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This webpage reproduces an article in
American Journal of Archaeology
Vol. 38, No. 1 (Jan.‑Mar. 1934), pp81‑82.

The text is in the public domain.

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 p81  Byzantine Gold Mosaic

Throughout the entire Church of Hagia Sophia at Constantinople the surfaces of all vaults and arches above the level of the marble revetment were encrusted originally with gold mosaic. Upon this as a background were developed not only a wealth of deeply glowing ornament, lending sparkle and accent to architectural lines, but also great figure compositions instinct with the awful dignity of a dogmatic and powerful faith, a faith imperial and triumphant, which thought it no more than fitting to people the halls of heaven with a vast hierarchy of deities,º saints and angels, modeled upon the grave pomp and glittering ceremony of the imperial court of all-powerful Byzantium. These rich designs and gravely daunting figures were worked upon the golden background with tiny cubes or tesserae of colorful glass and marble, silver, blue, red, green, black, yellow, brown, purple and rose, each cube set individually upon the matrix of moist plaster as the compositions were slowly and laboriously developed.

And yet, however august and inspiring these great scenes and solemn figures may once have appeared, it is the vast and sparkling expanse of the gold mosaic to which poets and historians refer again and again. "The entire ceiling," writes Procopius, "is covered with real gold,"1 while other mediaeval authors descant upon the great extent and dazzling beauty of this decoration,2 likening it to gleam of the noonday sun.3 And since it indeed ranks among the most remarkable and characteristic manifestations of the Byzantine artistic genius, a detailed study of the technique involved may perhaps not prove amiss.

Upon a comparatively thin slab of glass, transparent and of a yellow or greenish tinge, was spread a delicate layer of beaten gold, or sometimes silver, fastened smoothly in place by an adhesive such as gum arabic. Over this was flowed a final and very thin protective coating of glass which not only held the gold leaf firmly but also greatly enhanced its brilliance. Upon the under surface of the slab there seems further to have been smeared a peculiar red substance the purpose of which has not heretofore been understood. The slab was next broken up into tiny irregular cubes measuring less than a quarter of an inch on a side and was then ready for use. In setting the tesserae, the various horizontal rows, located for the most part upon the curving surfaces of vaults and arches and hence only to be seen obliquely and from below, were not placed contiguous one to another but were separated by an appreciable extent of plaster, often equivalent to several times the width of the individual cube; the tesserae moreover were not set normal to the surface of the plaster but inclined outward and downward, in order to catch the light and reflect it with a shimmering, twinkling vibration to the eye of the spectator below. By means of those simple devices was effected not only a considerable saving of labor and of the costly gold itself4 but the sparkle and brilliance of the entire surface was also vastly enhanced.

 p82  According to certain authorities5 the mysterious reddish substance mentioned above, being reflected through the almost transparent tissue of the beaten gold, imparts to the mosaic that marvellous depth and richness of tone which constitutes the crowning glory of Byzantine gold mosaic. Yet an examination of the plaster ground to which gold tesserae still adhere, or from which they have recently fallen, seems to indicate that such an effect is unlikely, since the very small amount of light which might penetrate to the plaster surface could have but little effect when reflected.6 The suggestion has also been made that this red substance, perhaps brick dust, was applied primarily for the purpose of giving the gold tesserae a better hold upon the plaster;7 but since it was very finely powdered it could have been of little use in this respect.

A possible solution of the problem was suggested of that writer by a minute study at close range of the mosaics of the sanctuary apse and elsewhere in the vaults of Hagia Sophia. This showed that, whereas the matrix in which the gold tesserae were set always exhibited a red impress, the impress of the other tesserae, i.e., of the polychrome cubes forming the ornamental motifs, was generally dark, or at least considerably darker than the tone of the matrix itself. Furthermore it was obvious in both cases that the coloring matter, red or blue-gray as the case may be, had been spread not upon the under side of the cubes but rather upon the surface of the moist plaster in which the tesserae were thereafter embedded. If these observations be accurate and if it be recalled also, that rarely if ever were the tesserae so placed in the matrix as to be actually contiguous one to another, but rather in most cases surprisingly widely spaced, the purpose of the coloring matter becomes at once apparent. It was to alter the light grayish buff of the plaster surface to a tone more closely in harmony with the ornament to be applied, in other words to provide that the interspaces of plaster between the cubes might reflect the abundant light in a manner richer, more subdued and completely in key with the varying tones of the tesserae themselves. Had the latter been set upon an untinted matrix of natural plaster the effect would have been almost as disturbing as that of a stained-glass window with buff leading and armature.

Emerson H. Swift

Columbia University

The Author's Notes:

1 De Aedificiis Imp. Justiniani, ed. Bonn, p178.

2 Cf. A. Banduri, Imperium Orientale, Paris, 1711, I, 3, p73.

3 Paulus Silentiarius, Explicatio S. Sophiae, vss. 671‑672.

4 Estimated by Salzenberg, Altchristliche Baudenkmäle von Constantinopel, Berlin, 1854, folio ed. p29, at from one-half to three-quarters of the total.

5 Lethaby and Swainson, S. Sophia, London, 1894, p289.

6 W. S. George, The Church of Saint Eirene at Constantinople, Oxford, 1912, p53.

7 Lethaby and Swainson, loc. cit.

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