Educational Linkage Approach In Cultural Heritage
Educational Toolkit
Teaching Material
Basic Course
Module
2
Knowing the built heritage
Topic
2.5
Historic building materials:
Stones, ceramics, mortars
Prof. Antonia Moropoulou - NTUA – National Technical University of Athens
Educational Linkage Approach In Cultural Heritage
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Prof. Antonia Moropoulou - NTUA – National Technical University of Athens
Educational Linkage Approach In Cultural Heritage
Abstract
The current presentation examines the basic categories of the historic building materials
used for the construction of monuments throughout the centuries. It examines their
evolution and their selection criteria. The main categories, stones, ceramics and mortars
are then described in detail, regarding their provenance, manufacturing-forming, basic
properties and characterisation
Prof. Antonia Moropoulou - NTUA – National Technical University of Athens
Educational Linkage Approach In Cultural Heritage
Content
Table of contents of this presentation
Historic materials
2.5.1 Stones
2.5.2 Ceramics
2.5.3 Mortars
Prof. Antonia Moropoulou - NTUA – National Technical University of Athens
Educational Linkage Approach In Cultural Heritage
2.5.1. Historic building materials - Stones
Mineral: Natural, homogeneous solid material that forms usually through inorganic procedures
Rock: The material of the solid layer of earth, which is a product of geological actions. It consists of minerals that
influence the physicochemical properties of the rock
Stone: A rock that has been treated and formed into shape by hand or mechanical means
Rock Categories based on their geological formation path
Igneous: They are formed when molten magma cools
Sedimentary: They are made by deposition of either clastic
sediments, organic matter, or chemical precipitates followed
by compaction of the particulate matter and cementation
during diagenesis
Metamorphic: are formed by subjecting any rock type to
different temperature and pressure conditions than those in
which the original rock was formed.
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials: Stones, Ceramics, Mortars
Educational Linkage Approach In Cultural Heritage
2.5.1. Historic building materials - Stones
Igneous rocks
Santiago di Compostela, Spain
(built mostly with granite)
Granite (upper: photograph,
below: polarized microscopy).
Plutonic rock with holocrystalline
matrix, leucocratic, acidic. It
contains alkali feldspars, biotite
and/or muscovite (micas) and
plagioclase
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials: Stones, Ceramics, Mortars
Educational Linkage Approach In Cultural Heritage
2.5.1. Historic building materials - Stones
Igneous rocks
Basalt: The extrusive rock of
gabbro. It is melanocratic with a
glassy or crystalline matrix. The
crystals are mainly basic feldspars,
augite, diopside or olivine (Below
polarized microscopy image)
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials: Stones, Ceramics, Mortars
Educational Linkage Approach In Cultural Heritage
2.5.1. Historic building materials - Stones
Sedimentary rocks
Medieval City of Rhodes:
Fossiliferous calcareous highly
porous stone, where
microcrystalline calcitic binder
is embedded by calcareous
aggregates
Scanning
Electron
Microscopy
Conglomerate: is a clastic sedimentary
rock that contains large (greater than 2mm
diameter) rounded clasts. The space
between the clasts is generally filled with
smaller particles and/or a chemical
cement that binds the rock together
Process of formation of sedimentary rocks
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials: Stones, Ceramics, Mortars
Educational Linkage Approach In Cultural Heritage
2.5.1. Historic building materials - Stones
Sedimentary rocks
Limestone of Attica
Fossiliferous Sandstone
Fossiliferous limestone
Shelly limestone
Limestone: is a sedimentary rock composed largely
of the mineral calcite (calcium carbonate: CaCO3).
Like most other sedimentary rocks, limestones are
composed of grains, however, around 80-90% of
limestone grains are skeletal fragments of marine
organisms such as coral or foraminifera. Other
carbonate grains comprising limestones are ooids,
peloids, intraclasts, and extraclasts. Some limestones
are formed completely by the chemical precipitation
of calcite or aragonite. i.e. travertine.
Photos courtesy of Museum of Archaeology & Art History – University of Athens
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials: Stones, Ceramics, Mortars
Educational Linkage Approach In Cultural Heritage
2.5.1. Historic building materials - Stones
Metamorphic rocks
Marble: A metamorphic rock resulting from the
metamorphism of limestone, composed mostly of
calcite (a crystalline form of calcium carbonate,
CaCO3). It has a hardness 3 Mohs and specific
gravity 2,7 g/cm3. It is very durable against decay
factors due to its microstructure. However, it can be
damaged by fire since at 900οC, it decomposes
calcium carbonate (CaCO3) into CaO and CO2.
