Most organic glassware uses "ground glass joints," which have a frosted appearance. They are precisely ground to a certain size (which makes them expensive) and have outer (female) and inner (male) joints so that pieces can be connected together with a tight fit (Figure 1.1a). Common joint sizes are 14/20, 19/22, and 24/40. The first number refers to the inner diameter (in millimeters) of a female joint or outer diameter of a male joint. The second number refers to the length of the joint (Figure 1.1b).
It is best if ground glass joints are free of chemicals when pieces are connected, or else the compounds may undergo reactions that cause the joints to "freeze" together, or become inseparable. Solid in the joint can also compromise the seal between the pieces. If chemical residue were to get on the joint during transfer (Figure 1.1c), the joint should be wiped clean with a KimWipe (lint-free tissue, Figure 1.2a) before connecting with another piece. Spillage on the joint can be minimized by using a funnel.
Figure 1.1: a) Connections of ground glass joints, b) Ground glass joint measurements (for a 14/20 piece of glassware), c) Pouring liquid into a flask.
Figures 1.2 b+c shows a "frozen" joint (notice the residue on the frosted joint), where benzaldehyde crept into the joint during storage and probably oxidized to seal the round bottomed flask and stopper together. To separate a frozen joint, first try to gently twist the two pieces apart from one another. If that fails, gently tap on the joint with a spatula or other piece of equipment (Figure 1.2c). If that fails, next try heating the joint in a hot water bath (heat may cause expansion of the outer joint), or sonicating the flask if a sonicator is available. As a last resort, see your instructor, and they may heat the joint briefly with a heat gun. The frozen joint in Figure 1.2 had to be heated to separate the pieces.