Plaster is a building material used for the protective and decorative coating of walls and ceilings and for moulding and casting decorative elements.
What is plaster?
Full coat interior plaster begins with a base. A hundred years ago it was wood lath, brick, or clay tile. Thirty years ago it might have been rock lath (a 3/8” x 16” x 48” sheet that looks a little like drywall), metal lath, block or tile. Today it is mostly metal lath or 1/2”- 5/8” gypsum base (also called blue board), similar to drywall but with a special type of paper to ensure a good bond.
Next a scratch coat (made of gypsum plaster and sand) “keys” into the base and stiffens it for the subsequent coats. It is very rough so that the next coat can get a good bond.
After curing for several days, the brown coat (consisting of gypsum plaster and sand) is applied. If thicker plaster is desired, it can be left to “set” (harden) then “double back” and put another coat of the same material over the top of it to the desired thickness. The brown coat is also left rough, although not as rough as the scratch coat. The finish coat is then applied to a thickness between 1/16”- 1/8”. The finish usually consists of lime mixed with an accelerant to make it set faster. The finish can be perfectly smooth or textured. Sand may also be added for further variety.
info and image source:http://www.oren-usa.com/about/what-is-plaster/
How does Plaster form?
The primary component of Plaster is the mineral gypsum. Each molecule of gypsum is composed of two molecules of water and one of calcium sulfate.
Because the water present in gypsum is in crystalline form, the material is dry.
Gypsum, called gypsos by the ancient Greeks, is one of the most useful minerals known to man. In its pure form, it is white, but impurities often give it colours like grey, brown, pink, or black. Today, pulverized gypsum is used for a wide variety of applications.
info source: http://www.madehow.com/Volume-2/Drywall.html
image source: http://geology.com/minerals/gypsum.shtml
What are the types of Plaster?
- Gypsum plaster, or plaster of Paris, is produced by heating gypsum to about 150 °C. 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 130 °C, hemihydrate is formed, which will also re-form as gypsum if mixed with water. On heating to 180 °C, the nearly water-free form, called γ-anhydrite is produced. γ-Anhydrite reacts slowly with water to return to the dihydrate state, a property exploited in some commercial desiccants. On heating above 250 °C, the completely anhydrous form called β-anhydrite or dead burned plaster is formed.
- 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 above approximately 850 °C 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 before use. The paste may be stored in airtight containers. When exposed to the atmosphere, the calcium hydroxide very slowly turns back into calcium carbonate through reaction with atmospheric carbon dioxide, causing the plaster to increase in strength.
- 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.
info source: https://en.wikipedia.org/wiki/Plaster
What are the stage of Plastering?
This is where you apply your first coat of plaster, which should be about 2mm thick. When you’ve loaded the plaster onto your trowel start at an angle with the leading edge of the trowel away from the wall, moving the trowel in a long motion, gradually flattening it to the wall as you go. On this first coat it’s more important to try and get a flat even coat with no bulges, don’t worry too much about any trowel marks or ridges because these will come out later.
As the plaster begins to firm up this is when you can flatten off the first coat with a clean trowel, making sure that the bulges are pushed out. The secret to this (as with the majority of plastering) is to use firm pressure as you move your trowel across the wall in long movements, holding the leading edge of the trowel at about 10-15mm away.
Straight after you’ve finished stage two, the second coat of plaster is applied. It’s very similar to the first stage, but this time you’re only using half the plaster, applying half the thickness and even firmer pressure to try and reduce any holes in the plaster that might be caused. If you do create holes don’t worry too much, hopefully, the next stage will get rid of them. The main reason for not spending too much time trying to fix the holes at this point is because you don’t want parts of your plaster to dry while you do.
This is where you can trowel the plaster flat and remove any holes. Hopefully, the plaster will be tacky and pliable enough to be pushed around into the holes, although make sure it isn’t too wet or this could create more. Again remember to run the trowel over the wall at the correct angle of around 10-15mm and reduce the risk of any dragging or scraping. Use firm pressure and don’t worry too much about any ridges that are caused as these can be removed in the next step. As you go over the wall look closely for holes and if some aren’t filling despite the pressure of your trowel, then more plaster can be applied to the hole and flattened. While it’s important to get the majority of holes filled at this stage, again don’t take too much time over it and let your plaster dry.
This is where you remove any ridges in the plaster by using firm pressure with your trowel. If the plaster is quite firm and not easy to push around you can widen the angle of your trowel against the wall, but be careful not to scrape the plaster off.
The final part of the six stages of plastering should leave you with a nice even slightly polished surface. It begins when the plaster starts to darken as it dries. This is where you can run the trowel with firm pressure over the entire wall to finish. And you’re done.
info and image source: https://www.gypsumtools.com/blog/6-stages-of-plastering/
What are Plaster characteristics?
- fire protective
- regulates sound
- thermal insulator when combined with insulation materials
- equilibrates humidity and heat peaks
- impact resistant
- easy to install and dismantle
- multifaceted, multipurpose, supple and aesthetic
info source: http://www.eurogypsum.org/about-gypsum/properties/
How was Plaster used throughout history?
Plastering is one of the most ancient building techniques. Evidence indicates that primitive peoples plastered their reed or sapling shelters with mud, thus developing more durable structures and more effective screens against vermin and inclement weather. More lasting and sightly materials in time replaced mud. Some of the earliest plastering extant is of a quality comparable to that used in modern times. The Pyramids of Egypt contain plasterwork executed at least 4,000 years ago that is still hard and durable. The principal tools of the plasterer of that time were in design and purpose like those used today. For their finest work, the Egyptians used a plaster made from calcined gypsum that is identical to the plaster of Paris.
Very early in the history of Greek architecture (e.g., at Mycenae), plaster of a fine white lime stucco was used. Greek artisans had achieved high quality earlier than the 5th-century bc. Plaster was frequently used to cover the exteriors of temples, a technique is commonly known as stucco, in addition to covering the interiors, in some cases even when the building was made of marble.
The ornamental plaster ceilings of England during the reigns of Henry VIII, Elizabeth I, and James I still are admired. Earlier extant specimens of the plasterers’ skill in England are the pargeted and ornamented fronts of half-timbered houses.
Plaster as a medium of artistic expression waned by the 19th century when imitation and mechanical reproduction displaced this creative art. As a surface material for interior walls and ceilings and to a lesser degree for exterior walls, plaster remains in common use. It facilitates cleanliness and sanitation in building and is a retardant to the spread of fire.
info and image source:https://www.britannica.com/technology/plaster