If the ceiling is flat all ornament placed upon it must not only be flat also, but must not fictitiously represent relief, for no shaded ornament can be pleasant when placed as the decoration of a flat architectural surface.
I have already noticed that the decoration of a room should be in character with its architecture, but that while this should be so, the ornament applied by way of enrichment should not be a servile copy of the decorative forms employed in ages gone by, but should be such as is new in character, while yet of the spirit of the past.
Many circumstances tend to determine the nature of the decoration which should be applied to a ceiling: thus, if a ceiling is structurally divided into square panels, the character of the ornament is thereby restricted, and should these panels be large it will probably be desirable that each be fitted with the same ornament; while if they are small three or four different patterns may be employed, if arranged in some orderly or methodical manner.
A ceiling may also have the joists or beams visible upon it: in this case the decoration would have to be of a very special character. The bottoms of the joists might have a string pattern upon them (a running pattern), as the "Greek key," or guilloche; whilst the sides might have either a running pattern, or a pattern with an upward tendency, as the "Greek honeysuckle;" and the ceiling intervening between the joists might have a running pattern, or better, a star, or diaper pattern, or it might have bands running in the opposite direction to the joists, so as, with them, to form squares, which squares might be filled with ornament.
If, however, the ceiling is flat, and is not divided into sections structurally, almost any "setting out" of the surface may be employed, as Fig. 53; or a large centre ornament, as Figs. 54 and 55; or a rosette distributed over the entire surface, as Fig. 56. In any case it is not necessary or even desirable that the ornament be in relief upon the ceiling. Flatly treated ornaments may be employed with advantage, and all fictitious appearance of relief, as we have already said, must be avoided.