= pan scale n.

(as a mass noun) a hard whitish deposit that tends to accumulate on surfaces in contact with water, esp. heated water (as in kettles and pipes)…

The hard deposit or ‘fur’ which gathers in boilers and other vessels in which water is habitually heated. (Rarely plural.)

spec. A deposit of coke on the inner surface of a gas retort.

An incrustation formed in boilers or retorts.

The process of becoming furred or encrusted; the state of being furred; also, a coating of fur.

The accumulation of combustion waste, for example cinders or ash, in the tubes of a boiler, resulting in a loss of efficiency.