3.1 Fabric oxidation

Oxidation is the step after dyeing in which a dye that has been dissolved in a chemically reduced form is converted back into its insoluble, coloured form inside the fibres. It applies mainly to vat dyes such as indigo and to sulphur dyes. In the dye bath these dyes are made soluble by a reducing agent, so the fabric comes out of the bath in a pale, often yellow-green colour. Oxygen from the air, or an oxidising chemical, then develops the true shade and locks it into the fabric. If oxidation is skipped or incomplete, the dye is washed out in the next stage and the colour is weak, patchy or lost.

Example: A textile department in a technical college dips a cotton batik panel in an indigo vat. The panel comes out yellowish-green, and after about ten minutes hanging in the shade it turns deep blue as the dye reacts with oxygen in the air.

Air oxidation procedure

Air oxidation is the simplest method and needs only space, ventilation and time. The fabric is exposed to the air so that oxygen reaches every part of the dyed area. The steps below suit a small workshop using an indigo vat.

1. Lift the fabric slowly: Raise the fabric out of the dye bath with gloved hands. Squeeze out excess dye liquor back into the vat, keeping the fabric low to the surface so that you do not splash air into the vat.
2. Open out the fabric: Unfold any folds, ties or bundles so that no part stays enclosed. Dye trapped inside folds cannot meet oxygen and stays pale or green.
3. Hang or spread the fabric: Place it on a clean line or rack in a well-ventilated, shaded area. Direct strong sun can dry the surface unevenly before oxidation is finished.
4. Watch the colour change: The fabric moves from yellow-green to blue (for indigo) as oxidation proceeds. Turn the fabric over or reposition it so both sides are exposed.
5. Allow enough time: Leave the fabric until the colour stops deepening. The time depends on the dye, the thickness of the cloth and the air movement, so judge by colour and not by the clock alone.
6. Re-dip for deeper shades: If a darker colour is needed, return the fabric to the vat for a short dip, then oxidise it fully again. Several short dips with full oxidation between them give a richer and more even colour than one long dip.
7. Check before rinsing: Confirm that the colour is uniform and that no green or yellow patches remain. Only then pass the fabric on to rinsing.

Chemical oxidation

Chemical oxidation uses an oxidising agent in solution when air oxidation is too slow, or when the dye type (such as sulphur dyes) needs it. The agent supplies oxygen quickly and evenly through the whole fabric. It is more controlled than air oxidation but demands more care in handling.

• Hydrogen peroxide: A common oxidising agent used in dilute solution. It acts quickly, but it must be used at the strength recommended by the dye supplier because strong solutions damage fibres and skin.
• Sodium perborate: A powder that releases oxygen when dissolved in water. It is easier to store than liquid peroxide and gives a gentler action.
• Fabric immersion: The fabric is fully submerged in the oxidising bath and moved gently so that all areas are treated equally. Crumpled or floating cloth oxidises unevenly.
• Controlled treatment time: Leave the fabric in the bath only as long as the dye supplier advises. Too long a treatment can dull or change the shade.
• Thorough removal of the bath: After treatment the fabric must be rinsed well so that no oxidising chemical remains in the cloth.

Factors affecting oxidation

Several conditions decide how quickly and evenly oxidation takes place. A decorator who understands them can predict problems before they spoil a batch.

Example: A fabric decoration workshop dyeing thick cotton throws finds that they need much longer in the air than the thin cotton napkins dyed in the same vat.

• Air circulation: Moving air brings fresh oxygen to the fabric surface. In a still, closed room oxidation slows down and the colour develops unevenly.
• Fabric thickness: Thick or heavy cloth holds more dye liquor and takes longer to oxidise through. Thin cloth oxidises quickly.
• Temperature: Warmth speeds up the reaction, while cold slows it. Very hot conditions can dry the fabric before the dye has fully developed.
• Fabric handling: Folds, creases and overlapping layers keep air away from the dye. Spreading the cloth out gives every area equal exposure.
• Dye bath condition: A vat that is not properly reduced, or that has weakened over time, produces fabric that develops pale or uneven colour however well it is oxidised.

Common oxidation faults and remedies

Faults during oxidation are usually visible straight away, which allows correction before the fabric is rinsed and dried. Recognising the cause saves cloth, dye and time.

Example: A workshop dyeing cotton aprons sees green streaks along the fold lines and corrects them by re-opening and re-hanging the cloth.

• Green or yellow patches: Oxidation is incomplete in those areas, often because of folds. Open the cloth out and expose it to air for longer.
• Uneven shade: Some parts were exposed to air sooner or longer than others, or the fabric was dipped unevenly. Handle and hang the cloth uniformly and use shorter, repeated dips.
• Pale colour after rinsing: The dye was washed away before oxidation was complete. Always check that the colour has fully developed before rinsing.
• Streaks and drip marks: Excess liquor ran down the cloth while it hung. Squeeze out the excess before hanging and hang the fabric evenly.
• Rubbing off (crocking): Too much dye sits on the surface rather than in the fibre, often from over-long dipping. Use several shorter dips and oxidise between them.
• Weakened fibres: Strong oxidising solutions or overlong treatment damage the cloth. Use the recommended strength and time.

Safety during oxidation

Oxidation involves alkaline dye liquor, and sometimes strong oxidising chemicals, so the work area and the worker must be protected. Simple habits prevent most injuries.

Example: A fabric decoration studio working with indigo keeps a dyeing corner with gloves, aprons and eye protection laid out before any dipping starts.

• Protective clothing: Wear rubber gloves, an apron and closed shoes. Dye liquor and oxidising solutions irritate the skin and stain clothing.
• Eye protection: Wear goggles when handling oxidising chemicals such as hydrogen peroxide, since splashes can cause serious eye injury.
• Ventilation: Work in an open or well-ventilated area, both to oxidise fabric well and to avoid breathing fumes.
• Chemical handling: Keep oxidising agents in labelled containers away from heat, fuel and other chemicals. Follow the supplier's instructions for dilution.
• Spills and waste: Wipe spills at once and dispose of used dye and oxidising solutions as advised by the supplier or the college, not into drinking-water sources or open drains.
• First aid: Rinse skin or eyes with plenty of clean water immediately after any splash and report the incident to the supervisor.