Discharge Dyeing

Discharging removes the color from fabrics and is the opposite of dyeing. This process is used to create natural or controlled patterns in textiles. Other common names are, bleaching or resist. It is known to bring dark colored fibers lighter. You can purchase specialty chemicals made to remove color from fabric in a liquid based form or paste. Such as industrial discharge processes that use reducing agents like asthiourea dioxide, sodium formaldehyde sulfoxylate, zinc formaldehyde sulfoxylate or tin chloride. You can also use the common household item, chlorine bleach.

When using the discharge dyeing technique, most of the time another dye is applied to the area that is discharged to highlight it. Sometimes the new color is included in the discharge paste. Some dyes are easier to discharge, others are very difficult to discharge. Commercially dyed black fabric is known to produce unpredictable results in discharge. It is probably because black dyes are mixtures of several colors. Some discharge techniques decolorize the dye but leave remnants fixed to the fiber, while others break the bond between the dye and the fiber. Discharging is better on 100% cellulose.

There are two different ways to chemically remove the dye. Oxidation is one where the electrons are removed and the other is by reduction, in which electrons are added. Commonly by hand dyers, chlorine bleach or peroxide is used for the oxidation discharge. The active ingredient in chlorine bleach is sodium hypochlorite. It removes or changes the color of many dyes but some dyes will resist changing color no matter what you do to them. Typically it is because a metal ingredient was added to the dyes.

If you immerse/dip the fabric into a discharge bath, you would dilute the bleach into a liter of water for every 5% hypochlorite (household bleach). Have anti-chlor ready to stop the discharging process before the fiber bonds break. Anti-chlor is a substance used to remove the excess of chlorine and are typically sodium bisulfate or sodium thiosulfate. Watch the fabric to see it lighten to the color you want it to be. Then rinse it thoroughly with clean water. Squeeze out the excess, then immerse the fabric into the anti-chlor solution. Allow it to sit in the anti-chlor for ten to fifteen minutes so it reacts with the residual bleach. Then wash the fabric with a mild detergent, usually Synthrapol is great for a pre and after-wash.

There are other ways of applying the discharge. You can spray, paint or stencil the discharge. In order to paint or stencil on the discharge you must thicken the solution. If using bleach, do not use dye thickeners like alginate because the hypochlorite will break it down immediately. Monagum is a modified starch gum that is specifically used with hypochlorite solutions and it can stay thick for up to a day.

Other oxidative discharge agents that aren't recommended to use at home are sodium dichloroisocyanurate, sodium chlorite and potassium permanganate. Sodium dichloroisocyanurate is used for discharging indigo-dyed denim. The chemical reacts with the indigo by oxidating and reduces the color. Indigo can only be discharged with the oxidation process. It's because reductive discharges react with vat dyes to form the water-soluble leuko form of the dye. Sodium Chlorite, NaCIO2, is related to sodium hypochlorite. It becomes dangerous at low pH levels because it can produce lethal chlorine gases. The household bleach has NaOH added to keep the pH high for a less hazardous environment. Potassium permanganate is also an industrial discharge for indigo-dyed denim. It's poisonous and becomes explosive if the solution is allowed to dry up.

Reductive discharges can give you different results then the oxidation discharges. Because this process reduces or fades the dye rather then stripping or breaking the bond of the dye and fabric. Sulfur dioxide, SO2, is the active chemical ingredient that reduces the dye. All reductive discharges require heat. This activates the reaction that breaks the double bonds in the dye. There are different ways to apply heat to the process. It depends on what discharge technique you use. One can either apply heat by using hot tap water in the washing process, high temperatures of a discharge bath (such as boiling it in a pot), use of a steamer or heat set it with an iron.

Some reducing agents like, Thiourea dioxide, H2NC(=NH)SO2H, is also known as formamidine sulfinic acid, is used in indigo, vat dyeing and discharge. Sodium hydroxymethanesuflinate, CH3NaO3S, is also known as formaldehyde sodium sulphoxylate and is great to use to reduce acid and basic dyes. Zinc formaldehyde sulfoxylate, Zn(HOCHSO2)2, is used for commercial screen-printing on t-shirts. Calcium formaldehyde sulfoxylate, Ca(HOCH2SO2)2, is a paste form discharge used for painting or stenciling on.

Both techniques are used for artistic reasons but the reductive discharge technique seems to give a wider range of creativity. The reductive discharge process has been used throughout history specifically in different ethnic cultures. Japan, Indonesia, India and Africa are amongst those cultures that have been using this process for thousands of years. This technique is a longer thought-out process rather then the, quicker free-spirited oxidation discharge process.

Attached is a video clip that I found at www.Metacafe.com for at home discharge dye. Provided by a Metacafe user, greeenpro.

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