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Plasterers in North America have relied on two materials to create their handiwork—lime and gypsum.
Until the end of the 19th century, plasterers used lime plaster. Lime plaster was made from four ingredients: lime, aggregate, fiber, and water.
The lime came from ground-and-heated limestone or oyster shells; the aggregate from sand; and the fiber from cattle or hog hair. Manufacturing changes at the end of the 19th century made it possible to use gypsum as a plastering material. Gypsum and lime plasters were used in combination for the base and finish coats during the early part of the 20th century; gypsum was eventually favored because it set more rapidly and, initially, had a harder finish.
The builders of this midth century house installed the baseboard molding first, then applied a mud and horse hair plaster. Lime was used for the finish plaster. Not only did the basic plastering material change, but the method of application changed also.
In early America, the windows, doors, and all other trim were installed before the plaster was applied to the wall. Generally the woodwork was prime-painted before plastering. Obtaining a plumb, level wall, while working against built-up moldings, must have been difficult.
But sometime in the first half of the 19th century, builders began installing wooden plaster "grounds" around windows and doors and at the base of the wall. Installing these grounds so that they were level and plumb made the job much easier because the plasterer could work from a level, plumb, straight surface. Woodwork was then nailed to the "grounds" after the walls were plastered. Evidence of plaster behind trim is often an aid to dating historic houses, or to discerning their physical evolution.
When building a house, plasterers traditionally mixed bags of quick lime with water to "hydrate" or "slake" the lime. As the lime absorbed the water, heat was given off. When the heat diminished, and the lime and water were thoroughly mixed, the lime putty that resulted was used to make plaster. When lime putty, sand, water, and animal hair were mixed, the mixture provided the plasterer with "coarse stuff. But the best plaster was done with three coats.
The first two coats made up the coarse stuff; they were the scratch coat and the brown coat. The finish plaster, called "setting stuff," contained a much higher proportion of lime putty, little aggregate, and no fiber, and gave the wall a smooth white surface finish. Schifferstadt, a simple house of German origin that dates to , utilized plaster for both flat and curved walls.
Additives were used for various finish qualities. For example, fine white sand was mixed in for a "float finish. If the plasterer raked the sand with a broom, the plaster wall would retain swirl marks or stipples. Or marble dust was added to create a hard-finish white coat which could be smoothed and polished with a steel trowel. Finally, a little plaster of Paris, or "gauged stuff," was often added to the finish plaster to accelerate the setting time. Although lime plaster was used in this country until the early s, it had certain disadvantages.
A plastered wall could take more than a year to dry; this delayed painting or papering. In addition, bagged quick lime had to be carefully protected from contact with air, or it became inert because it reacted with ambient moisture and carbon dioxide. Around , gypsum began to be used as a plastering material. Gypsum begins to cure as soon as it is mixed with water. It sets in minutes and completely dries in two to three weeks. Historically, gypsum made a more rigid plaster and did not require a fibrous binder.
However it is difficult to tell the difference between lime and gypsum plaster once the plaster has cured. Despite these desirable working characteristics, gypsum plaster was more vulnerable to water damage than lime. Lime plasters had often been applied directly to masonry walls without lathing , forming a suction bond. They could survive occasional wind-driven moisture or water winking up from the ground.
Gypsum plaster needed protection from water. Furring strips had to be used against masonry walls to create a dead air space. This prevented moisture transfer.
In rehabilitation and restoration projects, one should rely on the plasterer's judgment about whether to use lime or gypsum plaster. In general, gypsum plaster is the material plasterers use today. Different types of aggregate may be specified by the architect such as clean river sand, perlite, pumice, or vermiculite; however, if historic finishes and textures are being replicated, sand should be used as the base-coat aggregate. Today, if fiber is required in a base coat, a special gypsum is available which includes wood fibers.
Lath provided a means of holding the plaster in place. Wooden lath was nailed at right angles directly to the structural members of the buildings the joists and studs , or it was fastened to nonstructural spaced strips known as furring strips. Three types of lath can be found on historic buildings. Wood lath is usually made up of narrow, thin strips of wood with spaces in between.
The plasterer applies a slight pressure to push the wet plaster through the spaces. The plaster slumps down on the inside of the wall, forming plaster "keys. Metal lath, patented in England in , began to be used in parts of the United States toward the end of the 19th century.
The steel making up the metal lath contained many more spaces than wood lath had contained. These spaces increased the number of keys; metal lath was better able to hold plaster than wood lath had been. A third lath system commonly used was rock lath also called plaster board or gypsum-board lath.
In use as early as , rock lath was made up of compressed gypsum covered by a paper facing. Some rock lath was textured or perforated to provide a key for wet plaster.
A special paper with gypsum crystals in it provides the key for rock lath used today; when wet plaster is applied to the surface, a crystalline bond is achieved. Rock lath was the most economical of the three lathing systems. Lathers or carpenters could prepare a room more quickly. By the late s, rock lath was used almost exclusively in residential plastering. When plaster dries, it is a relatively rigid material which should last almost indefinitely.
However, there are conditions that cause plaster to crack, effloresce, separate, or become detached from its lath framewor.
These include:. Stresses within a wall, or acting on the house as a whole, can create stress cracks. Appearing as diagonal lines in a wall, stress cracks usually start at a door or window frame, but they can appear anywhere in the wall, with seemingly random starting points. Builders of now-historic houses had no codes to help them size the structural members of buildings. The weight of the roof, the second and third stories, the furniture, and the occupants could impose a heavy burden on beams, joists, and studs.
