Why do fine bubbles or seam marks appear on the surface of glass cups?

The exterior is transparent, but thin lines and small bubbles are revealed when light shines through it. These are marks left from the molding process. During the making process of glassware such as Toyo Sasaki sake cups, shot glasses, seasoning jars, or plates, fine bubbles, joint marks, and small irregularities can occur^{[1][2][3][4][5]}. This is because when shaping glass by putting it into a mold, seams are left where the molds meet, and air that enters between the molten glass is trapped in the form of small bubbles as it solidifies.

It is not a defect^[6]. Because this is a phenomenon that naturally occurs in glass processing, the brand also does not judge this as a product defect^[7][8]. It is not eligible for exchange^[9]. Since it is classified as a normal process trace, you cannot request exchange or return processing on the grounds of fine marks or bubbles on the surface^[10]. As it is a normal product with no problem in use, it is good to know in advance before purchase the finishing characteristics that remain on each piece of glassware.