The Wall Street Journal Article

Historic Remake: If These Walls Could Talk, They’d Say ‘Plaster Please’

http://blogs.wsj.com/developments/2012/07/20/historic-remake-if-these-walls-could-talk-theyd-say-plaster-please/
by: Constance Mitchell Ford/The Wall Street Journal

I had walls that were cracked and chipped.
Nothing makes an old house feel old and decrepit like cracked and bumpy walls.
But transforming walls from drab and dreary to bright and cheerful isn’t difficult. It’s messy, but not complicated.
When I began the renovation on my 85-year-old Tudor revival-style house, updating and repairing the walls was first on our list. Like most old homes built before World War II, my walls are made of plaster, not the drywall found in most post-WWII homes.
Preservationists insist that plaster is a better product than modern drywall. After all, plaster has stood the test of time. Archeologists say that lime plaster was used as a type of mortar in Egyptian pyramids.
Plaster is a mud-like substance that, when it dries, leaves a hard surface that is smooth to the touch, keeps firm for many decades and holds sound better than drywall. That’s good for parents who don’t want conversations in the master bedroom overheard by the kids in the next room.

Claudio Papapietro for The Wall Street Journal

This is the same area after two skim coats, but before painting.

Plaster is also less susceptible to mold, which was a big problem in my kitchen, the only room in my house that had drywall. (More on that in a later column.)
For a more high-end affect, some plasterers add marble dust to the mixture, which leaves an even harder wall finish that can be made to resemble the marble or granite walls found in historic Italian palazzos. (Some call this Venetian plaster, not to be confused with faux Venetian plaster that some paint companies sell.)
Drywall is made from gypsum wrapped in cardboard. It comes in large pre-made wall-size boards that are screwed or nailed in place. Even though drywall is considered a lower-end product than plaster, it has a major benefit: it’s much easier and less expensive to install.

“For every 500 guys who do drywall, you can find only a few who can do plaster,” said Kieran Quilligan, president of Bestwall Plastering Inc. in New York. If the plaster repair job includes intricate crown molding, that’s even a rarer skill that only a small number of plasterers can manage.
While the Internet is filled with debates about which wall construction is better, most specialists believe that both are good products for the right job. “Drywall for modern structures, it’s modular and fits into that scheme beautifully,” said Rory Brennan, chief executive of Preservation Plastering Ltd. in Brattleboro, Vt. “Old houses have their own set of rules predicated upon the materials and techniques used to build them.” Mr. Brennan provides plastering advice to the television program, “This Old House.”

Claudio Papapietro for The Wall Street Journal

I had just one plaster wall that needed to be replaced. Here it is after the work.

Yet preservationists say too many owners of old homes are tearing out their plaster and replacing it with drywall.
Not me. I took great pains to repair plaster, although I used a less expensive plastering method. The die-heart preservationists prefer that plaster walls are repaired or replaced using the most historically accurate — and most expensive — method, which is a three-coat system that requires a skilled plasterer to actually re-build the wall from the laths. A faster method is to install blue board over the laths and cover the boards with a plaster veneer.
The least expensive method, which works well on walls that aren’t badly damaged, is a simple skim coating, where a layer of plaster, sometimes combined with joint compound or spackle, are layered over the entire wall and sanded smooth.
Lucky for me, I had only one plaster wall — above the bay windows in the master bedroom — that needed to be replaced. For that, Edgar, my contractor, used the blue board system. But the remaining walls were just badly cracked and chipped, which was easily addressed by receiving two skim coats.
The end result was better than I expected. The walls are beautiful, rock hard and have a slight shimmer. When painted, they were somewhat luminescent.

Steve Reed Photography

Examples of other Tudor-style homes: slide show.

My delight at the results helped lessen the pain of the job itself, which was by far the messiest portion of my home renovation. It also took longer than any other job to complete and created loads of dust.
From start to finish, it took four weeks to complete most of the house, at a cost about $200 a day. But the results are so pleasing that I can’t complain.

What is Lath?

