No matter what type of collectible is your specific area of interest, you are very likely going to see a great deal of glass during your quest to build your collection or find items for resale. Understanding glass terminology can help you avoid expensive pitfalls along the way.
Here is a chart that illustrates some examples of the glass that Iíll be describing:
Stained vs. Flashed:
These two terms are often used interchangeably, but there is a distinct difference between the two. The term flashed refers to a very thin layer of colored glass applied over the top of another color glass, most often clear (crystal) and usually with a cut decoration exposing the color underneath. This is a highly skilled labor-intensive process, almost exclusive to blown glass, and therefore expensive.
Stained glass, as the term applies to collectible glassware, is glass that has a transparent colored coating painted on, usually over just a portion of the piece to highlight pattern detail. It is then fired to make the colored coating semi-permanent. (Stained glass windows are an entirely separate subject.)
Iridescent vs. Opalescent:
Since these terms sound somewhat similar, they are often confused. Opalescent glass is transparent, colored or clear, with portions that are milky and translucent. This is accomplished by adding heat sensitive chemicals to the glass then reheating after the piece has been pressed. The portions closest to the heat turn milky first.
Iridescent glass has a surface coating that usually glossy and seems to change color depending on the viewing angle. The coating is a produced by spraying a solution of mineral salts onto the piece and reheating. All Carnival Glass is iridescent, but not all iridescent glass is Carnival glass.
Stretch vs. Swung:
The confusion over these two terms is very understandable, since swung items look as if they have been stretched, which they have. Most commonly used on vases, a swung item has literally been held by the base after being pressed, using a special handlers tool, and swung in a circle while the glass is still pliable, elongating the piece.
Stretch Glass is another form of iridized glass that differs from Carnival Glass in a couple of key ways. The first difference is the complexity of the patterns. Most Carnival Glass items tend to have fairly prominent pattern details, while Stretch Glass items are usually quite plain. The second and most important difference in the point in the process where the mineral salts is applied. Carnival Glass is pressed, then shaped (crimps, ruffles, flared edges etc. added), and lastly the mineral salts are applied and the piece reheated. Stretch Glass has the mineral salts applied before the piece is shaped. This causes the iridized coating to stretch, giving it an onionskin appearance.
Etching vs. Cutting:
Many people refer to both of these decoration methods as cuttings. Cuttings are applied to the piece by a mechanical means, most often a cutting wheel. Etchings are usually much more elaborate than cuttings and are produced by putting a protective coating on the piece, leaving some areas exposed. Hydrofluoric acid is used to finely pit the surface of the exposed areas, leaving a frosted design on the glass.
This is probably the most misused glass term, often incorrectly used to describe any piece of colored pressed glass. Only massed-produced machine-pressed glassware made during the Great Depression of the 1930s, give or take a year or two, can be correctly called Depression Glass. Not all glass made during 1930s can correctly be called Depression Glass. Cambridge, Fostoria, Imperial, Fenton, and many other U.S. companies continued to produce quality hand-pressed glass during this period.
The usage of this term varies regionally, and not everyone is going to agree with me, but the definition I've used below is what I've found most commonly used by Vaseline Glass collectors. Many people call any glass that glows under UV (black) light Vaseline Glass, but that isn't entirely correct. To truly be Vaseline Glass, it must glow green under UV light, and it must also be a yellow to yellow-green color under normal light.
The name comes from the color of the petroleum jelly product, the older formula, not the whitish product you can buy today. Depression-era green glass quite often will also glow under UV light, but this may or may not be due to the presence of uranium oxide in combination with other coloring elements. All Vaseline Glass glows, but not all glass that glows is Vaseline Glass.
As you may have noticed when viewing some of the photo examples, the terms I've tried to clarify are not mutually exclusive. A piece of Vaseline glass can also be, and quite often is, opalescent. Stretch glass is also iridescent. Flashed glass can have, and usually does have, a cutting.