Snake-thread decoration:

Glass objects were sometimes decorated with trails of
molten glass. These trails were applied in curved, snakelike designs. They were then worked with tools to produce notches or crisscross patterns.

Later trailed vessels included objects that were decorated before they were blown to their full size and shape. The trails were dragged up and down with a tool. The objects were then rolled on a smooth surface until the trails were flat.

#1 Far left: Dropper Flask, 3rd century A.D.
#2 Dropper Flask Shaped like Helmet, 3rd century A.D.
#3 Dropper Flask, 3rd century A.D.
#4 Goblet, 2nd-3rd century A.D.

LARGER SIZE beneath top photo