Until the beginning of the 18th century the glass bulbs on sand or hour glasses were seperate, ground true at the necks, puttied and bound with linen. From the mid 18th century the glass was blown as a single unit.
COMPARATIVE LITERATURE:
Edward H. Pinto, Treen and other Wooden Bygones, Bell & Hyman, London 1969. Section XIX, pages 279-80. A similar example is illustrated plate 300, fig. A.