by
Walter E. Parham, Ph.D.
August, 2012
Introduction
“The ruins of St. Paul’s in Macau,” a UNESCO World Heritage Site since 2005, sits atop sixty-six, wide, granite stairs near the center of old Macau (Figure 1). Fire left its mark on the granite remains of Macau’s famous St. Paul’s Church and can be used to help reconstruct the fire history.
Figure 1. The remains of the front wall, facade, and granite stairs of the St. Paul Church in Macau after the fire of 1835, today a UNESCO World Heritage Site. (W. Parham photo, 1967)
Portuguese Jesuits settled in Macau in 1562 and constructed their first church in 1580 situated next to the College of St. Paul. It is not clear whether this church had a rammed-earth front wall or one constructed of granite blocks. A fire in 1595 however destroyed most of the church’s wooden structure. The Jesuits rebuilt the church, but a second fire in 1601 destroyed the building again. Rebuilding began again in 1602. The remaining granite-block front wall was enlarged and a granite façade carved by Japanese Christians was added between 1620 and 1627 and completed in 1634 (Anon. 2012). During a typhoon in 1835, a third fire again destroyed the church damaging parts of the granite façade (Figure 2) and the supporting wall behind. The wall and façade have been strengthened since the last fire but the church proper has not been rebuilt.
Figure 2. Facade of St. Paul Church, Macau; architect Genovese Carlo Spinola. (Wikipedia credit)
Small parts of what the Portuguese call rammed-earth walls still exist at the outside edges the front wall of the church (Figure 3). The composition of the thick, rammed-earth walls consists of a mixture of clay, soil, sand, rice straw, crushed rock, and oyster shells compacted in successive layers. Locally such building material is called chunambo or taipa.
Figure 3. The remains of a rammed-earth side wall stands directly above white Chinese characters in lower part of the photo. (W. Parham photo, 1967)
Alteration of St. Paul’s granite building-stones
When I first visited the site in 1967 I noticed that many of the granite blocks in the wall backing the façade displayed spheroidal/onion-skin characteristics especially those directly above the eastern arch (left) and the middle arch. I thought at first that the patterns might be related in some way to the intense chemical weathering that over geologic time produces the numerous granite core-stones so common to Hong Kong, Macau and surrounding areas. In general, three sets of joints in a rock such as granite leads to the formation of rhomb-shaped blocks that when weathered ultimately form core stones (Figure 4).
Figure 4. Core-stone formation from chemical weathering. (Credit: N. King Hubbert)
Subsequent erosion leaves the land surface littered with core stones. However, the weathered core-stone explanation falls short in explaining how certain granite blocks in the St. Paul’s Church wall developed their spheroidal/onion-skin characteristics. First, the distribution of the granite blocks exhibiting such characteristics does not support the normal top to bottom profile of a weathering origin. Second, assuming that the stone masons who built this wall used only fresh granite for construction, the maximum amount of time available to convert the blocks to core stones would be 400 to 450 years, a period of time far too short geologically to produce such a major alteration of a resistant rock like granite. Another mechanism however that could be responsible for the spheroidal/onion-skin characteristics in the granite blocks is fire (Figure 5).
Figure 5. Granite blocks exhibit spheroidal/onion-skin characteristics decreasing upward above the eastern arch on the church’s back wall. The archway has been narrowed from reinforcement construction. A curved, metal insert supports stones exhibiting a rounded form. (W. Parham photo, 2005)
Spheroidal/onion-skin features develop in granite building-stones where they have been subjected to severe building fires (Gomez-Heras et al. 2009; Sippel et al., 2007). Dr. Gomez-Heras commented (personal communication, Aug. 24, 2011) on seeing a photo of mine of the back wall of the St. Paul Church that “It seems plausible the origin of the damage in the picture you sent is caused by fire, as it combines the ‘onion-skin’ structures and reddening, although it would be interesting to see a more detailed picture.’ Reddening of granite blocks is said to be associated with the conversion of ferrous to ferric iron-compounds in the granite as a consequence of fire (Winkler, 1975). Many granite building-stones exposed on the rear side of the church wall exhibit spheroidal/onion-skin characteristics and some show reddening. The granite blocks showing the most well developed spheroidal/onion-skin characteristics are those immediately above the east (left) arch. Some blocks further to the west (right) show these patterns also but in a less well-developed form. I drew the white line in Figure 6 to include obvious blocks showing spheroidal/onion-skin features. The resulting pattern suggests that its likely cause was a high-temperature fire centered below the top of the eastern-most arch and that the flames were fanned westward across the wall by a strong wind.
