32
BCD SPECIAL REPORT ON
HISTORIC CHURCHES
21ST ANNUAL EDITION
The rising ground level in this church graveyard, perhaps in combination with
other factors, has created a depression along the foot of the exterior walls
encouraging water to collect in this vulnerable area.
A church wall which has been partly cement-rendered, forcing moisture to rise by
capillary action at the interface
SCIENTIFIC PRINCIPLES
To consider the movement of water in
masonry it is necessary to briefly touch on
the following scientific principles (each is a
subject area in its own right so only brief,
simplified definitions are given here):
POROUS BUILDING MATERIALS
The porosity of a material is defined by the ratio
of area occupied by its pores to its total cross-
sectional area. All building materials are porous
to some extent, with pores forming either
continuous networks or closed systems. The
principle of pore formation in building materials
is similar whether they are man-made or
naturally occurring. When water is trapped and
then driven out, usually by heat, voids are left
behind (brick production is a good example).
Plasters and mortars have well-developed
pore structures by default, as high volumes of
water (which will eventually evaporate) must
be added to make them sufficiently workable.
Pore space geometry and characteristics
Porosity in natural building stone is a little
more complex, although a rule of thumb is that
rocks generally become less porous with age
and depth of burial. Consolidation of all rock
types (by heat and/or pressure) can produce
complex dual porosities with open and closed
systems. Stones with well-sorted grains tend
to produce very coherent pore networks
which enable efficient hydraulic transport.
A material with a high porosity ratio
isn’t necessarily going to be conducive to
effective hydraulic transport. A brick fired
at high temperature will have large pores
within the clay body, but the formation
of a fireskin effectively closes off many
of the pore openings at the surface.
The heterogeneous nature of naturally
occurring building stones makes them
more difficult to categorise as the pores
often contain crystalline deposits which
affect their porosity. This phenomenon is
also seen in mortars, with complex crystal
growth within the pore structures of both
lime and cement-based compounds.
Pore networks constitute effective capillary
networks.
Capillary action
The process which ultimately defines how water
is able to move against gravity is capillary action.
Although this is not the most complex scientific
field, it is interesting to note that capillarity
was the subject of Albert Einstein’s first paper,
submitted to Annalen der Physik in 1901.
Capillary movement of water (above left)
occurs in a narrow tube because the adhesion
of the water molecules to many solid objects
is stronger than the cohesive forces between
them (this is why the water droplets in the
photograph above are sticking to the leaves).
Surface tension is a product of inter-molecular
cohesion. Water confined in a small tube or
capillary results in the formation of a concave
upper surface or ‘meniscus’ (from the Greek
for crescent). The smaller the tube diameter,
the greater the ratio of circumference to area
and the more efficient the capillary action.
The height to which water is able to
move in a capillary is therefore limited
by its surface tension, the size of the
capillary and the effects of gravity.
Understanding how and why water
molecules behave in this way, is fundamental to
an appreciation of how moisture behaves both
inside and on the surface of building materials.
There are many other factors which
affect the movement of water (in both liquid
and gaseous phases) in capillaries, such as
moisture storage characteristics, vapour
and capillary transport coefficients, vapour
diffusion and effusion in micro pores,
sorption and adsorption isotherms. These
and other factors are of specialist interest
only and so are not expanded upon here.
IN PRACTICE
Some churches have exposed stone walls
internally and some of these buildings have
problems associated with low-level damp
and/or the crystallisation of mineral salts
(which may be the reason why the plaster
was originally removed). Most churches,
however, are plastered internally so the
discussion below deals exclusively with them.
Wall construction
The scientific principles touched on above
would generally have been appreciated (in
a practical rather than an academic sense)
by the surveyors and masons originally
responsible for the construction of a
church. They did not have the benefit of
hindsight with regard to the use of some
stones that have proved especially prone to
weathering, particularly those damaged by
the products of the industrial revolution.
In general, however, they demonstrated a
Capillary action: water confined in a capillary forms
a concave upper surface or ‘meniscus’. The water
travels up the capillary because the adhesion of the
water molecules to the capillary walls is stronger than
the cohesion between them.
Similarly, water droplets cling to a leaf because the adhesion of the water molecules to the surface of the leaf is
stronger than the cohesive forces between the water molecules.