Color is white, grey, pink or green with various veins
Pentelic marble, Acropolis of Athens
Photo courtesy of A. Moropoulou
ELAICH Athens Experimental Course
Acropolis of Athens, Parthenon marbles
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials: Stones, Ceramics, Mortars
Educational Linkage Approach In Cultural Heritage
2.5.1. Historic building materials - Stones
Metamorphic rocks - Representative Greek Marbles
Marble from Penteli
Marble from Naxos
Marble from Skyros
Marble from Evia
Photos courtesy of Museum of Archaeology & Art History – University of Athens
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials: Stones, Ceramics, Mortars
Educational Linkage Approach In Cultural Heritage
2.5.1. Historic building materials - Stones
Metamorphic rocks - Representative Greek Marbles
Marble from Arcadia
Marble from Eretria
Marble from Volos
Marble from Thasos
Photos courtesy of Museum of Archaeology & Art History – University of Athens
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials: Stones, Ceramics, Mortars
Educational Linkage Approach In Cultural Heritage
2.5.1. Historic building materials - Stones
Stone properties
Porosity
Porosity and pore size distribution play an important role as they directly influence the fluid flow through the stone. Decay
phenomena such as soluble salts efflorescence and subefflorence, freeze-thaw decay and biological decay are highly dependent
on the stone’s porous network.
Density
Density, and in particular apparent density, is a good indicator of a stone’s decay status. It can decrease if cracks and
microcracks are developing, or it can increase if the porous network is filled with salts. In both cases, this has detrimental effects
on the mechanical properties of the stone
Water absorption
It controls the water transport phenomena and thus all the decay mechanisms associated with the presence of water (salt decay,
biological decay, freeze-thaw decay)
Hardness
It is important in application where wear resistance is required as the primary function of the stone (e.g. floors)
Thermal expansion coefficient
Very important property especially at climates that demonstrate high temperature fluctuations. A large thermal expansion
coefficient can result in mechanical stresses in the structure if the stone is not allowed to expand or contract freely. Structures are
more susceptible to failure when building materials of very different thermal expansion coefficient are combined, as this can lead
to differential mechanical stresses
Mechanical properties
The mechanical properties of interest are compression strength (since building stones are usually placed such that they are
subjected to compression stresses) and the modulus of elasticity
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials: Stones, Ceramics, Mortars
Educational Linkage Approach In Cultural Heritage
PROPERTIES OF STONES






Porosity
Density
Sorptivity
Mechanical properties
Hardnes
Coefficient of thermal expansion
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials: Stones, Ceramics, Mortars
Educational Linkage Approach In Cultural Heritage
Porosity
Porosity may exist in igneous rocks for various reasons:
 Gas dissolved in the magma that is released during crystallization, which then
becomes trapped in pockets between the mineral grains in the rock. These pores
are most often closed, that is, fluids cannot enter them from the surface of the
stone
 The rock might originally contain relatively soluble
grains that are dissolved by weathering, so that a
network of pores is created when they are removed
 As the rock cools the grains of different minerals
may contract by different amounts, leading to
cracking between the grains
In sedimentary rocks, the pores are created when the particles of sediment pack together. The amount of porosity in
the final rock depends on the amount of cementing material that is deposited. As the cement has to move through the
open pores, it is unlikely to obtain a structure with completely closed pores. Typically, sedimentary rocks have
interconnected pores
Metamorphism will usually reduce the porosity of the stone, due to the high pressures involved. The size of the pores
might simply be reduced, or some pore collapse might occur. In such a case, the interconnected pores might be
disrupted, and converted to isolated pockets
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials: Stones, Ceramics, Mortars
Educational Linkage Approach In Cultural Heritage
Porosity
The porosity P of a porous medium (such as a rock or a sediment) describes the
fraction of void space in the material, where the void (pore) may contain, for example,
air or water. It is defined by the ratio:
P
Vp
V

Vp
Vs  VP
where VP is the volume of pores (void-space) and V is
the total or bulk volume of material, including the solid
and void components, and Vs is the volume of the
solid phase
Porosity is defined as:
- Total porosity: refers to all pores (in this case Vp refers to all pores)
- open or active porosity: refers to those pores that allow the transfer – flow of
water (in this case Vp refers to those pores that are accessible, i.e. the open pores
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials: Stones, Ceramics, Mortars
Educational Linkage Approach In Cultural Heritage
Density
The density ρ of a porous medium (such as a rock or a sediment) is the ratio of the
mass (m) of a body to its volume (V):

m
V
There are two types of density to distinguish: the density of the minerals that constitute the rock,
and the bulk density of the rock that includes a mixture of minerals plus porosity. The difference
between them has to do with whether we include the volume of the pores in V
m
s 
Vs
m
m
b  
V Vs  V p
True density: The ratio of the mass (m) of a sample over its true
volume (Vs) i.e. without empty spaces
Apparent density: The ratio of the mass (m) of a stone sample over
its apparent volume (V) i.e. including empty spaces
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials: Stones, Ceramics, Mortars
Educational Linkage Approach In Cultural Heritage
Sorptivity
The sorptivity is a measure of the rate at which a liquid rises into a porous body. Capillary pressure
draws the liquid into the pores, just as it draws water into a capillary tube. The smaller the pores, the
higher the liquid will rise.
