Tissue swelling may occur. 5. Toxic and volatizes at room temperature producing harmful vapour. This vapour is toxic to the eye and respiratory tract. 6. Expensive.  Secondary fixation: Following fixation in formalin it is sometimes useful to submit the tissue to second fixative e.g. mercuric chloride for 4 hours. It provided firmer texture to the tissues and gives brilliance to the staining.  Fixation Artefact: Formalin pigment: Insoluble brownish-black granular retractile birefringent pigment due to reaction of formalin with hemoglobin derivatives. • Mercury pigments: Dark-brown irregular deposit. • Fuzzy staining: Due to improper fixation. • Prolonged fixation: Shrinkage of the tissue causes tissue separation and empty spaces.  Formalin Pigment • Colour: Brownish black. • Position: Extracellular granules . • Mechanism of formation: Formic acid reacts with hemoglobin derivatives of the blood and produces acid formaldehyde haematein. • How to avoid: Use buffered formalin. • How to remove: Treat with 1.8% picric acid in absolute ethyl alcohol for 15 min.