Study of historical mortars and plasters

 

Gregerová M., Department of Mineralogy, Petrology and Geochemistry, Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic, e-mail: mirka@sci.muni.cz

Pospíšil P., Department of Geotechnics, Faculty of Civil Engineering, Brno University of Technology, Veveří 95, 662 37 Brno, Czech Republic, e-mail: pospisil.p@fce.vutbr.cz

 

 

Abstract:

The presented paper summarizes results of historical plasters and mortars research of the Church of Saint Wenceslas in Ostrava, the Church of the Translation of Virgin Mary in Brantice and from archaeological survey of the basement of the Church of Saint Catherine in Kelč.

The relative dating of particular building stages of said churches is based on micropetrographic identification of the sandy fraction of mortars (in each stage, a different sand was used) and on the degree of re-crystallization (ageing) of the original micritic carbonate. The assessment of plasters and mortars relates to the localization (inner, outer plaster) and the height level of the sampling site above ground and position of the mortar in the wall (plaster, bedding mortar). The azimuthal orientation of the sampled wall has been also documented.

Micropetrographic analysis of sandy fraction of mortars and plasters, together with the assessment of the degree of matrix re-crystallization, can in almost 95% cases confirm or exclude the assumed age of the particular construction phases.

 

Keywords: plasters, mortars, durability, degradation, re-crystallization

 

1.       Introduction

Many factors affect ageing of plasters and mortars. Besides the age of the structure they are composition of mortars and plasters, stage of matrix hardening, position in the structure, type of building material, climatic conditions, capillary elevation of ground water, level above ground, azimuthal orientation, insulation of structure, quality of maintenance, reconstructions, utilization of object etc.

Decreasing of quality of outer parts of structure is first result of ageing. Plaster alters its texture, color, hardness and cohesion with wall construction material are decreasing and during the latest period parts of plaster peel off and fall down. Plaster does not supply protection and aesthetic function.

Employees of Museum in Ostrava offered the best structures for study. They are churches in Ostrava (Saint Wenceslas) and Brantice (Translation of Virgin Mary).

Medieval core of Saint Wenceslas church forms polygonally closed presbytery, outer walls and square shaped Western Tower. Author of building historical project D. Líbal supposes construction phase during second half of 14th century. Archaeological research (V. Sikulová) during 70’ discovered relicts of basement of walls in presbytery and in both aisles, which belong to former construction phase at the end of 13th century eventually in the beginning of 14th century. Archaeological research in 1998 discovered basement of originally Gothic sacristy built during the second construction phase and relicts of foots of columns. Columns probably held up flat beam ceiling up to 16th century when nave and aisles were covered by vault.

Samples of mortar of Saint Catherine church in Kelč were taken from outer side of presbytery basement. It was built according to archive data in 80’ of 16th century.

Mortars and plasters represent collection of samples of church of Translation of Virgin Mary in Brantice.

 

2.        Plaster and mortar durability

Strength, hardness and plaster or mortar diffusion depend on type and properties of applied sources, component mixture ratio and hardening conditions. Some historical plasters are better in properties after several centuries than building stone in structure. Example of high quality mortar is firm, consistent lime mortar (3 m2 of pre-Romanesque age), which was discovered in Mikulčice (Southern Moravia in Czech Republic).

Formation of firm lime mortar or plaster depends on drying of lime suspension, which is in close relation to shrinkage of mortar. Calcium silicates are formed by dissolution of quartz in alkali environment. Last process is carbonation.

The third above-mentioned process – carbonation – is the most significant for mortar strength. Lime matrix in mixture with water (together with soluble calcium alkali silicates) fill pores among particles of filling material (sandy grains). Reaction with atmospheric carbon dioxide forms calcium carbonate, which is identical with micrite by the texture. Rate of carbonation is the highest in relative humidity from 50% to 60%. Process does not occur in dry conditions and in higher relative humidity is almost stopped for difficult penetration of CO2 through the pore system filled by water (Hošek, Muk 1990). Original high alkalinity of lime mortar (pH 12,5 to 13,5) is gradually decreased to approximately pH 8. It may be said that strength of lime mortar depends on partial pressure of CO2 (its usual content in atmosphere is 0,03%) on amount, kind and stage of lime slaking (quick lime accelerates process of solidification but it blocks penetration of CO2 into mortar) on porosity of mortar (determined by hydraulic coefficient, ratio of matrix and filling material, size of sandy particles and particle size distribution curve) by moisture and temperature of surrounding environment.

