Bubbles

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Bubbles
Gaseous inclusions in the glass melt which are removed by refining (see "fining"). Fining agents are introduced to encourage the formation of larger bubbles which rise more rapidly to the surface of the melt, attracting smaller bubbles on their way.
Larger bubbles which are not removed by fining are known as "blisters", smaller ones as "seeds" and longitudinally stretched bubbles as "air-lines". Bubbles in glass are generally considered as defects but may also be intentionally created and used as a form of decoration (see "air twist").