Gypsum is a large medical encyclopedia. The history of the plaster cast Criticism: reviews of doctors

In the treatment of fractures, a plaster cast is most often used, which is applied by an average medical worker alone or together with a doctor.

Medical plaster obtained from gypsum stone (lime sulphate), calcining it in special furnaces at a temperature not exceeding 130 ° C. As a result, the gypsum stone loses water, becomes brittle and is easily ground into a fine white powder. The quality of gypsum depends on a number of conditions, in particular, on the residence time in the oven, the calcination temperature, and the mesh size of the screening sieves. Gypsum must be stored in a dry place, as the degree of its humidity depends on it.

Medical plaster should be white in color, finely ground, soft to the touch, free of lumps, should harden quickly and be durable in products.

When performing plaster work, you need to take two parts by weight of gypsum to one part of water. With an excess of water, the hardening of gypsum slows down. Gypsum hardens faster at high temperatures, slower at low temperatures. In some cases, for faster hardening of gypsum, alum is added to the water (20 g per bucket of water).

Plaster test. Upon receipt of a cast or before applying a cast, check the quality of the cast using the following steps.

1. Prepare a two- or three-layer splint and impose it on the forearm or hand. If the gypsum is benign, then it hardens in 5-7 minutes, the removed splint retains its shape and does not crumble.

2. Prepare a gypsum gruel (consistency of liquid sour cream) and smear it with a thin layer on a saucer or tray. Good gypsum hardens in 5-6 minutes. If you press on the hardened mass with your finger, then it is not crushed and moisture does not appear on its surface. A piece of such gypsum will not warm up, but break. A bad cast will loosen up.

How to improve the quality of gypsum. Sometimes you have to use not quite benign gypsum. In such cases, you can try to improve its quality. If the gypsum is damp and contains an excessive amount of moisture, then it can be dried. To do this, gypsum is poured in a not very thick layer on an iron sheet, which is placed for several minutes in a heated oven, oven, or simply on a stove. It is necessary to ensure that drying is carried out at a temperature not exceeding 120 ° C. After drying, warm gypsum should not release moisture. This is checked as follows. A mirror is held over the plaster for several minutes. If the mirror fogs up, then moisture is released and the plaster is still wet. Insufficiently well ground gypsum, in which there are lumps, should be sieved through a fine sieve.

In clinics and emergency rooms, plaster bandages are applied in the dressing room. In polyclinics and emergency rooms, bandages are often applied to the lower leg, foot, forearm, and hand. A paramedic or nurse working in the dressing room of a polyclinic or emergency room should ensure that it has everything necessary for applying a plaster cast, including a sufficient number of plaster bandages of various sizes and a set of special tools for processing and removing a plaster bandage (Fig. 126). Dressing staff should be trained in dressing procedures.

Rice. 126. Tools for circumcision and removal of plaster bandages.

Dubrov Ya.G. Outpatient traumatology, 1986

And you say: slipped, fell. Closed fracture! Lost consciousness, woke up - plaster. (film "Diamond Hand")

