C. Fiori Italy During the restoration of mosaics on the presbyterial · 2014. 5. 20. · Stens = tensile strength; Kic = fracture toughness; L = taken from literature. In addition

$Page 1: C. Fiori Italy During the restoration of mosaics on the presbyterial · 2014. 5. 20. · Stens = tensile strength; Kic = fracture toughness; L = taken from literature. In addition$
Ceramic anchorages for consolidation of

finishing elements and layers of monuments

and historical buildings

C. Fiori

Italy

Abstract

During the restoration of mosaics on the presbyterialarch in the basilica of S. Vitale in Ravenna, asolution was required for consolidating areas of themosaic supporting layer that had become detached fromthe loadbearing masonry. Our knowledge of ceramicmaterials combined with the restorers' understandingof special mortars for filling in masonry cavitiesmade us decide on the use of ceramic anchorages andthe injection of special mortar around their edges asa solution to that particular consolidation problem.After this initial application, it became apparentthat ceramic anchorages could offer wide-rangingpossibilities for use in consolidating plasterwork orother finishing elements in monuments or historicalbuildings. There followed further applications ofceramic anchorages made with various materials andtechniques, as we shall describe in this article.

1 Introduction

The idea of using ceramic anchorages as a substitutefor the traditional clips or metal anchors came aboutas an attempt to solve the problem of consolidatinginsecure wall mosaics with sections detached from theloadbearing masonry, in the basilica of S. Vitale inRavenna (Fiori & Vincenzini^) . The cavities betweenthe masonry and the layers that supported the mosaicdecoration had to be filled with special mortar inorder to bond the detached sections, but only afterfirst creating anchoring points between these sec-tions. Without this precaution the insecure layers

Transactions on the Built Environment vol 15, © 1995 WIT Press, www.witpress.com, ISSN 1743-3509

198 Dynamics, Repairs & Restoration

would be even more liable to collapse when the cavityfilling mortar was injected.Through our knowledge of the properties of advancedceramic materials for structural applications (Onoda& Hench^, Davidge^, Richerson^) we became convincedthat they could be used as an alternative to metalsor other materials. They do of course have somedisadvantages, especially with respect to metals. Theclearest, or most obvious, of these is the poorcapability ceramic materials have of deforming understress: they tend to be fragile and lack toughness.The mechanical properties of some advanced ceramicmaterials are, however, of great interest andsignificantly better than those commonly attributedto traditional ceramic materials.Table 1 shows the mechanical parameters of ceramicmaterials for structural applications; the resultswere in part obtained from tests conducted by IRTECand partly taken from literature.

Table 1 - Mechanical properties of advanced ceramics

material

ZrC>2-3yTZPAlpOiAl?Oi+30TiB?Al2C>3 + 30TiNAl?0]+30TiCAl?Oi+30ZrO?SiiNaSi]Na+40TiNTiB2ElectricPorcelain

E(GPa)

20539642441541033030536055584

Hv(GPa)

1219191921172115248L

Sflex(MPa)

800436711729785613895104371690

Scomp(MPa)

-3595----

3500L--650

Stens(MPa)

759L250L----700--60

KlC(MPa.mO.5)6. OL3.25.75.23.24.94.86.35.11-2.5L

E = Elastic modulus; Hv = Vickers hardness; Sflex =flexural strength; Scomp = compressive strength;Stens = tensile strength; Kic = fracture toughness; L= taken from literature.

In addition to their good tensile and compressivemechanical properties, the use of some ceramicmaterials in the structural restoration of finishinglayers in buildings can also be considered advanta-geous because of their almost unlimited stabilitywith time, their good adhesion to binding mortars andthe fact that their expansion coefficients are com-patible with those of the surrounding materials.Furthermore, it will be interesting to follow the


Dynamics, Repairs & Restoration 199

future development of ceramic-ceramic composites andceramic matrix composites, which will include evenmore effective products than those currently avail-able for this kind of application. These materials infact offer significantly superior mechanical proper-ties and will discredit yet further the arguments infavour of alternative materials that are capable ofdeforming before they break.

