A study has been made of the equilibrium with, and rate of diffusion into, wool keratin fibres of (i) hydrochloric acid and (ii) the free acid of Orange I1 @-naphthol-azo-p-benzene sulphonic acid). The effect of temperatures and concentration has been examined and the results analyzed from a thermodynamic viewpoint. The entropy term TAS was found to provide the major contribution both to the equilibrium free energy AG" of desorption and to the activation free energy AG* of diffusion at the temperatures (60"-100" C ) which are usual for the acid dyeing of wool, These phenomena can be attributed to the effect of the solvent (water) whose negative affinity for the dye accounts for the fibre having a large positive affinity, and whose hydration of the polar groups adds to the barrier to diffusion.

Textiles can be dyed using appreciably less water than is normal in conventional dyeing if satisfactory distribution of the dye‐liquor can be obtained, e.g. by padding. Padding techniques are, however, unsuitable for many types of material, because of their construction or the economics of the process. However, by converting small volumes of water into large volumes of micro‐foam in the body of the substrate, by the addition of a foaming agent to the dye‐liquor, even distribution of the liquor can be obtained in a relatively short time. Subsequent heating fixes the dye to give level well‐penetrated dyeings. The principle is also applicable to preparatory and finishing processes. Because of the efficiency of the distribution system, dyes can be applied by this technique (the Sancowad process) which, because of their properties, cannot be applied evenly in long liquors. The method has proved satisfactory on garments, half‐hose, and short lengths of fabric in bulk application with loads up to 50 kg.

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