community of dyers

As far as the chemical reaction that bonds the dye molecules to the fibers, it makes no difference whatsoever when the fabric is introduced to the soda ash. The difference comes in the way you want your dyed fabric to look... pre-soaking fabric in soda ash will force the molecules to begin bonding immediately, which gives- in my experience- a softer patterning. Your final result can still be quite dramatic, but you won't get as many sharp, clean lines and edges if you pre-soak. Also remember that in fabric, water behaves as a resist so it will take more time for the dyes to wick through all the fibers.

With soda ash applied afterwards, you'll likely see stronger, crisper lines and patterning.

Either way, if allowed to batch in the proper temperature and with the proper timing, both methods will bond the dye to the fabric.

Judi Hurwitt
Approachable Art
http://approachable-art.blogspot.com