What Is Mercerised Cotton?

How is mercerised cotton made?

Mercerised cotton is made by soaking cotton fibre, yarn or fabric briefly in concentrated sodium hydroxide (caustic soda), usually while it is held under tension, then neutralising it with water or acid. Britannica gives the soaking time as usually less than four minutes. The alkali makes each fibre swell, and held taut, its flat, kidney-shaped cross-section becomes rounder.

Published conditions vary by method:

• Full mercerisation: roughly 20–30% caustic soda by weight, according to a 2022 study in the journal Cellulose. A dyeing handbook describes the traditional bath as a cold solution of 25–26% by mass.
• Sewing thread: passed under tension through a cold 20% solution, according to a textbook on joining textiles.
• Hot mercerisation: a variant run at 70–100 °C for 4–60 seconds.
• Causticisation: a milder treatment at about 10–16%, used mainly to improve dye uptake rather than shine.

The word was coined for caustic soda treatments and later extended to treatments with liquid ammonia, which also swell the fibre.

From Mercer to Lowe

John Mercer, an English calico printer, patented the caustic soda treatment of cotton in 1850. Sources differ on when he first observed the effect: Britannica dates his discovery to 1844, while a finishing textbook places his observation of the swelling in 1850. The shine came later. Horace Lowe filed British patents in 1889 and 1890, and a finishing textbook credits him with showing that keeping cotton under tension during treatment gave it high lustre.

What are the properties of mercerised cotton?

Mercerised cotton is stronger and more absorbent than untreated cotton and takes dye more readily; when treated under tension, it is also shinier. The changes come from the swollen, rounder fibre, and the swelling is permanent.

• Lustre: applying tension during or straight after the caustic bath gives the fibres what a dyeing handbook calls a silky lustre. Britannica notes that the degree of lustre depends on the method used.
• Strength: mercerised sewing thread gains approximately 12% in tensile strength over a non-mercerised equivalent, according to the joining-textiles textbook.
• Colour: treated cloth takes brighter, longer-lasting colours from less dye. One handbook puts the saving at about 10–15% in light shades and 25–30% in deep shades.
• Absorbency: the fibre gains absorptive capacity, because its crystallinity drops.
• Shrinkage during treatment: without tension, cotton shrinks considerably along its length and may shrink by as much as one-fourth, according to Britannica. Tension largely prevents this.

How is mercerisation measured?

Mercerisation has no grade printed on a label; laboratories assess it, and one well-known method is the barium activity number (BA No.). The test compares a sample with an untreated control in barium hydroxide solution. Norms proposed in 1981 by researchers at India’s Textiles Committee class yarn as fully mercerised at 150 and above and unmercerised below 120. For fabric, their threshold for full mercerisation is 135 and above. Readings in between indicate causticised or inadequately mercerised material.

How does mercerised cotton appear on a clothing label?

On a US fibre content label, mercerised cotton appears as cotton, because the label must give the generic name of each fibre and mercerisation is a treatment, not a fibre. Under US rules, “100% cotton” means the product contains only cotton, apart from any non-cotton trim.

The word “mercerised”, where it appears, is a finishing claim. The Federal Trade Commission’s cotton guidance covers names such as Pima and Egyptian: any such term must be truthful and appear alongside the word “cotton”. The guidance does not mention mercerisation, so treat the word as a claim to check by eye and hand.

How do you tell mercerised cotton from ordinary cotton?

Look for a soft, even sheen, unusually deep colour and, in thread and yarn, a plied construction; none of these proves the treatment on its own.

• Tilt it under a light. Tensioned mercerisation gives the silky lustre. Causticisation, a weaker treatment with no tension across the weft, is done mainly to improve dye uptake, so expect less shine.
• Untwist a yarn end. Pull a loose tail or cut thread and roll it between your fingers. If it separates into two or more strands, that fits the claim: Cotton Incorporated notes that most mercerised yarns are plied to withstand the process.
• Compare the colour. Set it beside untreated cotton in the same shade. Britannica notes that mercerised cloth takes brighter colours from less dye.
• Know where the shine comes from. Sateen gets its sheen from a satin weave with long floats on the surface; mercerised cotton gets it from the fibre itself. Brushed cotton goes the other way, with a raised, matt surface.

Britannica notes that higher-quality cotton goods are usually mercerised. A “mercerised” label on dull, flat-coloured cotton is worth questioning, since Britannica ties the finish to brighter colour and, depending on the method, lustre. Liquid ammonia treatment is harder to confirm: in a 2022 study in Cellulose, infrared spectroscopy (FTIR-ATR) could tell it apart from caustic soda mercerisation, while X-ray diffraction struggled to detect differences between treated and untreated samples.

How do you care for mercerised cotton?

Care for mercerised cotton by following the garment’s care label, as for any cotton; the swelling is permanent, so the treatment does not wash out. Britannica describes its colours as longer-lasting, though how colourfast a garment is still depends on its dye. Shrink-control finishes such as Sanforized and pre-shrunk treatments are separate processes from mercerising, so check the label for those.

Is mercerised cotton sustainable?

Mercerisation adds a chemical step to cotton processing, so it is not a sustainability feature in itself. A dyeing handbook notes that the used caustic solution, especially from grey (loom-state, unscoured) cloth, is soiled and creates pollution problems.

Liquid ammonia is one alternative. The same handbook says water consumption falls to less than half, and the ammonia can be recovered and reused after distillation. Its improvements in lustre and dye uptake are, however, smaller than those of caustic mercerisation. How the fibre was grown, for example as organic cotton, is a separate question from how it was finished.