The table below lists the main classes of dyes used on wool and gives some of their properties. Within each dye class there are variations between individual dyes and there are therefore overlaps between some of the classes.

Wool Dye Classification

From a technical standpoint the better the level dyeing performance, the easier the dyes are to apply to wool. However, other properties, such as fastness, shade limitations and price will all influence the decision, when selecting dyes. There is a general trend in wool dyes for the wet fastness properties to increase as the migration (level dyeing) performance decreases.

The exception to this rule is chrome dyes. These dyes are very similar to level dyeing acid dyes and therefore readily give shade uniformity. However, they are then treated with chromium salts, which form large complex molecules with the applied dye inside the fibre. These large molecules are difficult to remove and therefore exhibit good wet fastness. A negative aspect to chrome dyes is that chromium is regarded as an environmentally unacceptable heavy metal and legislative restrictions on its discharge are restricting the use of these dyes. Additionally, the application process is very long and can lead to fibre damage, which adversely affects the efficiency of subsequent processing. However, because of their good economy and wet fastness they are still widely used for deep shades, such as black and navy.

Dyeing application methods obviously vary from shade to shade and between different substrates and dye classes. However, the general principles are the same and are illustrated in the figure below:

Dyeing is normally started in a “warm” dyebath at around 30-50°C and the chemicals (auxiliaries) required for dyeing are added. When the auxiliaries have been well circulated in the dyebath the dyes are added and they too are allowed to circulate fully. The dyebath temperature is then raised slowly, typically at 1°C per minute to around 95-100°C, which allows the dyes to be taken up uniformly by the fibre. Dyeing is continued at 95-100°C for 30-90 minutes, under which conditions the dyes penetrate the fibre. The dyeing cycle is completed by draining the dyebath and rinsing the material or by giving an after treatment to remove loose dye (particularly for machine washable materials).