Originally Posted by katesnanna
I have found and taught my BFF that if the blighter keeps bleeding, line dry, iron with a hot iron, then dip again. May have to do it a couple of times but it works better than salt or vinegar.
I agree with you.
I'm a hand dyer and I always laugh a little when I see the recommendations to "set" dye by washing cotton fabric with salt or vinegar.
Salt is sometimes used in the dyeing process to chemically drive the fibre reactive dye molecules away from the water molecules (with which they will happily bond and thus be wasted) and towards the receptor sites on the cotton fibres. Once whichever chemical that creates an alkaline environment has been removed, adding salt is not going to get stray dye particles to do anything.
Plus, the amounts of salt usually recommended are pitifully inadequate even if the fabric were prepared to receive more dye molecules. Quantities required vary but are usually around one cup of salt per gallon of water. That would mean around 20 cups of salt for a washing machine load!
Vinegar won't work because cotton is not dyed with acid dyes. Being exposed to an acid dye may stain the fabric but it will not be truly dyed and so will never be truly wash- or lightfast. Acid dyes are used on protein fibres like wool and silk.
So why do some fabrics bleed?
Fibre reactive dyes have a limited effective life span once they have been activated by an alkaline environment. The lifespan is dependent upon temperature but as a general matter, the dye molecules have all bonded either to fabric or to the water in 24 hours or so. After that time, the dye molecules are no longer capable of bonding with anything.
They can, however, stain fibres. Staining happens when a dye molecule gets caught between cotton fibres. This differs from dyeing, where the dye molecules actually bond with receptor sites on the cotton fibres and become part of the fibre itself.
Depending on the physical properties of the cotton and the fabric (whether the cotton was mercerised, tightness of weave, etc), staining can be permanent. An example of a stain that is quite permanent is indigo. Indigo doesn't form a molecular bond with the cotton fibres, it just lodges in the tiny spaces between fibres.
There is a product called Retayne, which acts to chemically fix certain types of fibre reactive dyes. The decision whether or not to use Retayne should be made keeping the ultimate purpose of the fabric in mind, since Retayne does contain formaldehyde and that formaldehyde is permanently added to the fabric.
Everyone should make up their own minds as to what their own safety standards are because there is nothing in life that comes without risk. Personally, I would not use Retayne on any fabric used for a quilt for a baby or small child because they do tend to put things in their mouths... but that's just my opinion.
Chemically, turquoise and fuchsia are the fastest striking dyes. This means that they bond the fastest, whether it is to the receptor sites on the cotton fibres or to the water molecules in the dye bath. They are also both quite temperature sensitive and won't bond properly at temperatres below 70 degrees F. If the manufacturer did not get the dye solution alkaline enough or warm enough, turquoise and fuchsia dyes will show the deficiency first.