Message: How long can you keep Procion MX dyes, after you mix them with water and urea?

Procion MX are the fastest-reacting of all of the fiber reactive dyes. This means that you can use them at room temperature, anything over 70°F, without having to apply additional heat, but it also means that they spoil more quickly.

As a general rule, you can store Procion MX dye mixtures at room temperature for a week or two without problems, as long as not even a tiny drop of soda ash has gotten into any of them. (They will spoil within an hour or two if you get soda ash in them, even if you just dip a fine paintbrush into the dye repeatedly to apply it to a dry soda-ash-soaked cloth.) Temperature has a big effect on how quickly the dyes will spoil. If you refrigerate your dye mixtures, they can stay good longer, up to a couple of months (and in some cases longer). The strength of the dye mixtures will gradually decline, for use on cotton, as the dye reacts with the water. You cannot tell whether the dye has spoiled by looking at it; before any large or important project, if your dye solutions are old, it's a good idea to do a small test to see if the dye will bond to cotton fabric.

The different Procion MX dyes have differing reactivities, so some spoil faster than others. Fuchsia, or red MX-8B, is the fastest-reacting of the dichlorotriazine dyes, and therefore it spoils the most quickly. Turquoise, or turquoise MX-G, is the slowest-to-react of these dyes, so it should last the longest. However, turquoise is a special case, in the the hue of the dye mixtures gradually becomes more blue and less green with time. This is because the copper phthalocyanine structures stack up together, forming larger dye particles that absorb light differently. See "A Beautiful Blue: Procion Turquoise MX-G", for more information about this phenomenon.

Even when Procion MX has completely spoiled for use on cotton, it can still be used as an acid dye. Acid dyes do not work on cotton or rayon, but they work well on silk, wool, and nylon. Instead of using soda ash as an auxiliary chemical, you would use an acid such as vinegar. See "Fiber reactive dyes on protein fibers".

If you detect the smell of ammonia from your dye mixtures, this means that the urea has begun breaking down. Because ammonia alters the pH of dye solutions, you should not use any dye mixtures after the urea has started to decompose to ammonia. At that point, they are no longer useful as reactive dyes or as acid dyes.

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