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Analysis of deteriorated sandstone at the Royal Palace,StockholmAnders G. Nord aa Institutionen for konservering , Riksantikvariembetet, Box 5405, Stockholm, S114 84,Sweden Fax:Published online: 09 Dec 2009.
To cite this article: Anders G. Nord (1995) Analysis of deteriorated sandstone at the Royal Palace, Stockholm, GFF, 117:1,43-48, DOI: 10.1080/11035899509546195
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GFF volume 117 (1995), pp. 43^8. Article
Analysis of deteriorated sandstone at the Royal Palace, Stockholm
ANDERS G. NORD
Nord, A.G., 1995: Analysis of deterioratedsandstone at the Royal Palace, Stockholm. GFF,Vol. 117 (Pt. 1, March), pp. 43-48. Stockholm.ISSN 1103-5897.
Abstract: The Royal Palace in central Stock-holm has faades built of calcite-cementedGotland sandstone, with socle stones of quartz-cemented Roslagen sandstone. A total of 197samples were analysed by SEM/EDS, XRD,and ICP. The Gotland sandstone shows manysigns of serious deterioration: gypsum forma-tion, pulverized surface, exfoliation, contourscaling, discolouration, and black gypsumcrusts. The Roslagen sandstone is somewhatmore resistant, displaying discolouration,cracks, and exfoliation. Three 'micro areas'were studied in detail, showing relationshipsbetween chemical composition and observeddamage. The chemical analyses and S isotopedata indicate that the stone decay to a large partmay be attributed to acid rain, car traffic, andother air pollutants.
Keywords: Gotland sandstone, Roslagen sand-stone, deterioration, weathering, air pollution.
Anders G. Nord, Institutionen for konservering,Riksantikvariembetet, Box 5405, S-114 84Stockholm, Sweden. Fax: 08-6614277. Manu-script received 8 July 1994, revised and ac-cepted 8 February 1995.
It is well known that air pollution causesdeterioration of stone materials (e.g. Fas-sina 1988; Feilden 1988; Del Monte 1991)and that rocks display different resistancein this respect. One of the least resistantrocks used as building stone in Sweden isthe calcite-cemented, upper Silurian Burgs-vik Sandstone from southern Gotland,herein referred to as 'Gotland sandstone'.Since this stone is easy to work, it has beenextensively used in the Baltic Sea areasince the Middle Ages.
Smaller buildings and sculptural deco-rations of deteriorated sandstone have ear-lier been investigated at the Swedish Cen-tral Board of National Antiquities (e.g.Nord & Tronner 1991; Lagerlof & Anders-son 1993; Nord et al. 1994). The RoyalPalace in central Stockholm, with seriouslydeteriorated facades of Gotland sandstoneand socle base of quartz-cemented Ros-
lagen sandstone, was another object forstudies of stone weathering.
Heavy traffic is passing along two fa-cades of the Palace, and up till about 1970the air was strongly polluted from househeating, car traffic, and other sources. Be-sides, the Stockholm climate is harsh withstrong winds, high relative humidity, snowand frost in the winter. The main purposeof this study was to find if there is any cor-relation between observed deterioration ofcalcite-cemented sandstone and sulphur/calcium (gypsum/calcite) fractions at thestone surface. Possible correlation betweenanthropogenic sources and observed dam-age was another aspect of interest
Building, material, andenvironmentThe Royal Palace in Stockholm was builtduring the first half of the 18th century. Itis a large square, with the north and southf acades prolonged by wings to the east andwest, and with an additional courtyard tothe west framed by two circular loggias (cf.Olsson 1940). The south facade is 176 m.
