Aluminum is not usually found free in tissues, and rarely needs to be demonstrated. Its most common histological use is in the form of "alum", i.e ammonium or potassium aluminum sulphate. These compounds are used in alum hematoxylins, or hemalums. The aluminum functions as the mordant, forming a lake with the dye, hematein. Hemalums stain nuclear chromatin blue, and are extensively used in the very common hematoxylin and eosin (H&E) staining method.
There are very many formulas for hemalums, most of which incorporate ammonium or potassium alum, reputedly used originally because they were a convenient source of relatively pure aluminum at the time these mixtures were introduced. Nowadays this is not an important consideration, since very pure chemicals are easily available. Even when this was a concern, some simple aluminum salts were also used similarly. Although there is no longer any reason to avoid simple aluminum salts, new formulas for hemalums usually still include the double sulphates as the source of the mordant.
Very strong alum hematoxylins can also stain acid mucins, and one such solution (Mayer's mucihematein) was specifically designed for that purpose, although not commonly used currently as there are superior methods readily available.