Hematoxylin is the most widely used natural dye. However, it has little natural affinity for tissues, and needs to be oxidized to encourage tissue staining. The oxidation product of hematoxylin is hematein. This conversion process is called "ripening" and can occur naturally or chemically.
The natural oxidation process takes place upon exposure to air and light and can take as long as 3-4 months before ripening is complete. However, the end product retains its staining ability for a long period of time.
Chemical oxidizing agents convert the hematoxylin to hematein almost instantaneously. However, the resultant solution has a shorter shelf-life.