Lythological f eatures and stonemasonry J. A. M. da Luz & C. A. Pereira Department of Mining Engineering of Ouro Preto School of Mines – Federal University of Ouro Preto, Minas Gerais, Brazil ABSTRACT : Stonework imparts some solemnity atmosphere to a buildi ng . The lythological aspects of raw material do condition the rock use and the stonework technique to be applied for. This paper deals with a Bra- zilian case, the colonial styled stonemasonry erected since XVII Century in Minas Gerais, then the richest mining province in Brazil. Two types of metasandstones (quartzites) and a quartz-chlorite-schist were studied. 1 INTRODUCTION In the past stonemasonry was a very important eco- nomic activity, specially linked to great civilizations. In deed, since hominids began making tools in order to make tools, stones have played a major role in our culture. From the Ancient World, buildings made with trimmed and faced stone have served since long ago as postcard of the improperly referred golden era. Nowadays stonework imparts some solemnity atmosphere to a monument or building. Although with an old-fashioned bulky imponency, stonecraft does have place in modern architecture yet. Figure 1. Stonework at Ouro Preto: “São Francisco de Assis” This article displays the influence of lythological Churchill at foreground and “Nossa Senhora das Mercês” features conditioning the stonework technique to be church in second plane. Itacolomi Peak is displayed at back- applied. Brazilian stonemasonry is quoted as exam- ground (left). ple. Ouro Preto Federal University’s project “Oficina de Cantaria” has taken part in an effort to renew the stonecraft skillness in Brazilian historical cities. Ouro Preto and Mariana house a great collection of important barroque buildings. Visitors can have im- mersive experience in an outstanding collection of barroque art (figures 1 and 2). These cities are in the southeast part of the geological province called Iron Quadrangle, in Minas Gerais State. Stonemasonry in such cities was done taking in ac- count the local rock availability. Metasandstone steatite and quartz-chlorite-schist are the natural choice for dimension stones and architectonic mate- rial (trimmed or carved pieces). Granite (cobbles) Figure 2. Stonework at Mariana: “Nossa Senhora do Carmo” and quartz pebbles instead were preferred for street church (Mariana is far 12 km from Ouro Preto,). Frontispiece paving (lining). in soapstone; columns and arch in metasandstone, street paving of quartz pebbles from rivers. (like volutes and arabesques, which are typical fea- Metasandstone (orthoquartzite) source is primarily tures of baroque style). See frontispieces above the from Itacolomi Group (banded, massif and moder- portals in figures 1 and figure 2 and balcony pre- ately friable, used as faced and trimmed pieces) and sented in figure 4. Taquaral Hill (layered orthoquartzite, used for pav- ing linked to Moeda Formation). Both can be in- cluded in the same former alluvial complex in deep and carbonatic plateau, with deltaic facies (Nalini Jr., 1993) Quartz-chlorite-schist is used preferentially in build- ings at Mariana, where there is no need for many de- tails (handrail in figure 3 foreground). Figure 4. A stonework balcony at Mariana, comprising a bal- ustrade in soapstone. The balcony is supported on quartzite (metasandstone) console. Pillared doorways in the ground store are in quartzite. Steatites (soapstones) occur as lens in gneisses or in metasedimentary sequence, the main constituent is talc (Mg 3Si 4O10 (OH) 2). This mineral imparts to soapstone low hardness, easing the stonework. Sev- eral near sites are source of this kind of rock, like: Catas Altas da Noruega, Cachoeira do Brumado, Santa Rita District. Figure 3. in foreground baluster and handrail in chlorite schist at Mariana Chamber of the Councilmen’s stair (the oldest city Although steatites in the Iron Quadrangle is classi- in Minas Gerais State). In background the “São Francisco” cally considered associated to the greenstone belt of church. In between it is depicted the “pelourinho” (quartzite Nova Lima Group (Cava Brasil, 2008), they seem to and iron pillory), used in colonial times for punishment for occur like plutons and younger intrusions as well slaves and other people accused of breaking the law. (Nalini Jr., 2008). Anyway there were intense me- tasomatic and metamorphic events (leading to a The geological schist source in Mariana is the Nova chlorite, serpentine and pyrite evolution). Lima Group (Archean eon, circa 2700 million years old) and in Ouro Preto is the Saramenha Formation Another rock of that region under study tested un- (Almeida et alii, 2005) of the Sabará Group (vulca- successfully for stonemasonry was dolomitic marble nosedimentary sequence, top of Paleoproterozoic from Gandarela Formation, commonly used nowa- era, between, 2500 and 1600 million years ago). In days as aggregates in building construction. Figure 5 both