Masonry in Cenotaphs / Monuments

« Conservation Guidelines Menu

Masonry used to construct Cenotaphs/Monuments is most often stone, though brick may also be present. The most common stone type used is granite, though sandstone, limestone and marble are also used. Brick types can include clay, terracotta or occasionally cement. These different types of masonry have different physical properties, will perform differently, and will require different techniques for conservation.

Granite, an igneous rock formed by the crystallization of magma, is relatively impervious to moisture and highly resistant to weathering. In heavily polluted environments, granite can be damaged by run-off from calcareous stones such as limestone. Small natural inclusions, such as pyrite or iron sulphide are frequently present. These can react with precipitation causing local staining and occasionally the break down of the stone. Cracks or micro-cracks in the stone can accelerate this deterioration.

Sandstone is a sedimentary rock composed of grains of sand held together by natural binders. Sandstone will gradually deteriorate when exposed to the elements, because the binders tend to weather more rapidly than the sand grains, resulting in erosion. Sandstone is also susceptible to exfoliation of its surface and swelling of its layers when exposed to moisture and freezing temperatures. If its layers are not properly oriented in a masonry assembly, exfoliation problems can be significant. If limestone or marble is placed above sandstones, the run-off of chemicals from these stones can react with acid rain to cause deterioration of the sandstone. Natural inclusions, such as deposits of minerals or clay, can also weaken the stone and produce staining. A common form of deterioration is contour scaling, in which a relatively impermeable, dense, brittle crust forms on the surface of the stone as a result of chemical reactions with airborne pollutants. Moisture trapped behind this crust will expand when frozen, forming blisters and causing spalling. The cycle is then repeated on the freshly exposed stone surface, and can lead to significant loss of detail.

Limestone, like sandstone, is sedimentary, relatively permeable, and susceptible to pollution damage which can result in erosion of surface detail. Limestone may possess inclusions that weaken the stone. It can be susceptible to differential weathering of its bedding planes, which can result in fracture or splitting of the stone.

Back to Top

Marble can include a wide variety of mineral compositions. Marble is soft and can be readily carved. It is soluble in acids and not durable when exposed to moisture. Reactions with sulphuric acids from the atmosphere can cause marble surfaces to be converted to gypsum, which may combine with carbon or soot to form dark crusts which will expand, converting more marble to gypsum, eventually eroding the surface and any carved details or inscriptions.

Brick and terracotta are similar in that they are both fired clay products. However, each material has its own characteristics and uses. Brick is a solid or hollow masonry unit, made of clay, with sand and other materials added as binders before being moulded, dried and fired in a kiln. Brick is used for both cladding and structural work. Terracotta is also made of clay mixed with sand, moulded but is fired at a higher temperature making it harder and more compact than brick. Terracotta is used for ornamental work, roof and floor tiles, and is not a load-bearing material.

Brickwork deterioration can result from: prolonged exposed to moisture and freeze-thaw cycles; crystallization of water-soluble salts which cause the surface to crumble; acid rain causing lime or calcium carbonates in the brick to dissolve; masonry ties corroding and failing; mortar failure, resulting in water infiltration which when frozen displaces brick units.

Failure of terracotta can be attributed to: problems with the manufacture of the units; corrosion of abutting metal causing loss of oxides; and water infiltration. Resulting problems include crackling of the glaze surface, accumulation of organic growth causing glaze deterioration, and cracking and shattering of the terracotta units themselves.

The most common masonry assemblies for Cenotaph/Monuments use one massive stone, or a system of several large stones. Less common are assemblies of smaller stone or brick units. Assemblies using large stones will typically have mortar placed between the stones to evenly distribute their weight sometimes with shims for levelling purposes. Assemblies of smaller stones or brick are built with a lime or cement based mortar to hold the units together and to evenly distribute their weight. The surface pointing for these is almost always mortar. It is possible that some assemblies may be dry laid, with no bedding mortar. A surface pointing of mortar may sometimes be applied to a dry laid masonry assembly. The surface of the joints are also sometimes filled or 'pointed' with putty, caulking, or lead.

Back to Top

Mortar is a mixture of a binder (lime and/or cement), aggregate (sand) and water, that acts to bind the masonry units together and evenly distribute the forces in the assembly. Mortar should always be slightly weaker than the masonry units in order to permit the units to expand or contract without damage in response to moisture and temperature variations. Mortar should also generally be more permeable than the stone, so water entering the masonry assembly can evaporate out through the joints. Portland cement based mortars are generally too hard and impermeable for historic masonry assemblies. The profile of a mortar joint is also an important factor in the mortar's performance. The joint should be designed to shed water away from the masonry.

Additional information on masonry conservation can be found in the following sources:

Weaver, Martin E., 1993. Conserving Buildings: A Guide to Techniques and Materials. New York: John Wiley and Sons, Inc.

De Teel, Paterson, and Tiller, 1979. Preservation Briefs 7: The Preservation of Historic Glazed Architectural Terra Cotta. U.S. Department of the Interior, National Park Service.

Ashurst, Nicola, 1994. Cleaning Historic Buildings, Volumes 1 and 2. London: Donhead Publishing.

Ashurst, John, and Ashurst, Nicola, 1988. Brick, Terracotta and Earth (Practical Building Conservation, English Heritage Technical Handbook, Vol. 2) . UK: Gower Technical Press.