The
materials used in 19th and 20th century plasters: from lime and gypsum
to Portland cement
By: JoAnn Cassar
Institute for Masonry and Construction
Research
University of Malta
The earliest plasters known to us were
lime-based. Around 7500 BC, the people of 'Ain Ghazal in Jordan used lime
mixed with unheated crushed limestone to make plaster which was used on
a large scale for covering walls, floors, and hearths in their houses.
Often, walls and floors were decorated with red, finger-painted patterns
and designs. In ancient India and China, renders in clay and gypsum plasters
were used to produce a smooth surface over rough stone or mud brick walls,
while in early Egyptian tombs, walls were coated with lime and gypsum plaster
and the finished surface was often painted or decorated. Modelled stucco
was employed throughout the Roman Empire. The Romans used mixtures of lime
and sand to build up preparatory layers over which finer applications of
gypsum, lime, sand and marble dust were made; pozzolanic materials were
sometimes added to produce a more rapid set. Following the fall of the
Roman Empire, the addition of marble dust to plaster to allow the production
of fine detail and a hard, smooth finish in hand-modelled and moulded decoration
was not used until the Renaissance. Around the 4th century BC, the Romans
discovered the principles of the hydraulic set of lime, which by the addition
of highly reactive forms of silica and alumina, such as volcanic earths,
could solidify rapidly even under water. There was little use of hydraulic
mortar after the Roman period until the 18th century.
Plaster decoration was widely used
in Europe in the Middle Ages where, from the mid-13th century, gypsum was
used for internal and external plaster. Hair was employed as reinforcement,
with additives to assist set or plasticity including malt, urine, beer,
milk and eggs. In the 14th century, decorative trowelled plaster, called
pargeting was being used in South-East England to decorate the exterior
of timber-framed buildings. This is a form of incised, moulded or modelled
ornament, executed in lime putty or mixtures of lime and gypsum plaster.
During this same period, terracotta was reintroduced into Europe and was
widely used for the production of ornament. In the mid-15th century, Venetian
skilled workers developed a new type of external facing, called marmorino
made by applying lime directly onto masonry.
In the 16th century, a new highly
decorative type of decorative internal plasterwork, called scagliola, was
invented by stuccoists working in Bavaria. This was composed of gypsum
plaster, animal glue and pigments, used to imitate coloured marbles and
pietre dure ornament. Sand or marble dust, and lime, were sometimes added.
In this same century, the sgraffito technique, also known as graffito or
scratchwork was introduced into Germany by Italian artists, combining it
with modelled stucco decoration. This technique was practised in antiquity
and was described by Vasari as being a quick and durable method for decorating
building facades. Here, layers of contrasting lime plaster were applied
and a design scratched through the upper layer to reveal the colour beneath.
The 17th century saw the introduction of different types of internal plasterwork.
Stucco marble was an artificial marble made using gypsum (sometimes with
lime), pigments, water and glue. Stucco lustro was another a form of imitation
marble (sometimes called stucco lucido) where a thin layer of lime or gypsum
plaster was applied over a scored support of lime, with pigments scattered
on surface of the wet plaster.
The 18th century gave rise to renewed
interest in innovative external plasters. Oil mastics introduced in the
UK in this period included a "Composition or stone paste" patented in 1765
by David Wark. This was a lime-based mix and included "oyls of tar, turpentine
and linseed" besides many other ingredients. Another "Composition or cement",
including drying oil, was patented in 1773 by Rev. John Liardet. A similar
product was patented in 1777 by John Johnson.
In 1774, in France, a mémoire
was published on the composition of ancient mortars. This was translated
into English as "A Practical Essay on a Cement, and Artificial Stone, justly
supposed to be that of the Greeks and Romans" and was published in the
same year. Following this, and as a backlash to the disappointment felt
due to the repeated failure of oil mastics, in the second half of the 18th
century water-based renders gained popularity once more. Mixes for renders
were patented, including a "Water Cement, or Stucco" consisting of lime,
sand, bone-ash and lime-water (Dr Bryan Higgins, 1779).
