Scale is formed by salts that have limited solubility but are not totally insoluble in boiler water. These salts reach the deposit site in a soluble form and precipitate when concentrated by evaporation. The precipitates formed usually have a fairly homogeneous composition and crystal structure.

High evaporation rates, due to high heat transfer rates, concentrate the remaining water in the area of evaporation. A number of different scale-forming compounds can precipitate from the concentrated water. The nature of the scale formed depends on the chemical composition of the concentrated water. Normal deposit constituents are calcium, magnesium, silica, aluminum, iron, and (in some cases) sodium.

The exact combinations in which they exist vary from boiler to boiler, and from location to location within a boiler. Scale may form as calcium silicate in one boiler and as sodium iron silicate in another.

Compared to some other precipitation reactions, such as the formation of calcium phosphate, the crystallization of scale is a slow process. As a result, the crystals formed are well defined, and a hard, dense, and highly insulating material is formed on the tube metal. Some forms of scale are so tenacious that they resist any type of removal-mechanical or chemical.

Sludge is the accumulation of solids that precipitate in the bulk boiler water or enter the boiler as suspended solids. Sludge deposits can be hard, dense, and tenacious. When exposed to high heat levels (e.g., when a boiler is drained hot), sludge deposits are often baked in place. Sludge deposits hardened in this way can be as troublesome as scale.

Once deposition starts, particles present in the circulating water can become bound to the deposit. Intraparticle binding does not need to occur between every particle in a deposit mass. Some nonbound particles can be captured in a network of bound particles.

Binding is often a function of surface charge and loss of water of hydration. Iron oxide, which exists in many hydrated and oxide forms, is particularly prone to bonding. Some silicates will do the same, and many oil contaminants are notorious deposit binders, due to polymerization and degradation reactions.

In addition to causing material damage by insulating the heat transfer path from the boiler flame to the water, deposits restrict boiler water circulation. They roughen the tube surface and increase the drag coefficient in the boiler circuit. Reduced circulation in a generating tube contributes to accelerated deposition, overheating, and premature steam-water separation.