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The Preservation and Repair of Historic Stucco
1. Historical Background
Stucco has been used since ancient times. Still widely used
throughout the world, it is one of the most common of
traditional building materials. Up until the late 1800s,
stucco, like mortar, was primarily lime-based, but the
popularization of portland cement changed the composition of
stucco, as well as mortar, to a harder material.
Historically, the term "plaster" has often been
interchangeable with "stucco"; the term is still favored by
many, particularly when referring to the traditional
lime-based coating. By the nineteenth century "stucco,"
although originally denoting fine interior ornamental
plasterwork, had gained wide acceptance in the United States
to describe exterior plastering. "Render" and "rendering"
are also terms used to describe stucco, especially in Great
Britain. Other historic treatments and coatings related to
stucco in that they consist at least in part of a similarly
plastic or malleable material include: parging and
pargeting, wattle and daub, "cob" or chalk mud, pise de
terre, rammed earth, briquete entre poteaux or bousillage,
half-timbering, and adobe. All of these are regional
variations on traditional mixtures of mud, clay, lime,
chalk, cement, gravel or straw. Many are still used today.
The Stucco Tradition in the United States
Stucco is primarily used on residential buildings and
relatively small-scale commercial structures. Some of the
earliest stucco buildings in the United States include
examples of the Federal, Greek and Gothic Revival styles of
the eighteenth and the nineteenth centuries that emulated
European architectural fashions. Benjamin Henry Latrobe,
appointed by Thomas Jefferson as Surveyor of Public
Buildings of the United States in 1803, was responsible for
the design of a number of important stucco buildings,
including St. John's Church (1816), in Washington, D.C.
(Fig. 2). Nearly half a century later Andrew Jackson Downing
also advocated the use of stucco in his influential book The
Architecture of Country Houses, published in 1850. In
Downing's opinion, stucco was superior in many respects to
plain brick or stone because it was cheaper, warmer and
dryer, and could be "agreeably" tinted. As a result of his
advice, stuccoed Italianate style urban and suburban villas
proliferated in many parts of the country during the third
quarter of the nineteenth century.
Revival Styles Promote Use of Stucco
The introduction of the many revival styles of architecture
around the turn of the twentieth century, combined with the
improvement and increased availability of portland cement
resulted in a "craze" for stucco as a building material in
the United States. Beginning about 1890 and gaining momentum
into the 1930's and 1940's, stucco was associated with
certain historic architectural styles, including: Prairie;
Art Deco, and Art Moderne; Spanish Colonial, Mission,
Pueblo, Mediterranean, English Cotswold Cottage, and Tudor
Revival styles; as well as the ubiquitous bungalow and
"four-square" house. The fad for Spanish Colonial Revival,
and other variations on this theme, was especially important
in furthering stucco as a building material in the United
States during this period, since stucco clearly looked like
adobe.
Although stucco buildings were especially prevalent in
California, the Southwest and Florida, ostensibly because of
their Spanish heritage, this period also spawned
stucco-coated, revival-style buildings all over the United
States and Canada. The popularity of stucco as a cheap, and
readily available material meant that by the 1920's, it was
used for an increasing variety of building types. Resort
hotels, apartment buildings, private mansions and movie
theaters, railroad stations, and even gas stations and
tourist courts took advantage of the "romance" of period
styles, and adopted the stucco construction that had become
synonymous with these styles.
A Practical Building Material
Stucco has traditionally been popular for a variety of
reasons. It was an inexpensive material that could simulate
finely dressed stonework, especially when "scored" or
"lined" in the European tradition. A stucco coating over a
less finished and less costly substrate such as rubblestone,
fieldstone, brick, log or wood frame, gave the building the
appearance of being a more expensive and important
structure. As a weather-repellent coating, stucco protected
the building from wind and rain penetration, and also
offered a certain amount of fire protection. While stucco
was usually applied during construction as part of the
building design, particularly over rubblestone or
fieldstone, in some instances it was added later to protect
the structure, or when a rise in the owner's social status
demanded a comparable rise in his standard of living.