Various experiments mixing different
limes with volcanic earths took place in the 18th century. John Smeaton
(from 1756) experimented with hydraulic limes and concluded that the best
limes were those fired from limestones containing a considerable quantity
of clayey material. In 1796, Revd James Parker patented Parkerís "Roman
Cement". This was a hydraulic cement which, when mixed with sand, could
be used for stucco. It could also be cast to form mouldings and other ornaments.
It was however of an unattractive brown colour, which needed to be disguised
by surface finishes.
Natural cements were frequently
used in stucco mixes during the 1820s. The popularisation of Portland cement
changed the composition of stucco, as well as mortar, to a harder material.
The development of artificial cements had started early in the 19th century.
In 1811, James Frost took out a patent for an artificial cement obtained
by lightly calcining ground chalk and clay together. The French Engineer
Vicat in 1812-1813 experimented with calcining synthetic mixtures of
limestone and clay, a product he introduced in 1818. In 1822, in the UK,
James Frost patented (another?) process, similar to Vicat's, producing
what he called "British cement". Portland cement, patented in 1824
by Joseph Aspdin, was called so because it was supposed to resemble Portland
stone. Aspdinís son William, and later Isaac Johnson, improved the production
process. A product, very similar to modern Portland cement, was available
from about 1845, with other improvements taking place in the following
years.
Thus, after about 1860, most stucco
was composed primarily of Portland cement, mixed with some lime. This made
it even more versatile and durable. No longer used just as a coating for
a substantial material like masonry or log, stucco could now be applied
over wood or metal lath attached to a light wood frame. With this increased
strength, it ceased to be just a veneer and became a more integral part
of the building structure. Early 19th century rendered façades were
colour-washed with distemper; oil paint for external walls was introduced
around 1840.
The 19th century also saw the revival
of the use of oil mastics. In the UK, patents were obtained for "compositions"
in 1803 (Thomas Fulchner), 1815 (Christopher Dihl) and 1817 (Peter Hamelin).
These oil mastics, as the ones before them, also proved to be short-lived.
Moulded or cast masonry substitutes,
such as cast stone and poured concrete, became popular in place of quarried
stone during the 19th century. However, this was not the first time "artificial
stone" had been widely used. Coade Stone, a brand name for a cast stone
made from fired clay, had been developed and manufactured in England from
1769 to 1843 and was used for decorative architectural elements. Following
the closure of the factory in South London, Coade stone stopped being produced,
and the formula was lost. By the mid 19th century manufacturing centres
were preparing cast stones based on cement for use in buildings. These
were made primarily with a cement mix often incorporating fine and coarse
aggregates for texture, pigments or dyes to imitate colouring and veining
of natural stones, as well as other additives. Also in the 19th century,
various mixtures of modified gypsum plasters, such as Keene's cement, appeared.
These materials were developed for use as internal wall plasters, increasing
the usefulness of simple Plaster of Paris as they set more slowly and were
thus easier to use.
The terms "plasterwork", "plaster",
"stucco" and "render" are often used interchangeably, but have regional
variations. In this paper, "stucco" is taken to be a composition that can
be applied to the whole or part of a building and can be used equally for
a plain covering or for mouldings and ornaments. Compositions range from
lime and aggregate mixes (with other additives) to gypsum mixes, to cement
and sand renders.
Up until the late 1800's, stucco,
like mortar, was primarily lime-based, consisting of hydrated or slaked
lime, water and sand, with straw or animal hair often included. Interior
stucco was generally made with gypsum, marble dust and glue. It was often
moulded into ornamental shapes and polished to a marble-like finish. During
the 17th and 18th centuries, elaborate painted figures and ornaments were
made with stucco. After ca 1860, as already stated, Portland cement largely
replaced lime for external renders.
2.1 Binders
Lime plaster is composed of calcium
oxide (lime), obtained by heating calcium carbonate (limestone, marble,
chalk and shells) in a kiln to between 900°C and 1200°C.
Carbon dioxide (and any water) is driven off leaving anhydrous calcium
oxide or quicklime, sometimes described as unslaked lime or, misleadingly,
as lump lime. Quicklime soaked in water changes to calcium hydroxide or
slaked lime. When dried and ground to a fine powder, it is called hydrated
lime or lime hydrate. The addition of highly reactive forms of silica and
alumina, such as volcanic earths, ash or rock (such as tuff or pumice),
brick dust, or ground iron slags, produces a hydraulic set. Here, the lime
does not set by carbonation and can solidify rapidly even under water.
