Two six inch plates with identical centers but different borders reveal a major innovation in early American pressing - the cap ring.
Early collectors of American glass were astonished at the sheer variety in pressed glass design. Close comparisson would show that two plates with radically different borders had identical centers. How could factories afford to create so many molds and, since it appeared that they did, why go to so much trouble to replicate the designs so precisely?
Jim Rose figured it out. As seen above, the patterned surface of the mold consisted of two parts: a round, central plunger that bore the majority of the design, and an outer ring carrying the shape and pattern of the border. The narrow octagonal border on the left bears scrolls while the round border bears a series of bullets. In each case the mostly flat border arrives at a wall above which starts the elaborate central design. The border and wall were created by the cap ring while the design of the center was at the base of the mold's plunger.
As seen below, molds with a cap ring (right) had several advantages over earlier molds consisting of only a plunger and receiver (left). Any variations in the quantity of glass dropped into an earlier mold were evident all the way to the edge of the plate; however, a cap ring moved variations in thickness from the edge and guaranteed borders of uniform thickness. Perhaps more importantly, with the complex border shape handled by the cap ring, an easily shaped round plunger could fit neatly into the round center of the cap ring, making mold manufacture considerably easier and simplifying the machine press operator's task of aligning the plunger to the rest of the mold.
While the cap ring may have overcome these technical challenges, its more important contribution was to increase variety, giving customers a choice not only of what pattern to buy but whether to buy it in round or octagonal.