Weld is a natural dyestuff obtained from the plant Dyer’s Rocket. It is the oldest European dye plant and was commercially cultivated well into the twentieth century; it is particularly valued for the clear and intense yellow it produced when dyeing silk.
The pigment made from Weld is known as Arzica. To manufacture, the whole plant, owers, stems and all, are dried, broken up and stewed in weak solution of alum. This extracts the colour. Then a solution of Potash (Potassium Carbonate) is added and the insoluble lake pigment is formed. This is allowed to settle, washed, then nally ltered and dried.
Since Arzica is very transparent, it is an ideal pigment for glazing. Medieval color-makers considered weld lakes with high esteem but were often used in a more unobtrusive manner than being used straight. They would be mixed with a range of blues to give greens and duller yellow pigments, such as a yellow ochre, to add life.
Sometimes in the making of Arzica chalk, egg-shells or white lead were included to add opacity. This bright, solid yellow provided a good (and harmless) substitute for poisonous orpiment.
Unfortuntaely, Arzica, like most vegetable-based lakes, is fugitive and fades under direct sunlight.