What is the purpose of "batching" of procion dyes? Is it for heat setting? or just to intensify the dyes color (uptake)? Am I mistaken to think that it is only the soda ash that fixes the dye to/in the fiber? or does it take some heat to do so?


The "batching" is just the time allowed for the dye to react. A fiber reactive dye actually reacts chemically with the fiber to become one molecule. If you don't allow enough time, not all of the dye molecules will become connected to the fiber. All of the unreacted dye will just wash out, because it is not attached to the fiber. Only the dye molecules that have reacted with the fiber will stay.

The reaction will not proceed without both a high pH and a certain amount of warmth, plus at least a little moisture. The soda ash provides the high pH; almost any chemical that raises the pH enough will do the same job, but soda ash is the safest and easiest to use.  

The reaction proceeds more quickly at higher temperatures. If more warmth is provided, less time is needed. Overnight at 70°F works, for Procion MX dyes, or a few hours at 90°F, or a few minutes at 120°F. Temperatures below 70°F are just too low and won't work very well no matter how long you leave it. Other types of fiber reactive dyes require a little more warmth.

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