Vat dyes lose their sharpness and become dull

Name: Amina

—ADVERTISEMENTS—

Books with instructions for dyeing with vat dyes

Books with more information about dyeing with vat dyes

Country: Nigeria

Message: Hi there,

Hope this won't be too much of a bother but I am at my wits end.

I am self taught and have been dyeing for a few years now and I have noticed a recurring problem with my finishing. What I dye is primed canvas fabric and the preferable type of dye easily and cheaply accessible here is the vat dye. Once dyed the finished fabric looks sharp and bright but after a couple of uses the fabric loses its sharpness and becomes dull. I have tried using scotch guard fabric protector on the fabric hoping it will help in fixing the dye but the fabric still looks dull after a while. What would you suggest I do? Your advice will be greatly appreciated seeing as you are an expert in the field. To get a clear picture of how I mix my "chemicals" I use 2 times more hydrosulphite than Soda eg. 2 tablspns soda and 4 tblspns Hydros and probably 4 or more full tspns of vat dye. I let my waxed fabric soak for at least 6 hrs and when rinsing, the dye stops running after 2 or 3 tries.

I am eagerly awaing your response and I will be happy to answer any questions you might have.

It's sadly true that Scotchgard does not fix vat dyes. Neither do Retayne and other cationic dye fixatives, which work so well to improve the washfastness of cheap direct dyes. There is no dye fixative that you can add to fix vat dyes in place after dyeing, since their fixation must occur during the dyeing itself, not afterwards. I think that the answer in your case may lie in the step known as "soaping off", which is done after the last washing, but there are other possibilities worth examining, as well.

Vat dyes work differently from most other types of dye. In order to make them truly dissolve, you must chemically reduce them, which is the exact opposite of oxidizing them; you must exclude oxygen, as far as is practical (avoid vigorous stirring, which may introduce too much air to the dyebath), and instead use a chemical that will add electrons to the dye molecules. As a result, the dissolved dye particles can penetrate inside the fiber of the fabric or yarn. After this, when you expose the wet dyed fabric to air, the oxygen in the air acts to oxidize the dyes, turning them back to their insoluble state. Since, at this point, the dye particles cannot dissolve in water, those that are located inside the fiber are stuck fast and will remain there.

Unfortunately, some of the dye particles at this point are instead found on the surface of the individual textile fibers, not stuck inside at all; the step known as "soaping off" helps to remove this non-fixed vat dye after dyeing. If the non-fixed vat dyes are not removed, they give only a temporary effect of brightness, but they will tend to fade quickly, and, worse, to rub off onto other items even when dry, a serious dyeing fault known as "crocking". If too much vat dye is applied in one dip, more than is called for by the recipe, then, again, too much of the dye particles will remain on the outside of the fibers, instead of being fixed within the fibers; for more intense colors, it is better to do multiple dips with a weaker dyebath than to do a single dip in a dyebath that contains too much dye.

The first question to consider is always the fabric. Do you know that your fabric is 100% cotton (or other natural fiber), and free of surface finishes such as stain resistance? You have probably already made sure of this, but it's always worth asking. You can't get good bright colors with vat dye on a fabric that has a significant content of synthetic fiber, and I have seen canvas made with synthetic fibers such as acrylic. What do you mean by "primed canvas fabric"? Priming can be a problem, if it means anything other than thorough cleaning. Canvas that is primed for use by oil painters has been coated with a sort of glue that will prevent good dyeing.

A related question is your preparation of the fabric. Step one, as always with any dye, is to prepare the fabric by washing it in the hottest available water, preferably 140°F or higher (60°C), using laundry detergent plus some extra soda ash for added cleaning power. It is most important to remove any finishing chemicals that are on the fabric, as they dull the colors of any dye you apply, by preventing the dyes from fully penetrating the fiber. Some such finishes cannot be removed by washing, but the spinning oils and many sizings can be. If your fabric has been sized with starch, it will be impossible to remove it all, and the starch can interfere badly with dyeing.

Most of the vat dye recipes I see call for a certain number of grams of each ingredient, by weight, rather than the volume measures to which you are accustomed. You will get much more repeatable results if you can weigh your materials instead of using spoonfuls, since one lot of dye or any other chemical may be more or less dense than another. A certain number of grams is always the same amount, but the number of tablespoons per gram may vary. Would it be possible for you to buy a scale that is good for measuring quantities as small as one gram? I recommend that you do so, if you can find one.

