Mercerized vs Unmercerized Cotton

What the treatment does

Raw cotton fiber is a flattened, twisted ribbon. Mercerizing swells it into a rounder, straighter cross-section and holds it there while it is under tension. The change is permanent and it is structural rather than a coating.

• A rounder fiber reflects light more evenly, which is the sheen.
• A straighter fiber packs more tightly, which is the strength and the stitch definition.
• A swollen fiber takes dye more readily, which is why mercerized cottons come in more saturated colors.
• A tighter surface absorbs less water, which is the trade.
• Nothing washes out, because the fiber itself has changed shape.

Telling them apart

1. Read the band. Mercerized cotton says so, because it is a selling point.
2. Look at the sheen. Mercerized cotton is visibly lustrous next to a matte ball.
3. Feel the stiffness. Mercerized is firmer and squeaks slightly against itself.
4. Look at the color saturation. Deep, clear colors in cotton usually mean mercerized.
5. Crochet thread is essentially always mercerized, which is why doilies have that characteristic shine.

Which to use for what

For anything going near heat, cotton is a requirement rather than a preference: best yarn for potholders and trivets covers why, and is acrylic yarn flammable covers the fiber that is wrong for the job.

What neither one fixes

• Cotton has no elasticity, mercerized or not. Ribbing will not grip and a stretched garment stays stretched.
• Both are heavy for their bulk, which is why cotton sweaters hang off the shoulders.
• Both show uneven tension, because there is no loft to even it out - see cotton yarn pros and cons.
• Both take a long time to dry, and a cotton blanket laid flat is a multi-day job.
• Neither blocks permanently. Cotton flattens rather than setting into a new shape.

It is not a quality grade

Mercerized cotton costs more and looks more expensive, so it is easy to read the treatment as an upgrade. It is not - it is a change in behavior, and half of what it changes is a loss.

1. You lose absorbency, which is the entire point of a dishcloth or a towel.
2. You lose softness, which matters against skin and for babies.
3. You gain strength, which matters for bags, warp and anything under load.
4. You gain stitch definition, which matters for amigurumi and anything textured.
5. You gain color depth, which is why the brightest cottons are mercerized.
6. The fiber is the same plant either way. Nothing about mercerizing makes the cotton itself finer or better grown.

The practical test is to ask what the finished object has to do. If it has to hold water, buy the cheaper matte one and do not feel you have compromised.

Care

Both machine wash, and both publish temperature limits worth respecting - the mercerized cotton we list specifies water not above 105F, and the unmercerized one a delicate cycle not above 86F. Mercerized cotton shrinks less, because the treatment has already put the fiber through its dimensional change.

Yarn care symbols covers reading the label.