Removal of formalin pigment (formaldehyde haematin)
- Dewax sections, rinse in alcohol, rinse in water.
- Treat with saturated alcoholic picric acid for 20 minutes in a closed coplin jar.
- Wash well in tap water for 5 minutes.
- Continue with required staining technique (usually H&E).
Compare untreated section with treated section.
If the pigment was formaldehyde haematin it will have been removed.
NB: some other methods do not work after the application of picric acid - notably those for calcium which is also removed.
Treated and untreated controls should be used if there is any doubt about how a method will be affected.