Hydrogen Peroxide can be used to neutralize after bleaching
There are two different ways to attack a dye, chemically, to remove its color. One is by oxidation, in which electrons are removed, while the other is by reduction, in which electrons are added. On this page I describe each of the discharge chemicals that are used by hand dyers to discharge dye. Please note that household bleach is a toxic chemical. All discharge agents should be used with appropriate safety precautions.
The most familar discharge agent is ordinary household bleach, whose active ingredient is sodium hypochlorite. It will remove or change the color of many dyes, but not all dyes can be bleached. Some dyes will resist changing color no matter what you do to them.
Try diluting ordinary household bleach, which is 5% hypochlorite, 60 ml in one liter of water, or one cup in a gallon. Before you begin, prepare a basin (or washing machine) containing your Anti-chlor so that you will be ready to use it immediately, or set out your bottle of 3% hydrogen peroxide ready to hand if that is what you are going to use. Dip your fabric in the bleach solution, or paint the solution onto the fabric, or spray it on (if you can take adequate precautions to avoid exposure to the bleach mist or damage to your furnishings). Watch to see when the color changes. Once the color has lightened sufficiently, immediately plunge the fabric into clean rinse water or rinse under the faucet, wearing rubber gloves, squeeze it out, then immerse it in your anti-chlor solution, or pour your 3% hydrogen peroxide over until it is throughly saturated. After allowing fifteen minutes for the Anti-chlor to react with the residual bleach, wash the item thoroughly.
Use thickened bleach. For printing or painting with bleach, you will want to use thickened bleach so that it does not run and creep on the fabric. The most convenient method, for small projects, is to use a Clorox Bleach Pen, which contains a thickened formula of hypochlorite bleach. These pens are extremely convenient to use, and limit your exposure to bleach fumes considerably, although they are expensive compared to the price per ounce of the large jugs of liquid hypochlorite bleach. They also have a limited shelf life after opening; I would recommend you check after six months to make sure your bleach pen still works, before embarking on another project, as you may need to buy a new one. I have found an old Bleach Pen to be entirely ineffective in discharging dye.
Dishwashing machine detergent gel with bleach is said to be an ideal substance to use for discharge printing, because it is already thickened. I've read that you can use a pastry bag to dispense it in lines (my usual substitute for a pastry bag is a ziplock bag with a tiny corner snipped off - I wonder if that would work in this case), or simple rubber stamps, or applying it through a stencil with a sponge or foam brush.
To thicken your own liquid bleach, you cannot use ordinary dye thickeners such as alginate, because they will be broken down quickly by the hypochlorite. Instead, use a thickener that is sold specifically for this purpose. Monagum is, according to PRO Chemical & Dye, a modified starch gum that is the only thickener for discharge printing with hypochlorite bleach that stays thick for up to one day, rather than breaking down and becoming thin quite soon after mixing with the bleach. Dharma's equivalent product is called Bleach Thickner.
Chlorine bleach is extremely damaging to both synthetic fibers, such as nylon or polyester, and to animal fibers such as wool or silk. Never use chlorine bleach on any fiber that is not 100% cellulose, such as cotton, linen, or hemp. Chlorine bleach will cause a permanent unattractive yellowing of polyester fiber.
Even after chlorine bleach has been rinsed from a garment, either the chlorine itself, or perhaps the free radicals produced by chlorine's reaction with your fabric, will continue to eat away at your fiber. You should use a good bleach stopping chemical on your garment after bleaching. The most economical choice is Anti-Chlor (sodium metabisulfite); other good choices are Bleach Stop (sodium thiosulfate) or hydrogen peroxide. Do not use vinegar to neutralize bleach, as the reaction of acid with hypochlorite produces dangerous chlorine gas. For more information on this very important subject, please read How can I neutralize the damaging effects of chlorine bleach?.
Hydrogen peroxide is used to whiten wool, but it will not remove the color of most dyes. Peroxides are the basis for chlorine-free "colorsafe" bleaches for use in the laundry. (PRO Chemical & Dye has instructions for Bleaching Wool using Hydrogen Peroxide.)
