Hematoxylin is a naturally occurring stain extracted from the logwood tree. For use in staining, the hematoxylin is oxidized to hematein which combines with the trivalent mordant and is the active staining ingredient. The usual mordants are aluminum salts, which give a bluer stain, and iron salts, which give a blacker stain. The counterstain is usually eosin Y or eosin and phloxine. The latter gives a sharper staining then eosin alone. With Harris hematoxylin the oxidizer is mercuric oxide and the mordant is aluminum sulfate. Harris hematoxylin, catalog # 617A, is frequently used in histology and is usually used regressively, i.e. the slide is over stained and then decolorized. This item is manufactured in the United States by Medical Chemical Corp. Medical Chemical corp. also makes a mercury free Harris hematoxylin, catalog # 618A.