The maximum height He that a liquid will rise in a capillary depends on the density of the liguid, ρ, the gravitational
constant, g = 9.8 m2/s, the liquid/vapor surface tension (interfacial energy), σLV , of the liquid, the contact angle (angle
made by the surface of the liquid where it makes contact with the wall of the capillary), θ , and the radius of the tube:
2 cos( )
H e  LV
 gr
For water:
σLV = 0.072 Joules/m2
θ ≈ 0 -> cos(θ) ≈ 1
ρ = 1000 kg/m3
The rate of humidity rise can be described by a first order kinetic model*, where H is the height at time t, He is the
equilibrium height and tcr is a time constant, and H0 is a correction factor that relates to the humidity portion that is
confined in the stone pores and cannot be removed by drying
dH 1
 (He  H )
dt tcr
H  H e  ( H e  H 0 ) exp(
t
tcr
)
*M. Karoglou, A. Moropoulou, A. Giakoumaki, M.K. Krokida “Capillary rise kinetics of some building materials” J of Colloid
and Interface Science, 284 [1] 260-264 2005
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials: Stones, Ceramics, Mortars
Educational Linkage Approach In Cultural Heritage
Sorptivity
One normally measures the increase in weight as liquid enters a stone, rather than directly
measuring the height of rise. Suppose a stone sample with a bottom area A is placed in contact
with water. The amount of water adsorbed, ΔW depends on the contact area and time
Sorptivity is then defined as follows
W
C

A t
P 3 r cos(  )
5
It depends:
-
From the porosity: More porous stones absorb more water
-
From the shape and size of the pores: Pores that are large and rather straight, allow the easy
entrance of water. In contrast, pores of small diameter and with complex shapes and
interconnections as well as closed pores (that do not connect between each other) makes
wetting of stone more difficult
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials: Stones, Ceramics, Mortars
Educational Linkage Approach In Cultural Heritage
Mechanical Properties
The most important property for a building material, from the point of the engineer, is the compressive
strength, because the material must support the weight of the building.
The compressive strength, σc is typically measured by squeezing a
cylinder or cube of material between the jaws of a testing machine.
Regardless of the shape of the sample, the strength is given by the force, F,
at which the fracture occurs, divided by the area, A, of the surface on which
the force is applied: σc = F/A
Brittle materials are weaker in tension (i.e., when they are pulled
apart) than in compression (i.e., when they are crushed). This is true
because tension pulls the flaws open, whereas compression tends to
close them. When salt crystals grow inside a stone, they push on the
pore walls and create tensile stress. Therefore, in evaluating durability,
we are most interested in the tensile strength of stone. This is
measured by the Brazilian test, or splitting test, which uses a cylindrical
specimen (d) and applies a force along its length (L) . The tensile
strength is then given by:
σt = 2F/(π L d)
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials: Stones, Ceramics, Mortars
Educational Linkage Approach In Cultural Heritage
Mechanical Properties
When a solid object is subjected to tensile stress, its length increases from L to L + DL.
The fractional change in length is called the strain, ε
Material
Steel
Elastic Modulus (GPa)
200
Window Glass
60-70
Marble
30-70
Limestone
10-50
Concrete
15-25
The ratio between the stress, σ = F/A, and
the strain is called the elastic modulus
(or, Young's modulus), and is usually
denoted by E:

  E
Rather than stressing a material to
measure its modulus, the elastic
modulus can be found
from the velocity of sound waves
passing through it.
The elastic modulus can be
calculated from the bulk density, db,
and the pulse velocity, v:
E  db v2
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials: Stones, Ceramics, Mortars
L
L
Educational Linkage Approach In Cultural Heritage
Hardness
Rock
Hardness
Mohs scale
Granite, Gneiss
5,5-7,5
Basalt
4,5-6,6
Argillaceous schist
1,5-3,5
Psammite
1,5-7,5
Limestone, marble
2,5-3,5
Dolomite
2,5-4,5
Phyllite
2,5-5,5
Quarzite
6,5-7,5
Mohs' scale of mineral hardness quantifies the scratch resistance of minerals by comparing the ability of a harder
material to scratch a softer material.
