Fenton Art Glass, Hobnail Pattern, Identification & Value Guide
In Fenton Art Glass: Hobnail Pattern Identification and Value Guide by Margaret and Kenn Whitmyer, the authors note that after some testing, the Hobnail pattern was introduced in the general line in 1940. Colors included blue opalescent, french opalescent, green opalescent and cranberry. Other colors were added in the years that immediately followed.    There are more than 800 color photographs included in the Whitmyers' grand set, along with hundreds of current prices and an extensive index.