As rare as hen's teeth, this 5 1/4" high amber Ohio flask has a pattern known to collectors as "popcorn."
Blowers got considerable mileage from an ancient tool — a small, cup-shaped mold surrounded by deep vertical grooves.
As the hot, smoothly shaped gather of glass was forced down into this mold, it became alternately thick and thin, in the simplest case leading to a bubble with vertical ribs that could be expanded and finished as a flask:
However things could get fancier. By grabbing the base of the patterned-but-unexpanded gather and twisting, the vertical ribs were swirled, and then preserved as the glass was expanded and finished as this bottle:
After ribbing and twisting, but before expanding, the patterned gather could be dipped again, resulting in what collectors call "broken swirl" or "basket weave", with vertical ribs made up of segments of the diagonal ribs created earlier by swirling:
Here the pattern is emphasized in a second layer of glass, softer than the inner "core" layer and therefore more strongly patterned by the mold.
The final step in making "popcorn" was to take an unexpanded basket-weave bubble and give it an extra twist, taking it half way back, so that the first and second (breaking) sets of ribs were now equally swirled, one left, one right:
Nobody knows why, but that extra half twist was rarely ever added, making this kind of popcorn a rarity indeed!