parameter you need to consider.
