BLOCK (AKA) jef by Jefferson Glass Company (Steubenville-OH/Follansbee-WV, Co. Operated: 1900-1930), c1902 Introduced
Bowls including rare cobalt opalescent. Also found with frit decoration.
Reference: Heacock/Elmore. Opalescent Glass from A to Z page 32
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Shape:
Contributor:

Shape: Novelty Worked
Contributor: Gene Serbus

Shape: Vases, Rose Bowls, Ferners
Contributor: Joann Spears notte strong frit in view

Shape: Bowls & Berry Sets
Contributor: Gene Serbus (1-3), Aaron Judd, Cathy M Cavalcante, Laura Thompson

Shape: Novelty Worked
Contributor: Jim Thielen (1-3), Gene Serbus

Shape: Novelty Worked
Contributor: Gene Serbus (1,3), Sandy Wilkin (2,4), Amy Lynn Vincent (5,6)

Shape: Catalogue/Ads
Contributor: Jim Thielen

Shape: Vases, Rose Bowls, Ferners
Contributor: Jason Burgess, Jim Masterson, Bruce Preston, Gene Serbus, Pam Wojcik, Nancy Dawson

Shape: Novelty Worked
Contributor: Gene Serbus, Bill Banks, Jim Masterson