Page 31 - Curriculum Visions Dynamic Book

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Making glassware on a core
Early glassmakers did not blow glass, so they either had to make a solid piece and then drill out the middle or form a piece of glass around a core.
The core was put on the end of a rod.
It was made of clay that was moulded to the desired shape. The core was then pushed into the molten glass. As the core was turned in the glass, it became coated with glass. The longer it was kept in the glass, the thicker the coating became.
Different coloured glasses could be wrapped around each other, often to make
a raised pattern of one coloured glass on the other. The glass could be worked while it was soft to produce patterns.
The glass was then cooled and the clay core scraped out, leaving a hollow vessel. Even though they were often very small bottles, they took a long time to make and so were used for oils and perfumes rather than to hold large amounts of liquids.