Alum is used at 25% by weight of fabric, or use less if you are dyeing a light tone. For silk dyeing, it is possible to use hot tap water and not have to heat the mordant bath. Just dissolve the alum well in hot water from the tap, add the fabric and let it sit overnight, with occasional stirring.
Food grade Alum. Alum is commonly used in baking powder, pickle juice (keeps them crisp), antiperspirants, and to treat water supplies to “clarify”, by combining with dissolved tannins and precipitating them.
Always wear gloves, such as dish washing gloves, when working with any mordant.