The ones I have removed from atomizers have been a non burning material (will not burn, melt or discolour even when heated to the point that they are glowing in a blue flame) which suggests to me glass fibre or silica fibre is the more likely option. They are certainly not cotton which burns easily. I have read some claims that they can be made of aramid fibres (an example of this type of material is Nomex, often used in flameproof clothing) although these materials do eventually break down with exposure to extreme heat.