Figure 6. The white line encloses those granite blocks which show obvious spheroidal/onion-skin characteristics. The origin of the pattern seems compatible with that produced by a large fire centered beneath the eastern-most arch with its flames blown strongly westward. (W. Parham photo, 2005)
Description of fire damage to the granite wall and facade:
Damage to the granite façade is visible at all three arches. Literature tells us that Macau was being subjected to a typhoon at the time of the 1835 fire. An 1854 painting by Wilhelm Heine (Figure 7) entitled, Jesuit Convent, Macao, shows the three arches open with a single bell hanging in the east arch as well as one in the west arch.
Figure 7. Wilhelm Heine 1854 painting of St. Paul’s Church, Macau shows bells mounted in east (R) and west (L) arches. One closed door shown below. Credit: Jesuit Convent Macao
Open arches would have allowed strong winds to draw flames outward and to blow them westward. Spheroidal/onion-skin characteristics are most evident in the granite blocks above the east arch and above the middle arch of the façade, likely sites to have been subjected to the highest fire temperatures.
West arch: The west (left) arch when viewed facing the front of the church (Figure 2), shows that fire caused the granite trim to spall (the process in which a fragment or chip is released from a piece of stone), immediately above the arch and along both sides of the opening. The two Corinthian granite pillars on either side also are spalled and cracked.
Middle arch: Fire damage to the middle arch shows a similar pattern to that of the west arch except spheroidal/onion-skin features are evident in the trim immediately above the middle arch. The granite of the west pillar of the middle arch is spalled more than the east pillar. In addition, the second pillar to the west of the middle arch also is spalled.
East arch: The eastern-most arch shows the trim spalled immediately above the arch and in the trim along the west part of the opening whereas the trim along the east side of the opening shows little damage. The pillar just west of this arch exhibits cracking toward its top. [Note: to see damage to the St. Paul Church granite façade go to Google Images and enter St. Paul Church Macau. Here there are hundreds of photos of the façade many of which show the damage.]
The damage pattern visible on the granite façade combined with the damage pattern visible in granite blocks on the supporting wall behind support the existence of strong typhoon winds during the fire. Easterly typhoon winds during the 1835 fire would place the center of the typhoon south of Macau.
At ground-level below the arches are rectangular openings that probably once held tall, double doors. The Wilhelm Heine 1854 painting shows the west door as closed. All three doors probably were closed during the typhoon when the fire broke out. A floor of the church, now missing, existed above the level of the doors as evidenced by a row of granite corbels i.e., supportive blocks projecting from the wall. The missing floor was likely breached by the fire from below. The greatest intensity of the fire above would have been where the oxygen supply was most abundant i.e., at the open arches above the floor, the area where the granite in fact shows the greatest damage.
Effects of fire on granite building-stone
Literature on fire’s effects on building stones shows that fire temperatures above 530o C (986o F) have a significant adverse effect on the strength of granite building-stone. Granite’s two major mineral components are alkali feldspars and quartz. Feldspar normally makes up at least two-thirds of granite and quartz at least another 10 percent. Alkali feldspars expand one to two percent between 530o C (986o F) and 800o C (1472o F) whereas quartz expands about 4.5 percent (Winkler, 1975). The difference in expansion between feldspar and quartz with increasing temperature causes the two minerals to break loose from one another and to weaken the strength of the rock.
As early as 1906, W.E. McCourt subjected granite and other common building stones to controlled firing in laboratory furnaces to evaluate the effect of heat on their behavior. McCourt cited similar work conducted by E.R. Buckley in 1898 entitled “Building and ornamental stones of Wisconsin.” In the Buckley tests, after granite was heated to 600o C (1112oF) for 30 minutes the stone was “ready to crumble” and that after heating to 816o C (1501o F) Buckley reported “granite will crumble when handled.” Buckley also reported that some of the granites he tested showed cracking and that the greatest damage occurred in the more coarsely-grained granites, a finding confirmed later by McCourt’s work. McCourt fired his granite test samples at 550o C (1022o F) and 850o C (1562o F) for 30 minutes. Some of the granite samples fired at 550o C (1022o F) and that were slowly air cooled displayed minute cracks in the feldspars. However, when McCourt cooled the rock samples rapidly with a stream of cold water some cracking also became evident at mineral boundaries. McCourt’s samples
fired at 850o C (1562o F) and then air cooled showed considerable cracking but when heated to 850o C (1562o F) and cooled by a steam of cold water the resulting damage was so great that the granite crumbled.
