Stucco
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Stucco or render is a material made of an aggregate, a binder, and water. Stucco is applied wet and hardens to a very dense solid. It is used as a coating for walls and ceilings and for decoration. Stucco may be used to cover less visually appealing construction materials such as concrete, cinder block, or clay brick and adobe.
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[edit] Composition
The difference in nomenclature between stucco, plaster, and mortar is based more on use than composition. Until the later part of the nineteenth century, it was common that plaster, which was used inside a building, and stucco, which was used outside, would consist of the same primary materials: lime and sand (which are also used in mortar). Animal or plant fibers were often added for additional strength. In the later part of the nineteenth century, Portland cement was added with increasing frequency in an attempt to improve its durability. At the same time, traditional lime plasters were being replaced by gypsum plaster.
Traditional stucco is made of lime, sand, and water. Modern stucco is made of Portland cement, sand, and water. Lime is added to decrease the permeability and increase the workability of modern stucco. Sometimes additives such as acrylics and glass fibers are added to improve the structural properties of the plaster. This is usually done with what is considered a one-coat stucco system, as opposed to the traditional three-coat method.
Lime stucco is a relatively hard material that can be broken or chipped by hand without too much difficulty. The lime itself is usually white; color comes from the aggregate or any added pigments. Lime stucco has the property of being self-healing to a limited degree because of the slight solubility of lime (which in solution can be deposited in cracks. where it solidifies). Portland cement stucco is very hard and brittle and can easily crack if the base on which it is applied is not stable. Typically its color was gray, from the innate color of most Portland cement, but white Portland cement is also used. Today's stucco manufacturers offer a very wide range of colors that can be mixed integrally in the finish coat.
[edit] Traditional stucco
Traditionally, stucco was applied directly to a masonry surface such as brick or stone. In wood-framed buildings, stucco was applied over a wood lath, similar to the lath and plaster technique used in the interiors of buildings. Modern stucco is usually applied over an expanded metal lath that is fastened with staples or screws through wall sheathing into studs. The material is generally applied in three coats — the scratch coat, the brown coat and the finish coat. The finish is usually an integral color and is where texture is also achieved.
Stucco can be hand-applied or sprayed. The finish material can be troweled smooth, hand-textured, floated to a sand finish or sprayed.
In some parts of America (California, Arizona, New Mexico and Florida), stucco is the predominant exterior for both residential and commercial construction.
Stucco has also been used as a sculptural and artistic material. Baroque and Rococo architecture makes heavy use of stucco. Examples can be found in churches and palaces, where stucco is mostly used to provide a smooth, decorative transition from walls to ceiling, decorating and giving measure to ceiling surfaces. Stucco is an integral part of the art of belcomposto, the Baroque concept that integrates the three classic arts, architecture, sculpture, and painting.
Since stucco can be used for decorative purposes as well as for figurative representation, it provides an ideal transitive link from architectural details to wall paintings such as the typically Baroque trompe l'oeil ceilings, as in the work of the Wessobrunner School. Here, the real architecture of the church is visually extended into a heavenly architecture with a depiction of Christ, the Virgin Mary or the Last Judgment at the center. Stucco is used to form a semi-plastic extension of the real architecture that merges into the painted architecture.
Islamic art makes use of stucco as a decorative means in mosques and palaces. Indian architecture knows stucco as a material for sculpture in an architectural context. It is rare in the countryside.
Because of its "aristocratic" appearance, Baroque-looking stucco decoration was used frequently in upper-class apartments of the 19th and early 20th century.
Beginning in the 1920s, stucco, especially in its Neo-Renaissance and Neo-Baroque materialization, became increasingly unpopular with modern architects in some countries, resulting in a widespread movement to remove the stucco from tenements.
Stucco was still employed in the 1950s in molded forms for decorating the joints between walls and ceilings inside houses. It was generally painted the same color as the ceiling and used in designs where a picture rail or rat rail was in use.
[edit] Modern stucco
Modern stucco usually consists of one layer of wire lath and two layers of Portland cement-based plaster. However, cement's crystalline structure cannot accommodate significant movements in the building structure (as lime does) and is thus prone to cracking. Thus an additional acrylic finish, which adds a waterproof flexibility, is often applied on top.
If stucco is applied to a wood-framed structure or light-gauge steel frame structure, a wire mesh is first attached to vapor-permeable, water-resistant tarpaper which protects the sheathing and interior of the wall from outside moisture without trapping vapor inside. A wide variety of stucco accessories, such as weep screeds, control and expansion joints, corner-aids and architectural reveals are sometimes also incorporated into the lath. Wire lath is used to give the plaster something to attach to and to add strength. Types include expanded-metal lath, woven-wire lath, and welded-wire lath.
The first layer of plaster is called a "scratch coat," consisting of plastic cement and sand. A brush is used to scratch the surface horizontally or in a crisscross pattern to provide a key for the second layer. The first coat is allowed to dry (cure) before the second layer is applied.
The next layer is called the "brown coat" or leveling coat. It also consists of sand, cement, and lime. It is leveled with tools called "darbies," "rods," and "federeges," scraped smooth, and floated to provide a smooth, even surface onto which the finish coat is applied. It is then allowed to dry (cure) for 7-10 days minimum to allow "checking" (shrinkage) and cracking to take place.
If applied during very dry weather, the layers of stucco are sprayed with water for one or more days to keep a level of moisture within the stucco while it cures, a process known as "moist curing." If the stucco dries too soon, the chemical hardening ("hydration") will be incomplete, resulting in a weaker and brittler stucco.
The final, exterior layer is the final layer is the "finish coat," of which there are two recommended types:
1. Color Coat is a colored sand, cement, and lime mixed finish and is typically 3 mm (1/8") thick. It is applied over the second coat (brown coat) and can be floated with water for a sandy finish or textured over with a trowel to create various styles of finishes. Premixed, bagged stucco is gaining in use and is available in coarse graded sand and finer graded sand for creating a variety of troweled finishes; it is available in a variety of colors.
2. Acrylic Finish is an acrylic-based finish from 1.0 mm to 4.0 mm thick. It can be applied in many ways; it is the most recommended finish for long-lasting quality. It also can be ordered in any color.
3. Hard Coating is another method of adding a finish to the stucco wall, although no longer recommended. In the 1960s and 1970s people added a variety of materials like glass chunks, stones or marble into the wet stucco wall. This kind of finish coat is very heavy and inflexible and is hard to repair.
[edit] See also
- Cement render
- Exterior Insulation Finishing System
- Pargeting
- Plaster
- Plasterwork
- Polished plaster
- Tadelakt
[edit] External links
- Anne Grimmer, The Preservation and Repair of Historic Stucco, published by the Technical Preservation Services, Heritage Preservation Services Division, National Park Service [1], and duplicated all over the web
- About.com article on stucco
