Here's a Knowles cable with swirled surface etching. The shiny areas are where the "good" glass is showing through. The swirls are most likely surface scum from the soda in the glass mix. Glass with too much (unmixed, floating) soda has very poor resistance to chemical and water attack, and etches easily.

Most glass furnaces made in the last 100 years or so have a outlet near the bottom of the tank, so the only glass that flows out is of a nice clean quality. A dam attached to the ceiling keeps the surface glass from flowing down to forming equipment. If more glass is leaving the lower opening than new ingredients added to the top end, then level in the tank lowers to the level of the dam, then floating surface material can enter the forming equipment.

If I remember correctly, it was Robert Good who designed the first furnace of this type, now the only type used by "big" glass.