Abstract: According to this method, the adjuvant is delivered from a supply means (12) placed above the casting bowl (4), a target zone (Z) for the theoretical passage of this adjuvant is chosen, said zone lying downstream of the supply means (12), and the actual passage of the adjuvant in the target zone (Z) is detected optically, by means of at least one camera (14). At least one image of the target zone (Z) is taken by means of the or each camera (14), this image being taken with an exposure time short enough to identify the density of particles of this adjuvant that are present in the or each image, and a warning signal is generated if the measured particle density is below a predetermined value.