Surprising properties of plaster attracted people since ancient times: trimmed with plates from this mineral walls of houses in Persia, and to residents of Egypt stacked in hand-made article tombs from selenit. There passed centuries, but plaster remains demanded in many fields of construction, medicine and processing of the soil. And sculptors and carvers of a stone continue to create fine products from this soft material.
It is unknown when for the first time representatives of mankind learned about surprising properties of plaster. But it is clear that else in antique times this mineral was applied to production various sculptural and objects for life. And its texture which can be processed so easily allowed our ancestors to apply plaster plates and blocks when finishing interiors of buildings. Even walls of the ancient Persian city of Risafa were put up from the plaster plates shining under beams of the sun.
Continue to apply plaster and in the modern industry: in some countries from it produce sulfuric acid. And the main amount of the extracted and burned mineral is used for production of different grades of construction cement. Except the sphere of construction, plaster is necessary in medicine, in agriculture (as raw materials for nitrogen fertilizers), for production of expensive types of paper. And from a beautiful translucent kind of plaster, selenit, do unique figurines, vases and jewelry.
Structure and properties
Plaster - one of the most widespread minerals on our planet, sometimes its crystals form the whole picturesque caves.
Its chemical formula Ca(SO4) 2H2O, the hydrated calcium sulfate. In the nature enough large crystals meet in the form of transparent or slightly turbid. The impurity received at emergence give to mineral a pink, blue, greenish or yellowish shade. Formation of the dual crystals which grew together with the bases and forming similarity of ""a tail of a swallow"" is possible. In emptiness Druzes of plaster of different degree of transparency are sometimes formed.
Except the crystals of a tablitchaty and prismatic form which grew together among themselves the layered structure consisting of thin flexible leaflets is characteristic of plaster. Visually it can remind the surface of a brain. At small effort, it is powerful to separate a similar plate pressing of a nail.
Sometimes plates under pressure of breed are twisted and bent, forming effective joints, so-called ""plaster flowers"". If in the course of formation of such form mineral took the smallest grains of sand, ""the desert rose"" which is very difficult for finding turns out surprising beauty.
Types of plaster
It is very fragile mineral (density 2.32 on Moos's scale), it is easy to scratch it even a finger. Depending on availability of impurity, density and time of origin distinguish three types of mineral:
- Anhydrite: absolutely waterless sulfate of calcium. In the nature meet white, blue, gray and reddish samples are more rare. It is formed in sedimentary rocks, usually at the expense of dehydration of plaster deposits. It is possible to receive anhydrite in the artificial way, having calcinated usual plaster at a temperature above 110 °C. At addition of water can increase in volume approximately by 30%. Sometimes for simplification of work with anhydrite it is milled in powder.
- Alabaster: the cleanest, almost without impurity, a kind of plaster. In the nature meets in the form of massive accrete crystals dense, fine-grained structure. Most often it is mineral of white color, there are pink, greenish or gray sites. Natural alabaster of uniform peach tone is most of all appreciated, it is used as an ornamental stone. Because of small hardness, alabaster well gives in to processing by means of any construction tools.
- Selenit or silky spar. This fibrous mineral with a smooth wax surface to be appreciated as an excellent ornamental stone. It has rich color scale, most often selenit happens a gentle pink, blue or yellowish shade, snow-white selenit meets nacreous gloss much less often. The first jewelry of selenit was found at excavation of treasuries of ancient Egypt therefore among jewelers this kind of plaster is called ""the Egyptian stone"".
Origin and regions of production
Experts find plaster in different forms on all continents of our planet. The most powerful deposits are located in mouths of the ancient rivers or in places where there were sea coasts and small salty lagoons millions years ago. Most of all plaster is extracted in regions of the Mediterranean, leaders can also be considered the USA, Canada and Central Asia. In our country the rich fields are in the Urals, in the Caucasus Mountains and in the Volga region.
Though it is much simpler to list areas where never extracted plaster, it is not enough such regions. Universal distribution of this mineral is connected with features of its origin which began in the old Permian Period of the Palaeozoic Era. In total distinguish five possible ways of formation of plaster in the nature:
- Adjournment in the sedimentary way in ancient salty reservoirs. At the same time together with plaster also rock salt was formed therefore plaster layers together with the hardened salt and insignificant amount of anhydrite often meet;
- In emptiness of breeds the large Druzes formed due to concentration of the plaster dissolved in atmospheric waters often meet;
- Under the influence of a surface water there is a hydration of anhydrite in friable sedimentary deposits. Saturated with moisture, it considerably increases in volume, forming rich plaster deposits over time;
- It is quite often formed in layers of calcareous breeds at hit on them of the water rich with the dissolved sulfates or sulfuric acid;
- In droughty desert areas with their considerable temperature difference and the plastered soils, the streak or the formations which appeared on a surface thanks to aeration of the next rocks meets in a look.
Application
This mineral is well dissolved in water, has low heat conductivity and quite high resistance to influence of high temperatures. At the same time the dissolved plaster quickly dries and stiffens that does it very demanded by production of construction materials. Exactly thanks to plaster form equal walls and ceilings in houses, it is the most important component of cement.
Most often apply alabaster (the plaster subject to heat treatment crushed then in powder) to needs of construction. Depending on technology, produce about 12 types of industrial alabaster. They differ in different degree of durability and time of the beginning of a skhvatyvaniye. Besides, from sheets of dense cardboard and the divorced plaster produce such popular construction material as gypsum cardboard.
Plaster is irreplaceable at production of jewelry and decorative elements for internal and exterior finish of houses too. From it it is possible to create a modelled product of any complexity, as according to author's sketches, and mass production. The decor from plaster is completely eco-friendly and natural product therefore such elements of a decor are very demanded among admirers of natural style.
The bandages impregnated with solution from plaster in traumatology and orthopedics are widely applied. They are done of usual gauze bandage, applying a thin film of plaster powder on material. Still this mineral is used in stomatology and front surgery, forming molds of tooth alignments, jaws and exact masks of the person.
At sculptors since ancient times plaster is considered one of the most demanded materials for work. It so easily gives in to influence of tools that the artist can create any form voluntarily. In old times the transparent lamellar crystals of plaster especially white with nacreous gloss used for decoration of icing frames and images of saints. This material was called ""Maryino flew down"" and highly appreciated up to the end of the 19th century.
And figures and caskets from silky selenit continue to please fans of products from a stone. And it is considered that hand-made articles from this stone help the owners to clean reason and to master negative emotions.