Why is the rim of the whiskey shot glass fire-finished?

Fire finishing refers to a heat-treatment method where the cut rim of the glass is lightly melted with high-temperature flames to round off and smooth sharp edges.

TOYO SASAKI Toyo Sasaki's whiskey shot glass is finished by smoothing the part where the lips touch and then baking it with fire^[1]. Fine clear soda-lime glass is used as the material for the glass^[2]. Generally, fine irregularities are likely to remain on the edge of glass cut by machine, but melting the surface evenly with flames rounds off the edges and creates a smooth texture. Due to the nature of whiskey shot glasses where lips are placed directly on the rim to drink, it serves to enhance the drinking experience by making the touch against the lips soft.