Full report to be published in: Hortus Artium Medievalium (1998), forthcoming
Conservation History. The mosaics were restored twice in the 19th century and have been repaired at least twice in the 20th century. The first restoration (1886), by the Neuhauser Mosaic Firm in Innsbruck, was relatively limited. Luigi Solerti restored the panel of the Annunciation, on the south side of the apse. The second restoration (1890-1900), by Pietro Bornia of the Vatican, was the most extensive (apse, triumphal arch, side apses). By contrast, the 20th century work was minor, relating mainly to protective measures meant to ensure the safety of the mosaics during World War II (1944/48) and the recent war in Croatia (1994). Additional, minor restoration occurred during architectural work on the basilica in the late 1950s.
Pre-restoration Descriptions and Visual Records. This section reviews material predating the earliest restoration of the mosaics. These records offer valuable clues, particularly about original iconographical details, but they also vary in trustworthiness, and raise as many questions as they answer. Three main conclusions were drawn: the motif at the crown of the apse was changed by restorers from a chrismon (painted) to a lamb; the shape of the Virgin's footstool was changed by restorers; and the recording of the figures on the window piers may differ from what appears today. The substitution of a lamb for a chrismon is well documented from other sources, but the documents are silent about the Virgin's throne and the window-pier figures.
Visual Analysis. The on-site visual analysis, done with the aid of scaffolding, was limited to the lower apse (up to the level of the inscription). Particularly detailed analysis was done of 30 "sondages", areas of ca. 25 x 40 cm, examining the tesserae, setting beds and setting techniques. Three phases were found. (1) The original 6th century mosaics consisted of 50+ hues, primarily of glass but also including some marble, limestone and brick, set into a gray mortar setting bed. (2) Solerti's restoration from 1886, identified in one area of the Annunciation, introduced at least 9 colors not found elsewhere in the mosaics, and set the tesserae very closely together. (3) Bornia's restoration of 1890-1900 used 30+ colors set into a pink mortar setting bed. A hallmark of the Bornia restoration was the careful integration of original areas of mosaics with areas of restoration.
Tesserae. Since the vast majority of the mosaics are either original or from Bornia's restoration, the discussion of tesserae, as well as that of setting bed and technique was focused on the differences between the original mosaics and Bornia's restored mosaics. The original tesserae are irregular in shape, have uneven surfaces, high porosity, appear worn, and exhibit great variation in coloring. By contrast, the new tesserae are more regular in shape, have crisply cut edges, flat and even surfaces, low porosity, and homogeneity in color. In fig. 11, all tesserae to the left of the white disks are old while those to the right are new.
Original and Restored Settings. The mortar are more complex to study, partly because mortar, whose composition changes little over time, is difficult to assess by color, and partly because Bornia did many small repairs in the mosaics, where he often tried to match the color of the original setting bed. The original settings, which survive in patches rather than in broad swaths, are generally very worn, and have a gray to gray-white color, often with inclusions of a white substance. The original setting technique is markedly irregular, with tesserae set askew and at angles to one another, creating a jumbled appearance. Fig. 15 illustrates the original setting bed and technique.
Of the settings of Bornia, the most prominent is a well preserved pink mortar, with a fine consistency and some minute inclusions of red particles. Bornia's setting technique is highly regular with equally-sized tesserae consistently spaced in even rows. Figs. 16, 17 and 18 illustrate Bornia's settings. The contrast between the old and new settings may be seen in Fig. 11, where the left side of the jeweled band is old while the right side is new.
Bornia's General Methods. In the areas examined, most of Bornia's interventions were minor, consisting mainly of repairs and patching. He went to great lengths to integrate the old with the new, and the article discusses various combinations of old and new tesserae and settings. Attempts were made to match the color of original tesserae. Not only was Bornia sensitive to salvaging original tesserae and mortar, he also took care with iconographical details. For example, in the figure of Zacharias (north window pier) several key details, such as the censer and box remain significantly original, even if tesserae around them were altered.
Gold Tesserae. The gold tesserae represent a special case. Much of the gold in the mosaics today is the product of restoration. The original gold tesserae were made from an amber colored and semi-translucent glass, which was covered first with a layer of gold leaf and then with a thin layer of translucent glass. Many of the original gold tesserae had lost their surfaces of gold leaf, and thus appeared brown or black. Bornia and the Austrian officials debated a solution to the problem. Some areas were set with new gold tesserae, but, since the new gold tesserae had objectionally brilliant mirror finishes, Bornia also introduced a different solutions. One was to layer the old gold tesserae with gold leaf and then covering them with varnish. Certain areas of the sondages found traces of the gold leaf clinging to the edges of adjoining white stones ( Fig. 13).
In summary, the restoration by Bornia was painstaking, especially for its time. In most areas we examined, the fabric of mosaic was a careful mixture of old and new tesserae, but with the old generally predominating. Usually at least 75% of the cubes were old, and sometimes up to 98%. Even heavily restored areas, such as the window embrasures, may contain up to 50% old tesserae. Some areas were so heavily restored that they are essentially new, such as St. John the Baptist, and certain areas of gold.