Composition of Historic Stucco
Before the mid-to-late nineteenth century, stucco consisted
primarily of hydrated or slaked lime, water and sand, with
straw or animal hair included as a binder. Natural cements
were frequently used in stucco mixes after their discovery
in the United States during the 1820's. Portland cement was
first manufactured in the United States in 1871, and it
gradually replaced natural cement. After about 1900, most
stucco was composed primarily of portland cement, mixed with
some lime. With the addition of portland cement, stucco
became 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, stucco
ceased to be just a veneer and became a more integral part
of the building structure.
Today, gypsum, which is hydrated calcium sulfate or sulfate
of lime, has to a great extent replaced lime Gypsum 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.
The composition of stucco depended on local custom and
available materials. Stucco often contained substantial
amounts of mud or clay, marble or brick dust, or even
sawdust, and an array of additives ranging 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. Waxes, fats and oils were included to introduce
water-repellent properties, 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 color 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 color-washed after stuccoing was
completed. Brick dust could provide color, and other
coloring 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 marbleized--stained
to look like stone by diluting oil of vitriol (sulfuric
acid) with water, and mixing this with a yellow ochre, or
another color. As the twentieth century progressed,
manufactured or synthetic pigments were added at the factory
to some prepared stucco mixes.
Methods of Application
Stucco is applied directly, without lath, to masonry
substrates such as brick, stone, concrete or hollow tile.
But on wood structures, stucco, like its interior
counterpart plaster, must be applied over lath in order to
obtain an adequate key to hold the stucco. Thus, when
applied over a log structure, stucco is laid on horizontal
wood lath that has been nailed on vertical wood furring
strips attached to the logs. If it is applied over a wood
frame structure, stucco may be applied to wood or metal lath
nailed directly to the wood frame; it may also be placed on
lath that has been attached to furring strips. The furring
strips are themselves laid over building paper covering the
wood sheathing. Wood lath was gradually superseded by
expanded metal lath introduced in the late-nineteenth and
early-twentieth century. When stuccoing over a stone or
brick substrate, it was customary to cut back or rake out
the mortar joints if they were not already recessed by
natural weathering or erosion, and sometimes the bricks
themselves were gouged to provide a key for the stucco. This
helped provide the necessary bond for the stucco to remain
attached to the masonry, much like the key provided by wood
or metal lath on frame buildings.
Like interior wall plaster, stucco has traditionally been
applied as a multiple-layer process, sometimes consisting of
two coats, but more commonly as three. Whether applied
directly to a masonry substrate or onto wood or metal lath,
this consists of a first "scratch" or "pricking-up" coat,
followed by a second scratch coat, sometimes referred to as
a "floating" or "brown" coat, followed finally by the
"finishing" coat. Up until the late-nineteenth century, the
first and the second coats were of much the same
composition, generally consisting of lime, or natural
cement, sand, perhaps clay, and one or more of the additives
previously mentioned. Straw or animal hair was usually added
to the first coat as a binder. The third, or finishing coat,
consisted primarily of a very fine mesh grade of lime and
sand, and sometimes pigment. As already noted, after the
1820's, natural cement was also a common ingredient in
stucco until it was replaced by portland cement.
Both masonry and wood lath must be kept wet or damp to
ensure a good bond with the stucco. Wetting these materials
helps to prevent them from pulling moisture out of the
stucco too rapidly, which results in cracking, loss of bond,
and generally poor quality stuccowork.
Traditional Stucco Finishes
Until the early-twentieth century when a variety of novelty
finishes or textures were introduced, the last coat of
stucco was commonly given a smooth, troweled finish, and
then scored or lined in imitation of ashlar. The illusion of
masonry joints was sometimes enhanced by a thin line of
white lime putty, graphite, or some other pigment. Some
nineteenth century buildings feature a water table or raised
foundation of roughcast stucco that differentiates it from
the stucco surface above, which is smooth and scored. Other
novelty or textured finishes associated with the "period" or
revival styles of the early-twentieth century include: the
English cottage finish, adobe and Spanish, pebble-dashed or
dry-dash surface, fan and sponge texture, reticulated and
vermiculated, roughcast (or wet dash), and sgraffito. |