2 First application of ceramic anchorages

The first ceramic anchorages used in the restoration(1988-1990) of the mosaics on the presbyterial archof the basilica of S. Vitale in Ravenna were made ofalumina (Gitzen^). This material was chosen becauseat that time in Italy alumina anchorages producedusing extrusion techniques and workable when unfiredand before sintering were the only ones available onan industrial scale and at low cost.The unfired cylindrically shaped anchorages weremachined in order to obtain a finished surface withthread or grooving that would improve bonding betweenthe anchorages and the mortar used to fix them inplace. The machined pieces were subsequently fired attemperatures of about 1500°C. The finished anchorageshad a flexural strength of 130 ± 10 MPa, consideredsufficient for this type of application.A scheme showing the installation of a cylindricalceramic anchorage in a mosaic is shown in Fig.l anddescibed also in an earlier paper (Fiori&).The first operation (Fig.lA) involves removing one ormore tesserae (unless this is a "lacuna" where thetesserae are absent) and drilling a hole to a depthof 4-5 cm in the masonry; the anchorage is then fixedinto the hole in the masonry and the mosaicsupporting layer using a bonding mortar, which may beof the same type as that used to fill the cavity;when the bonding mortar has set, the cavity is filledby injecting mortar around the anchorage (Fig.IB) andthe tesserae that were initially removed are replacedin their original positions. The anchorage istherefore not visible when the restoration work hasbeen completed.Laboratory tests carried out on an 8 mm diameterceramic anchorage inserted to a depth of 4 cm in abrick and fixed using the same mortar used in therestoration work showed that a tensile force of atleast 150 kg was required in order to extract theanchorage from the brick. More than 80 anchorageswere fixed in the presbyterial arch of S. Vitale(Alberti & Tomeucci?). These were of the threaded



bonding mortar filling mortar

A B

Figure 1: Scheme showing the application of a ceramicanchorage in a wall mosaic detached from the masonry:A) removing one or more tesserae; drilling a hole andconnecting the detached layer to the loadbearingmasonry using a ceramic anchorage; B) filling thecavity by injecting special mortar and replacing thetesserae.

Figure 2: Anchorage installed in the mosaic of thepresbyterial arch of S. Vitale; external view of thehead of the anchorage before replacing the tessera.



cylindrical type, 10 cm long and with two diameters:8 mm on the vault and 10 mm further down towards thevertical walls of the mosaic decoration. Anchoragesof greater diameter were used on the vertical wallsin order to withstand any bending or shearing forces.After installing the ceramic anchorages, all theclips and other metal type anchors set in placeduring restoration work at various times in the pastwere removed. Fig. 2 shows a ceramic anchorage in-serted in mosaic, viewed from the outside.

3 Ceramic anchorages for frescoes

Encouraged by the success of the work carried out onmosaics, an initial application of ceramic anchorageson wall paintings was performed as part of re-storation work on the frescoes in the Galleria delleBattaglie in the Castello di Spezzano, Modena (Fiori& Costantini^). The frescoes dating from the firsthalf of the sixteenth century, of the greatesthistorical and artistic value, were badly damaged andentirely covered up by plasterwork. They wereuncovered and restored during the period 1986-1991.Over a limited surface area of restored paintedsurface, which presented serious detachment problems,10 ceramic anchorages of diameter 8 mm and length 10cm - the same type as those used in S. Vitale - wereinstalled and fixed with special hydraulic mortar.The anchorages were mounted so that their heads wereperfectly level with the surface of the painting.This meant that during functional restoration theycould be treated chromatically just as if they werean ordinary gap; the microtexture and the colour ofthe ceramic anchorage material were such that it waseasy to apply the normal watercolour restorationtechnique to it.After this initially limited, experimental applica-tion, consolidation using ceramic anchorages and theinjection of special mortars was adopted as therepair method for the fourteenth century frescoes inthe so-called "Cappella Giottesca" of the church ofS. Giovanni Evangelista in Ravenna. The restorationwork was carried out from February to September 1994.The frescoes on the cross vault, depicting theEvangelists and the Doctors of the Church, were badlydamaged and, above all, had clearly come away fromthe masonry of the vault and were close to fallingoff altogether. The phenomenon was particularly ac-centuated on two of the four cells of the vault andin the fragments on the end wall of the chapel. Thefrescoes on the vault were consolidated by insertingflat-headed alumina ceramic anchorages of 6 mm diame-



