Dip or immersion dyeing. Used for dyeing of tanmono (bolts) and skeins (threads). Mainly used for dyeing the plain base color of fabrics (i.e. iromuji・色無地・plain color). The process is also used for color corrections and re-coloring old fabrics.
The process consists of dipping/immersing the material to be dyed in a large tub or vat containing the dye. Depending on the type of dye the vat can be heated. Additional process steps might include setting the color after dyeing and drying the material.
Shinzen used to be the main process for dyeing but with the introduction of decoration techniques that require more sensitive handling, like yuzen, some of the base color dyeing has been replaced by hikizome.
Nowadays the process often involves a machine where the material is draped over a rotating barrel that immerses the material in a heated vat containing the dye solution with each rotation. The color can be adjusted during the process by increasing the immersion time. Some care needs to be taken to make sure all material is evenly exposed.
The same immersion machine that is used for dyeing can also be used to remove the color using chemicals (like sodium hydrosulfite) before applying a new color.