cases the schists display a very metamorphised shows the appearance of this rock. The cause of the texture. This rock have high grade of weathering and poor behavior (poor workability) of Gandarela mar- several colors: violaceous, greenish, grayish, light ble is its strong tendency to chip. This is due to per- creamy. fect cleavage of dolomite on {1011} crystallo- graphic plane. After Almeida et alii (2005), Saramenha Formation consists of metadiamictites, metaritmites, metape- 2 MATERIAL AND METHODS lites, mostly chlorite-schists, phyllites and lenses of dolomite. In order to characterize some typical rocks used in stonemasonry at Ouro Preto and Mariana, samples The use of soapstone in Ouro Preto and Mariana is of orthoquartzite and quartz-chlorite-schist were col- preferred for pieces with many geometrical details lected and submitted to tests. Specific gravity, ap- parent porosity, water absorption and hardness were spotted meta-sandstone (quartzite ITA B) from Itacolomi Peak determinated. (scales’ minor pick gaps are millimeters). In quartzite B the dark grains are magnetite showing some degree of oxidation to hematite. This weather- ing is responsible for color zoning in itacolomites. Especially in itacolomite B some samples display an interesting pattern of cross stratification typical of deltaic environment. This kind of feature (visually intensified due weathering) can be perceived in the columns seen in the church façades depicted in fig- ures 1 and 2 and seen in detail in figure 7. Figure 5. Crushed particles of dolomitic marble (from Bemil Quarry , Ouro Preto) used as coarse aggregate for concrete (scale in millimeters). A gray chlorite-schist (hereafter labeled as schist: CX; 2.1 km far Ouro Preto at Ouro Preto – Ouro Branco State Road), a faint yellow quartzite (hereaf- ter labeled as itacolomite A: ITA A; sample UTM coordinates UTM7742607) and a spotted quartzite (hereafter labeled as itacolomite B: ITA B; sample UTM coordinates UTM7742607) from Itacolomi Peak region were studied. The compression strength was studied using cubic probes of 50 mm in edge. In order to analyze the rock resistance to abrasion (and consequently its du- rability in stonework application) this property was studied elsewhere (Luz et alii, 2005) by tumbling. The hardness was determined by Schmidt sclerome- ter elsewhere (Luz et alii, 2005). Figure 7. Textural features of quartzite in foreground. Soap- stone was used above doorway in the “São Francisco” church 3 RESULTS at Mariana. The visual aspect of samples under analyses can be seen in the scanned slides in figure 6. The specific gravity of samples is presented in table 1. The sample behavior in water absorption by sim- ple immersion (24 h) is presented in table 2. The property associated to water absorption, the apparent (open) porosity is presented in table 3. Table 1: Densities of studied rocks. Density Standard Samples [kg/m³] Deviation ITA A 2640 0.02 ITA B 2700 0.01 CX 2760 0.02 Figure 6. Textural features of three rock types used in stone- work at Ouro Preto and Mariana. From left to right: gray chlo- rite schist, faint yellow meta-sandstone (quartzite ITA A) and Table 2: Apparent water absorption of rocks. The consequence of strength, hardness and resilience was appreciated comparing the mean force applied Absorption Standard Samples by stonemason per stroke, using as work table a [%] Deviation scale pad. Itacolomite and Gandarela marble were ITA A 0.84 0.16 compared using two typical tools: hand pick and ITA B 0.5 0.05 mallet-headed chisel (a pointed chisel) percussed by CX 1.15 0.3 mallet (Campos and Luz, 2004). The mean effort per stroke using pick was 6.5 new- Table 3: Apparent porosity of the samples. tons for itacolomite and 42.2 newtons for Gandarela Porosity Standard marble. The mean effort using pointed chisel was Sample [%] Deviation 8.9 newtons for itacolomite and 45.3 newtons for ITA A 2.21 0.41 Gandarela marble. As the contact area was about the same for both tools, these results explain why usu- ITA B 1.34 0.12 ally pick is applied for more detailed work and mal- CX 3.15 0.79 let-headed chisel for rougher preliminary conform- ing steps. When it come to hardness, this property is The fine granulation of rocks also imparts resistance displayed in table 4, showing similarity in because the difficulties of intergranular crack evolu- tion. So, rocks like conglomerates (figures 8 and 9) all type of rock studied. are prone to rupture and seldom are used in stone- masonry. Table 4: Rockwell hardness of the samples. As an example, figure 8 shows a conglomerate that Standard nowadays is used sometimes like dimension stone in Sample Hardness Deviation Minas Gerais, because of its ornamental aspect. ITA A 78.5 7.8 Sometimes there would be need of resin impregna- ITA B 86.8 6.4 tion in order to enhance the resistance of this kind of CX 87.0 4.5 rock. Similarities with the famous Crestone con- glomerate (figure 9) should be observed.