How do you give artificial age to wool and cotton fibers? What causes the wool to oxidize?
Not sure if this is precisely what you mean, but in some theatre productions I've worked on, I've seen a couple of very good ways to age and weather cloth - though I'm not sure how it specifically works to wool and cotton fibres.
One way to age fabric is to rub soap up against it (no water) which gives a sort of greasy, worn-out look. Obviously it washes straight off.
The other way - and this is quite obvious - is with a cheese-grater. Cheese-graters are great to make clothes look worn out and worn through and lightly taking it over the surfaces will make something look bobbly and used. Rubbing harder you can even wear realistic looking grazes and holes in fabric.
Hope that helps!