With the Mohs Scale, the hardness of a material is measured against the scale by finding the hardest material that it can
scratch, and/or by identifying the softest material that can scratch it. If a given material can be scratched by topaz, but
not by quartz, its hardness on Mohs scale is 7.5.
Note: The table above shows a comparison with absolute hardness measures using a sclerometer. The Mohs scale is a
ordinal or successive scale and therefore, does not measure or compare actual hardness. For instance, corundum is
twice as hard as topaz, but diamond is almost four times as hard as corundum yet there is only one step between each
of these three minerals
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials: Stones, Ceramics, Mortars
Educational Linkage Approach In Cultural Heritage
Coefficient of thermal expansion
Materials expand because an increase in temperature leads to greater thermal
vibration of the atoms in a material, and hence to an increase in the average
separation distance of adjacent atoms.
The linear coefficient of thermal expansion α describes by how much a material
will expand for each degree of temperature increase, as given by the formula:
dL

L dT
where: dl is the change in length of material in the direction being measured
l is the overall length of material in the direction being measured
dT is the change in temperature over which dl is measured
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials: Stones, Ceramics, Mortars
Educational Linkage Approach In Cultural Heritage
2.5.1. Historic building materials - Stones
Stone properties
Hardness
Mohs
Hm
Hardness
Specific weight
Porosity %
Compression
strength
103 psi
Fracture
toughness
103 psi
Grinding
hardness
Ha
Thermal
expansion
10-7 / oC
Granite
5.80-6.60
85-100
2.54-2.66
0.4-2.36
14-45
1.3-5.5
37-88
37-60
Syenite
5.68-6.58
82-99
2.72-2.97
0.9-1.9
27-63
2.3-3.2
(37)
Gabbro
4.76-6.21
40-92
2.81-3.03
0.3-2.7
18-44
2-8
20-30
Basalt
4-6
50-92
16-49
2-8
22-35
Limestone
2.79-4.84
10-60
2-37
0.5-5.2
2-24
17-68
Psammite
2.40-6.1
20-70
5.-36
0.7-2.3
2-26
37-63
Gneiss
5.26-6.47
74-97
2.64-3.36
0.5-0.8
22-36
1.2-3.1
13-44
Quarzite
4.2-6.6
55-83
2.75
0.3
30-91
1.2-4.5
60
Marble
3.7-4.3
45-56
2.37-3.2
0.6-2.3
10-35
0.6-4
7-42
27-51
45-58
2.71-2.9
0.1-4.3
20-30
5-16
6-19
45-49
Rock
Schist
Sh
1.79-2.92
0.26-3.60
Quartz
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials
180
Educational Linkage Approach In Cultural Heritage
2.5.1. Historic building materials - Stones
Decay of stones
Salt crystallization
It refers to the mechanical decay of porous stones, through the
development of mechanical tensions in the interior of the stones (pores)
from salt crystals and disruption of the material when these tensions
surpass its strength. The main salt sources in masonries are rising damp
(from the ground), neighboring materials, such as cement and usually the
binding mortar itself. If the evaporation takes place in the interior of the
mass of the material, the decay appears in the form of alveolation)
Mechanism of alveolation
Metamoprhic rock (marble)
Archaeological site of Eleusis
P. Theoulakis, A. Moropoulou
“Salt crystal growth as weathering
mechanism of porous stone on
historic masonry”, J. Porous
Materials, 6 (1999) pp. 345-358
Sedimentary rock (biocalcarenite)
Medieval City of Rhodes
Black crust
It is the result of gypsum formation in the calcite surface and of the
absorption of black smoke particles, Η/C and other particles of
atmospheric origin that act as active catalysts in the transformation
of calcite into gypsum. Its surfaces are protected from direct
washout from rain water
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials
Photo courtesy of A. Moropoulou
Educational Linkage Approach In Cultural Heritage
2.5.1. Historic building materials - Stones
Decay of stones
Metamoprhic rock (marble)
Detail of the Parhenon frieze
Gypsum formation
The continuous peeling of the weathered surface reveals fresh material that in turn, is
exposed to the creation of gypsum layer reaction and subsequent peeling, with the result
that the phenomenon develops in depth
The deterioration of the gypsum layer at the surface of the pentelic marble removes the
details from the face and body of the Caryatides statues. In order to avoid further
deterioration, for their protection, the Caryatides were placed in the Acropolis Museum.
Replicas replaced the original ones at the Erectheion.
