The convex torus covered in the last post has its corresponding concave forms: the simple hollow, and the closely related compound scotia.

The hollow is a concave half-circle; it can also be formed from deeper or shallower arcs or elliptical curves. Its scalloped profile makes it a separating moulding: it produces a strong, smooth shadow gradient, darkest at the top.

The scotia, from the Greek skotia meaning ‘dark, shadowy,’ is a compound curve consisting at its most basic of two arcs, arranged with the smaller radius arc at the top and the larger at the bottom, so that the lower end of the curve ‘runs out’ past the upper end. The two simplest scotia are (1) where the lower arc is twice the radius of the upper, sometimes referred to as the Greek scotia, and (2) where the lower arc is three times the radius of the upper, sometimes referred to as the Roman scotia. Note: you will find different definitions of and formulas for Greek and Roman scotia from different sources; in any case a reasonable generalisation for this and indeed for mouldings in general is that Greek moulding profiles tend to be ‘flatter’ with a sturdier presentation, whereas Roman mouldings are ‘deeper’ and more dramatic.

Bear in mind that these are only the two simplest scotia, and that more complex profiles can be constructed with three, four or even more arcs to achieve a variety of effects.

The scotia is often seen at the base of columns, as in the sequence found in the classical Ionic column, which goes, from top to bottom: fillet-torus-fillet-scotia-fillet-smaller torus-plinth. The out-in-out rhythm produces a dynamic effect that is harmonious without being monotonous or repetitive; the separating fillets between the curves give some spine to the composition and prevent it from becoming too ‘soppy.’ Note also the vertical hollows, called flutes, carved into the shaft of the column.