Irish glass is generally very heavy, though there are exceptions to
this rule, markedly in the blown specimens from about 1735 to 1750,
which are extremely light. These pieces were never cut, but either
engraved only or left perfectly plain. They can be distinguished
from foreign pieces of similar weight, as they never show the little
specks of sand in the metal peculiar to the latter. On the other hand,
air-bubbles often appear in the Irish glass, which were caused by the
faulty stirring of the molten metal. These are sometimes so minute that
they appear like sand specks to the naked eye, and it is only possible
definitely to identify them as bubbles by the use of a magnifying glass.