Benzoyl peroxide (properly pronounced ben-zo-EEL peroxide) is an oxidative bleach often inadvertently used to remove dye. It is the answer to the common question of "Why are my towels getting light spots?" Many skin care regimens include benzoyl peroxide for its acne-fighting abilities, but it is also noted for its ability to remove the color of common commercially used blue dyes. This can happen even for those who take great care in washing their hands after use. Interestingly for a skin care product, benzoyl peroxide is also an explosive and has been used as a rocket fuel. Users of benzoyl peroxide skin care products, if bothered by ruined towels, should either use white towels only, or consider switching to another acne-fighting agent such as Retin-A gel.
Other oxidative discharge chemicals. There are several oxidative discharge agents used in the textile industry which are not recommend at all for use at home. Sodium Chlorite, NaClO2, is closely related to sodum hypochlorite; solutions of it become particularly dangerous at low pHs. (Household bleach has NaOH added to keep the pH of its hypochlorite at a less hazardous high pH; low pHs are hazardous primarily because they lead to the production of lethal chlorine gas.) Potassium permanganate is used industrially to discharge indigo-dyed denim, but it is extremely poisonous and also becomes dangerously explosive if a solution of it is inadvertantly allowed to dry up.
Just as for chlorine bleach, described above, not all dyes can be removed or changed in color by reductive discharges, though the results are often quite different for reductive discharges than for chlorine bleach.
Sulfur dioxide, SO2, is the chemical that does the reducing, no matter which of the following chemicals you use to reduce your dye. Originally it was produced by burning yellow sulfur in the presence of the fiber to be bleached. This is probably not a good approach, for your own health; better to use one of the other reducing discharge agents to produce it on the fabric.
Thiourea dioxide (Colour Index Reducing Agent 11), also known as aminoiminomethanesulfinic acid or formamidine sulfinic acid, is sold under the brand names Thiox, Spectralite, Jacquard Color Remover, and Dharma Dyehouse Color Remover. The chemical formula is H2NC(=NH)SO2H. It is used in indigo dyeing and other vat dyeing as well as for discharge. It costs more than sodium hydrosulfite, but you use only one-fifth as much. Jacquard Color Remover contains Thiourea dioxide and soda ash. (Here's a link to PRO Chemical & Dye's instructions for using Thiox on cellulose, wool, and silk.) The solubility of thiourea dixodide is only 37 grams per liter at 20°C.
Sodium hydrosulfite (Colour Index Reducing Agent 1), also known as sodium dithionite, sodium sulfoxylate, and sodium sulphoxylate, is the active ingredient in Rit Color Remover, Tintex Color Remover, Dylon Run away for Whites, and Carbona Color Run Remover, all of which also contain sodium carbonate (soda ash). Its chemical formula is Na2O4S2. This is the chemical used by Carter Smith in Kate Broughton's book Textile Dyeing: The Step-By-Step Guide and Showcase. Storage of large quantities is unsafe due to its flammability, but it is easy to find this product at local drugstores or sewing stores, so there is no need to buy a year's supply at once. You can use sodium hydrosulfite on the stovetop or in the washing machine; the latter is less effective but far more convenient for use on clothing.
Sodium hydroxymethanesulfinate (Colour Index Reducing Agent 2), also known as sodium formaldehyde sulfoxylate, sodium hydroxymethanesulphonate, sodium hydroxymethanesulfonate, and formaldehyde sodium sulphoxylate, is the chemical used in Formosul and Rongalit (most likely including BASF's Rongalit C and Rongalit ST, also sold as Jacquard Rongolit ST). Update: Also the active ingredient in deColourant Mist and deColourant Paste; see ProChem. Its chemical formula is CH3NaO3S. It can be used under acid as well as basic conditions. (Here is a link to PRO Chemical & Dye's instructions for using Formosul to strip color and discharge print on cotton, silk, wool and nylon.)
