How were fabrics dyed?

This is what Celtic dyers’ work might have looked like. Wool was dyed using plant, animal, and mineral dyes. Among the most commonly used plants were woad, which provided blue color, and madder, which gave red shades. Yellow tones were obtained from herbs such as tansy or onion skins. The process began with the preparation of wool, which was first thoroughly cleaned and then mordanted to better accept the dye. Mordants, such as alum or ash, increased the permanence and intensity of colors. Dyeing took place in hot water, where the wool along with natural dyes was soaked for a long time until the color absorbed into the fibers. This process was time-consuming, and colors often had symbolic significance, for example, blue and red were considered a sign of prestige.