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Removing Formalin Pigment After Tissue Fixation

Removing Formalin Pigment

After Tissue Fixation

Formalin pigment is a brown, granular, doubly refractile deposit seen both intracellularly and extracellularly in tissues which have been fixed with a simple formalin solution, such as formal-saline. In small amounts, formalin pigment does not interfere with slide examination, but large amounts can be distracting. Removal from sections prior to staining is quite easy, as outlined below.


  1. Bring sections to water via xylene and ethanol.
  2. Place into saturated picric acid in absolute ethanol for 1 hour.
  3. Optionally, treat with saturated aqueous lithium carbonate to remove picric acid discoloration.
  4. Wash well with water.
  5. Continue with the staining method.


  • Most formalin pigment will be removed fairly rapidly, and the time given should be adequate.
  • Heavy deposits may take longer. Sections may be left overnight if necessary.
  • Treatment with lithium carbonate is only required if picric acid discoloration interferes with staining.

Safety Note

Prior to handling any chemical, consult the Safety Data Sheet (SDS) for proper handling and safety precautions.


  1. Drury, R.A.B. and Wallington, E.A., (1980)
    Carleton’s histological technique Ed. 5
    Oxford University Press, Oxford, UK.
  2. Kiernan. J.A., (1999)
    Histological and histochemical methods: Theory and practice, Ed. 3
    Butterworth Heinemann, Oxford, UK.