In the Ashvalayana Ghriya Sutra, during a munja of a Kshatriya it is dictated that the munja/girdle should be made out of a bowstring. Since it was written when Kshatriyas or solders used bows and arrows and they might not have access to synthetic materials like Nylon etc., how was bowstring constructed in ancient times and with what material? Does any hindu scripture mentions bowstring's composition? I think Mahabharata or Ramayana might have these details as these books mentioned big wars and great kshatriyas.
Here is what chapter 2 of the Manu Smriti says about the girdle:
mauñjī trivṛt samā ślakṣṇā kāryā viprasya mekhalā ।
kṣatriyasya tu maurvī jyā vaiśyasya śaṇatāntavī ॥
The girdle of a Brahmana shall consist of a triple cord of Munga grass, smooth and soft; (that) of a Kshatriya, of a bowstring, made of Murva fibres; (that) of a Vaisya, of hempen threads.