Blenko Glass Company was started in 1921 in West Virginia, as the Eureka Art Glass Company. By the mid-1900s, Blenko was one of the most prominent producers of artistic glassware in the country. Blenko glass stemware and tableware are all hand-blown, and are known for their vivid colors and clean, modern lines. The years from 1947 to 1974 are known as Blenko's "Historic Period"--the golden era of Blenko design--and pieces from this period are the most valued. If you find a piece of glass that you believe may be Blenko, do a quick screening for typical Blenko details before you start scouring the old Blenko catalogs to identify your piece.
Check for Blenko's signature mark or label. See the Signatures and Labels section of Blenkoarchive.org for examples of the various marks used for certain lines or periods. Note that only about 10 percent of Blenko glass pieces are marked, so an absence of a mark does not necessarily indicate inauthenticity.
Look for tooling marks, bubbles and striations in the glass: all signs that the glass has been hand-blown. Blenko glass is all hand-blown.
Verify the absence of mold lines. Blenko glass is blown into a wood mold, but then it is spun, which smooths out mold lines completely. One exception to this is the 384 double-spout water bottle, which was made from a metal mold and was very widely produced.
Check for a rough mark on the base of the glass. This the pontil mark, left from where a rod was affixed to the bottom of a piece of molten glass after the base has been shaped. The pontil mark on Blenko glass is usually unpolished, and may still feel rough, or even sharp.
Look for a thick rim. For the most part, Blenko glass rims are at least ¼ inch thick. This factor is not foolproof, but Blenko glass is seldom thinner than this.
Check the Colors section of Blenkoarchive.org and compare your piece to the color samples found there.
Check the transparency by holding the glass up to the light. Blenko glass is always transparent. It will never be opaque or even translucent. The only exception to this rule is the Rialto Specialty Line, which has a translucent white body with ruby elements. In some cases, a few of the yellow pieces will have some opaque areas, but this is a random artifact of glasswork, and not part of the design.
Look for a rounded and smooth rim. Blenko rims are fire-polished: the rims are reinserted briefly into the fire to smooth away shear marks or signs of tooling, so the rims will be smooth and rounded, though they may be a little uneven.
If your piece meets the above criteria, then you can try to authenticate it as Blenko by matching it with a Blenko design number in one of the company's annual catalogs. Check Blenkoarchive.org for catalog resources and some popular design numbers with photos.