The critical factor in the operation of pressure filters is the quality of feed. Dregs and overliming cause blinding of the white liquor filter media, requiring more frequent acid washes. Overliming happens when the molar ratio of CaO/Na₂CO₃ is larger than that required to reach the equilibrium causticity, for a given ionic strength of the liquor (CaO+ Na₂CO₃+H₂O?2NaOH+CaCO₃). However, in actual mill practice, problems associated with overliming tend to occur when the molar ratio is 3-5% below the equilibrium causticity. Improved green liquor clarification and good control at the slaker are required to prevent problems in filters.  Mills may increase the use of fresh lime due to poor control, or in order to increase causticizing efficiency. To improve causticizing efficiency without adding excess lime, advanced causticizing control or partial borate autocaustcizing is recommended.   Poor settling of lime mud is attributed to a surge of fresh lime added as make-up. The fresh lime make-up should be added on a steady basis, dependent on the losses experienced by the system. The quality of the purchased lime is also important. The slaking reaction (CaO+H₂OàCa(OH)₂) generates a lot of heat, and proceeds quickly with a maximum temperature rise in 2-8 minutes, depending on the type of reburned lime*.  Reburned lime with a longer slaking time always has better settling and filtering properties than fresh lime. Research has shown that reburned lime is an agglomeration of small CaO crystals, with large voids between the crystals. Since slaking and causticizing reactions take place on the surface of solid particles, the reaction rates depend on the available surface and are inversely related to the degree of agglomeration due to sintering. This indicates that producing a kiln product with a high degree of sintering is desirable for optimum results in causticizing.  * C.F. Cornell, Current recausticizing practice, Part 2-Principles, 1996 Tappi Kraft recovery short course