Another area in a kraft mill which suffers from solubility problems is the evaporation of black liquor. Evaporator scaling problems are mostly due to the precipitation of sodium salts. The two important components are sodium sulphate and sodium carbonate; they precipitate together to form a range of double salts, including burkeite (2Na₂SO₄.Na₂CO₃). Sodium carbonate is another component of the scales formed. The solubility limit depends on the solids content, the total sodium content, the ratio of carbonate to sulphate, as well as the soluble organics and the operating temperature. Sodium carbonate content is influenced by the efficiency of causticizing, and the sodium sulphate content depends on the reduction efficiency in the recovery boiler and the sodium/sulphur make up. Other components such as Ca, Al, Si, fibre and soap also contribute to the fouling problem. For example, if the soap removal efficiency is poor, Ca scaling can be a problem.

Exessive scaling and frequent cleaning reduce evaporator capacity, and place a limit on the recovery boiler capacity. To prevent scaling, it is important to achieve a steady operation, to make sure that the evaporators always operate below the solubility limit of burkeite. To prevent precipitation on heat transfer surfaces, homogenous crystal nucleation in the bulk liquor is required.