Neutral cellulase stone washing: contrast without the stones

What pumice actually does, and why mills want out

Denim warp is ring-dyed. The indigo sits on the outer shell of the yarn and the core stays white. The whole logic of stone washing is to abrade that shell in places and expose the white underneath. Contrast is the difference between the abraded and the unabraded areas.

Pumice does this mechanically. It tumbles with the garments, rubs against the high points of the fabric and scrapes indigo off. It works. The bill comes in several parts.

Drums and bearings wear; a machine running stones has a shorter service interval than one that does not. The stone breaks down into sludge, and pumps, filters and the effluent inlet all have to deal with it. On the garment, damage concentrates at the seams, hems and pocket openings. Stones stay behind in pockets and turn-ups, and getting them out is hand labour. A single missed stone is a customer complaint. And stone takes up drum volume, so fewer garments fit in the same machine.

The argument for the enzyme is that it delivers the same effect without those line items.

How does neutral cellulase get contrast?

Cellulase hydrolyses cellulose at the fabric surface. Fibre ends and microfibrils on the face loosen, and when the mechanical action in the machine tears them away, the indigo they carry goes with them. The enzyme prepares chemically what the stone did by brute force; the movement finishes the job.

The part that gets missed is that the enzyme creates no contrast on its own. Without mechanical action the loosened fibre stays where it is and no effect develops. Load factor, drum speed and time decide the result more than dosage does.

Neutral cellulase works in a higher pH band than acid cellulase, and in garment washing that buys two practical things. First, alkali the garment carries over from desizing does less damage in the neutral band, so pH control is more forgiving. Second, back-staining is less severe in the neutral band than in the acid one. Acid cellulase gets there faster, but in denim the price of speed is usually back-staining.

As a class, neutral cellulases run around pH 6-7. That is a class range. Derin Kimya's own figures sit on each product page: pending from the client.

Why is back-staining the number one complaint?

The enzyme lifts indigo off the surface. It does not take it out of the bath. Freed indigo stays suspended in the liquor and re-deposits anywhere it gets the chance.

Where it lands is predictable: pocketing, labels, sewing thread and the weft yarn. All of those are undyed, so the smallest amount of staining is immediately visible. The garment comes out greyish and muddy with the contrast flattened. Pocketing going blue is the one the customer spots straight away.

The mechanism has two legs. Indigo is insoluble in water, so released particles aggregate and have an affinity for cellulose. The second leg is less well known: the cellulase protein itself adsorbs onto cotton and carries adsorbed indigo onto the fabric with it. The enzyme both releases the indigo and acts as its vehicle.

This is the point a dispersing agent is dosed against. Its job is to keep released indigo particles dispersed in the liquor and stop them aggregating and attaching to the surface. Some anti-back-staining products also suppress the protein-fibre interaction and close off the carrier route.

It is worth being clear about what it does not do. A dispersing agent will not strip indigo that has already fixed onto the cloth; that needs a hot alkaline wash or an oxidative step. It does not raise contrast either, because it is a protective chemical rather than an effect chemical. Overdosed, it can settle on the fibre surface and slow the enzyme down.

None of these substitutes for another. Dose the dispersing agent and leave the bath to cool and you will still get back-staining.

When it enters the bath matters as much as how much goes in. A product added after the indigo is already loose cannot take back what has already attached, so the protection is built before the enzyme or alongside it. Topping up mid-cycle is the most common reason a dispersing agent gets written off as ineffective.

What has to be done before the enzyme bath?

Garments arriving at the stone wash are not greige. A desizing step and a rinse came first, and the quality of those two decides what the enzyme bath can achieve.

Residual size blocks the enzyme from reaching the fibre and the effect comes out patchy. Alkali carried over from the desizing bath pushes the enzyme bath pH up; neutral cellulase is more forgiving than acid cellulase, but that tolerance is not unlimited. Liquor trapped in pockets and seam allowances is what produces differences between individual garments in the same load.

Load factor is set here too. An underloaded drum means the garments do not rub against each other, mechanical action drops, and the same enzyme in the same time gives less contrast. Overloaded, the movement drops again and the liquor no longer reaches every part of every garment. Each machine has its own workable load range, and that range belongs to the recipe, not to the operator's judgement on the day.

Cold or hot neutral cellulase?

Both types do the same job. The difference is the temperature at which they do it.

The hot type gives stronger abrasion in less time and is preferred where heavy contrast is wanted. It is paid for in steam and in heating and cooling time.

The argument for the cold type is operational. Washing machines heat slowly; taking the heat-up and cool-down out of the recipe shortens the cycle directly and raises the number of loads per day. Where the steam line is weak or steam is expensive, that gap is larger than the difference in chemical cost. Lower temperature also reduces back-staining and the dimensional risk on stretch denim, and it is less likely to cause trouble on garments carrying prints or leather patches.

The Derin Kimya range covers hot and cold working neutral cellulases plus a variant that gives a bluer cast. Which product sits in which band is on the product pages: pending from the client.

One warning. The conditions needed to stop a low-temperature enzyme are not the same as those for a hot one. When the recipe changes, the deactivation step gets re-proven along with it.

Abrasion or strength loss?

Cellulase cuts cellulose wherever it makes contact. Contrast and strength loss therefore move in the same direction, and the skill in the job is knowing where to stop while still holding the look.

The loss is not even across the garment. Seams, hem folds, the waistband and back pocket openings take the most punishment in the drum. The enzyme does more work there, and mechanical load piles on top. In an over-processed batch the first thing to fail is not the middle of the leg panel; it is the corner of the back pocket.

Three measurements get read together: weight loss on a sample from the leg panel, tensile and tear strength periodically on that quality, and a visual check of the seam areas. On a quality being run for the first time, step the time and pull samples. Hold the dosage, because time is the variable that adjusts the effect linearly.

Target weight loss depends on the wash look and the fabric: pending from the client.

Was the enzyme stopped before the softener?

The enzyme does not stop when the clock runs out. Even after the bath is dropped, enzyme left on the garment keeps working, and in garment washing a load can sit wet for hours before it reaches the dryer. The loss that accumulates in that time appears in no record anywhere.

The softener bath makes the risk worse. Softener baths are usually set slightly acid or neutral, which puts them inside the working band of a neutral cellulase. An enzyme that was never stopped works comfortably there.

Sequence:

1. At the end of the enzyme time, take the bath pH clearly outside the enzyme's working range with soda ash.
2. Raise the temperature above the denaturation threshold and hold it. pH alone is not enough, because the pH effect is reversible.
3. Drop the bath hot.
4. Run a hot rinse with a dispersing or washing-off agent; loosely held indigo leaves at this step.
5. Rinse cold and set the pH for softening.
6. Dose the softener.

Which pH and which temperature are enough depends on the product: pending from the client.