The supposed to be 'original flemish bowstring' would be made from very long fibred Hemp. Due to anti drug politics this plant is long gone from where I live. Manila hemp may be used if good quality is available, but Silk and Flax (Linen) are stronger and should be easier to get anyways. However, it is still difficult to find the correct size and quality for string making.
Flax is rather low stretch material. Due to the mass needed for a strong enough string, there is no advantage though. For your task you need to be aware of the fact that linen (hemp too) gains up to about 20% in strenght when moist - or will loose the same amount when dried out. If the string breaks, the bow might follow and you're a dead man :( ELMER (1952: 55) descriebs how Homer Taylor broke 6 strings in a row, because they were dried out
I do have some valuable 18/3 linen thread left. I've always been to much of a sissy to use it on my 70# Yew ELB because of the above mentioned problems, but I will do some experiments on lesser bows. Maybe even today ...
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edit: did some breaking strenghts testing for the 18/3 linen thread I spoke of above and got 11# (dry!) - which is about okay. BTW: Saxton Pope used Irish linen no.12, which broke at 6# per strand.
You would want your string to hold at least 4x the draw weight; for a 70# bow this are 27 strands, going into a three ply string. My own string thickness formular calculates me a final diameter of 4.5mm (.178") with standard serving material. This is about the minimum for your nocks throaths ...