Blued Hairsprings

Reading between the lines a bit here. The balance wheel is mono-metallic, and the rim is seemingly not split at the balance arms as near as I can see. So the balance wheel is not designed for temperature compensation. That would indicate to me that, on a watch of the (apparent) vintage of this one, the hairspring is likely alloy, not steel. An alloy hairspring is usually not as affected by temperature change and the effects of magnetism as a steel hairspring would be. Alloy hairsprings are often made of a white alloy. I am assuming that the hairspring on this watch is alloy that has been “blued” for aesthetic reasons. Hamilton did this in the early days of the alloy Elinvar hairsprings which were white. They “blued” them because people preferred the look of the earlier blued steel hairsprings. This might answer your question. Does anybody really know?

Most if not all of the hairsprings on my Longines are blued.
But I too am curious to know if there were any functional reasons for blueing them— or whether it was pure aethetics. Blued MAINsprings are usually more brittle and fragile, according to my watchmaker at least.

When Hamilton introduced the early Elinvar hairsprings circa 1931, the hairsprings were the colour of the alloy- a dull grey colour. Customers (particularly retailers, watchmakers, and some customers) hated the look of them. Hamilton dyed the hairsprings to resemble the earlier blued steel hairsprings. By the time of the introduction of Elinvar Extra (circa 1941}, Hamilton discontinued the practice. About the tendencies of Longines and other makers, I don’t know.