Laboratory hot plates often serve a dual purpose by incorporating both the stirring assembly and a heating element. The smaller sizes (less than about 10mm) are often referred to as 'fleas'. The smallest are only a few millimeters long and the largest several centimeters. Many stir bars have a pivot ring around the center on which they rotate.

A variety of special shapes exist for more stable or efficient stirring in different conditions or to conform to the shape of small vessels. Magnetic stirrers are bar-shaped and usually octagonal or circular in cross-section, a pointed oval shape is also common for use in round-bottom flasks. Glass can be attacked by strong alkali (such as lye) depending on heat, exposure time, and concentration. Birch reductions (a common dissolving metal reduction) are often conducted in a glass vessel, thus indicating that a glass stir bar would likewise be compatible. In dissolving metal reductions that use an alkali metal dissolved in a primary amine, PTFE may be attacked to some extent. Glass may be viable as an alternative if PTFE is unsuitable due to high temperature or chemical attack.

Stir bars are typically coated in PTFE, or, less often, in glass the coatings are intended to be chemically inert, not contaminating or reacting with the reaction mixture they are in.

The stir bar's motion is driven by another rotating magnet or assembly of electromagnets in the stirrer device, beneath the vessel containing the liquid. Four magnetic stir bars next to a metre stickĪ magnetic stirrer consists of a magnetic bar placed within the liquid which provides the stirring action.

Hot plate stirrer