Lath

Lath seen from the back with brown coat oozing through

Traditionally, plaster was laid onto laths, rather than plasterboard as is more commonplace nowadays.

Wooden laths are narrow strips of straight-grained wood depending on availability of species in lengths of from two to four or five feet to suit the distances at which the timbers of a floor or partition are set. Laths are about an inch wide, and are made in three thicknesses; single (⅛ to 3⁄16 inch thick), lath and a half (¼ inch thick), and double (⅜–½ inch thick).

The thicker laths should be used in ceilings, to stand the extra strain (sometimes they were doubled for extra strength), and the thinner variety in vertical work such as partitions, except where the latter will be subjected to rough usage, in which case thicker laths become necessary. Laths are usually nailed with a space of about ⅜ of an inch between them to form a key for the plaster.

Laths were formerly all made by hand. Most are now made by machinery and are known as sawn laths, those made by hand being called rent or riven laths. Rent laths give the best results, as they split in a line with the grain of the wood, and are stronger and not so liable to twist as machine-made laths, some of the fibers of which are usually cut in the process of sawing.

Laths must be nailed so as to break joint in bays three or four feet wide with ends butted one against the other. By breaking the joints of the lathing in this way, the tendency for the plaster to crack along the line of joints is diminished and a better key is obtained. Every lath should be nailed at each end and wherever it crosses a joist or stud. All timbers over three inches (76 mm) wide should be counter-lathed, that is, have a fillet or double lath nailed along the centre upon which the laths are then nailed. This is done to preserve a good key for the plaster.

Walls liable to damp are sometimes battened and lathed to form an air cavity between the damp wall and the plastering.

Lathing in metal, either in wire or in the form of perforated galvanised sheets, is now extensively used on account of its fireproof and lasting quality. There are many kinds of this material in different designs, the best known in England being the Jhilmil, the Bostwick, Lathing, and Expanded Metal lathing. The two last-named are also widely used in America.

Lathing nails are usually of iron, cut, wrought or cast, and in the better class of work they are galvanized to prevent rusting. Zinc nails are sometimes used, but are costly.

What is Plastering?

Plastering is one of most common forms of interior wall materials in your home. 

Plastering refers to the finishing step that must be done once the sheets of drywall are hung. Ornamental plastering of ceilings and walls, using decorative architectural plasterwork, gives a rich and stylish feel to any room. Plasterwork is one of the most ancient of handicrafts employed in connection with building operations, the earliest evidence showing that the dwellings of primitive man were erected in a simple fashion with sticks and plastered with mud. Today, we use much more long-lasting and stylish materials perfected over years of scientifically testing various drywall and plaster compositions.

What is Plaster?

Plaster is a building material used for coating walls and ceilings. Plaster starts as a dry powder similar to mortar or cement and like those materials it is mixed with water to form a paste which liberates heat and then hardens. Unlike mortar and cement, plaster remains quite soft after setting, and can be easily manipulated with metal tools or even sandpaper. These characteristics make plaster suitable for a finishing, rather than a load-bearing material.

The term plaster can refer to gypsum plaster (also known as plaster of Paris), lime plaster, or cement plaster.

Types

Gypsum plaster (plaster of Paris)  Gypsum plaster, or plaster of Paris, is produced by heating gypsum to about 300°F (150 °C):

    2CaSO4·4H2O + Heat → 2CaSO4·H2O + 3H2O (released as steam).

When the dry plaster powder is mixed with water, it re-forms into gypsum. The setting of unmodified plaster starts about 10 minutes after mixing and is complete in about 45 minutes; but not fully set for 72 hours. If plaster or gypsum is heated above 392°F (200°C), anhydrite is formed, which will also re-form as gypsum if mixed with water.

A large gypsum deposit at Montmartre in Paris led gypsum plaster to be commonly known as "plaster of Paris".

 Plasterers often use gypsum to simulate the appearance of surfaces of wood, stone, or metal, on movie and theatrical sets for example. Nowadays, theatrical plasterers often use expanded polystyrene, although the job title remains unchanged.

