In a method of applying a permanent form or set to, or removing stress from fibrous substance containing keratin such as wool or hair, the substance is treated with a reducing agent at such pH, concentration and temperature as to effect relaxation of the fibre structure by disruption of the constituent disulphide or cysteric bonds of keratin, and thereafter permanence is given to the desired form of the substance by treating it in the released condition while maintaining the desired form, with a solution, e.g. a 5 per cent solution of a polyvalent metal salt or compound or with an organic compound containing two or more reactive halogen atoms such as phosgene whereby sulphur bonds containing a metal or organic grouping are formed in or between the fibre molecules.  Released fibres may be deformed without the development of significant internal stress and offer little or no resistance to deformation.  Suitable reducing agents are sodium, ammonium or potassium bisulphite, sodium sulphide, sodium hydrosulphide, titanous chloride, cysteine hydrochloride, sodium hypophosphite, ammonium sodium or potassium sulphite, sodium thiosulphate, sulphurous acid, sodium hydrosulphite, sodium dithionate or potassium metabisulphite.  The reducing agent may be formed in situ, e.g. by the action of alkali, e.g. baryta on the fibre when hydrosulphide is formed.  Reducing agents other than those yielding SO3" ions may be employed at normal, body or elevated temperatures, and the pH and concentration may be adjusted to give the desired degree of relaxation.  Reducing agents yielding SO3" ions disrupt the disulphide bonds, but at certain ranges of pH and temperature, the disruption is immediately followed by formation of further bonds, so that the solution of itself yields a set which is permanent to hot water and no appreciable duration of the relaxed state is obtained. These conditions which do not occur below 50 DEG C. but which occur with their particular reducing agents at 50 DEG C. with a pH value of 6, over the range 50--100 DEG  C. at varying pH values, and always at boiling temperatures are to be avoided.  The best results are obtained according to the invention with the reducing solution at a pH of 4 or 10 or more.  Suitable polyvalent metal salts are those of calcium, barium, zinc, copper or nickel. They may be oxidizing agents, e.g. nitrates or mixed with oxidizing agents. If an alkaline reducing agent is employed, the salt solution is desirably acid.  In an example, human hair after the usual shampoo is treated with a 5--20 per cent sodium bisulphite solution at pH4 and containing 10 per cent alcohol, for about 15 minutes at 40 DEG C. The hair may be wound on a curler before or after treatment with the solution which may be circulated in a tube surrounding the curler.  The hair is then treated with a 5 per cent copper sulphate solution.  In a modification, a half-molar solution of sodium sulphite brought to pH11 by addition of sodium carbonate or metasilicate, or a 6 per cent solution of sodium hydrosulphide at pH10 is employed for about 15 minutes at 30 DEG  C.  The set is made p  permanent by use of a fourth normal zinc sulphate solution containing an oxidizing agent and preferably acidified.  An acid reducing agent used at pH4 may contain an agent to reduce ionization and aid swelling, e.g. 45 per cent of ethyl alcohol. The reducing agent may be employed with a thickener such as agar-agar or kieselguhr in the form of a paste, and the hair formed into waves or curls on the head.  Wetting agents may also be added.  The reducing agent may be removed before the final treatment by washing or in the case of bisulphite by treatment with an aldehyde or ketone solution. This is particularly necessary if, e.g. a sulphide is used as a reducing agent followed by the salt of a metal which forms insoluble coloured sulphides. The invention may be applied to the usual crabbing or blowing operations on woollen fabrics or to the removal of stress in knitted fabrics or to the manufacture of yarns for curl fabrics, artificial astrakhan or persian lamb fabrics.  The treatment increases the resistance of keratin containing substance to alkali so that they may be treated, e.g. with vat or sulphur dyes without deterioration and also serves to mordant them for subsequent dyeing operations.