The principal facade stone is Gotlandsandstone, showing great variation in col-our and quality. In the main it is composedof 10-100 um quartz grains, cemented byca. 7-10 wt% calcite, and with a porosityaround 15 per cent by volume. It containslow amounts of potassium feldspar, pyrite,glauconite, micas, and clay minerals. Theporosity facilitates the penetration of acidrain into the stone, where calcite is trans-formed into gypsum. This transformationis catalyzed by nitrogen oxides, ozone,soot, and certain transition metals (Changet al. 1981; Johansson et al. 1988; GrgiC etal. 1992,1993). When gypsum is formed,the molar volume is increased by a factorof 2, causing internal strain, cracks, andexfoliation. Moreover, the solubility ofgypsum in water is about 200 times larg-er (ca. 2 g/1 at 20 C) than for calcite, sothat part of the gypsum may be dissolvedby rain-water. However, calcite may alsobe dissolved according to the reactionCaCO3(s) + H2O(1) + CO2(g) -H= Ca
2+(aq)+ 2HCO3" (aq). There is also dry deposi-
tion of particles and gaseous pollutantsoriginating from traffic and other sources,to further increase the deterioration andmake the facades dirty. All these effects,in combination with the climatic factors,cause the stone surface to fall off as a loosepowder or as flakes. Occasionally, hardthin gypsum crusts are observed on thepalace. Serious deterioration is shown inFigs. 1-2. The original sandstone colourhas usually darkened, and partly the sur-face is black from soot and grime. This isalso true for the harder and more resistantRoslagen sandstone.
Large restaurations took place in thebeginning of the 19th century, around 1900,and between 1923 and 1929. The presentrestauration, which started in 1985, in-cludes cleaning, consolidation with silicaester, replacement of stone, and patinationof the new stone blocks. Dust and dirt havecarefully been removed with brushes,manual tools, and airbrasive units. Thewest, south, and east fafades and the outerwings have now been restored. Our sam-ples were taken prior to the restaurationand constitute a stone materal which, onthe average, has weathered for approxi-mately a century.
The palace is situated in central Stock-holm (Old Town). A main road runs closeto the north and east facades, causing ex-haust gases, particles, and vibration. Thereis a parking-lot for cars and buses alongthe south facade, and some neighbouringboat traffic. Besides, air pollutants aretransported to the area by wind. The airpollution situation in Stockholm was formany years quite bad, with average SO2concentrations around 200 ng/m3 (Statis-tics Sweden 1970-1993). The situation isnow much improved, and the SO2 concen-tration seldom exceed 10 ng/m3. A de-crease for NOX and soot particles has alsobeen noted, but not as pronounced as forSO2. The 1991 average concentrations atthe palace, in |i.g/m\ are 7 for sulphur di-oxide, 28 for nitrogen dioxide, and 38 forozone (Simon 1992).
The climate in Stockholm is also harm-ful to stone. There is often frost and snowin the winter, with an almost constant rela-tive humidity of 80-90 % and a tempera-
44 Nord: Analysis of deteriorated sandstone at the Royal Palace, Stockholm GFF117(1995)
Fig. 1 (left).Example of severecontour scaling (type"cs") of Gotlandsandstone. SampleG-36 from the southloggia of the RoyalPalace.
vt % S
Fig. 2 (below).Severe deteriorationof Gotland sandstonein the form ofscaling and exfolia-tion. Sample G-101on the south facade.
ture usually in the interval 0 to -10 CC. Atother seasons the relative humidity is highat night (80-90 %) and lower (40-50 %)at daytime. It is often windy around thepalace due to its position on a small islandbetween Lake Malaren and the Baltic Sea.The annual precipitation (rain and snow)is around 550 mm.
Sampling and analysesIn total 153 samples of Gotland sandstonewere taken from all facades, mainly 0.5-1.5 m above ground level. In addition, three'micro areas' were chosen for closer study.The Gotland sandstone samples are either
. - - - I
a powder scratched off the outermost milli-metres of an apparently undamaged surface(henceforward abbreviated 's'), a loosepowder of decaying stone ('p'), or small,thin (1 mm) flakes of deteriorating/exfoli-ating stone (T) . Occasionally a samplefrom a severe contour scaling ('cs') hasbeen taken, or a piece of thin hard gypsumcrust ('cr'). The Roslagen sandstone sam-ples are scratched powder or exfoliatedflakes. We have also sampled marble,mortar, and salt efflorescence.