Since ancient times, various materials have been used to immobilize damaged bone fragments in order to maintain immobility in the fracture area. The very fact that bones grow together much better if they are immobilized relative to each other was obvious even to primitive people. The vast majority of fractures will heal without any need for surgery if the broken bone is properly aligned and fixed (immobilized). Obviously, in that ancient time, immobilization (limitation of mobility) was the standard method of treating fractures. And how in those days, at the dawn of history, you can fix a broken bone? According to an extant text from the papyrus of Edwin Smith (1600 BC), hardening bandages were used, probably derived from bandages used in embalming. Also in the excavation of the tombs of the Fifth Dynasty (2494-2345 BC), Edwin Smith describes two sets of immobilization splints. Before the advent of the first plaster cast was very far ...
Detailed recommendations for the treatment of fractures are given in the Hippocratic Collection. The treatises “On Fractures” and “On Joints” give a technique for repositioning joints, eliminating limb deformities in fractures, and, of course, immobilization methods. Hardening dressings made from a mixture of wax and resin were used (by the way, the method was very popular not only in Greece), as well as tires made of "thick leather and lead."
Later descriptions of methods for fixing broken limbs, in the 10th century AD A talented surgeon from the Caliphate of Cordoba (the territory of modern Spain) suggested using a mixture of clay, flour and egg white to create a tight fixing bandage. These were materials that, along with starch, were used everywhere until the beginning of the 19th century and technically underwent only minor changes. Another thing is interesting. Why was plaster not used for this? The history of the plaster cast as we know it today is only 150 years old. And gypsum as a building material was used as early as the 3rd millennium BC. Has no one thought to use plaster for immobilization for 5 thousand years? The thing is that to create a plaster cast, you need not just gypsum, but one from which excess moisture has been removed - alabaster. In the Middle Ages, the name "Parisian plaster" was assigned to it.

History of plaster: from the first sculptures to Parisian plaster

Gypsum as a building material was used 5 thousand years ago, and was used everywhere in works of art, buildings of ancient civilizations. The Egyptians, for example, used it to decorate the tombs of the pharaohs in the pyramids. In ancient Greece, gypsum was widely used to create magnificent sculptures. In fact, the Greeks gave the name to this natural material. “Gypros” in Greek means “boiling stone” (obviously, due to its lightness and porous structure). It was also widely used in the works of the ancient Romans.
Historically, the most famous building material was used by the architects of the rest of Europe. Moreover, the manufacture of stucco and sculpture is not the only use of gypsum. It was also used for the manufacture of decorative plaster for processing wooden houses in cities. A huge interest in gypsum plaster arose because of the misfortune quite common in those days - fire, namely: the Great Fire of London in 1666. Fires were not uncommon then, but then more than 13 thousand wooden buildings burned out. It turned out that those buildings that were covered with gypsum plaster were much more resistant to fire. Therefore, in France they began to actively use gypsum to protect buildings from fires. An important point: in France there is the largest deposit of gypsum stone - Montmartre. Therefore, the name "Paris plaster" was fixed.

From Parisian plaster to the first plaster cast

If we talk about hardening materials used in the "pre-gypsum" era, then it is worth remembering the famous Ambroise Pare. The French surgeon impregnated the bandages with an egg white composition, as he writes in his ten-volume manual on surgery. It was the 16th century and firearms began to be actively used. Immobilizing dressings were used not only for the treatment of fractures, but also for the treatment of gunshot wounds,. European surgeons then experimented with dextrin, starch, wood glue. Napoleon Bonaparte's personal physician, Jean Dominique Larrey, used bandages impregnated with a mixture of camphor alcohol, lead acetate and egg white. The method, due to the complexity, was not massive.
But who first guessed to use a plaster cast, that is, a fabric soaked in plaster, is unclear. Apparently, it was a Dutch doctor - Anthony Mathyssen, who applied it in 1851. He tried rubbing the dressing with plaster powder, which, after being applied, was moistened with a sponge and water. Moreover, at a meeting of the Belgian Society of Medical Sciences, he was sharply criticized: the surgeons did not like the fact that the plaster stains the doctor's clothes and quickly hardens. Mathyssen's dressings were strips of coarse cotton fabric with a thin layer of Parisian plaster applied. This method of making a plaster cast was used until 1950.
It is worth saying that long before that there is evidence that gypsum was used for immobilization, but in a slightly different way. The leg was placed in a box filled with alabaster - a "dressing projectile". When the gypsum set, such a heavy blank was obtained on the limb. The downside was that it severely limited the patient's mobility. The next breakthrough in immobilization, as usual, was the war. In war, everything should be fast, practical and convenient for mass use. Who in the war will deal with boxes of alabaster? It was our compatriot, Nikolai Ivanovich Pirogov, who first applied a plaster cast in 1852 in one of the military hospitals.