Šujanová (1981) determined rate of carbonation of standard lime mortar in usual atmospheric conditions as follows: 59% of calcium hydroxide changed to calcium carbonate during 2 days, after 5 weeks it was 75% and after 2 years 86%. We have to find explanation stage of slaking of lime matrix (all studied mortar samples contain relicts of quick lime).

 

3.        Relative age determination

It was verified by study of more than 300 samples of historical plasters and mortars in many medieval structures. It is possible to determine within the frame of a structure relative age of mortars and plasters.

Method is based on following:

 

Identification of mineral composition of sandy fraction.

State of re-crystallization of carbonate matrix.

Height level of mortar and plaster above ground together with azimuthal orientation.

 

Gregerová, Vlček (1994) studied relative age determination on the base of  identification of sandy fraction and its relation to matrix.

Solidification of lime mortar as it was mentioned above is initiated by changing of calcium hydroxide to calcium carbonate affected by atmospheric CO2. Forming CaCO3 is sub-microscopic in size of crystals and relates to micrite. Micro-crystallized carbonate (sparite) is formed (at first in pores of mortar and later in matrix) by partial dissolution and following re-crystallization. Amount of sparite increases in time but increasing is not linear and depends on many factors.

Mortar degradation caused by atmospheric humidity, rainwater, snow thawing, capillary elevation of groundwater in relation to position of studied mortar in the structure (height level above ground, azimuthal orientation) causes quicker or slower dissolution of micrite and crystallization and gradual crystallization - increasing of calcite crystals – formation of sparite (Figure 1,2). It has been verified by long-term study that these processes are quicker in mortars and plasters prepared of slaking of incompletely burned lime (CaO). Crystallization pressure of newly formed calcite crystals (according to physico-chemical conditions in place of formation) leads to reduction of mortar and plaster strength and follows up by their falling down from face of wall.

 

They are identifiable according to presented methods mortars of two time periods in the structure of church of Translation of Virgin Mary in Brantice. Classification of mortars shows Table 1.

 

Table 1: Classification of studied mortars of Translation of Virgin Mary church in Brantice within individual construction phases.

Older construction phases

Sample No.:

localization

1

k. 908, bedding mortar of aisle basement

2

bedding mortar of face of Victory arch 1. construction phase

3

bedding mortar of outer face of basement of eastern nave wall

4

bedding mortar of inner face of basement of southern nave wall (place A3, k. 956)

5

bedding mortar of southern face presbytery basement (place C2, k 902)

6

place A3, k 906,  bedding mortar of inner face of southern nave wall

Younger construction phases

7

bedding mortar of top of northern nave wall

8

inner plaster of top of northern nave wall (approximately 1593)

9

younger inner plaster of northern nave wall

 

Mortars of older construction phase are macroscopically the same in colour, granularity and sandy fraction composition. They belong to unsorted mortars by granularity and composition. Fragments of pelosiderites and bricks are observable in all samples. Typical is also high amount of clay minerals. All samples contain admixture of organic matter similar by optical parameters to white of the egg. Re-crystallization of micritic matrix is visible in thin sections. Size of sparite calcite crystals is within the interval 0.1 to 0.05 mm.

Mortars of younger construction phase differ in stage of re-crystallization of micrite to sparite. The sample No. 9 is exceptional within younger construction phase. Two layers of plaster form it. But the difference in matrix re-crystallization between both layers is very small and it is not possible to accurately identify if it is formed during one construction phase or if the outer layer is younger reconstruction.

Collections of studied mortar samples show Tables 2, 3 and 4 of St. Wenceslas church in Ostrava. They can be divided into three groups – three construction phases.

 

Table 2: Material composition of studied mortars and plasters of St. Wenceslas church in Ostrava.