Zinc formaldehyde sulfoxylate (Colour Index Reducing Agent 6), Zn(HOCHSO2]2, is another discharge chemical, used commercially for screen-printing t-shirts.
Calcium formaldehyde sulfoxylate (Colour Index Reducing Agent 12), Ca(HOCH2SO2]2, is a discharge chemical that is manufactured in paste form. It is also the main ingredient in a different Rongalit product, Rongalite H, though I have not found a suitable source for it.
Other reductive discharge chemicals. Both sodium sulfite (Na2SO3) and sodium bisulfite (NaHSO3) are listed in some documents as reductive bleaches, but I cannot find a recipe for either's use for this purpose, though it's easy to find recipes for the use of the other reducing agents above. At our level it seems that these are only used to neutralize hypochlorite bleach. Do NOT confuse them with sodium bisulfATE (Na2SO4), which is used to destroy cellulose; remember the mnemonic, "bisulfATE ATE my fabric."
Jacquard Discharge Paste apparently contains Rongalit ST (see above), along with water, urea, and ammonia. This product is highly recommended by dyers who like the convenience of not having to make their own paste and like its stable shelf life. To use it, you paint or print your design on fabric, allow to dry, and then steam-iron to activate the reaction. (Here is a link to Jacquard Products' instructions for using three different kinds of discharge paste: thiourea dioxide print paste, Rongalit print paste, and Jacquard Discharge paste.)
Tin(II) chloride is unlike the above reductive discharges in that it does not involved sulfur dioxide at all. Also known as "tin salt", it has a formula of SnCl2•2H2O, and has been used in discharge printing on wool. It reacts to form hydrochloric acid in the steamer, which is corrosive to the equipment. It must not be used with thiodiglycol as a dye solvent, because the reaction of hydrochloric acid with thiodiglycol produces deadly mustard gas. (Source: David M Lewis, Wool Dyeing.)
All of the reductive discharge chemicals require heat to activate the reaction that breaks the double bonds in the dye chemicals. Some recipes call for hot tap water in the washing machine; most call for the higher temperatures of a discharge bath (heated in a cooking pot), a steamer, or heating with a steam iron.
An alternative way to provide the moist heat required to activate discharge agents is to steam your treated items in a spare microwave oven that has been placed out-of-doors during use. It is recommended that you use a microwave oven that you will not be using for food. Do not microwave dry fabric, as it will burn. Wrap damp fabric in plastic so that it will not dry out as it cooks, or place a cup of water in the microwave while you use it. Do not use a microwave that is indoors for heating discharge chemicals, as the irritating fumes produced can be bad for your lungs.
Although the reductive discharge chemicals are less toxic, in general, than chlorine bleach, they all produce sulfur dioxide, which may be particularly dangerous for people who have asthma. All should be used only with gloves and with care to avoid overexposure to any vapors produced. Be sure to use proper ventilation and/or an acid gas respirator while working with them. Note that a dust mask provides no protection at all. Obtain and read the MSDS (materials safety data sheet) for each chemical you work with.
Sulfur dioxide fumes may also have undesirable effects on fabrics that have been dyed with indigo or other vat dyes, causing blues to turn yellow or green. Do not leave indigo-dyed fabrics exposed to the air in a room in which you are using any reductive discharge chemical.
Karren Brito wrote an excellent article on the subject of discharge agents entitled Honorable Discharge: Decolorization of Natural Fabrics [PDF].
Jane Dunnewold's book, Complex Cloth: A Comprehensive Guide to Surface Design, gives good instructions for using chlorine bleach to discharge dye designs on cotton fabric.
Kate Broughton's book, Textile Dyeing: The Step-by-Step Guide and Showcase, shows Carter Smith's illustrated step-by-step instructions for using a heated bath of hydrosulfite (the chemical in Rit Color Remover) to discharge silk in beautiful designs.
Last updated: November 3, 2010
Page created: July 17, 2007
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Parts of this page originally appeared in the form of a posting by Paula Burch on the Dye Forum on May 17, 2006.