Name: Dorcas

Country: Nigeria

Message: Here in Nigeria, we are used to tie-dye with caustic soda and hydro-sulphite among others. Please how safe are these chemicals or what are the precaution I should take as a fabric designer? I will appreciate your answer. Thanks 

The chemicals you're concerned about are used with vat dyes. The caustic soda and sodium hydrosulfite are used to reduce and solubilize vat dyes in order to apply them to textiles; the solubilized dyes in the fibers are then oxidized by exposure to air, so that they become insoluble and fixed in place inside the fiber. Dark colors are obtained by repeated applications, rather than by strong dye solutions, because heavy application of vat dyes can result in dyes that are deposited on the outside of the fibers, resulting in the problem of dry-rubbing, which is called crocking. For more information on these dyes, see my page, "About Vat Dyes".

With care, these chemicals can be used safely. You will need protective gear to prevent the chemicals from contacting your skin or eyes, and you will need excellent ventilation (such as a fume hood, or working out-of-doors, preferably with a large electric fan blowing air away from you, in case there is no breeze). You will need a dust mask for use when working with chemical powders, and you should consider a cartridge respirator to keep out fumes. I'll describe the safety considerations for caustic soda, sodium hydrosulfite, and the vat dyes themselves, below.
 
Caustic Soda (sodium hydroxide)

Caustic soda, also known as lye, is known to chemists as sodium hydroxide, which is abbreviated in every language by the letters NaOH. Sodium hydroxide is used to increase the pH of the dyebath. As the common name implies, it is very caustic to your skin and eyes. If it contacts your skin or hair, it reacts with the natural oils to create soap. Major exposures can cause severe burns. Minor exposures can cause the skin to become very dry, reddened, and sore. If caustic soda is splashed into the eyes, it can cause permanent blindness.

A very important thing to know about sodium hydroxide is that you should never add water to it. Instead, add the sodium hydroxide to water. The first few drops of water poured into sodium hydroxide become very hot and may boil, causing dangerous splashing. If sodium hydroxide is added to a container of water, instead of the other way around, it is much safer, because the first drops of sodium hydroxide are immediately diluted.  Use cold water to start with, because dissolving sodium hydroxide produces heat. Also, be sure to replace the cap on the bottle of sodium hydroxide immediately, to reduce the risk of spills. Do not use aluminum containers, because sodium hydroxide will react badly with aluminum.

When working with sodium hydroxide, it is important to wear safety glasses or goggles to prevent an accidental splash from endangering your sight. A plastic face mask, similar in shape to a welder's mask, will protect your whole face from burns. You should wear protective clothing (such as a lab coat plus an apron), which, if penetrated by a solution of caustic soda, should be immediately removed and the skin flushed with water. Water must be available at all times for pouring over any burns produced by caustic soda. You'll want to cover your hair to protect it from fumes, and wear proper gloves, not flimsy disposable gloves that tend to develop holes. Good ventilation is important, too, of course. It goes without saying that there must be no children in the area where sodium hydroxide is being used, because someone who foolishly eats sodium hydroxide will suffer severe and sometimes fatal damage to the esophagus, and it is important not to breathe dust from the powdered chemical.

The Materials Safety Data Sheet for each chemical, required by law in the US for potentially dangerous chemicals, gives more information on the precautions to be taken with sodium hydroxide. Here's a link to an MSDS for sodium hydroxide provided by the J.T.Baker chemical company.

Hydro-sulphite (sodium hydrosulfite or sodium dithionite)

Sodium hydrosulfite or sodium hydrosulphite is an older name of a chemical which chemists now prefer to name as sodium dithionite. It is a sulfur-containing reducing agent; sometimes another reducing chemical, thiourea dioxide, is used instead. These and other chemicals suitable for reducing vat dyes are listed on my page of "What chemicals can be used to remove dye?", because they will also remove the color from many dyes in other classes, such as direct dyes, acid dyes, or reactive dyes. (The most traditional methods for dyeing with indigo, a natural vat dye, do not use these chemicals, but instead a fermenting dyebath, in which microbes produce the chemicals that reduce the dye.)

Sodium hydrosulfite is not as hazardous as sodium hydroxide, but it is far from innocuous if appropriate precautions are not taken. It can trigger serious breathing difficulties, ranging, depending on the level of exposure, from triggering an asthma attack to potentially deadly pulmonary edema, in which fluid builds up in the lungs. It is important not to breathe this chemical. Sulfur-containing reducing agents such as sodium hydrosulfite and thiourea dioxide can produce gaseous sulfur dioxide, which is damaging to the lungs. The fire hazard from sodium dithionite can be serious; stored sodium hydrosulfite will ignite on contact with small amounts of moisture and air, so it is unsafe to store any significant quantity. Substituting thiourea dioxide is the simplest way to reduce this fire hazard; you can use one-fifth as much thiourea dioxide as you would use of sodium hydrosulfite. Here are links to MSDS pages for sodium hydrosulfite (=sodium dithionite) and for thiourea dioxide [PDF], which is also known as aminoiminomethanesulfinic acid or formamidinesulfinic acid.

Side effects of vat dyeing

An interesting interview with an American tie-dyer of 1970, before the fiber reactive dyes preferred by most American dyers became widely available, described the ill effects of working with sodium hydroxide, sodium hydrosulfite, and vat dyes without proper protection, fortunately without accidents resulting in serious burns:

It is to be hoped that the heavy chest congestion she reported does not indicate permanent cumulative damage to the lungs. You should do everything you can to avoid the development of this symptom, because in the long run it may be very dangerous. If you experience this lung congestion yourself after using vat dyes, you must improve your ventilation and use a cartridge respirator to protect your lungs against gases.

Vat Dyes

The dyes themselves are relatively non-toxic. The  original vat dye, indigo, has been approved in the US for use as a food additive, whether it is synthetic or natural in origin. It is always important to avoid breathing any dye powder, as this may result in the development of severe respiratory allergies. Wearing a dust mask while working with dye powder (before it is dissolved), wearing safety glasses and reliable gloves, avoiding and quickly cleaning up any spills, and practicing good cleanliness should be sufficient for safety when working with vat dyes. The main concern is not the dyes themselves, but the caustic soda and sodium hydrosulfite that are used with them. Here is a page from PRO Chemical & Dye that has links to a great many different MSDS pages; scroll down to the section on the vat dyes they carry.

If you have other questions about how to apply these safety precautions with the resources you have available, please write again.

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