The first ever use of a plaster cast

But why is it gypsum? Gypsum is one of the most common minerals in the earth's crust. It is calcium sulfate bound to two water molecules (CaSO4*2H2O). When heated to 100-180 degrees, gypsum begins to lose water. Depending on the temperature, either alabaster (120-180 degrees Celsius) is obtained. This is the same Parisian plaster. At a temperature of 95-100 degrees, low-fired gypsum is obtained, called high-strength gypsum. The latter is just more preferable for sculptural compositions.

He was the first to use the familiar plaster cast. He, like other doctors, tried to use different materials to create a tight bandage: starch, colloidin (a mixture of birch tar, salicylic acid and colloid), gutta-percha (a polymer very similar to rubber). All these funds had a big minus - they dried out very slowly. Blood and pus soaked the bandage and it often broke. The method proposed by Mathyssen was also not perfect. Due to the uneven impregnation of the fabric with gypsum, the bandage crumbled and was fragile.

For immobilization in ancient times, there were attempts to use cement, but the long curing time was also a minus. Try sitting still with a broken leg all day...

As N.I. Pirogov in his "Sevastopol Letters and Memoirs" he saw the action of gypsum on canvas at the workshop of the famous sculptor N.A. Stepanov in those days. The sculptor used thin linen strips soaked in a liquid mixture of Parisian plaster to make models. “I guessed that it could be used in surgery, and immediately put bandages and strips of canvas soaked in this solution on a complex fracture of the lower leg. The success was wonderful. The bandage dried up in a few minutes ... The complex fracture healed without suppuration and any seizures.
During the Crimean War, the method of using plaster casts was widely put into practice. The technique for preparing a plaster cast according to Pirogov looked like this. The injured limb was wrapped in a cloth, and the bone protrusions were additionally wrapped around. A gypsum solution was being prepared and strips from shirts or underpants were immersed in it (in war there is no time for fat). In general, everything was suitable for bandages.

In the presence of a plaster solution, you can turn anything into an immobilizing bandage (from the movie "Gentlemen of Fortune")

The gypsum gruel was distributed over the tissue and applied along the limb. Then the longitudinal stripes were reinforced with transverse stripes. It turned out to be a solid construction. Already after the war, Pirogov improved his method: a piece of tissue was cut out of coarse canvas in advance, corresponding to the size of the injured limb and soaked in a plaster solution before use.

Abroad, the Matissen technique was popular. The fabric was rubbed with dry gypsum powder and applied to the patient's limb. The gypsum composition was stored separately in sealed containers. In the future, bandages sprinkled with the same composition were produced. But they wetted them after bandaging.

Pros and cons of a plaster cast

What are the advantages of a gypsum-based fixing bandage? Convenience and speed of application. The gypsum is hypoallergenic (only one case of contact allergy is remembered). A very important point: the bandage "breathes" due to the porous structure of the mineral. A microclimate is created. This is a definite bonus, unlike modern polymer dressings, which also have a hydrophobic substrate. Of the minuses: not always sufficient strength (although a lot depends on the manufacturing technique). Gypsum crumbles and is very heavy. And for those who have been affected by misfortune and had to turn to a traumatologist, the question is often tormented: how to scratch under a cast? Nevertheless, under a plaster cast, it itches more often than under a polymer one: it dries out the skin (recall the hygroscopicity of gypsum). Various devices made of wires are used. Who faced, he will understand. In a bandage made of plastic, on the contrary, everything “fades”. The substrate is hydrophobic, that is, it does not absorb water. But what about the main bonus of polymer dressings - the ability to take a shower? Of course, here all these disadvantages are devoid of bandages created on a 3D printer. But so far, such bandages are only in development.

Polymer and 3D printer as a means of immobilization

Will the plaster cast become a thing of the past?

