Full information for using colour modifiers is given in most of my books. For details of my latest book, “Colours from Nature”, see under “My Books” on this blog. Some information on modifiers is also given here in the entry “Colours from Dahlias”.
Basically, colour modifiers are used after the initial dyeing process to alter or modify the shades. So if you dye 5 skeins in the same dyebath, leave 1 skein unmodified, then apply a different modifier to the remaining 4, you will end up with 5 differently coloured skeins that all tone together. Modifiers may be pH alterants: acids like clear vinegar, citric acid, lemon juice; or alkalis like washing soda, soda ash or wood ash water. (See below) The other modifiers are solutions of iron or copper. A small quantity of the modifier is added either to a pot of water or to some of the used dyebath. The dyed materials are then added to these solutions (1 to each) & left to soak until a colour variation is achieved. If nothing much seems to be happening, just add some more modifier & continue with the soaking. Sometimes the colour variations are dramatic, sometimes they are more subtle & on occasions it’s difficult to notice much change at all, especially with acidic modifiers. In my experience, the most useful modifiers are the alkalis & iron. You can also heat all the modifier solutions EXCEPT THE ALKALINE MODIFIER. If you heat this, you may damage or even destroy animal fibres such as wool. BE WARNED!
NOTE: Wood ash water is made by removing the ashes from a wood-burning stove & soaking them in water for a week or two. The ashes sink to the bottom, leaving a yellow liquid which is strongly alkaline. You can tell when it’s ready because it will feel slick or slimey to the touch. Just remove the solution without disturbing the ashes & use half to one cupful as your alkaline modifier.