Daniel Oliver 
The specific temperature a steel part is tempered to corresponds to a specific balance of toughness and hardness the function of the part demands. In the early days of watchmaking, when precision thermometers were not available, finding out an accurate reading of the temperature was a difficult task. But that’s where color comes in.
As we temper carbon steel, an oxide layer will form on the surface of the steel. Depending on the temperature to which the steel is heated, the oxide layer will have a certain thickness that result in a certain color due to an phenomenon called thin film interference. Conveniently, the color, being directly linked to the tempering temperature, serves as a temperature indicator for watchmakers. The color blue shows that the steel part was heated to the right temperature and would result in the right physical properties. The oxide layer also helps with corrosion resistance. For this reason, tempering, a.k.a. thermal bluing, becomes an integral part of traditional watchmaking.
Bluing Nowadays
While steel bluing had important functional significance for watchmaking back in the day, it is more about aesthetics and the preservation of traditional craftsmanship today. For the same requirement of mechanical properties and corrosion resistance, modern watchmakers have far easier and effective solutions in alloys and electroplating. Consequently, thermal bluing is now largely relegated to a cosmetic role. And even for cosmetic purpose, there are more cost effective solution with paint. Manually flame blued hands and screws are a luxury in terms of time and manpower.
That doesn’t deter some luxury brands and independent watchmakers to insist on using manually flame blued parts in their watches. Properly blued steel parts are still very much desired as the blue color resulting from the beautiful reflection of the oxide layer possesses a sense of depth unrivaled by that of plating or painting. And the parts that are individually blued by hands show a subtle difference in hue as a result of the human variance. At a luxury price point, this is less perceived as a defect but rather a mark of the human craftsmanship that goes behind each part of the watch.
With flame bluing becoming more of a niche craft, it is good to remind ourselves about the lineage of craftsmanship that exists behind each blue screws, and appreciate the work and craftsmanship that went into it. It’s not just a color, it’s a craft.
