What is the bubble on the bottom of my Stagg Double Wall Tasting Glasses/Carafe? What is the bubble on the bottom of my Stagg Double Wall Tasting Glasses/Carafe?

What is the bubble on the bottom of my Stagg Double Wall Tasting Glasses/Carafe?

Each piece of our glassware is individually hand-blown. This process requires using a tube to inflate molten glass. The bubble marks the spot where the tube was inserted and later plugged to avoid moisture becoming trapped between the two walls.

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