The decay patterns of building stones
are governed by their composition,
microstructure, physicochemical and
physicomechanical properties
For more information:
See Module 3 – Topic 3.4
Phenomena and mechanisms of
decay
Photo courtesy of K. Labropoulos
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials
Prof. A. Moropoulou, NTUA
Educational Linkage Approach In Cultural Heritage
2.5.2. Historic building materials - Ceramics
As described in topic 2.1 the use of ceramics as building materials date back to the
neolithic period when mud brick houses started appearing that were coated with plaster.
Since then, ceramics have been used as the main building material of many
civilizations, even when other building materials (e.g. stones) where used as the
building material of choice for structures of capital importance (e.g. temples). The
Romans in particular, followed by the Byzantines, used fired bricks as their main
building element in their architecture. The use of ceramic bricks has decreased with the
introduction of more modern materials such as steel and Portland cement both of
which, however, do not exhibit the durability that the historic ceramic material has
demonstrated in the span of centuries.
Unfortunately, the shift to more modern materials has resulted in much of the
manufacturing technology and especially the compositions of the traditional ceramics
being lost. Today, significant efforts are focusing on reverse engineering these
materials, either in order to manufacture ceramics compatible with the traditional ones,
or in order to improve and impart greater durability to ceramic materials for modern
applications. Sustainability and the concept of “green building” has revived the interest
in the historic ceramic technology
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials
Educational Linkage Approach In Cultural Heritage
2.5.2. Historic building materials - Ceramics
Ceramics are defined as non metallic inorganic solids with crystalline or amorphous structure.
They are usually, tough, brittle, with a high melting point , with low electrical and thermal conductivity,
with good thermal stability and with high compression strengths
•
•
Classic (natural): They are made from natural materials, usually from clay-earth, lime and
sand. They are used for pottery, kiln bricks, tiles, glass and cement
Technical: They are made since 1900 with various properties (high strength, electronic
applications, biological applications, insulation, resistances, semi-conductors)
Advantages of technical ceramic materials
The structural ceramics offer certain advantages over the
natural structural materials: Generally, they are lighter from
equivalent size structural stones. It is possibly to obtain
ceramic structural units of any shape and size, since the
ceramic material are typically processed in a fluid state, thus,
through the use of molds they can be formed accordingly
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials
Educational Linkage Approach In Cultural Heritage
The Hagia Sophia Bricks
Moropoulou, A., Cakmak, A.S., Polykreti, K., “Provenance and technology investigations of the Agia Sophia
bricks”, J. American Ceramic Society, 85 [2] (2002) pp. 366-372
Lightweight - High Strength - Microporous bricks
The Great Basilica of Hagia Sophia (532–537 A.D.)
is famous for its architectural and artistic
magnificence. According to Diegesis (Narration), a
9th century text, the great dome was constructed
with specially ordered brick from Rhodes: “. . .
Special light bricks have been ordered from the
island of Rhodes, weighing one-twelfth the weight
of normal brick, to use them for building the four
main arches and the dome. The structure is thus
completed. . . . ” After a strong earthquake in 557
A.D., the great dome collapsed and was rebuilt
with lightweight bricks from Rhodes
Within the framework of a trilateral protocol
agreement among Princeton University, Bogazici
University, and the National Technical University of
Athens concerning the structure and materials of
Hagia Sophia and other monuments of historic
significance in Byzantine Istanbul provenance and
technology investigations of the Hagia Sophia
bricks were performed.
Sample bricks were studied from Hagia Sophia,
from Byzantine monuments in Istanbul, and from a
6th century Basilica in Rhodes
Description of the
sampled bricks and tiles
Neutron Activation
Analysis combined with
multivariate statistics
allow the calculation of
probabilities of Hagia
Sophia samples
belonging to the Istanbul
or Rhodes groups
The possibility the bricks from the dome of Hagia Sophia originating from
Rhodes is up to 97% compared with the raw materials of ceramics used in
other Byzantine Monuments of Istanbul
Prof. Antonia Moropoulou – Topic 3.6: Diagnosis of decay: Mechanisms, criteria and techniques
Educational Linkage Approach In Cultural Heritage
The Hagia Sophia Bricks
Moropoulou, A., Cakmak, A.S., Polykreti, K., “Provenance and technology investigations of the Agia Sophia
bricks”, J. American Ceramic Society, 85 [2] (2002) pp. 366-372
Lightweight - High Strength - Microporous bricks
Hagia Sophia Masonry Technology
Dome construction:
• compact bricks bound with strong,
hydraulic, crushed brick mortars
Narthex and Outer-Narthex
• Compact bricks bound with strong
hydraulic crushed brick mortars
• Reinforcing zones of large
sculptured stones at the arches
base support and curvature
Brick dimensions:
Length 30 - 36cm
Height 3.5 - 4.0cm
Joint mortar: Height 6 - 7cm
The bricks are similar to those used in the Great
Basilica of Rhodes. However, the Hagia Sophia
bricks are lighter (45% porosity) than the other
bricks (35% porosity).
