Denim casts
To achieve the classic blue hue of denim, indigo dye is used. Originally this would be natural indigo, derived from the plant, however with synthetic indigo offering a more stable and uniform colour this has become the staple.
Chemist Adolf von Baeyer formulated the chemical structure of indigo in 1883 and it began commercial use in 1897. By 1914, synthetic indigo had almost replaced natural indigo due to it’s higher productivity and lower costs. Although synthesised indigo is produced from petroleum products, it is agreed that it is more sustainable than using natural indigo for mass denim production. This is due to the volume that would be needed to keep up with the demand, therefore taking up a considerable amount of the world’s arable land.
At first glace all raw denims can look very similar, with a deep blue shade. However denim varies in cast, typically red or green, which usually becomes more evident after washing and wear. Variances in shade can occur due to the dye recipe used, such as the concentration of dye, temperature, amount of reducing agent and pH level. It is also caused by special additives, such as sulphur, which are often added to tint the yarns creating different hues or denim casts.
Synthetic indigo typically gives a red cast, which appears as a slight purple tint. This is in comparison to natural indigo dye which usually has a distinctive green cast.
GREEN CAST
Green cast denim achieved by synthetic indigo dye, is created when the yarns have been dyed with green sulphur first before being dyed with indigo. This is known as a sulphur bottom and helps reduce the amount of indigo for darker tones. The effect of this dyeing becomes more apparent as the jeans are worn and the indigo dye fades to expose the green dye underneath.
Alternatively a sulphur top can be dyed, meaning the sulphur is added after the indigo dye. Again this will create a more green or grey denim cast which often gives a vintage appearance. In this case the denim becomes more blue after each wash because indigo is the base layer.
RED CAST
Red cast denim is denim that has only been dyed with indigo. Unworn red cast denim will have an almost purplish tint. As the jeans are worn, the indigo will fade to a brilliant blue hue.
SULPHUR
Sulphur is the next most popular dye stuff after indigo for use in denim. Using sulphur results in dull and deep shades, with a limited shade range of black, blue and khaki. The range of shade after washing is not as wide as created through indigo dyeing.
