The effects and shapes achieved by winding, pressing,
and sagging of glass could be improved and
accelerated when the craftsmen employed a turning,
or potter’s wheel. By rotating the glass object on such
a wheel, an even distribution of the molten material
and a smooth surface were more readily obtained. It
has been suggested that the rotary scratches on
ancient glass vessels are indicative of a turning motion
employed during the working of hot glass. Previously
thought to be the product of grinding or polishing
these rotary scratches were probably introduced into
these glass vessels unintentionally and represent
production defects caused by irregularities in the
material of the mold used, regardless of whether the
technique employed was winding, pressing, or sagging
of the glass material.
The ribbed bowl, because it was manufactured on a
turning wheel, represents one of the most important
types of ancient glass pottery. To create such a vessel
a hot batch of glass was placed onto the top of a
convex half-mold in the shape of the interior of the
vessel. The mold was carefully centered upside down
on a turning wheel. Due to the force of gravity the
viscous glass coursed down the mold, forming a thick
walled vessel. As the wheel completed one rotation, a
cool tool was pressed to the glass in order to create
the rim. As the rim stiffened, the flow of the glass was
stopped. While being slowly turned on the wheel a
second time, the vessel was evidently tooled with a
lath pressed into the wall at intervals, thereby shaping
the ribs. This procedure required less than a minute
of labor, as experiments have demonstrated. It was
for this technical reason that such ribbed bowls must
have been hand-crafted in such exponentially large
numbers which explains the survival rate of some
many complete examples and thousands of preserved
fragments.
Stages (slideshow)
The making of a ribbed bowl.
1. A hot batch of glass placed onto the top of a convex
half-mold.
2-3. A glass bowl is made by sagging of a hot glass cake
or disk (2) or by natural flowing (3) or over a core
mold on a slowly rotating wheel.
4. The rim is pressed flat during one rotation of the
wheel.
5. During one more rotation of the wheel the
interstices are pressed, the ribs emerge from their
background.