The Hagia Sophia bricks are made of a
noncalcareous, fine paste with quartz temper,
fired at low temperature (750°C). Exceptional
characteristics are their homogeneity and small
pore-size distribution (0.3 – 0.8 μm), as well as
the unexpectedly high (up to 1.3 MPa) tensile
strength of the dome bricks as opposed to their
high porosity.
Τhe very narrow pores, of an almost standard
diameter, probably are the result of a fine
sieving of the clay mix or pressing of the brick
molds or of levigation or grinding, as well as of
accurate-stable
firing
temperature
and
controlled furnace atmosphere.
Hagia Sophia, Upper
South Gallery, Arch
area where bricks,
mortars and stone are
visible after removal
of the mosaic layer
during conservation
interventions
Photo courtesy A. Moropoulou
This reveals the existence of organized workshops, with mass production and a level of experience and
know-how sufficient to satisfy the demand for standardized product quality.
Prof. Antonia Moropoulou – Topic 3.6: Diagnosis of decay: Mechanisms, criteria and techniques
Educational Linkage Approach In Cultural Heritage
2.5.2. Historic building materials - Ceramics
Historic ceramics manufacturing process
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Selection of clay based on the desired use of the ceramic
Cleaning and precipitation process depending on the required quality of the final product
Mixing of clay with sand or ground limestone depending on the desired properties of the
ceramic product
Forming of the ceramic article with hands, ceramic wheel of molds -> green ceramic
Drying of the green ceramic under controlled conditions
Sintering in a kiln with the appropriate atmosphere depending on the desired properties of the
final ceramic product
Forming of ceramics
Forming of the raw material to its final shape is accomplished either manually (e.g. during
the manufacture of classic pottery) or at an industrial or semi-industrial scale with
appropriate devices or molds. In both cases, pressure is applied to form the final shape. The
pressure depends on the forming technique:
Wet method: Clay contains enough water so that it has adequate plasticity
Dry method: Clay is almost dry and requires high pressure in order to form the final shape
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials
Educational Linkage Approach In Cultural Heritage
2.5.2. Historic building materials - Ceramics
Drying of ceramics
Drying aims to remove the water that was necessary for
forming the “green ceramic”. It is a process that needs to be
careful designed so that cracks and shape deformations are
avoided. Diffusion of water from the interior of the ceramic
body to its external surface, where it can evaporate, needs to
be performed gradually. Two methods are usually employed:
Natural drying: The green ceramics are placed under well
ventilated sheltered areas, and when they have dried
adequately, they are placed on open air, under direct sunlight
to dry completely
Technical drying: The green ceramics are placed in specially
designed chambers where either hot air from the kiln is fed, or
their floors and walls are heated by the kiln exhaust air.
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials
Educational Linkage Approach In Cultural Heritage
2.5.2. Historic building materials - Ceramics
Sintering of ceramics
Sintering aims to stabilize the shape of the ceramic and allow it
to obtain its final physical and mechanical properties, such as
mechanical strength, impermeability, surfacial hardness etc.
Sintering removes the pores from the ceramic matrix, in effect
shrinking the ceramic article, while in parallel, “gluing” the grains
together, creating a strong body.
It is a process that has to be carefully designed, and it still has
the form of an “art”. The design of the kiln, the temperature it can
reach, the placement of the ceramic articles inside the kiln, the
atmosphere of the kiln (oxidizing or reducing), the heating rate
and heating pattern (temperature plateaus), the cooling rate and
cooling pattern, all play an important role in obtaining the desired
properties.
Schematic of the sintering process:
(Top left - Green material) the porosity of the green body is between 40-60%.
(Top right - Initial stage ) as the temperature increases to the sintering temperature,
“necks” between the grains are formed and grow.
(Bottom left - Intermediate stage) material diffuses into the pores, the pores are
changing from irregular to a cylindrical shape, and grain growth takes place
(Bottom right - Final stage) the cylindrical pores become spherical and closed.
