The Herringbone Apparatus

Most of the recorded herring-bone bonds is located between the second and third corridor, where a very irregular laying of the bricks and relevant variation of their inclination are visible.

After the dome intrados survey, the zenith angle of the herring-bone traces has been measured. It has been verified a very wide range of the inclination (from circa 31° to 55°) among the eight faces of the dome and among the herring-bone bonds of the same face, with the prevalent inclination from 36° to 40°. It depends on the different way of laying the bricks, using more or less quantity of mortar in the joints, as an evidence of the unequal way of working of the different teams of masons.

In particular, in the South-East face of the dome much more inclined herring-bone bonds than elsewhere have been recorded. It seems to prove the

Figure 4. The eight faces intradós (by stereophotogrammetric survey) with the traces of herring-bone lines to point out their different inclination, ranging from 31° to 55°.

Figure 5. View of dome intrados with the lines of identified herring-bone traces.

Figure 4. The eight faces intradós (by stereophotogrammetric survey) with the traces of herring-bone lines to point out their different inclination, ranging from 31° to 55°.

to build the corners, at least where the plaster has been detached and the wall apparatus has been visible.

It has to be considered that the stone ties of the third corridor probably determined some interruption of the inclined lines of the herring-bone bricks. The same interruptions should be caused by the correspondent stone ties of the second corridor; but at this level it was impossible to record them because of the absence of wide visible herring-bone bond segments.

Also the extrados of the inner dome has been examined, at least in many parts with no plaster. The same characteristics as the intrados have been found: often there is no continuity in the herring-bone lines, adjoining bricks have different rotations, and thick mortar joints are used.

The removal of part of the tiles, made possible to take a stereo-photogrammetric survey of a meaningful sample of the outer dome extrados. The resulting information on the herring-bone bonds is comparable with that previously acquired: different inclinations, discontinuities, variable thickness of the joints, irregular brick disposition even in short segments of the herring-bone bonds.

The information now available on the herring-bone bonds is insufficient to make a comparison between the wall apparatus of the intrados and the extrados of the same dome portion. The available surveys discontinuous, limited to only few samples and related to parts located on different levels. Nevertheless they appear really meaningful to make some considerations on the herring-bone structure in the whole of the dome. It is interesting the wide variation in the horizontal distance (measured along the brick courses) of two adjacent herring-bone bonds. At the intrados of the inner dome this measure is known in all the parts where the plaster was detached: in the corner between the East and the North-East faces, at the height of about 15 meters above the inner gallery, these distances are cm 95,100 e 110; in the North and the North-West corner, at the height of 25 meters above the gallery, are permanence of the same team in the same worksite area for a long construction period.

The knowledge ofthe wall apparatus has been deepened thanks to the detachment of plaster parts executed during the restoration works of the wall paintings. The detailed survey drawings (Dalla Negra 2004) and the casts of the masonry prove the irregularity of the herring-bone bricks, laid with continuous and varying rotations on the plane of the dome face, using mortar joints often thick as much as the bricks. The rows of the vertical bricks are discontinuous: somewhere the inclined herring-bone row is interrupted by a number of horizontally disposed bricks, and then it starts again inclined. The interruption of the herring-bone pattern is located both in the middle of the dome faces, and in the corners. In this point there is nearly no wall toothing: nearly all the bricks, unbroken or fragmented, arrive just at the corner. In the few cases where a single brick bridges the faces, it is not a special element but a ordinary brick cut to adapt to the corner; so there is no evidence that special V-shaped bricks have been used

Figure 6. The georadar diagram of the inner dome displays two symmetrical discontinuities (at the intrados and the extrados) in the brick masonry, and a different inner core.
Figure 7. The drawing represents the wall structure of the Cu-pola, basing on georadar investigations. The inner dome is made by two outer layers where herring-bone bonds are present, and by an inner core where bricks are probably laid on simple curved beds (corde blande) (Rocchi C.d.Y. 2006).

cm 45, 60, 90; in the North and the North-East corner, at the height of 27 meters above the gallery, are cm 45, 50, 60; in the North face, at the height of 29 meters, the greatest variability has been recorded, as the herring bones are distant cm 10, 15, 25, 30, 45, 50, 55. At the extrados, above the third corridor, at the height of the second horizontal arch (circa 26 meters above the gallery) the distances among the herring-bone bonds vary from 85 to 110 cm, so they are much greater than the ones recorded on the intrados at about the same level.

Also the wall apparatus recorded on the outer shell has great variability (in the same sub-horizontal brick course two adjacent herring-bone bonds may be divided by half brick or many bricks) and discontinuity in the herring-bone lines (short segments made by few bricks, sudden changes in their inclination, splitting of herring-bone bonds,...).

In the case of herring-bone bond crossing the dome masonry through its total thickness following a unique curve centre, very different dimensional data should be expected: two adjacent herring-bone bonds distant 60 cm on the intrados of the inner dome, on its extrados the distance should be 65 cm and on the outer dome respectively 68 and 70 cm, distances only limitedly greater than the ones recorded on the inner intrados. The dimensional heterogeneity that has been recorded in the building is totally different, even if the relevant slowness of the works should have made easy to control the geometry of the herring-bone lines.

For these reasons it seems improbable that the herring-bone bonds cross radially the whole of the dome following a unique alignment to the centre. It is more reasonable that the intrados and extrados herringbone bonds of the inner dome are not continuous, but belonging to two different masonry layers. The existence of two distinct layers, the inner and the outer parts of the wall, has been proved by georadar investigations, and can also be supported by considerations relating to the building technique used in the works. Moreover, it seems proved that the herring-bone bonds of outer dome are totally independent from those of the inner dome.

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