Even when houses were built properly, later remodeling efforts may have cut in a doorway or window without adding a structural beam or "header" across the top of the opening. Occasionally, load-bearing members were simply too small to carry the loads above them. Deflection or wood "creep" deflection that occurs over time can create cracks in plaster. Stress cracks in plaster over a kitchen door frame can be repaired using fiberglass mesh tape and joint compound.
Overloading and structural movement especially when combined with rotting lath, rusted nails, or poor quality plaster can cause plaster to detach from the lath.
The plaster loses its key. When the mechanical bond with the lath is broken, plaster becomes loose or bowed. If repairs are not made, especially to ceilings, gravity will simply cause chunks of plaster to fall to the floor. Cracks in walls can also result when houses settle. Houses built on clay soils are especially vulnerable. Many types of clay such as montmorillonite are highly expansive.
In the dry season, water evaporates from the clay particles, causing them to contract. During the rainy season, the clay swells. Thus, a building can be riding on an unstable footing.
Diagonal cracks running in opposite directions suggest that house settling and soil conditions may be at fault. Similar symptoms occur when there is a nearby source of vibration-blasting, a train line, busy highway, or repeated sonic booms.
Horizontal cracks are often caused by lath movement. Because it absorbs moisture from the air, wood lath expands and contracts as humidity rises and falls. This can cause cracks to appear year after year. Cracks can also appear between rock lath panels. A nail holding the edge of a piece of lath may rust or loosen, or structural movement in the wood framing behind the lath may cause a seam to open.
Heavy loads in a storage area above a rock-lath ceiling can also cause ceiling cracks. Errors in initial building construction such as improper bracing, poor corner construction, faulty framing of doors and windows, and undersized beams and floor joists eventually "telegraph" through to the plaster surface.
In addition to problems caused by movement or weakness in the structural framework, plaster durability can be affected by poor materials or workmanship. The proper proportioning and mixing of materials are vital to the quality of the plaster job. A bad mix can cause problems that appear years later in a plaster wall.
Until recently, proportions of aggregate and lime were mixed on the job. A plasterer may have skimped on the amount of cementing material lime or gypsum because sand was the cheaper material.
Over sanding can cause the plaster to weaken or crumble. Plaster made from a poorly proportioned mix may be more difficult to repair. Use of perlite as an aggregate also presented problems. Perlite is a lightweight aggregate used in the base coat instead of sand. It performs well in cold weather and has a slightly better insulating value. But if a smooth lime finish coat was applied over perlited base coats on wood or rock lath, cracks would appear in the finish coat and the entire job would have to be redone.
To prevent this, a plasterer had to add fine silica sand or finely crushed perlite to the finish coat to compensate for the dramatically differing shrinkage rates between the base coat and the finish coat.
The smooth-trowled lime finish has delaminated from the brown coat underneath. Photo: Marylee MacDonald. The finish coat is subject to "chip cracking" if it was applied over an excessively dry base coat, or was insufficiently troweled, or if too little gauging plaster was used.
Chip cracking looks very much like an alligatored paint surface. Another common problem is called map cracking—fine, irregular cracks that occur when the finish coat has been applied to an over sanded base coat or a very thin base coat. Retarding agents are added to slow down the rate at which plaster sets, and thus inhibit hardening. They have traditionally included ammonia, glue, gelatin, starch, molasses, or vegetable oil. If the plasterer has used too much retardant, however, a gypsum plaster will not set within a normal 20 to 30 minute time period.
As a result, the surface becomes soft and powdery. Plaster is applied in three coats over wood lath and metal lath—the scratch, brown, and finish coats. In three-coat work, the scratch coat and brown coat were sometimes applied on successive days to make up the required wall thickness.
Using rock lath allowed the plasterer to apply one base coat and the finish coat—a two-coat job. If a plasterer skimped on materials, the wall may not have sufficient plaster thickness to withstand the normal stresses within a building.
This minimum plaster thickness may affect the thickness of trim projecting from the wall's plane. Proper temperature and air circulation during curing are key factors in a durable plaster job.
The ideal temperature for plaster to cure is between 55 to 70 degrees Fahrenheit. However, historic houses were sometimes plastered before window sashes were put in. There was no way to control temperature and humidity. When temperatures were too hot, the plaster would return to its original condition before it was mixed with water, that is, calcined gypsum. A plasterer would have to spray the wall with alum water to reset the plaster.
If freezing occurred before the plaster had set, the job would simply have to be redone. If the windows were shut so that air could not circulate, the plaster was subject to sweat-out or rot.
Since there is no cure for rotted plaster, the affected area had to be removed and replastered. Plaster applied to a masonry wall is vulnerable to water damage if the wall is constantly wet. When salts from the masonry substrate come in contact with water, they migrate to the surface of the plaster, appearing as dry bubbles or efflorescence. The source of the moisture must be eliminated before replastering the damaged area. Some properties require rigorous fire standards, such as flats and other domestic dwellings.
Fire protection plasterboards are formulated to provide dependable resistance against fire. Thermal insulating plasterboards keep properties warm and ventilated, and keeps energy costs down.
It also reduces condensation, which can lead to damp. Acoustic plasterboard is designed to reduce sound travelling between rooms, floors and adjacent dwellings. Perfect for soundproofing floors, walls and ceilings. Jewson Plaster Guide.
Plaster types One coat plaster Designed to act as a 2-in-1 base coat and finish. Multi-finish plaster Provides a great, smooth coverage on a variety of surfaces. Bonding plaster Easy to apply and spread. Browning plaster Browning plaster is similar to bonding plaster in that it can be used as a base undercoat or backing coat. Hardwall plaster As the name might suggest, hardwall plaster is heavy-duty and durable. Tough coat plaster Extremely hardwearing, and perfect as a base coat.
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