All samples have been analysed with ascanning electron microscope (JEOL JSM-840A) equipped with a LINK/EDS unit forX-ray microanalysis. Crucial elements are
3 mmFig. 3. Typical sulphur concentration profile foran apparently undamaged piece of rain-shel-tered Gotland sandstone (type"sS"). Sample G-58 from the north loggia.
silicon, calcium, sulphur, iron, chlorine,and phorphorus, but also sodium, potas-sium, magnesium, and aluminium weredetermined. Cited values are given in percent by weight (wt%) assuming conven-tional oxide stoichiometry (H, C, N, Ocould not be analysed). For some selectedsamples we have also utilized other ana-lytical techniques like Guinier powder dif-fraction, plasma spectrography for heavymetals (Philips ICP PU-7450), or a carbonanalysing instrument (Carlo-Erba). Thesulphur isotope ratio "S/^S for gypsum-rich samples was determined with a VG-Micromass 602C mass spectrometer. Largepowder samples have been soaked withdistilled water and the respective anionsdetermined with a Perkin-Elmer HPLC-3Dion chromatograph, using tetrabutylammo-nium hydroxide as the mobile phase.
Thefaqad.esPrior to the present restauration, the con-ditions of the facades were observed. Thenorth f acade was most seriously damaged.Near the base, the Gotland sandstone waspowdered (often to a depth of 10-50 mm),exfoliated or had gypsum crusts, coveringabout 50 % of the surface. Closer to theroof only about 10-20 % of the surfacewere deteriorated. In exceptional casesstone blocks of extremely high qualityshowed no damage at all. Sculptural deco-rations were more affected than smoothstone blocks. The east part of the northfacade was covered by a thin black layerof soot and grime. The west, south, andeast facades as well as the two circularloggias were in slightly better conditionthan the north facade. The inner courtyard
GFF117 (1995) Nord: Analysis of deteriorated sandstone at the Royal Palace, Stockholm 45
was in somewhat better condition than theouter facades. The courtyard is protectedfrom strong winds which may have helpedto better preserve the sandstone.
The chemistry of the Gotland sandstonesamples is here focused on calcium andsulphur. In pure, undamaged Gotland sand-stone, calcium is basically bound in cal-cite, and the calcium content is around 3 -4 wt%; sulphur
46 Nord: Analysis of deteriorated sandstone at the Royal Palace, Stockholm GFF117(1995)
taken from ground to roof, show no significant chemical changewith height (cf. Table 1). On the other hand, observed damagewas most serious near the ground.
In previous papers (e.g. Nord & Tronner 1991) we have intro-duced an empirical factor DGF (Degree of Gypsum Formation),defined as DGF (%) = 100 x (wt% gypsum)/(wt% gypsum + wt%calcite), ranging from zero (pure calcite) to 100 % (only gyp-sum). The Royal Palace data indicate that the DGF average valueis slightly higher at the outer f acades (ca. 80 %) than in the innercourtyard (ca. 60 %). In particular, the northern facade displayshigh DGF values (average ca. 90 %).
The Cl content is usually 0.2 wt%, at rain washed as well as atsheltered parts of the facades. Correlation between excess Na andCl concentrations at the surface, compared to the undamaged bulkstone, indicates a substantial marine origin. The P surface con-centration is usually 0.3-0.6 wt%, with local maxima up to 2.2wt% near ground level due to contamination of urine. The ironconcentration is also significantly increased at the surface (often3-6 wt%) due to air pollution and, possibly, dissolution of iron-containing minerals in the bulk stone.
The three micro areasMicro area 1 is below a ground-floor window on the west wingof the prolongation of the south facade, as number 4 from the SWcorner. 27 samples of Gotland sandstone were taken, numberedG-61 to G-87 (Table 1). Fig. 4A shows that the observed damageis rather symmetric comparing left with right. Protruding or in-tensely rain-washed parts display a more pronounced state of de-terioration. It is noteworthy to also find a parallel analogy inanalytical data (Table 1). Rain-sheltered parts have acquired athin gypsum crust from dry deposition, with Ca and S con-centrations >9 wt%, whereas protruding, rain-washed parts, likesamples G-62 and G-82, have very low concentrations of theseelements, i.e., almost all gypsum that formed has been rinsed offby rainfall. Powder and flake samples are intermediate in thisrespect; part of the gypsum has been rinsed off from time to time.Little soot and grime was found in this micro area.
Micro area 2 is at the south facade, below the 8th ground-floorwindow from the SW comer of the palace. Again the deteriora-tion is symmetric comparing left with right (Fig. 4B), though withsome inexplicable differences in preservation such as betweensamples G-305 and G-322. In general, the stone was dirtier thanin micro area 1....