On another site, I asked about nettle bow string and had a nice chat with a lady who is the yarn/twine/string/cordage maker for a group of Scottish Heritage people at a heritage site village (i.e. she does it for a living).
She has said that only about 1-2% of the mass of nettle will actually be suitable for spinning into good thread, which can then be plyed up to make bow string.
She advocates stripping off the leaves and small branches, setting aside until the leaves start to wilt, splitting the stalks into four by flattening twice lengthwise, then retting in damp grass for a day or two. (keep in mind this is in England, where it is often wet). Point is, she said that the plant material can rot very quickly destroying the fibers. She said sometimes only a day or two is needed to ret the plant material away from the fibers.
Then, after retting, she takes bundles of stalks and works them in her hands under running water to "wash" away all the material that isn't good fiber. There is a "skin" that is attached to the fibers that will flake off when the cordage is dry, thus loosening the weave. You need to get rid of all those little bits before making the string for a good strong string. Then after that, she spins her threads, then adds them together to get a much stronger cord.
Hope this helps.
Edit - I found an excerpt from one of her posts regarding spinning nettle:
"Spin it damp like flax but don't leave it damp, it grows moldy very quickly. If you've kept the fibres in bundles or broken it out like flax it ought to be already lined up for spinning. If not, you can draw out lengths from the smashed stems to make a strick. Short lengths card well. The thing is that flax will give you about 12% fibre from dry weight of material but nettle will only give 1 or 2%. So you need a *lot*. Thereafter it's much the same. Cut, stack, dry out a bit, bundle and soak until it starts to decay then dry and beat or break out the fibres. (easier if you wrap them in a clean cloth). If you're in a hurry, cut the stems, strip the leaves and then pour boiling water over the stems. Walk up and down over them or beat them with rounded stick to seperate the skins from the woody core. Peel off the lengths and again soak to encourage retting, sometimes even a day can be enough....be careful watch out for a powdery mildew forming on the damp stems.
At this point it gets fiddly because you have no core to act as a stiffened length to break out the fibres. However, if you keep the lengths in bundles you can twist them in water until the detritus comes away and the fibres come out clean. Nettle fibres may break at the leaf nodes, unlike flax which carries it's leaves v.high and on unbranched stems, it's a knack to remove the leaves without compromising the 'skin' of the nettle. However even short fibre is still good stuff. Vigorous combing to remove debris only suceeds in breaking up the fibres too. Better to hold the retted bundle under running water and gently work your fingers through the lengths allowing the water to 'comb' the fibres free. This avoids snarls and gives excellent fine fibres in good workable lengths.
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