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), anti-perspirants, and to treat water supplies to “clarify”, by combining with dissolved tannins and precipitating them.
Always wear gloves, such as dishwashing gloves, when working with any mordant.
All mordants we supply and use are from naturally occurring minerals that come from the Earth. When the fabric biodegrades the minerals will return to the Earth from whence they came. There are no residuals left on the fabric as the minor amount of fixative is locked into the fibre.
For more information please see the article "The Myth of the Bad Mordant."