 

1

2

3

4

5

6

7

8

9

10

12

13

matrix

24[1]

44

36

46

33

41

25

43

39

45

67

53

pores

20

12

16

12

12

12

11

16

17

19

5,6

8,7

sand

56,1

50,3

46,9

42,3

55,3

45,5

63,7

40,9

43,3

36,3

26,4

38,3

Material composition of sandy fraction

quartz

25

14

17

17

5,9

7,2

9,7

8

14

13

12

2,9

ortho and metaquartzites

9,9

14,9

23,7

14

9,2

18

40

22

17

12

6,7

9

other rock fragments

20

11

2,7

8,3

12

4,2

6,4

9,7

8,2

8,5

6,8

8,9

feldspars

0,6

3,4

3,1

2,5

1,7

3,4

4

1,2

3,4

1,9

1,9

1,4

accessories

0,4

7

0,3

0,2

0,8

0,7

0,1

 

0,3

0,6

0,2

0,8

carbonates

0,2

 

0,1

0,3

23

10

3,2

 

0,2

0,1

 

14

micas

 

 

 

 

2,7

2

0,3

 

0,2

0,2

 

1,6

 

Table 3: Mixture ratio of lime and sand in studied mortars of St. Wenceslas church in Ostrava.

 

Mixture ratio (lime : sand)[2]

 

1

2

3

4

5

6

7

8

9

10

12

13

lime

1

1

1

1,1

1

1

1

1,04

1

1,2

2,5

1,3

sand

2,3

1

1,3

1

1,6

1,2

2,6

1

1,2

1

1

1

 

Table 4: Classification of studied mortars of St. Wenceslas church in Ostrava within individual construction phases.

The oldest construction phase

Group

Sample No.:

Localization in the structure

a

1

sample of outer face of basement of southern presbytery wall (k. 900)

 

3

sample  of basement of  buttress of northern nave wall in place of Victory arch prolongation (k 906)

 

4

sample of basement between first couple of  nave buttress (k. 908)

 

10

 sample of inner face of tower basement (k.917)

b

2

sample of outer face of Gothic sacristy basement ( k. 904)

 

9

sample of inner face of northern basement of Victory arch (k. 915)

 

7

 sample of inner face of southern nave wall ( k. 912)

 

8

 sample of inner face of southern Victory arch basement (k. 914)

Younger construction phase

 

5

sample of basement wall relict in northern aisle (k.909)

 

6

sample of basement wall relict in southern aisle (k. 910)

 

13

sample of the end of nave basement

The youngest construction phase

 

12

sample  of mortar of outer face of Classicism sacristy (1803-1805)

 

Groups a) and b), which were recognized within oldest construction phase differ in re-crystallization of micrite and thickness of sparite layers. The group b) has higher amount of sparite and crystals are bigger.

Samples of church in Kelč are unique. They are samples of bedding mortar taken of outer face of presbytery basement, which was built according to archive data during 80’ of 16th century. The thickness of sparite layer composed of fibrous calcite crystals is up to 5 mm (Figures 3 and 4). Based on archive data of construction and microscopic study the rate of calcite re-crystallization is 0.5 to 1.25 mm per 100 years.

 

Figure 1: Počátek rekrystalizace mikritové matrix na sparit. St. Wenceslas church in Ostrava (16th century), XPL. Photo M. Gregerová.

Figure 2: Re-crystallized matrix. Translation of Virgin Mary church in Brantice XPL. Photo M. Gregerová.

Figure 3: Re-crystallized matrix of calcareous mortar of 16th century. Fibrous structure of calcite. Church in Kelč. Mag. 200x, PPL. Photo M. Gregerová.

Figure 4: The same sample as on Fig. 1, XPL. Photo M. Gregerová.

 

 

4.       Conclusions

Results of study of collection of 300 samples of plasters and mortars verified that micropetrographic analysis of sandy fraction of mortars and plasters, together with the assessment of the stage of matrix re-crystallization, can in almost 95% cases confirm or exclude the assumed age of the particular construction phases.

Mixture ratio of lime and sand recommended by Šujanová (1981) for identification of Romanesque, Gothic and Renaissance mortars and plasters was not verified.

The research has been supported by Czech Grant Agency projects No.: 103/99/0941 and 103/00/0607.

 

5.      References

Gregerová M., Vlček R. (1994): Petrografická a geochemická charakteristika malt a omítek kostela sv. Petra a Pavla v Brně.- Geol. Výzk. Mor. Slez. V r. 1993, 104-105. Brno.

Hošek J., Muk J. (1990): Omítky historických staveb.- SPN, 143 str. Praha.

Šujanová O. (1981): Povrchové úpravy pamiatkovych objektov.- Sborník ze semináře SÚPSOP.

 


[1] Results are in volume %.

[2] Mixture ratio results from real modal composition.