Modern capabilities of a 3D printer in the creation of fixation dressings

Undoubtedly. But I don't think it will be very soon. Rapidly developing modern technologies, new materials will still take their toll. The plaster bandage still has a very important advantage. Very low price. And, although new polymeric materials are emerging, the immobilizing bandage of which is much lighter and stronger (by the way, it is much more difficult to remove such a bandage than a regular plaster one), fixing bandages of the “external skeleton” type (printed on a 3D printer), the history of the plaster bandage is not over yet.

Palamarchuk Vyacheslav

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Unfortunately, people quite often break something due to an unforeseen event or in winter, falling on ice. At the same time, the properties of gypsum and its correct application become an integral part of the fracture treatment.

As a rule, a plaster cast is applied in the first hour after the accident. Therefore, medical plaster plays an important role both in treatment and in medicine in general.

How to get medical plaster

Medical plaster does not immediately look like most people imagine it.

Before we see it as a free-flowing powder, it goes through several stages.

So, initially it is a simple gypsum stone, which is heated in a special oven, but the temperature should not exceed 130-140 ° C.

After that, the stone loses all moisture and becomes very brittle. This is done in order to turn the stone into a fine powder.

The properties of gypsum and its quality depend on several factors, but the main thing is the residence time in the oven and the correct exposure. It is very important to store such gypsum in a dry room so that it does not absorb moisture.

What should be the gypsum

The properties of gypsum are very simple, since it must be white, soft, well sifted, harden quickly, and most importantly, not have lumps.

When gypsum is applied, it is imperative to observe the proportions, as a rule, these are 2 parts of gypsum per part of water. If the proportion is not observed, then the plaster will not harden, and the treatment will not begin on time.

What to do if the quality of gypsum has deteriorated

Often, hospitals do not use all the cast on time, and it begins to become damp, but this is not a tragedy.

It happens that not the best cast is used, but you can always make sure that patients feel quality services.

To do this, you need to take gypsum, pour it onto an iron layer, and send it to the oven (the temperature should not exceed 120 ° C), so the gypsum will lose moisture.

If in doubt, then you need to take a mirror, bring it over the plaster, and if it fogs up, then moisture is still present, if not, then everything is in order.

Gypsum is most often applied to the lower leg, hand, forearm and foot. Applying a cast requires different sizes of bandages and appropriate tools.

So, having considered the properties of gypsum and its features, everyone realizes that it is not easy to get gypsum and you need to make an effort, and also make sure that it does not deteriorate.

But it's better that you just know about it, and never come across it on your body.

97. Plaster and plaster bandages. Plaster bandages, splints. The main types and rules for applying plaster bandages.

Plaster bandages are widely used in traumatology and orthopedics and are used to hold fragments of bones and joints in position.

Medical gypsum - semi-aqueous calcium sulfate salt, is available in the form of a powder. When combined with water, after 5–7 minutes, the process of gypsum hardening begins, which ends after 10–15 minutes. The gypsum acquires full strength after the entire dressing has dried.

Using various additives, you can speed up or, conversely, slow down the process of gypsum hardening. If the gypsum does not harden well, it must be soaked in warm water (35–40 °C). Aluminum alum can be added to the water at the rate of 5–10 g per 1 liter or table salt (1 tablespoon per 1 liter). A 3% starch solution, glycerin delay the setting of gypsum.

Since gypsum is very hygroscopic, it is stored in a dry, warm place.

Gypsum bandages are made from ordinary gauze. To do this, the bandage is gradually unwound and a thin layer of gypsum powder is applied to it, after which the bandage is again loosely rolled into a roll.

Ready-made non-shrinking plaster bandages are very convenient for work. The plaster bandage is designed to perform the following manipulations: anesthesia of fractures, manual reposition of bone fragments and reposition with the help of pulling devices, application of adhesive traction, plaster and adhesive bandages. In some cases, it is permissible to apply skeletal traction.

