Silk and Synthetic: What the Difference Actually Is — and What Washable Silk Means
May 20, 2026 — Mercer St.
Silk and synthetic fabrics can look similar on a hanger. Against the skin, they behave differently — because they are structurally different at the fibre level. Understanding what silk actually is helps explain why it costs more, why it requires more careful handling, and what the newer Washable Silk treatments are actually doing.
In this note
- What silk is, structurally — the protein fibre, the filament, and what that means
- How silk behaves in terms of moisture and temperature
- What synthetic and man-made fibres do differently
- Why silk requires careful care — and why
- What Washable Silk is and what it does and does not change
What silk is, structurally
Silk is a protein fibre. Most cultivated silk begins as continuous filaments produced by silkworms during cocoon formation. The filaments are composed primarily of fibroin (the structural protein) and sericin (a gum-like coating removed during processing).
The protein structure of silk is what distinguishes it from synthetic fibres at a molecular level. Silk fibroin contains amino acids with an affinity for moisture, meaning silk can absorb and release moisture differently from most synthetics. Polyester and nylon are generally hydrophobic — they repel water rather than absorbing it.
The long, continuous filament structure also gives silk its characteristic smooth surface. The subtle sheen and the way light moves across silk fabric both come from this filament character. The degree of sheen varies with weave or knit structure, yarn construction, and finishing.
What the protein structure means in practice
Silk’s moisture behaviour has practical consequences for how it feels to wear, though the extent varies with the specific fabric:
- Silk can absorb some moisture and release it as conditions change, contributing to comfort in everyday wear
- Silk is often described as temperature-aware — it can feel cool at first contact and comfortable as it warms against the body
- The smooth filament surface means lower friction against the skin than many woven or knitted fabrics
Mercer Note
These are properties of the fibre in general terms. The actual experience depends on fabric weight, structure, and finishing. Silk is not a performance fabric designed for high-output use. For close-to-skin garments worn through an ordinary day, the moisture and friction behaviour of silk is relevant — but not absolute.
What synthetic fibres do differently
Synthetic fibres — polyester, nylon, acrylic — are produced through industrial processes that allow precise control over diameter, strength, and surface character. The results are consistent, volume-scalable, and lower-cost than natural protein fibres. For many applications, this is entirely appropriate.
Regenerated cellulose fibres such as viscose, modal, and lyocell are man-made but derived from plant sources. They behave differently from synthetics and from silk — their moisture behaviour varies with the specific production process.
The surface of synthetic filaments can be engineered to be smooth, and some synthetic fabrics are designed to look similar to silk. The visual similarity can be close. The structural difference — in fibre formation, moisture behaviour, response to heat and wear — remains.
Why silk requires careful care
Silk’s protein structure, which gives it its useful properties, also makes it sensitive to conditions that damage proteins:
- Heat: denatures protein fibres, causing irreversible structural change — why silk is sensitive to high wash temperatures and tumble drying
- Alkaline detergents: designed to break down proteins; can damage silk if used at high concentration
- Mechanical agitation: creates friction and tension that can cause filaments to shift, fray, or felt
These sensitivities are not unique to silk — cashmere and other protein fibres share them. They are more pronounced in silk because the filament structure is finer and has less natural crimp to absorb mechanical stress.
What Washable Silk is
Washable Silk is not a different fibre. It is silk that has been treated or constructed to reduce its sensitivity to the conditions that normally damage it.
Several approaches exist:
- Tighter weave or more tightly spun yarn (increasing structural stability when wet)
- Controlled pre-shrinking during manufacturing (so the same contraction does not happen during home care)
- Resin or coating treatments that provide protection against water penetration and friction
Washable Silk is not a universal standard. The care label on each garment is the authority.
Mercer Note
What Washable Silk does not do is make silk equivalent to a synthetic in terms of durability or care freedom. It remains a protein fibre that benefits from gentle handling. The treatment varies by manufacturer, and the care label for each specific garment is the correct reference — not a general assumption about Washable Silk. Wash care is subject to each product’s care label.
What Washable Silk retains
A carefully developed Washable Silk may retain many of the qualities associated with untreated silk, though the exact handfeel, sheen, drape, and care performance depend on the specific treatment. Surface coatings may slightly reduce the natural sheen or alter the drape. Structural treatments (tight weave, pre-shrinking) are less likely to alter surface character.
Washable Silk represents a practical compromise: reduced sensitivity to home washing, at the cost of a small degree of the most delicate qualities of untreated silk. For a garment worn regularly — a tank top, a camisole, innerwear — this is likely a reasonable trade.
What to look for
- Does the product page or care label specify whether the silk is washable or dry-clean-only?
- Is the washable treatment described (construction-based or coating-based)?
- Does the care label align with the product claims?
- Is the silk specification — momme weight, origin — noted?
What this means for Mercer St.
The care requirements for each Mercer St. silk piece are noted on the product page and the care label, and are subject to each product’s care label. Where a piece is described as washable, this is based on supplier confirmation and the specific care label for that garment. Not all silk pieces are machine washable — the care label is the authority.
If you have a question about a specific piece, the care label is the correct reference. For questions not answered there, contact us at hello@mercer-st.com.