I found more precise information in the book on the history of Dugan/Diamond glass started by William Heacock, and completed by James Measell and Berry Wiggins and after Heacock’s death ( All information from here on comes from it, hereby referred to as HMW ). 
Dugan glass begins its history as the Indiana Glass Company, operating from Indiana, Pennsylvania, from 1892 to 1896 ( HMW 7 ). 
In 1896 Harry Northwood - accompanied by Samuel Dugan Sr. and his three sons: Thomas E. A., Alfred and Samuel Jr. ( HMW 14 ) - takes over the company, which changes its name to Northwood.  From 1900 to1903 it is known as the National Glass company, under the direction of Harry Bastow and Thomas e. A. Dugan (HMW 29).  From 1904 to 1909, it is called the Dugan Glass Company, creating the "Diamond-D" mark for some of its pressed ware, most notably, Carnival glass ( HMW 45 ).  Finally, from 1913 to its demise in 1931, due to a plant fire, it is known as the Diamond Glass Company ( HMW 169 ).

Venetian vase                      01

Second, Dugan iridescent was produced in a deliberate effort to compete with foreign, imported glass, particularly Webb and Loetz ( HMW 52 ).  In a 1906 Butler Brothers catalogue, pieces denominated "Venetian" were described as "exact reproductions of the expensive imported vases" ( HMV 50 ) and in a Sommers catalogue from the same year as "perfect imitations of relics found in the buried cities of Italy" ( HMW 51 ).

Third, to judge from the advertising in trade catalogues for 1905 and 1906 (HMW 50-51), and later 1907 and 1908 catalogues (HMW 68), it was impossible to distinguish between "Japanese," "Pompeian" and Venetian," since the drawings did not allow for careful differentiation between lines.  At that point, I only had two pieces of identifiable Dugan iridescent glass, which did not resemble each other except in color - both were amethyst. They had (a) no pontils and (b) quite visible mold marks, both facts enough to dismiss any Loetz attribution.

A variant appears in a Butler Brother's catalogue from 1902, item 4, listed as a "6 in. Large Crackle Vase" (HMW 34).  Mine has two distinguishing features: (1) an optic pattern of "polka dots" or, as the dealer from whom I bought it called them, "thumb-prints"; and an outside decoration of white glass in the manner of Northwood’s"Granite Ware", first introduced in 1893, and characterized by "Opal" (white) frit (HMW 35).
The "crackle" effect really refers to an optic pattern: "After the glass is gathered on the blowpipe and made smooth on a marver, it is rolled in frit ( the term frit refers to powdered glass ) and expanded by blowing into a spot mould which imparts narrow vertical ribs. Still on the blowpipe, the glass is then blown to its final shape in another mould. The ribs imparted earlier by the spot mould become an internal ‘optic’ pattern, and the frit is now part of the surface of the glass, often appearing thinner where the final shape is its greater diameter" ( HMW 34-35 ).

The optic vertical ribbing and the "opal frit" (white glass specks) appear in the Dugan iridescent lines, but so does the "polka dot" optic pattern, covered by amber/gold frit, and called "stars" by dealers. The honeycomb rose bowl ( Photo 3 - see also HMW 94:389 ) may be the ‘iridescent antique green’ mentioned in Dugan’s notebook and dated January 4, 1903" ( 94 ).  Mine has surface golden frit bands ( looking like applied glitter) over the honeycomb pattern, creating, in effect, "vertical ribs."

Thus, in two pieces preceding Dugan’s iridescent lines, one can already find some of their most distinguishing features.  I have identified five separate variants of Dugan’s iridescent glass which, for the purposes of this article, I shall label V.1, V.2, etc.

Three vases variant 1a                                                                                          04

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