'scale-forming' is a compound of 'scale' + 'forming'. 'scale' in this sense comes from earlier English terms for a thin plate or crust, and 'forming' is the present participle of 'form'.

'scale' developed from Old English/Old Norse words for a shell or thin plate (e.g. Old English 'scealu' / Old Norse words) into Middle English 'scale' and then modern English 'scale'. 'form' comes from Latin 'formare' via Old French 'former' into Middle English 'formen', and the present participle became 'forming'. The compound 'scale-forming' is a modern English formation combining these elements.

Initially, 'scale' referred to a thin plate or shell-like layer and 'form' meant 'to give shape'; over time the compound came to mean 'causing the formation of mineral or crusty deposits' in technical contexts.