Silk is a continuous protein filament and it is weaker than it feels

A filament, not a staple

Almost every natural fibre is a staple: short lengths that must be twisted together into a yarn, with the ends of each fibre working loose over time. Silk is the exception. The filament is extruded continuously by the silkworm as it builds its cocoon, and it can be reeled off in a single unbroken length.

That is why silk fabric is smooth. There are no protruding fibre ends to catch the light or the skin, which produces the characteristic surface and a good deal of the cool, slippery hand. It is also why silk resists pilling in a way that short-fibre cloths cannot, since pilling begins with loose ends.

Not all silk is reeled. Damaged cocoons and waste are processed into staple fibre and spun conventionally, producing spun silk, which is duller, softer, fuzzier and cheaper. Wild silks such as tussah come from different moth species, are naturally coarser and browner, and behave differently again.

Lustre comes from the shape of the filament

The raw filament emerges as two strands of the protein fibroin held together by a gum called sericin. Each strand has a roughly triangular cross-section with rounded corners, and that geometry is the origin of the lustre: light striking a prism-shaped fibre is refracted and reflected at angles rather than scattered evenly.

Removing the sericin, a process called degumming, is what reveals the shine and the softness. Undegummed silk is stiff and comparatively dull. How completely the gum is removed is a processing choice, and partly degummed silks retain more body.

Weave then does the rest. The same yarn produces a matte, textured crepe or a brilliant satin depending entirely on the interlacing, which is why silk garments vary so wildly in appearance while the composition label reads identically.

Strong in tension, vulnerable to almost everything else

Silk has excellent tensile strength for its weight, which is the property that gets quoted. What is quoted less often is that it loses roughly a fifth of that strength when wet, that it has poor abrasion resistance, and that it’s chemically among the more fragile fibres in ordinary use.

Being a protein, it’s damaged by alkali. Ordinary laundry detergents are alkaline, and the protease enzymes in biological detergents digest protein indiscriminately, which is precisely what silk is made of. Chlorine bleach destroys it outright. This isn’t fussiness on the part of the label.

Sunlight is the other quiet enemy. Silk degrades under ultraviolet light faster than most fibres, which is why a curtain or a scarf kept in a bright window becomes brittle and splits along the fold lines long before anything appears worn.

Water marks, and the reason they appear

A drop of water on some silks leaves a visible ring after drying. The mechanism is that water carries dissolved sizing, residual gum or dye into the surrounding fibre and deposits it in a concentration line at the edge of the wet area, exactly as a spilled drink dries into a ring on paper.

This is why a single splash is worse than a full immersion. Wetting the entire panel evenly gives the deposits nowhere to concentrate, which is the principle behind treating a water mark by dampening the whole area rather than dabbing at the spot.

Perspiration is a related problem and a more permanent one. It is both salty and, in combination with body oils and the alkaline residues of some products, chemically aggressive to a protein fibre, which is why underarm damage on silk shows as yellowing and eventual disintegration rather than mere staining.

What all this means for owning it

Hand washing in cool water with a detergent formulated for delicates, without enzymes and without bleach, is appropriate for many plain silks, and a mild acid rinse is a traditional way of neutralising alkaline residue. Structured or heavily dyed pieces are a different matter, and the label deserves more respect there.

Do not wring it, since wet silk is weaker and abrasion is what it handles worst. Roll it in a towel to remove water, dry it flat away from direct sun, and press on the reverse at a low temperature while slightly damp, because water spotting from a steam iron is a real risk.

The larger point is that silk is a luxury with genuine mechanical trade-offs rather than a superior fibre in general. It is cool, light, strong for its weight and beautiful, and it will not survive being treated like cotton. Both halves of that are worth knowing before buying.