When you mentioned soda, did you mean soda ash, or caustic soda? Soda ash contains sodium carbonate, or Na₂CO₃; washing soda is another form of sodium carbonate and can be used for the same purposes as soda ash. Caustic soda is a completely different chemical, also know as sodium hydroxide, or lye, known by the chemical abbreviation NaOH.

All of the recipes I know for using synthetic vat dyes call for the use of caustic soda. Soda ash (or washing soda) does create a high pH when you add it to water, but caustic soda produces a significantly higher pH. If you are using soda ash, you should get better results with caustic soda. Caustic soda is a hazardous material and has caused deaths among people who drank it, and blindness to those who splashed it in their eyes, so care is needed. It's safe to use with appropriate precautions. Expect the lye to make the water very warm when it dissolves. Wear waterproof gloves and safety glasses, and always add lye to water, never water to lye, and of course always keep it out of reach of children. Carefully label all bottles and jars that contain lye, and never store it near food or drink. Do not use reactive metal containers, such as aluminum, when working with lye, or soda ash or washing soda, because these high-pH chemicals will react with most metals. Stainless steel and enameled steel are both good.

A good way to troubleshoot your difficulties with vat dyeing is to look at some good recipes and compare your techniques. If your dye manufacturer has not supplied you with reliable instructions, look to other companies to see what they have. Start with my page, "About Vat Dyes". I like the detail of PRO Chemical & Dye's "Immersion Dyeing using PRO Vat Dyes" recipe [PDF] , but it calls for thiourea dioxide (Thiox), instead of the sodium hydrosulfite (Hydros) that you are using. It is necessary to use five times as much sodium hydrosulfite as thiourea dioxide, if they're reasonably pure; if either one is sold in a preparation that includes other chemicals such as sodium carbonate, measurement will be more complicated.

Aljo Mfg, in New York, has a simpler vat dye recipe, which calls for both caustic soda and sodium hydrosulfite, and for measuring with a tablespoon, as you have been doing, rather than by weight. For one pound (500 grams) of cotton or rayon fabric, they say to dissolve one tablespoon (15 ml) of caustic soda in a cup (240 ml) of cold water, then, in this mixture, dissolve a tablespoon of sodium hydrosulfite. In a separate container, mix one to two tablespoons of vat dye with a little warm water. Add the dye to 12 liters of warm water (30°C to 50°C). This is your dyebath. They say to add your pre-wetted fabric to the dyebath and add the lye/hydrosulfite solution, then raise the temperature of the dyebath to between 50°C and 70°C, stirring but not allowing the heat to approach boiling. Clearly you don't want to heat your dyebath to a high enough temperature to melt your wax, but they say "in batik, where lower temperature is used, dyeing time will be longer", so evidently this recipe can be used for your circumstances.

After dyeing with vat dyes, industrial recipes call for a "soaping off" step. This comes after you have completed dyeing and have washed out the fabric and exposed it to air to oxidize the dye to its final color. Apparently, in spite of the name, soap is not as important to this final step as the nearly boiling temperature of the water, suggested in some sources as being 95° to 98°C. Here is an explanation from J. Richard Aspland's 1997 book, Textile Dyeing and Coloration, p. 59:

"After sodium leuco-vat anions have diffused into cellulosic fibers, after the excess alkali and reducing agents have been washed off, after the oxidation has been accomplished and the pigmentary form of the vat dye is left embedded within the cellulosic fibers, then comes soaping.

"Soaping is peculiar to vat dyes and it is a misnomer in that it does not require the use of soap. It can describe any treatment, in near boiling aqueous solutions containing a surfactant, during which the isolated molecules of vat pigments are reoriented and associate into a different, more crystalline form. This new form can have a significantly different shade and gives the dyed goods materially improved fastness to light and washing."

Since you are applying wax, you may already be using a boiling water treatment at the end to remove the last traces of wax. If not, it's something to consider trying, since it is supposed to improve lightfastness and washfastness.

Your question is an interesting one. I hope that you will find this useful.

Also see the related question, "Safety of the caustic soda and hydro-sulphite used for tie-dyeing in Nigeria", posted in this blog on December 03, 2009.

(Please help support this web site. Thank you.)

Posted: Thursday - January 13, 2011 at 05:14 PM

Follow this blog on twitter here.
Tweet

getBlogName(); > getCategoryName("<$EntryCategoryFolder$>"); >