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials
Educational Linkage Approach In Cultural Heritage
2.5.2. Historic building materials - Ceramics
Characterization of ceramics
Schematic presentation of a DTA curve for clay
Scanning Electron Microscopy (SEM):
Analysis of the ceramic’s microstructure and the vitrification state
X-Ray Diffraction (XRD)
Chemical analysis and phase identiication
Mercury porosimetry
Assessment of the degree of sintering, and analysis of the ceramic’s
microstructure
Thermal Analysis
Differential Thermal (DTA) and Thermogravimetric (DTG) Analyses
essentially provide a thermal “fingerprint” of the ceramic and besides
revealing its composition they also allow the deduction of its
provenance and manufacturing process
Mechanical tests
The mechanical properties of interest are compression strength and
the modulus of elasticity
Moropoulou, A., Bakolas, A., Bisbikou, K., "Thermal analysis as a method of characterizing
ancient ceramic technologies”, Thermochimica Acta, 2570 (1995) pp. 743-753
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials
Educational Linkage Approach In Cultural Heritage
2.5.2. Historic building materials - Ceramics
Thermal Analysis as a method of characterization of historic ceramics
Moropoulou, A., Bakolas, A.,
Bisbikou, K., "Thermal analysis
as a method of characterizing
ancient ceramic technologies”,
Thermochimica Acta, 2570
(1995) pp. 743-753
Ca-rich ceramic
Ca-poor ceramic
The alternate presence of augite, anorthite (XRD results) gives indications as to the role of CaO transformations in the
ceramic matrix. From the two qualities distinguished according to texture and colour, generally, red bricks show
extensive vitrification with iron oxide phases dispersed almost homogeneously in the vitreous matrix and buff bricks
show fragmented vitrification, where lower concentrations of iron oxides are observed, allocated to large haematite
crystals surrounded by lemonite.
The behaviour of red bricks is governed by the CaO% transformed into the anorthite phase, releasing iron oxides by
firing in alternate oxidizing/reducing atmosphere. Buff bricks in contrast, present a higher CaO% transformed into an
extended calcium aluminosilicate microcrystalline development, which explains the inhibition and finally the more
fragmented vitrification observed, along with the colour difference, due to the trapping of iron in the augite lattice.
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials
Educational Linkage Approach In Cultural Heritage
2.5.3. Historic building materials - Mortars
Mortars - Components
Binding material
Lime putty:
Hydraulic lime:
It cures and hardens when it comes in contact with air (gypsum, air-cured lime)
It cures and hardens in the presence of water, with or without the presence of air
and remains hard even in water (hydraulic lime, cement)
Aggregates: materials in grainy form, that are used as fillers in mortars
Natural aggregates:
stones, pebbles, gravel
Industrial aggregates:
expanded argil, perlite and vermiculite
Additives: they are used to improve certain properties of the mortars
Inorganic - pozzolanas (natural, artificial pozzolanas, crushed bricks et als.)
Organic
Water
Mortars – Classification based on their use
Joint mortars:
Binding material of structural elements
Substrate mortars:
Substrate in the form of layers used in floors, mosaics, frescoes
Plasters:
A form of surface layering. It is a protective layer for corrosive factors (rain, humidity), or for decorative purpose
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials
Educational Linkage Approach In Cultural Heritage
2.5.3. Historic building materials - Mortars
Characterization of mortars
X-Ray Diffraction (XRD)
Mineral phases dispersion and mineral analysis of crystal phases
Petrographical Analyses / optical / fibre optics / electron microscopy
Structure textural and microstructural characteristics
Gradation
Binder to aggregates ratio
Thermal Analysis
Phases and composition (Classification to basic categories)
Mechanical tests
Mechanical properties mainly compression strength and modulus of elasticity
Mercury porosimetry, Digital image procesing on microscopical investigation et als
Microstructural analysis
Water capillary rise tests, water vapor permeability
% absorbed water, capillary rise coefficient
NDT (DIP, IR Thermography, Ultrasonics, colorimetry, etc.) in situ
Classification of mortars in monuments of the Mediterranean Basin
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Typical lime mortars
Hydraulic lime mortars
Lime-pozzolana mortars
Crushed brick-lime mortars
Rubble masonry mortars
Acceptability limits for the