Plaster bandages are dipped in cold or slightly warmed water, while air bubbles are clearly visible that are released when the bandages are wet. At this point, you should not press on the bandages, as part of the bandage may not be saturated with water. After 2-3 minutes, the bandages are ready for use. They are taken out, slightly squeezed and rolled out on a plaster table or directly bandaged the damaged part of the patient's body. In order for the bandage to be strong enough, you need at least 5 layers of bandage. When applying large plaster casts, do not soak all the bandages at once, otherwise the sister will not have time to use part of the bandages within 10 minutes, they will harden and will be unsuitable for further use.

Dressing rules:

- before rolling out the plaster, measure the length of the applied bandage along a healthy limb;

- in most cases, the bandage is applied in the position of the patient lying down. The part of the body on which the bandage is applied is raised above the level of the table with the help of various devices;

- a plaster cast should prevent the formation of stiffness in the joints in a functionally unfavorable (perverse) position. To do this, the foot is set at a right angle to the axis of the lower leg, the lower leg is in the position of slight flexion (165 °) in the knee joint, the thigh is in the position of extension in the hip joint. Even with the formation of contracture in the joints, the lower limb in this case will be supporting, and the patient will be able to walk. On the upper limb, the fingers are set in the position of slight palmar flexion with opposition of the first finger, the hand is in the position of dorsal extension at an angle of 45 ° in the wrist joint, the flexor forearm is at an angle of 90-100 ° in the elbow joint, the shoulder is retracted from the body at an angle of 15– 20 ° using a cotton-gauze roller placed in the armpit. For some diseases and injuries, at the direction of the traumatologist, for a period of not more than one and a half to two months, a bandage can be applied in the so-called vicious position. After 3-4 weeks, when the initial consolidation of fragments appears, the bandage is removed, the limb is set in the correct position and fixed with plaster;

- plaster bandages should lie evenly, without folds and kinks. Those who do not know the techniques of desmurgy should not apply plaster bandages;

- places subject to the greatest load are additionally strengthened (the area of ​​\u200b\u200bthe joints, the sole of the foot, etc.);

- the peripheral part of the limb (toes, hands) is left open and accessible for observation in order to notice the symptoms of compression of the limb in time and cut the bandage;

- before the plaster hardens, the dressing should be well modeled. By stroking the bandage, the body part is shaped. The bandage should be an exact cast of this part of the body with all its protrusions and depressions;

- after applying the bandage, it is marked, i.e., the fracture scheme, the date of the fracture, the date the bandage was applied, the date the bandage was removed, the name of the doctor are applied to it.

Methods for applying plaster bandages. According to the method of application, plaster bandages are divided into lined and unlined. With lining bandages, a limb or other part of the body is first wrapped with a thin layer of cotton wool, then plaster bandages are applied over the cotton wool. Unlined dressings are applied directly to the skin. Previously, bone protrusions (the area of ​​the ankles, femoral condyles, iliac spines, etc.) are isolated with a thin layer of cotton wool. The first dressings do not compress the limb and do not give bedsores from gypsum, but they do not fix bone fragments firmly enough, therefore, when they are applied, secondary displacement of fragments often occurs. Unlined bandages with inattentive observation can cause compression of the limb up to its necrosis and bedsores on the skin.

By structure, plaster bandages are divided into longet and circular. A circular plaster bandage covers the damaged part of the body from all sides, a splint - only from one side. A variety of circular dressings are fenestrated and bridge dressings. An end bandage is a circular bandage in which a window is cut out over the wound, fistula, drainage, etc. It is necessary to ensure that the edges of the plaster in the window area do not cut into the skin, otherwise the soft tissues will swell when walking, which will worsen the conditions for wound healing. The protrusion of soft tissues can be prevented if each time after dressing the window is closed with a plaster flap.

A bridge dressing is indicated in cases where the wound is located in the entire circumference of the limb. First, circular bandages are applied proximal and distal to the wound, then both bandages are connected to each other by U-shaped metal stirrups. When connected only with plaster bandages, the bridge is fragile and breaks due to the severity of the peripheral part of the bandage.

