Why does the inside look like foam? Five small dark cups in pieces 1901–1950 are lined with a frothy, bubbled cream-to-brown coating that laps up and over their rims — in the macros it reads like sea foam caught mid-fizz, sometimes breaking into leopard-spotted patches. 1912 and 1919 show the spotting most clearly; 1909, 1920 and 1921 wear quieter versions of the same boil.
The term groups these cups by the bubbled, sea-foam look, not by a named material. Related-but-different looks stay separate: 1918’s frothy bottle neck is described in prose on its own page, and smooth glassy pooled looks carry the registry’s glassy-pools term instead.
See it on real pieces
1909, 1912, 1919, 1920, 1921 — or browse the froth-interior listing.
Each piece page links to its live listing on the Shopify store.