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.:
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
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 ofnave 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.

[1] Results are in volume %.
[2] Mixture ratio results from real modal composition.
Design by © Ondřej Kovář 2004  , Author: