When a thread is made from sheep’s wool together with another material, such as cotton, its status follows the majority. If it is mostly cotton, the small amount of wool is disregarded, and the thread is considered cotton and may be joined to linen. Likewise, a thread made from hemp and flax may be joined to wool if it is mostly hemp. This principle applies even when raw wool and raw flax have already been mixed together: another material, such as cotton, may be added until it exceeds either the wool or the flax, thereby nullifying that component, and threads made from the resulting mixture are permitted. If the third material does not exceed either the wool or the flax, however, a garment woven from the resulting threads is prohibited by Torah law (Yerushalmi, Kilayim 9:1; SA YD 299:1).
Raw wool and flax can be nullified in a majority, but once they have been spun into thread, they acquire significance and are not nullified even in a thousand times their volume. Thus, a cotton garment sewn with a plied wool-and-linen thread is prohibited by Torah law even if the cotton is a thousand times greater. Likewise, a single wool thread mixed into a large linen garment – or a linen thread into a wool garment – renders the entire garment prohibited. Even if the thread can no longer be located, the garment remains prohibited until it is removed (SA and Rema YD 299:1).[4]
Recycled thread made from shredded worn-out garments is permitted. Although the worn-out garments may include both wool and linen fabrics, once they have been shredded and returned to the state of raw material, any wool or linen present is nullified in the majority. If most of the raw material were wool, the resulting thread would be considered wool, but in practice the likelihood of this is negligible (see Ha-sha’atnez Le-halakha 2, notes 28 and 46; 2:29).
Cushions and linings filled with scraps of worn-out garments are also permitted, even though the scraps were not shredded and may contain intact threads. This is based on a sfek sfeka, a double uncertainty: it is uncertain whether the filling contains both wool and linen – in practice, the likelihood is very low – and even if both are present, it is uncertain whether the sewing thread joins them. If it does not, most authorities maintain that no prohibition applies, even though the wool and linen are enclosed together in a single casing sewn shut on all sides, as explained in SA YD 300:5. Rema records that the accepted custom follows the lenient authorities, as discussed in note 5 (Tiferet Yisrael, Batei Kilayim §118; Ḥeshev ha-Efod 1:64).
Because of marit ayin – concern for appearances – the Sages prohibited threads known as shirayim and kalakh, which resembled wool and linen (Mishna, Kilayim 9:2). Today, however, only actual wool and linen are prohibited, since there is no concern that these other fibers will be mistaken for them. Those familiar with textiles can distinguish them, while those unfamiliar with textiles would not identify them as wool or linen in the first place (see SA YD 298:1; AHS YD 298:5–6).
