Natural Fibers · Protein · Fibroin
Stronger than steel by weight — destroyed by soap
Silk is the only natural filament fiber. One cocoon. One continuous thread. Up to 1,500 metres of it, wound by a caterpillar in three days. What it produces is one of the most structurally sophisticated materials in the natural world — and one of the most chemically fragile fabrics in your closet.
Silk fiber is composed almost entirely of fibroin — a structural protein produced by the silk gland of Bombyx mori, the domesticated silkworm. Unlike wool's α-keratin (which coils into helices), fibroin adopts a β-sheet secondary structure: the protein chains lie flat and stack in parallel sheets held together by hydrogen bonds.
This stacked sheet geometry is responsible for silk's signature properties. The chains are aligned, dense, and crystalline — giving silk a tensile strength of 300–500 MPa, comparable to mild steel by weight. The flat protein surfaces reflect light coherently, producing the characteristic lustre. And the thinness of the filament — 10–13 microns — gives it the drape and weightlessness nothing else quite replicates.
Raw silk fiber is actually two fibroin filaments bound together by sericin, a gum-like protein that acts as the adhesive. Most of silk's delicacy comes from what happens when water, heat, or chemistry interacts with this sericin layer.
Silk is unique among textile fibers in that it is a continuous filament rather than a staple fiber. Cotton, wool, and linen are all short fibers spun together into thread. Silk is unwound from a single source — a cocoon — and reeled into a continuous thread that can run to 1,500 metres without a single join.
This is why silk fabric behaves so differently from every other natural textile. There are no fiber ends. No join points. No mechanical weakness from staple spinning. The thread is, in a very real sense, one molecule wide all the way through.
Fibroin is a protein — and like all proteins, its structure is maintained by a precise balance of electrostatic forces between amino acid side chains. Change the pH, and you change those forces. The β-sheet hydrogen bond network that gives silk its strength is stable at mildly acidic to neutral pH. Above pH 8, the protein begins to hydrolyse — peptide bonds break, the β-sheet structure unravels, and the fiber loses tensile strength, lustre, and structural integrity.
Standard laundry detergent runs at pH 9–11. Bleach is pH 12–13. A single wash in regular detergent begins the process of destroying silk's molecular architecture — visibly, it yellows, loses its sheen, and becomes brittle. The damage is cumulative and irreversible.
| Substance / pH | pH range | Effect on silk fibroin | Verdict |
|---|---|---|---|
| White vinegar (diluted) | 3–4 | Mildly acidic — can restore some pH balance after alkaline damage. Occasional rinse only. | Use carefully |
| Pure water | ~7 | Neutral — safe. Minor sericin swelling possible; handle gently. | Safe |
| Silk-specific wash (Woolite, etc.) | 6–7 | pH-balanced for protein fibers. Minimal peptide bond disruption. | Recommended |
| Standard detergent | 9–11 | Alkaline hydrolysis begins. Peptide bonds cleave. Yellowing, brittleness over time. | Damaging |
| Baking soda solution | ~8.3 | Commonly suggested for stains — degrades silk protein at this pH. | Damaging |
| Chlorine bleach | 12–13 | Rapid, complete protein hydrolysis. Fiber dissolves. Irreversible destruction. | Catastrophic |
| Sweat (acidic) | 4.5–6 | Mildly acidic — damages dye more than fiber. Rinse promptly; acid dyes in silk are vulnerable to sweat salts. | Rinse promptly |
You've seen it: a drop of water lands on a silk blouse and leaves a visible ring or tide mark when it dries. This seems paradoxical — it's just water. But the mechanism is actually straightforward once you understand sericin.
Silk fabric almost always retains some sericin even after degumming — particularly along the yarn surface. When a water droplet lands, it partially dissolves and redistributes this surface sericin. As the droplet dries, surface tension pulls dissolved sericin and any mineral content toward the perimeter of the droplet — the classic "coffee ring effect". The sericin deposits there as the water evaporates, leaving a visible ring with a higher concentration of dried protein and minerals than the surrounding fabric.
The fix is counterintuitive: wet the entire garment evenly. When the whole surface dries uniformly, there's no concentration gradient, no tide-mark perimeter. Water spots on silk are almost always fixable — they're a surface phenomenon, not a fiber damage event.
Silk is a fiber. Satin is a weave structure. "Silky" is a texture description. These three things are routinely conflated in retail, resulting in consumers paying silk prices for polyester or spending years confused about what they're actually sleeping on.
A satin-weave silk is both silk (fiber) and satin (construction) — and is legitimately what most people mean when they say "silk sheets." But polyester satin, bamboo satin, and viscose satin all use the same weave structure on completely different fibers with completely different properties, price points, and lifespans.
Raw silk is graded on a scale from 6A (highest) down to A (lowest commercial grade). The grading assesses filament uniformity, cocoon quality, lustre consistency, and defect rate. Most silk sold at mid-market is Grade A or Grade B. Grade 6A is the domain of couture houses and specialist linen companies.
Silk's care rules are derived entirely from fibroin protein chemistry and sericin surface behaviour. Every rule has a molecular reason.