Rounding of granite building-stones
The geological explanation for the weathering origin of core stones in granite might be adapted as a model to explain formation of the spheroidal/onion-skin features that form in granite building stones as a result of fire. Natural, chemical weathering, attacks the corners of granite blocks first because here weathering is able to invade the rock corners from three sides whereas the edges of the granite blocks weather from just from two sides, and the flat surfaces only from one side. Chemical weathering weakens the rock as the original minerals transformed to more stable minerals. The weathered parts of the rock in time would separate from the fresher rock beneath to begin the rounding process which gives rise to an onion-skin structure. Erosion ultimately removes the softer, weathered rock leaving a core stone behind on the land surface (See small drawing at lower left in Figure 4).
It seems possible that fire might round granite blocks in a geometric fashion much like chemical weathering. The corners of a granite block when subjected to fire receive heat from three sides. Heat penetrates the edges of the block from just two sides, and penetrates the sides only from one side. High temperatures weaken the granite because of the differential expansion of feldspar and quartz. Expansion and weakening of the granite would be greatest at corners, less so at edges, and the least on rock faces. Expansion of the heated outer parts of the granite block causes them to separate from the cooler inner granite core giving rise to the onion-skin characteristics.
The granite firing-experiments suggest that temperatures reached 850o C (1562o F) during the 1835 fire at the top of the eastern arch on the rear wall and at the top of the middle arch on the front wall. Well-developed spheroidal/onion-skin features are most prominent at these two sites. Rain may have caused some rapid cooling here which would accentuate the rock degradation process. Granite crumbling may have been a factor considered later in the decision to preserve the arches by adding steel supports at the top of the arches.
Notes on changes of the size and shape of the wall
The St. Paul Church also suffered major fires in 1595 and 1601 but it is not clear whether any damage to the granite blocks that may have occurred then can be differentiated today from that of the 1835 fire. What is needed to make such a determination is an analysis and reconstruction of the size and shape of the wall at the times of the two earlier fires. For example, the location of the small, closed arch in Figure 8 suggests that it may have been the high point of a smaller, pre-façade wall and that it was closed off when the façade was added. The arch might have been used as a place to suspend a church bell. Because some of the granite blocks of the arch and of those filling it show spheroidal/onion-skin characteristics it seems likely that the arch was already closed at the time of the 1835 fire.
Figure 8. The small, closed, top-most arch shown here probably was open at one time and used to house a church bell. The arch will filled in prior to the 1835 fire probably when the wall was extended westward and the façade added. (W. Parham photo, 2005)
Summary
The granite building-stones forming the back wall of the St. Paul Church show a pattern of blocks with spheroidal/onion-skin characteristics that is consistent with their formation during the 1835 fire. The pattern lends support to strong, east winds at the time of the fire, likely typhoon winds. Some of the granite blocks show reddening, also a common result of such a fire. Damage to the granite façade also indicates a strong wind from the east that blew the flames out through the open arches and then to the west. The hot granite blocks expanded and contracted as they were subjected to fluctuating intense flames and cold rain. Differential expansion of alkali feldspars and quartz in the outer parts of the building stones during the heating and cooling episodes weakened the stone and caused those parts to separate from the inner, cooler rock giving rise to the spheroidal/onion-skin characteristics. These features suggest that temperatures of about 850o C (1562o F) were reached during the fire.
The small, closed arch still visible on the back wall may have been the highest point of the 1601 church. The east end of the wall shows considerable damage and parts of it may have been rebuilt or perhaps redesigned early in the church history. An architectural analysis would be a good first step to aid in our understanding of what the size and shape of the church wall was in 1595 and 1601 before greater detail of the effects of each fire can be determined.
References cited
Anon, Feb. 2012, Jesuits in Macau – 450 Years, Chinese Province News, p. 6-11, http://sjapc.net/sites/default/files/jesuits_in_macau_450_years.pdf
Gomez-Heras, M. et al., 2009, Impacts of fire on stone-built heritage: an overview, Jour. Arch. Conservation, v. 15, is. 2, p. 47-58.
McCourt, W.E., 1906, Fire tests of some New York building stones, http://www.state.nj.us/dep/njgs/enviroed/oldpubs/GS-Annual-Report-1906.pdf.
Sippel, J. et al., 2007, Decay of natural stones by fire damage, The Geol. Soc. London, Spec. Pubs., v. 271, p. 139-151.
Winkler, M.J., 1975, Stone: properties, durability in man’s environment, Springer, Wien, 230 p.
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