Weld is one of the three primary dyes in medieval ages (along with madder and woad). Also known as: Dyer's mignonette, dyer's weed.
In Rosetti (1548), weld is called "gualda". Woad, in this text, is "guado". Edelstein and Borghetty comment on Rosetti mixing them up in the text because they are so similar. Interestingly, I often slip up in talking about woad and weld, as they are so similar.
Edelstein and Borghetti translating Rosetti:
"70. To dye yellow
To mordant take 20 pounds of alum and 3 pounds of tartar. Take 100 pounds of weld and if you cannot have weld, take guilitia or cioretta. Under the weed put two fistfuls with both hands or two twin fistfuls of fustet and observe the rule above mentioned in the other recipes of dyeing and you have a yellow colour."
Guilitia and cioretta both refer to broom.
It is an incredibly bright yellow when mordanted with alum. Weld even dyes linens, although not as brightly as the protein fibres.
Weld is also the most light fast of the yellow dyes that I experimented on.
(silk, silk/wool, wool, wool, linen)
Weld with an iron mordant was far less satisfying- given to grey-greens.