Features of the pigment:
(1) brown or brownish-black
(2) birefringent (anisotropic)
Risk factors for formalin pigment formation:
(1) tissue rich in blood (spleen, thrombus, etc).
(2) prolonged fixation
(3) acidic pH
Ways to reduce formalin pigment:
(1) Use formalin that is well-buffered.
(2) Change the formalin fixative several times if the tissue contains a large amount of blood.
The best way to remove the pigment is to:
(1) Place a deparaffinized section in 70% ethyl alcohol.
(2) Place the section in 10% ammonium hydroxide made up in 70% ethyl alcohol for at least 5 minutes.
(3) Rinse in distilled water then stain.
Other ways to remove the pigment include:
(1) saturated alcoholic picric acid
(2) alcoholic solution of sodium or potassium hydroxide
