Mordants strengthen the positive ionic charge of the hematin. This aids the bonding of the hematin to the anionic tissue component, which is most commonly chromatin. The type of mordant also influences the final color of the stained components. Hematoxylins are typically classified by the mordant used in their composition. The most common mordant used in routine histology is aluminum ammonium sulfate (alum). With the use of an alum mordant, the nuclei is stained red in color, which is then changed to the more familiar blue color when the sample is later rinsed with a weakly basic solution.
