Aluminium potassium sulphate (or 'alum') is a mordant used to treat paper prior to marbling. This ingredient is key for making the paint stick!
Mix 10g alum with 120 ml freshly boiled water and use a paintbrush or sponge to coat the paper you wish to marble. Apply 15-20 minutes before marbling for best results. You don't need your paper to be completely dry to marble it.
