Evaluation of soot particles from different-sized rapeseed oil flames: Scientific focus on a traditional Japanese craft, Nara sumi

• Nara sumi, a traditional Japanese ink made from soot, is used for art and calligraphy. • The artisans' rule of thumb in Nara sumi production is scientifically validated for the first time. • Fine soot particles can be produced from a small flame by using a small wick. • Soot transfer from the innermost to the middle flame zone is important for formation. Nara sumi is a traditional Japanese craft, the skills and wisdom of which have been handed down through generations by artisans. Soot is a principal material determining sumi ink quality. Hand-made soot from rapeseed oil (lampblack) is still used in traditional Nara sumi instead of mass-produced carbon black. However, currently only one manufacturer continues to produce lampblack, and the artisan's production rules of thumb have not been scientifically studied. This study aims to clarify the effect of flame size variation on soot particle size, the most important rule of thumb in Nara sumi production. High-quality soot purportedly has a smaller particle size and can be generated from a smaller flame. We used three wick sizes to generate different sized flames and measured the particle size distribution of generated soot. The resulting soot particle size was found to increase as the flame increased. To additionally identify the flame area contributing to soot formation, a 2D analysis was conducted on the morphology and chemical state of soot collected from various heights within a rapeseed oil flame. The results show that soot formation progress is limited to the innermost zone of the flame, and only surface oxidation occurs in the middle zone and beyond. Soot transfer from the innermost to the middle zone was considered to be important in soot formation, based on both the morphology and chemical state changing rapidly and greatly at their boundary. These findings can help to preserve the knowledge for the traditional craft and Japanese cultural heritage.