Ringing is a procedure for sealing the edges of coverslips so that the mounting medium is no longer exposed to the air and evaporation of the solvent is inhibited. The ringing media harden fairly rapidly trapping the fluid mounting medium under the coverslip. The name comes from the way it was originally done, i.e. with circular coverslips and a turntable. The coverslipped preparation was centered over the turntable so that it slowly revolved with the edge of the coverslip rotating without any deviation from the centre. The ringing medium was then applied with a fine brush in a ring around the coverslip. With square and rectangular coverslips more common today the medium is often applied with a straight metal rod rather than a brush. Often this is to make the preparation aesthetically pleasing.
Traditional ringing media are applied either as a varnish which dries by evaporation or as a solid which is melted for application, then sets hard as it cools. Once ringed, preparations are considered permanent. An often substituted alternative for occasional use is nail varnish. This is available in a wide variety of colours, including black, and comes with a brush for application. Drying time is a few minutes as the solvent evaporates, but more than one coat may need to be applied. Keep in mind that trace amounts of the solvent may diffuse into the mounting medium before the varnish dries, so check its suitability first.
Ringing media are better purchased than compounded.