Glass threads are generally different than threads cut from non-glass materials, such as metal and plastic. Most commonly, we purchase glass threads from several manufacturers who offer them in various sizes and designs. These are typically produced by molding or tooling borosilcate glass tubing, and require a specialized setup due to the imperative of redistributing the glass, as opposed to cutting to remove material as is the norm for other materials. Glass tubing can also be cut by machining with diamond tooling, though custom threads in small quantities will be more expensive. This means a more limited variety of threads are available, but are more cost-effective as solutions for porting glassware.
Some glass threads are externally threaded and have corresponding threaded plastic caps, both open and closed. Some are internally threaded glass, historically called Ace threads, that mate with plastic bushings to hold tubes or plugs, and can connect with solid plastic bushings that can be machined for more options. See many examples of glass threads in these images.
These stock components, in conjunction with their various non-glass mating components, can be used to make a tight connection to a straight tube, seal with an o-ring, hold a silicone/Teflon septum for needle piercing, or be used to provide an interface with other materials. You can see a chart of the sizes of GL threads, a page of options for GL14 threads, and a comparison of the internal and external threads. Click the images to see more of these plastic fittings.
Shown below is an example of a machined glass 1/4-28 male thread, which is a standard size for connecting to assorted small tubing and valves via plastic and Teflon fittings.
Another example of a 1/4"-28 machined glass thread,
connecting to standard small diameter tubing