Figure 3: Insertion of flat-headed ceramic anchoragesin a fresco ("Cappella Giottesca", Chiesa di S. Gio-vanni Evangelista, Ravenna)

Figure 4: Scheme showing the installation of a flat-headed ceramic anchorage; the fixing mortar and thecavity filling mortar may be of the same type.



er and 6 cm length, which were then blocked usingpremixed hydraulic mortar with a low salt content;the cavities were then filled by injecting speciallow density mortar. On the end wall cylindricalceramic anchorages, headless and with a diameter of 8mm and length of 8 cm, similar to those describedabove for the previous restoration works, were used.Fig. 3 shows the insertion of ceramic anchorages inthe frescoed vault. Fig. 4 shows a scheme of the in-stallation of a flat-headed ceramic anchorage forconsolidating plasterwork. The flat-headed ceramicanchorages where made at IRTEC using a plaster mouldcasting technique.A completely different situation concerns the appli-cation of ceramic anchorages made of silicon nitrideby isostatic pressing for connecting stone blocksthat make up the structure of the fountain in Piazzadella Liberta in Faenza (built in the years after1617; and restored in the period 1992-1994). Theseanchorages are much more expensive than those madefrom alumina and they can only be used in specialcases where a small number of pieces with particular-ly high-level mechanical characteristics are requir-ed. Fig. 5A shows a ceramic anchorage of the typeused in the fountain, while Fig. 5B shows a diagramof how these anchorages were inserted in crossed-overpairs between the blocks and fixed with mortar.

. 20cm,

B

Figure 5: A) ceramic anchorage made of silicon ni-tride; B) design of the anchorages inserted incrossed-over pairs between two blocks of the fountainin Piazza della Liberta in Faenza.



4 Future prospects

The positive results of the first experiments inusing ceramic anchorages open up new possibilitiesfor the use of these products for in situ conserv-ation of surface treatments of historical architec-ture. In addition to mosaics and frescoes, ceramicanchorages may be applied to any other type of wallpainting or decorating technique, including graffiti,stucco, "marmorini" (imitation marble plaster) andhistorical plasterwork in general. Special casesinvolve the connecting of structural stone or firedclay blocks using extremely high mechanical per-formance anchorages. In the future the restorationsector may be able to use advanced ceramic materialsor composites with superior characteristics.The experience acquired by IRTEC has served as aspringboard for further restoration work usingceramic anchorages (for example: a mosaics from thiscentury at the EUR centre in Rome and plasterworks inthe "Galleria Borghese",Rome).

References

1. Fiori, C. & Vincenzini P. Restoration on the an-cient wall mosaics in basilica of S.Vitale with theuse of alumina ceramic anchorages, Industrialceramics, 1992, 3, 115-120.

2.Onoda,G.Y. & Hench, L.L. Ceramic processing beforefiring, John Wiley & Sons, New York, 1978.

3. Davidge, R.W. Mechanical behaviour of ceramics,Cambridge University Press, Cambidge, 1979.

4.Richerson, D.W. Modern ceramic engineering. Pro-perties, processing and use design. Marcel DekkerInc, New York, 1882.

5. Gitzen, W.H. Alumina as a ceramic raw material, TheAmerican Ceramic Society, 1970.

6. Fiori, C. High-technology ceramic anchorages ofwall mosaic presenting areas of detachment, inScience, Technology and European Cultural Heritage(ed N.S.Baer, C.Sabbioni & A.I.Sors), pp 723-726,Bologna, Italy, Commission of the European Commu-nities, Butterworth-Heinemann Ltd., Oxford, 1991.

7. Alberti, L. & Tomeucci A. Intervento di restaurosui mosaici dell'arco di ingresso al presbiterio inS.Vitale, Restauri ai mosaici nella basilica diS.Vitale a Ravenna. L'arco presbiteriale, edC.Fiori & C.Muscolino, pp 90-130, Istituto di Ri-cerche Tecnologiche per la Ceramica, Faenza, 1990.

8. Fiori, C. & Costantini, G.M. Prima applicazione diancoraggi ceramici per la conservazione in si to diaffreschi, Ceramurgia, 1993, 6, 277-282.


Documents

C. Fiori Italy During the restoration of mosaics on the presbyterial · 2014. 5. 20. · Stens = tensile strength; Kic = fracture toughness; L = taken from literature. In addition