Fenton issued Aqua Crest 3 times: 1940-42; 1948-1953 and 1963-64. The pieces are distinguishable by the color of the crest and the density of the milk glass itself. The pale blue crest shown above dates from the 1948-1953 time frame, and the milk glass reflects the early, almost translucent milk glass Fenton called opal. Hold a piece of the early glass to the light and you'll see the 'fire'.
A slightly darker shade of blue accounts for the original issue, and the 1963-64 issue was a brighter blue on a much brighter, whiter milk glass. Fenton changed their formula from 'opal' to 'milk' in 1958 - that is another way to date a crest piece with a long production period.
