Alum (potassium aluminum sulfate) is a soluble metal that is the most common mordant used for natural dyeing, as it produces the clearest, brightest colours. Used since before the 15th century as a mordant, and as early as 1500BC in Egypt as a water purifier, alum continues to have many uses, including in modern day baking powder.
Mordanting your fibres before dyeing is one of the most important steps in natural dyeing. Alum can be used at 10-20% wof.
More information can be found on mordanting in our FAQs.