Gill's hematoxylin is an alum hematoxylin. It may be used as a progressive or regressive stain and is available in different concentrations. Because it is made with water and ethylene glycol, auto-oxidation of the stain is typically prevented over months. This characteristic makes it more stable than Harris hematoxylin. However, the nature of Gills is such that extra-nuclear staining may occur. Mucin and even adhesives used on the slide may become heavily contaminated with Gills.