In a method of applying a permanent set to or removing internal stress from fibrous substances containing keratin, such as wool or hair, the substance is treated with a reducing agent at such a pH, in such concentration and at such temperature as to effect relaxation of the fibre structure by reduction and disruption of the constituent disulphide or cystine bond of keratin, and thereafter permanence is given to the desired form of the substance by causing the formation of further disulphide bonds in or between the fibre molecules by subjecting the fibrous substance in the relaxed condition to the action of an oxidizing agent while maintaining it in the desired form.  In the relaxed condition, the fibres may be deformed without the development of significant internal stress.  Suitable reducing agents are potassium, sodium or ammonium sulphite or bisulphite, sodium sulphide, sodium hydrosulphide, titanous chloride, cysteine hydrochloride, sodium hypophosphite, sodium thiosulphate, sulphurous acid, potassium metabisulphite, sodium hydrosulphite, or sodium dithionate.  The reducing agent may be formed in situ, e.g. by moistening the fibres with alkali such as baryta, hydrosulphide is formed. Reducing agents yielding SO3" ions, under certain conditions of pH and temperature, give rise of themselves to a set which is permanent to hot water, i.e. a relaxed structure of appreciable duration is not obtained.  Under these conditions therefore, viz. at boiling temperature or at temperatures of from 50--100 DEG  C. and at certain concentrations and pH values, reducing agents yielding SO"3 ions are unsuitable for use according to the invention.  They may be employed generally at temperatures below 50 DEG  C.  Before the oxidizing treatment excess reducing agent may be removed if desired, either by washing or in the case of certain reducing agents by treatment with an aldehyde or ketone solution.  The reduction treatment is particularly effective at pH4, i.e. in acid solution when the fibres are swollen, thus causing strain in the S--S links of the keratin molecule or at pH 10 or 11, i.e. in alkaline solution, since alkalies can attach strained or unstrained disulphide bonds yielding sulphydryl and sulphuric acid groups, the latter being then reduced to sulphydryl groups.  In examples of the invention as applied to the permanent waving of hair: (1) the hair after a preliminary shampoo is treated before or after winding on a curler with a half molar solution of sodium sulphite brought to pH11 by addition of sodium carbonate or metasilicate, or with a sodium hydrosulphide solution similarly brought to pH10.  The solution may be circulated about the hairwound on a curler.  After 15 minutes at room temperature or above a solution of an oxidizing agent, e.g. 10 volume hydrogen peroxide or 10 per cent ammonium persulphate is applied; (2) the hair is treated at pH4 with 5--20 per cent bisulphite or metabisulphite solution containing 45 per cent ethyl alcohol for about 15 minutes at room temperature or above.  It is then wound on a curler or formed into curls on the head, and its configuration made permanent by treatment with 10 volume hydrogen peroxide or 10 per cent alkali metal persulphate solution preceded if desired by treatment with an aldehyde or ketone solution, or with sodium carbonate solution or ammonia in solution or as a gas. The hair may be wound on a curler before treatment.  In a modification, the hair before or after treatment with the reducing agent may be formed into waves or curls on the head, and the reducing solution may be formed into a paste by addition of a thickener such as agar-agar or kieselguhr.  Suitable wetting agents may also be added.  Combing of the hair may be facilitated by use of a water soluble lubricant, e.g. sulphonated castor oil. The invention may be applied to fabrics containing keratin. Thus, the usual crabbing or blowing treatments may be replaced by treatment with cold or warm solutions of reducing agents, e.g. 5--20 per cent bisulphite or 0,6 per cent sodium sulphide, followed by treatment to remove the reducing agent, e.g. with aqueous aldehyde or ketone in the case of a bisulphite, or by washing, and then by treatment with an oxidizing agent, e.g. 10 volumes hydrogen peroxide.  Distortion in knitted fabrics of wool due to uneven stress may also be removed and yarns for the manufacture of curl fabrics or artificial astrakhan and persian lamb fabrics may have a permanent set imparted to them by the process of the invention.