Name: Rain

Country: USA

Message: I started dyeing using food coloring because it was reasonably safe to have in the house with small kids. I got me a couple of small nosy ones. I have a friend who is also a professional fabric dyer, though, and we were having a debate of sorts. Her contention is that professional grade acid dyes will last longer than my food color based dyes, that they have a higher light- and color-fastness rating that will withstand more washings, etc. My contention is that food color evolved from the dye industry, that certain colors were rated as “safe” for use in food, but that those basic compounds, having originated in the dye industry, are still just as tough and long lasting.  I cannot find  the science to settle the argument as all my sources have done nothing to either confirm or deny it. Can you validate either position? Thanks in advance.

This is a good question, but I'm afraid your friend is right. Here's my explanation.

There are thousands of different acid dyes available. Some are better than others, in terms of washfastness or lightfastness. Only a very small number of acid dyes have passed safety tests for consumption as food; these are the food colorings. All of the synthetic food colorings are acid dyes; this is why food coloring performs so abysmally on cotton but works well on wool, like all acid dyes. Since so few of the acid dyes are safe to eat, you don't have a lot of choice as to what other properties a food dye may have, besides safety. You don't get to choose one that is particularly resistant to washing or light. 

There are only seven artificial food colorings allowed for use in the US; all other colors, unless marked as being "natural coloring", consist of some mixture of these seven. These are what you find in any color of Wilton cake colors, food colorings, Kool-aid drink mix, and other forms of food dye. For more information about these dyes, see "Using Food Coloring as a Textile Dye for Protein Fibers". For more information about their lightfastness ratings, see "Steaming Silk with Food Dyes".

For use in food, coloring does not have to be able to stay bright for years. Since food has a limited shelf-life, it's fine if the dye fades in brightness after only a few months, or in some cases after even just a few days in bright light. Washfastness is even less important for food dye than lightfastness. Foods that have been dyed don't have to withstand frequent laundering. Since the current selection of food dyes works well for food, there's no need for manufacturers to spend the money on expensive testing to find out whether other acid dyes, in particular dyes that are more washfast, are also safe to eat.

Dyeing wool and other protein fibers with food coloring is wonderful for use with children, and it's particularly handy when you want to use your good stainless steel cooking pots to dye in, without having to give up on using them for cooking food. The results of using food coloring as a textile dye for wool are surprisingly good. However, the food colorings we have available are not nearly as long-lasting and resistant to fading as the very best acid dyes. They cannot be washed in hot water, as most of the food dye will simply wash out when exposed to hot water.

In some cases, professional-grade acid dyes may not perform a lot better on textiles than food dyes. Not all of the professional acid dyes are particularly long lasting, even among the ones that are not safe to eat. Some, especially the fluorescent colors such as Rhodamine B or Flavine Yellow, fade rather quickly when exposed to light, as the energy that makes the dyes seem to 'pop' under ultraviolet light also makes the dye molecules fall apart quickly. Some of the WashFast Acid dyes, in spite of their name, are not all that washfast. However, many other dyes within the same line of dyes are much more long lasting, and definitely superior in permanency to food coloring. 

The very best performing of the textile dyes readily available to hand dyers of wool in the US are the Lanaset dyes. (See "Lanaset Dyes: A Range of Reactive and Acid Dyes for Protein Fibers".) None of the Lanaset dyes are safe to eat, but they are extremely washfast and very reasonably lightfast. You can even wash fabric or yarn that has been dyed with Lanaset dyes in hot water, without causing the dyes to fade. That's something you can never do if you're using food coloring.

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