Since it was introduced, the flash‐ageing process has been modified by the introduction of acetaldehyde sulphoxylate as the reducing agent, to overcome the difficulties of handling and of plant design associated with the low stability of sodium hydrosulphite. The universally used reducing agent for vat printing is sodium sulphoxylate formaldehyde, but this is insufficiently reactive for use in the flash‐ageing process as originally operated. It has now been shown that, by the incorporation of a suitable reduction catalyst into the print paste, sodium sulphoxylate formaldehyde can be sufficiently activated to render it suitable for use in flash ageing. This paper reviews the steps in the investigation leading to the use of such a catalyst. If the sodium sulphoxylate formaldehyde is incorporated in the vat print paste and the catalyst in a caustic alkaline padding liquor, vat‐dye discharge styles can be produced by flash ageing. This process can be extended to the fixation of a reactive dye background in the same flash‐ageing process as the fixation of the vat illuminating pigments.

The evolution of pad–steam processes for fixing printed vat dyes on cellulosic fabrics is traced. The behaviour of a wide range of vat dyes under laboratory “flash‐ageing” conditions with sodium dithionite (hydrosulphite)–caustic soda as the reducing system indicated the most satisfactory thickening agent, dyes, and padding and steaming conditions, and led to the design of a bulk‐scale steamer operating at about 7 yd./min. with a steaming time of about 20 sec. The percentage fixation of a selected range of dyes on viscose rayon and on cotton under flash‐ageing conditions compares favourably with that obtained by the “all‐in” sulphoxylate–carbonate printing process, where the steaming time is sixty times as long. Many vat dyes show improved brightness and/or colour value when steamed for short times, owing to the absence of leuco decomposition or over‐reduction. Such a flash‐ageing process is ideally suited to screen printers and to those machine printers of all‐vat styles who wish to augment their steaming capacity with little capital expenditure. A modified process is being worked out with thiourea dioxide as reducing agent in the printing paste, development being by padding with caustic soda before steaming. This is particularly suitable for designs of low coverage, or where it is essential to process other types of dyes alongside vat dyes. Preliminary experiments indicate that emulsion thickenings are unlikely to show much technological advantage over conventional thickenings in the flash‐ageing process.

An experimental method for the determination of the Formosul concentration on a print, described in the paper, has been used to investigate the rate of decomposition of Formosul during the various stages of the vat printing process. The effects of the constituents of the print paste, drying and storage conditions, temperature, and conditions during steaming are examined.

The use of azoic dyes for dyeing synthetic fibres is reviewed, and the application techniques are compared with those employed for dyeing cellulosic fibres. New information is given on the development of azoic combinations on secondary cellulose acetate and Terylene.