Factory glass is a term used by collectors of art glass to distinguish relevant items from the more individual or unique studio glass and by studio glass artists to distinguish their work from the more standardised items which are generally made in larger glassworks. 
It is difficult to specify how large a glassworks would be before it is considered a factory but size is not the key indicator. The crucial distinction would be where there is a significant degree of specialisation or "division of labour" as opposed to the more hands-on working methods used by a single glass artist, with perhaps an assistant, in a studio.
Not all glass made in factories counts because more individual, limited edition or one-off pieces were made for a variety of reasons. Examples would include; experimental, "end-of-day" and apprentice pieces and "friggers" (test or trial pieces) but also special orders and one-off commissions.
Factory "Studio" Glass"
At Fenton, Dave Fetty, a factory glass worker for most of his career, was allowed to use specialist skills, learned before joining the factory, in limited editions of "offhand" pieces without the use of moulds. Such pieces are more akin to Studio glass than "Factory Glass".
In the United Kingdom, Whitefriars and Caithness, have produced limited production "studio" lines alongside standard production in the same factory. At the ZBS glassworks in Czechoslovakia, there was a separate department for making more experimental studio type pieces which were later hived off and came to be called Libera. The small Skrdlovice works was used by designers in the communist era to test out their designs with short runs before some went on to full production at other factories. The limited test items are much more prized by collectors but it's often difficult to be sure which ones these are and which ones were replicated in full production runs elsewhere.
While, the exceptions would generally account for a very small proportion of overall production, the term applies, strictly speaking, only to the production which was standardised, where many workers would be involved in the making of each item.
Other Types of Glass made in Factories
The many other types of glass which are generally made in factories are usually referred to by their individual names; for example; Float glass (for windows) and "Glass Packaging" (bottles, jars, and containers) and domestic Glassware.
|Look up factory glass in Wiktionary, the free dictionary.|
- "20th Century Factory Glass" by Lesley Jackson
- "Fenton Art Glass Presents The Artistry of Dave Fetty" http://www.fentonartglass.com/aboutus/FettyBio.asp