Plaster of Paris can be used to impregnate gauze bandages to make a sculpting material called modroc. It is used similarly to clay, as it is easily shaped when wet, yet sets into a resilient and lightweight structure. This is the material which was (and sometimes still is) used to make classic plaster orthopedic casts to protect limbs with broken bones. Set modroc is an early example of a composite material.

Lime plaster

Lime plaster is a mixture of calcium hydroxide and sand (or other inert fillers). Carbon dioxide in the atmosphere causes the plaster to set by transforming the calcium hydroxide into calcium carbonate (limestone). Whitewash is based on the same chemistry.

To make lime plaster, limestone (calcium carbonate) is heated to produce quicklime (calcium oxide). Water is then added to produce slaked lime (calcium hydroxide), which is sold as a wet putty or a white powder. Additional water is added to form a paste prior to use. The paste may be stored in air-tight containers. Once exposed to the atmosphere, the calcium hydroxide turns back into calcium carbonate, causing the plaster to set.

Lime plaster was a common building material for wall surfaces in a process known as lath and plaster, whereby a series of wooden strips on a studwork frame was covered with a semi-dry plaster that hardened into a surface. The plaster used in most lath and plaster construction was mainly lime plaster, with a cure time of about a month. To stabilize the lime plaster during curing, small amounts of plaster of Paris were incorporated into the mix. Because plaster of Paris sets quickly, "retardants" were used to slow setting time enough to allow workers to mix large working quantities of lime putty plaster. A modern form of this method uses expanded metal mesh over wood or metal structures, which allows a great freedom of design as it is adaptable to both simple and compound curves. Today this building method has been partly replaced with drywall, also composed mostly of gypsum plaster. In both these methods a primary advantage of the material is that it is resistant to a fire within a room and so can assist in reducing or eliminating structural damage or destruction provided the fire is promptly extinguished.

Lime plaster is used for frescoes, where pigments, diluted in water, are applied to the still wet plaster.

Cement plaster

Cement plaster is a mixture of suitable plaster, sand, portland cement and water which is normally applied to masonry interiors and exteriors to achieve a smooth surface. Interior surfaces sometimes receive a final layer of gypsum plaster. Walls constructed with stock bricks are normally plastered while face brick walls are not plastered. Various cement-based plasters are also used as proprietary spray fireproofing products. These usually use vermiculite as lightweight aggregate. Heavy versions of such plasters are also in use for exterior fireproofing, to protect LPG vessels, pipe bridges and vessel skirts.

Uses

In architecture

Plasterwork

Plaster may also be used to create complex detailing for use in room interiors. These may be geometric (simulating wood or stone) or naturalistic (simulating leaves, vines, and flowers) These are also often used to simulate wood or stone detailing found in more substantial buildings.

In art

Many of the greatest mural paintings in Europe, like Michelangelo's Sistine Chapel ceiling are executed in fresco, meaning they are painted on a thin layer of wet plaster, called intonaco (in fact the general term for plaster in Italian); the pigments sink into this layer so that the plaster itself becomes the medium holding them, which accounts for the excellent durability of fresco. Additional work may be added a secco on top of the dry plaster, though this is generally less durable.

Plaster may be cast directly into a damp clay mold. In creating this piece molds (molds designed for making multiple copies) or waste molds (for single use) would be made of plaster. This "negative" image, if properly designed, may be used to produce clay productions, which when fired in a kiln become terra cotta building decorations, or these may be used to create cast concrete sculptures. If a plaster positive was desired this would be constructed or cast to form a durable image artwork. As a model for stonecutters this would be sufficient. If intended for producing a bronze casting the plaster positive could be further worked to produce smooth surfaces. An advantage of this plaster image is that it is relatively cheap; should a patron approve of the durable image and be willing to bear further expense, subsequent molds could be made for the creation of a wax image to be used in lost wax casting, a far more expensive process. In lieu of producing a bronze image suitable for outdoor use the plaster image may be painted to resemble a metal image; such sculptures are suitable only for presentation in a weather-protected environment.