Materials that induce this effect are called pozzolanic additives. Hydraulic
lime can also be made by calcining limestones such as lias, which have
a high silica content.
Gypsum plaster has been used for
renders and mortars since the time of the ancient Egyptians. It is also
used for moulds, sculptures and castings. The plaster is prepared by heating
gypsum minerals (calcium sulphate dihydrate) to about 150°C to partially
remove the chemically bound water, thus producing calcium sulphate hemihydrate,
the main component of Plaster of Paris. When this is mixed with water,
it converts to the hydrated calcium sulphate that rapidly sets to an impenetrable
solid, generating heat and expanding slightly. Its rapid setting necessitates
great skill in handling when used as a wall plaster. The set and workability
of gypsum plaster are controlled by various additives. As it is slightly
water soluble, its use in temperate climates was largely confined to interior
decoration, as a finish for walls and ceilings, although it was occasionally
used, on its own or mixed with lime, for external work such as pargeting
or as an infill in timber-framed buildings. In such situations the surface
had to be worked to a smooth finish and protected by effective roofing.
It was often protected by linseed oil paint. Today, gypsum has to a great
extent replaced lime for internal plasters. It is preferred because it
hardens faster and has less shrinkage than lime. Lime is generally used
only in the finish coat in contemporary stucco work.
In the 19th century, various mixtures
of modified gypsum plasters were patented in the UK as artificial marbles,
many of which were cast into slabs for interior decoration. These included
Martin's cement and Keene's cement, both patented in the 1830s, and Parian
cement, patented in 1846. These involved the modification of gypsum plaster
through heating and chemical treatments to produce a material which reliably
set more slowly than gypsum but more rapidly than lime. This obviated the
problems of difficulty in working and delay before decoration could be
carried out.
Portland cement was patented in
1824 by Joseph Aspdin. It was made by crushing and calcining a "hard limestone",
mixing the lime with clay and grinding the product into a fine slurry with
water. This was fired, broken into lumps and fired a second time. Although
this material was hydraulic, it was not as strong as modern cements because
it was manufactured at lower temperatures. In the late 1830s Isaac Johnson
discovered that overburnt lumps from the kilns produced a stronger, more
reliable product with a slower set. His process, patented in 1838, involved
heating limestone or chalk to 1300-1500°C, which converted it to quicklime.
This then reacted with the clay to form clinker that was ground and mixed
with gypsum to retard the set. For use it was mixed with water. The set
occurs through the formation of calcium silicates and calcium aluminates
in the film. These react with water to from starburst-shaped particles
that interlock to give cement its strength. Modern ordinary Portland cement
is a hard, grey, impervious material that is mixed with sand for use as
a mortar and a render.
Terracotta, a decorative building
material made by moulding and/or modelling clay and firing to obtain a
hard, durable product, was a very popular substitute material in
the late 19th and early 20th centuries. It simulated the appearance
of intricately carved stonework, which was expensive and time-consuming
to produce, and could be glazed to imitate a variety of natural stones,
from brownstones to limestones, or could be coloured for a polychrome effect.
The clay used was generally fine-grained to accept detail; sand or crushed
fired clay, called grog, were added to reduce shrinkage and distortion
during firing.
2.2 Additives
Stucco often contained substantial
amounts of mud or clay, marble or brick dust, or even sawdust. An
array of other additives ranged from animal blood or urine, to eggs, keratin
or gluesize (animal hooves and horns), varnish, wheat paste, sugar, salt,
sodium silicate, alum, tallow, linseed oil, beeswax, and wine, beer, or
rye whiskey. Additives, or admixtures, were usually added to enhance or
modify characteristics such as curing time, plasticity, colour, or volatility.
Also, without additives the plaster was weak and prone to shrinkage and
cracking. Often aggregates were added to increase the strength and
decrease porosity.
