Silk

Silk is a protein, chemically far closer to skin than to any synthetic, and there is nothing in it to leach - which is why it is the default thread for a floss dragged between teeth and gums. Silk allergy is uncommon. Historical suspicion fell on sericin, but current biomaterials work characterizes sericin as low-immunogenicity and studies it for wound healing, and degumming removes most of it regardless. Dyes and finishes are the variable worth asking about, not the fiber. One worry can be retired outright: weighted silk, the 19th-century practice of loading degummed silk with tin salts to put the lost weight back - roughly 8% per treatment cycle, and over 40% after four. It slowly destroys the cloth and is a serious problem in textile conservation, but it is not done to silk you can buy today. The genuine ethical question is the silkworm: cocoons are conventionally stifled in hot water or steam with the pupa inside, because letting the moth emerge breaks the single continuous filament. Peace or ahimsa silk lets the moth out and spins the broken fibers into a shorter, duller, more expensive yarn.

Silk is not a hot-path material and has no thermal safety story - it is a protein, so it chars rather than melting onto skin. What heat does is wreck the fabric. Silk loses up to 20% of its strength when wet, so a hot wash with agitation is where garments tear; wash cool and iron low if at all. Chlorine bleach destroys silk outright and sulfuric acid dissolves it, both worth knowing before reaching for an aggressive stain treatment.

The lookalike is polyester satin - satin is a weave, not a fiber, and most of what is sold under that name is polyester. Burn a thread from a hem: silk smolders, smells like burning hair and leaves a crushable ash, while polyester shrinks away, melts to a hard bead and smells chemical. Silk also warms in your hand and stays warm, where polyester feels slick and cool. On floss, read the thread and the coating as two separate claims - "silk floss" describes the thread, and the glide coating should name a plant wax rather than PTFE. If a spec sheet says only "natural fiber", ask which fiber, whether it is degummed, and what the coating is.

Hand wash cool with a pH-neutral detergent, do not wring, and dry out of direct sun - sunlight weakens silk over time and perspiration yellows it. Clothes moth larvae eat silk as readily as wool and are drawn to soiled cloth, so store it clean and sealed. Treated this way silk lasts years; treated like cotton it does not.

Silk is a protein and breaks down in soil, so an undyed silk floss with a plant-wax coating can go on a home compost heap and will eventually get there. Nothing about silk is curbside recyclable, and a blend with elastane or a garment with a synthetic lining is not compostable. The floss container, whether glass or plastic, outlives the floss by orders of magnitude and is usually the more consequential part of the purchase.