Plaster expands while hardening, then contracts slightly just before hardening completely. This makes plaster excellent for use in molds, and it is often used as an artistic material for casting. Plaster is also commonly spread over an armature (form), usually made of wire, mesh or other materials, a process raised details. For these processes, limestone or acrylic based plaster may be employed.

In medicine

Plaster is widely used as a support for broken bones; a bandage impregnated with plaster is moistened and then wrapped around the damaged limb, setting into a close-fitting yet easily removed tube, known as an orthopedic cast; however, this is slowly being replaced by a fibreglass variety.

Plaster is also used within radiotherapy when making immobilization casts for patients. Plaster bandages are used when constructing an impression of the patients head and neck, and liquid plaster is used to fill the impression and produce a plaster bust. Perspex is then vacuum formed over this bust creating an immobilization shell.

In dentistry, plaster is used for mounting casts or models of oral tissues. These diagnostic and working models are usually made from dental stone, a stronger, harder and denser derivative of plaster which is manufactured from gypsum under pressure. Plaster is also used to invest or flask wax dentures, the wax being subsequently removed and replaced with the final denture base material which is cured in the plaster mold

In fire protection

Plasters have been in use in passive fire protection, as fireproofing products, for many decades.

The finished plaster releases water vapor when exposed to flame, acting to slow the spread of the fire, for as much as an hour or two depending on thickness. It also provides some insulation to retard heat flow into structural steel elements, that would otherwise lose their strength and collapse in a fire. Early versions of these plasters have used asbestos fibres, which have by now been outlawed in industrialized nations and have caused significant removal and re-coating work. More modern plasters fall into the following categories:

    fibrous (including mineral wool and glass fiber)

    cement mixtures either with mineral wool or with vermiculite

    gypsum plasters, leavened with polystyrene beads, as well as chemical expansion agents to decrease the density of the finished product.

One differentiates between interior and exterior fireproofing. Interior products are typically less substantial, with lower densities and lower cost. Exterior products have to withstand more extreme fire and other environmental conditions. Exterior products are also more likely to be attractively tooled, whereas their interior cousins are usually merely sprayed in place. A rough surface is typically forgiven inside of buildings as dropped ceilings often hide them. Exterior fireproofing plasters are losing ground to more costly intumescent and endothermic products, simply on technical merit. Trade jurisdiction on unionized construction sites in North America remains with the plasterers, regardless of whether the plaster is decorative in nature or is used in passive fire protection. Cementitious and gypsum based plasters tend to be endothermic. Fireproofing plasters are closely related to firestop mortars. Most firestop mortars can be sprayed and tooled very well, due to the fine detail work that is required of firestopping, which leads their mix designers to utilise concrete admixtures, that enable easier tooling than common mortars.

In dentistry

Dentures (false teeth) are made by first taking a dental impression using a soft, pliable material that can be removed from around the teeth and gums without loss of fidelity and using the impression to creating a wax model of the teeth and gums. The model is used to create a plaster mold (which is heated so the wax melts and flows out) and the denture materials are injected into the mold. After a curing period, the mold is opened and the dentures are cleaned up and polished.

Safety issues

The chemical reaction that occurs when plaster is mixed with water is exothermic in nature and, in large volumes, can burn the skin. In January 2007, a student sustained third-degree burns after encasing her hands in a bucket of plaster as part of a school art project. The burns were so severe she required amputation of both her thumbs and six of her fingers.

Some variations of plaster that contain powdered silica or asbestos may present health hazards if inhaled. Asbestos is a known irritant when inhaled in powder form can cause cancer, especially in people who smoke, and inhalation can also cause asbestosis. Inhaled silica can cause silicosis and (in very rare cases) can encourage the development of cancer. Persons working regularly with plaster containing these additives should take precautions to avoid inhaling powdered plaster, cured or uncured. (Note that asbestos is rarely used in modern plaster formulations because of its carcinogenic effects.

 From Wikipedia, the free encyclopedia