Inorganic additives were used to
increase hardness, resistance, and durability and to retard setting. Sand
was added as filler, primarily to reduce shrinkage, increase strength,
improve working properties and generally enhance performance. Most natural
sands are at least 98% silica. Fine aggregate produced by crushing rock,
gravel, or slag commonly is known as manufactured sand. Marble powder was
sometimes added as an aggregate because it permitted the rendering of fine
detail.
The aim of adding organic substances
was either to improve the workability, to harden the mass, to retard setting,
or to influence the mechanical properties of the mass. Organic materials
(blood, glue, casein) and weak acids (citric acid, boric acid) can act
as retardants. Accelerants are compounds that speed up the rate of hydration
and thereby cause the mix to set or harden sooner. In medieval work gypsum
plaster was frequently added to lime as an accelerator of set. Additives
to provide water-repellency, such as waxes, fats and oils, or to entrap
air and thus improve the set, such as urine or beer, were traditionally
used. For hand-modelled ornament, gypsum was added, with a retardant such
as glue, sour milk or wine, to allow a long working time; sugary materials
reduced the amount of water needed and slowed down the setting time, and
alcohol acted as an air entrainer. All of these additives contributed to
the strength and durability of the stucco.
The appearance of much stucco was
determined by the colour of the sand--or sometimes burnt clay--used in
the mix, but often stucco was also tinted with natural pigments, or the
surface whitewashed or colour-washed after stuccoing was completed. Brick
dust could provide colour, and other colouring materials that were not
affected by lime, mostly mineral pigments, could be added to the mix for
the final finish coat. Stucco was also marbled or marbleised--stained to
look like stone - by diluting oil of vitriol (sulphuric acid) with water,
and mixing this with a yellow ochre, or another colour. As the twentieth
century progressed, manufactured or synthetic pigments were added at the
factory to some prepared stucco mixes.
Many formulations included the use
of hair, jute, flax or straw as extenders. Animal hair gives the
lime and sand greater toughness and cohesion; it has long been used as
a binding material in lime and gypsum plasters. Ox hair was commonly used,
but horse, goat, and even human hair were used as substitutes. The use
of human hair was however rare because of its fineness and poor strength.
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3. Examples of exterior plasterwork
Historic external renderings
include a wide range of aggregates, binders and reinforcement. A simple
classification includes types varying from very low strength daubs, usually
applied in a single thick coat to backing of wattle or lath, to high strength
renderings based on hydraulic cements and applied in two or more coats
to brick or stone or lime based undercoats. Other types include low to
medium strength renderings based on lime applied in two or more coats to
backing of brick, stone, unbaked earth, wattle or lath; and medium
strength oil mastics usually applied in one thin coat to brick, stone,
or lime based undercoats but also used to model detail. Medium strength
gypsum or lime/gypsum renderings in two or more coats on brick, stone or
lath are also to be found.
Historic external stucco includes
pargeting, sgraffito and marmorino. Treatments and coatings related to
stucco in that they consist at least in part of a similarly plastic or
malleable material include wattle and daub and "cob". These are regional
variations on traditional mixtures of mud, clay, lime, chalk, cement, gravel
or straw. Many are still used today.
Pargeting is external decorative
plasterwork often incised or modelled with ornamental impressions or patterns
made with a mould or comb. It consists of a tough lime plaster, made of
lime putty or mixtures of lime and gypsum plaster, reinforced with ox hair,
and with tallow sometimes added as a plasticizer and a water-repellent.
The surface of the pargeting was frequently limewashed; occasionally linseed
oil or wax was used as a protective coating. It was usually applied to
timber-framed houses, especially during 15th and 16th centuries in south-east
England.
Sgraffito is a decorative technique
used in various media, also known as graffito or scratchwork, in which
layers of contrasting colours are applied to a surface and a design is
scratched through the upper layer to reveal the colour beneath. In the
Renaissance, lime plaster, tinted with ash, was used as the under layer,
and then covered with white lime plaster. Most sgraffito, especially two
or three colour work, belongs to the 19th century. Then, the lowest, levelling
layer usually consisted of Portland cement, above which a layer of cement
coloured with earth pigments was applied. The final layer for indoor work
was Parian cement; Portland cement was used externally. With all materials
the layers were allowed to dry but not set completely before the design
was cut in with an iron stylus or knife, used to peel away the upper layer.
