Soluble dyes for wool
comprise acid, mordant, and metal-complex dyes. Acid dyes are classified broadly into the following three groups, according to their dyeing and wet fastness properties, though many dyes have intermediate properties:

• Leveling or equalizing acid dyes, which require a strong acid (usually H2SO4 is used) to give good exhaustion of the bath. They have poor fastness to washing, milling, and other wet processes, but good leveling properties, i.e. the dye distributes itself evenly throughout the fibres on continued boiling.

• Milling acid dyes,
which are dyed neutral or from a weakly acid bath (acetic acid is usually used). They have good fastness to washing, etc, but they usually have poor leveling properties.

• Super Milling acid dyes,
which are dyed from a weakly alkaline bath (ammonium sulphate is usually used). They have very good fastness to washing, etc, but they have very poor leveling properties.

All acid dyes are applied at the boil. Glauber salt is used to retard dyeing and thus to assist in obtaining level dyeing.

Procedure:

M: L 1:30
Dye bath additives for leveling type
10% Glauber salt + 3% H2SO4
+ dye solution

Dye bath additives for Milling type

10% Glauber salt + 1% CH3COOH
+ dye solution

Dye bath additives for Super Milling type
3% leveling agent + 3% CH3
COONH4 Solution

Adjust dye bath for 1% shade and 1:50 MLR. Start dyeing at room temperature. Raise temperature to boil. Keep at boil for one hour. Finally squeeze the hanks, rinse with cold water, squeeze and dry.