production of restoration mortars
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials
Educational Linkage Approach In Cultural Heritage
2.5.3. Historic building materials - Mortars
Characterization of mortars
Correlation between the tensile strength
(Fmt.k) and hydraulicity of mortars
(inverse CO2 / structurally bound water)
Moropoulou, A., Bakolas, A., Michailidis, P., Chronopoulos, M.,
Spanos, Ch., “Traditional technologies in Crete providing mortars
with effective mechanical properties”, Structural Studies of
Historical Buildings IV, ed. C.A. Brebbia, and B. Leftheris,
Computational Mechanics Publications, Southampton Boston,
Vol. 1 (1995) pp. 151-161
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials
Educational Linkage Approach In Cultural Heritage
2.5.3. Historic building materials - Mortars
Typical lime mortars
30
GRADATION
% Weight
25
20
15
10
5
0
< 0.063 0.063
0.125
0.250
0.500
1.0
2.0
> 4.0
Diameter (mm)
Binder/aggregates ratio: 1:2 – 1:3
Typical Lime
Mortar from the
Medieval City of
Rhodes
Fibre Optics Microscopy image from
a Typical Lime Mortar
MICROSTRUCTURE
Total Porosity:
Average Pore Radius:
Density:
Specific Surface Area:
30 – 35%
0.8 – 3.3 μm
1.5 – 1.8 g/cm3
1.3 – 3.3 m2/g
Μoropoulou, A., Bakolas, A., Bisbikou, K., “Investigation of the technology of
historic mortars”, J. Cultural Heritage, 1 (2000) pp. 45-58
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials
THERMAL ANALYSIS
Educational Linkage Approach In Cultural Heritage
2.5.3. Historic building materials - Mortars
Hydraulic lime mortars
35
GRADATION
30
% Weight
25
20
15
10
5
0
< 0.063 0.063
0.125
0.250
0.500
1.0
2.0
> 4.0
Diameter (mm)
Binder/aggregates ratio: 1:2 – 1:3
Arkadi Monastery
Fibre Optics Microscopy image from
a Hydraulic Lime Mortar
MICROSTRUCTURE
Total Porosity:
Average Pore Radius:
Density:
Specific Surface Area:
18 – 40%
0.1 – 3.5 μm
1.7 – 2.1 g/cm3
2.5 – 13.5 m2/g
Moropoulou, A., Bakolas, A., Michailidis, P., Chronopoulos, M., Spanos,
Ch., “Traditional technologies in Crete providing mortars with effective
mechanical properties”, Structural Studies of Historical Buildings IV, ed.
C.A. Brebbia, and B. Leftheris, Computational Mechanics Publications,
Southampton Boston, Vol. 1 (1995) pp. 151-161
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials
THERMAL ANALYSIS
Educational Linkage Approach In Cultural Heritage
2.5.3. Historic building materials - Mortars
Lime pozzolana mortars
30
GRADATION
% Weight
25
20
15
10
5
0
< 0.063 0.063
0.125
0.250
0.500
1.0
2.0
> 4.0
Diameter (mm)
Binder/aggregates ratio: 1:2 – 1:4
Image from
Cisterns /Lavrio
Fibre Optics Microscopy image from
a Lime pozzolana Mortar
MICROSTRUCTURE
Total Porosity:
Average Pore Radius:
Density:
Specific Surface Area:
30 – 42%
0.1 – 1.5 μm
1.6 – 1.9 g/cm3
3 – 14 m2/g
Μoropoulou, A., Bakolas, A., Bisbikou, K., “Investigation of the technology of
historic mortars”, J. Cultural Heritage, 1 (2000) pp. 45-58
Prof. Antonia Moropoulou – Topic 2: Knowing the built heritage
THERMAL ANALYSIS
Educational Linkage Approach In Cultural Heritage
2.5.3. Historic building materials - Mortars
Crushed brick lime mortars
30
GRADATION
% Weight
25
20
15
10
5
0
< 0.063 0.063
0.125
0.250
0.500
1.0
2.0
> 4.0
Diameter (mm)
Binder/aggregates ratio: 1:2 – 1:4
Crushed Brick Lime
Mortar / Hagia Sophia
masonry
MICROSTRUCTURE
Total Porosity:
Average Pore Radius:
Density:
Specific Surface Area:
Fibre Optics Microscopy image from
the same mortar
32 – 43%
0.1 – 0.8 μm
1.5 – 1.9
3
g/cm
3.5 – 15 m2/g
Moropoulou, A., Cakmak, A.S., Biscontin, G., Bakolas, A., Zendri, E.,
“Advanced Byzantine cement based composites resisting earthquake
stresses: The crushed brick-lime mortars of Justinian’s Hagia Sophia”,
Construction and Building Materials, 16 [18] (2002) pp. 543-552
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials
THERMAL ANALYSIS
Educational Linkage Approach In Cultural Heritage
2.5.3. Historic building materials - Mortars
Hot lime technology mortars
Rubble masonry mortars
Photo from Simonos Petra Arsenal in Mount Athos
Moropoulou et als,
“Hot lime
technology
imparting high
strength to historic
mortars”,
Construction and
Building Materials,
10, No 2 (1996)
pp. 151-159
Photo from the
Medieval City of
Rhodes
Fibre Optic
Microscopy image
shows adhesion
between the stone
and the mortar
Fibre Optic
Microscopy image
In situ slaking of
lime / day
admixture
Binder/aggregates ratio: 1:2 – 1:4
Moropoulou, A. et als.
“Technology and
behavior of rubble
masonry mortars”,
Constr. & Build. Mat., 11,
No 2 (1997) pp. 119-129
Prof. Antonia Moropoulou – Topic 2.5: Historic building materials
Binder/aggregates ratio: 1:2 – 1:4