After the plaster or cement had set fully, areas of the design could be
oil-gilded.
Marmorino (or marmorina) plaster
is an external facing made by applying lime directly onto masonry or, more
often, onto a layer of ground "Cotto", bound with air-hardening lime ("Cuogolo"
lime), a semi-hydraulic type ("Negra" or "Brovada" lime). Finishes included
treating with "Saponata", to which a further wax treatment was often added,
or by applying one or two coats of linseed oil. In more exposed areas,
additives such as metal grit (the so-called "Marogna" with a high Si and
Mg content) were added.
The low strength wattle and
daub is composed of interwoven sticks, twigs or branches plastered with
clay, grease or mud. Wattle and daub construction was used for roofs, walls
and fences, especially in the early 19th century. Traditional mixes also
include mixtures of lime putty, sand and slurried cow dung, reinforced
with chopped straw.
Cob (or clob) are walls composed
of clay, earth, straw, lime and sand, mixed with water. Wet clay or chalk
is tempered by the tread of horses with straw or reed. It is constructed
without shutters in layers upon a stone or brick plinth. It is lifted wet
on to walls, the surface is trimmed in position as it hardens. Shrinkage
is considerable. It is usually covered with protective limewash or plastered.
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4. Examples of artificial
stone
Moulded or cast masonry substitutes,
popular during the 19th century, were produced in a variety of materials
made readily available by technology at this time. These products were
cheap to fabricate and easy to use. Examples of some cast stone binders
included clay, plaster, water glass, oxychloride cement, epoxy and Portland
cement.
Cast stone is made with a cement
mix often incorporating fine and coarse aggregates for texture, pigments
or dyes to imitate colouring and veining of natural stones, as well as
other additives. Examples include Frear stone, Victoria stone and Benedict
stone. Various aggregates such as granite and other stone dusts, slag and
crushed brick were used to produce a range of colours and textures. Cast
stones were produced in many sizes and shapes, often with intricate patterns
and tracery. Manufacturing centres were preparing cast stones based
on cement (Pulham stone, Haddon stone) by the mid 19th century for use
in buildings, statuary and decoration including balustrades, columns, architraves,
arches and tracery windows. These fabricated units avoided expensive quarrying
costs, and were versatile in representing either ornately carved blocks,
plain wall stones or rough cut textured surfaces. The end result depended
on the type of patterned or textured mould used.
Coade Stone was developed by Eleanor
and George Coade in England and manufactured from 1769 to 1843. The formula,
which was lost following closure of the factory, has only been deduced
from recent analysis and experimentation. Analysis indicates the raw materials
were ball clay from Dorset or Devon with the addition of 5-10% flint, 5-10%
quartz sand, at least 10% grog and about 10% soda-lime-silica glass. The
material was fired at temperatures between 1100°C and 1150°C over
a four-day period, producing a hard, partially vitrified and durable material.
Minimal shrinkage and distortion in the kiln, owing to the presence of
the pre-fired grog, contributed to the materialís commercial success. These
terracotta cast stones were used for interior and exterior ornamental elements.
Coade Stone is acid resistant and weathers well. It was used for statues,
tombs, balustrades, vases, pillars and decorative architectural elements.
Rangerís artificial stone was patented
in 1832 and named after its inventor. This artificial building stone combined
a hydraulic lime with an aggregate such as beach shingle or broken flints
and boiling water, which apparently produced a rapid set.
Silicious Concrete Stone was an
artificial stone made with water glass. It was made by a process patented
by F. Ransome in 1856. In this process, water glass (sodium silicate) is
mixed with sand (and gravel, flint, chalk, or limestone) in an aqueous
solution of caustic soda. Then this alkaline solution is mixed with calcium
chloride to form an insoluble silicate stone.
This historical overview of materials
used in plaster and artificial stone, prior to and in the 19th and 20th
centuries is by no means meant to be exhaustive. However, it should shed
light on the variety of materials and mixes used over the centuries, and
in particular in the last two centuries. Their use has occasionally given
rise to problems of permanence, and we are now faced with the challenge
of their conservation. It is only by knowing accurately the composition
of these materials that the survival of remaining examples can be assured.
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