Bandages applied to various parts of the body have their own names, for example, a corset-coxite bandage, "boot", etc. A bandage that fixes only one joint is called a splint. All other dressings must ensure the immobility of at least 2 adjacent joints, and the hip - three.

A plaster splint on the forearm is most often applied for fractures of the radius in a typical place. The bandages are laid out evenly over the entire length of the forearm from the elbow joint to the base of the fingers. Gypsum splint on the ankle joint is indicated for fractures of the lateral malleolus without displacement of the fragment and ruptures of the ligaments of the ankle joint. Plaster bandages are rolled out with a gradual expansion at the top of the bandage. The length of the patient's foot is measured and, accordingly, 2 incisions are made on the splint in the transverse direction at the fold of the bandage. The longueta is modeled and strengthened with a soft bandage. Longuets are very easy to turn into circular bandages. To do this, it is enough to strengthen them on the limbs not with gauze, but with 4–5 layers of a plaster bandage.

The lining circular plaster bandage is applied after orthopedic operations and in cases where bone fragments are soldered by callus and cannot move. First, the limb is wrapped with a thin layer of cotton, for which they take gray cotton rolled into a roll. It is impossible to cover with separate pieces of cotton wool of different thicknesses, since the cotton wool falls off, and the bandage will cause a lot of inconvenience to the patient when worn. After that, a circular bandage in 5–6 layers is applied over the cotton wool with plaster bandages.

Removing the plaster cast. The bandage is removed using plaster scissors, saws, plaster tongs and a metal spatula. If the bandage is loose, then you can immediately use plaster scissors to remove it. In other cases, you must first put a spatula under the bandage in order to protect the skin from cuts with scissors. Bandages are cut on the side where there are more soft tissues. For example, a circular bandage to the middle third of the thigh - along the posterior surface, a corset - on the back, etc. To remove the splint, it is enough to cut the soft bandage.

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Introduction

Gypsum-based materials have various uses in dental practice. These include:

Models and stamps;

impression materials;

foundry molds;

Refractory molding materials;

Stamps- these are copies or models of individual teeth that are necessary in the manufacture of crowns and bridges.

Refractory molding material for the manufacture of cast metal dentures is a material resistant to high temperatures, in which gypsum serves as a binder or binder; such material is used for molds in the manufacture of prostheses from some gold-based casting alloys.

The chemical composition of gypsum

Compound

Gypsum- calcium sulfate dihydrate CaS04 - 2H20.

When calcining or roasting this substance, i.e. when heated to temperatures sufficient to remove some water, it turns into calcium sulfate hemihydrate (CaSO4) 2 - H20, and at higher temperatures anhydrite is formed according to the following scheme:

Obtaining calcium sulfate hemihydrate can be carried out in three ways, which make it possible to obtain varieties of gypsum for various purposes. These varieties include: burned or ordinary medical plaster, model plaster and super plaster; It should be noted that these three types of material have the same chemical composition and differ only in shape and structure.

Calcined plaster (ordinary medical plaster)

Calcium sulfate dihydrate is heated in an open digester. The water is removed and the dihydrate is converted to calcium sulfate hemihydrate, also called calcined calcium sulfate or HS hemihydrate. The resulting material consists of large porous particles of irregular shape, which are not capable of significant compaction. The powder of such gypsum must be mixed with a large amount of water in order for this mixture to be used in dental practice, since the loose porous material absorbs a significant amount of water. The usual mixing ratio is 50 ml of water per 100 g of powder.

Model plaster

When calcium sulfate dihydrate is heated in an autoclave, the resulting hemihydrate consists of small particles of regular shape, which have almost no pores. This autoclaved calcium sulfate is called a-hemihydrate. Due to the non-porous and regular particle structure, this type of gypsum gives a denser packing and requires less water for mixing. Mixing ratio - 20 ml of water 100 g of powder.

Supergypsum

In the production of this form of calcium sulfate hemihydrate, the dihydrate is boiled in the presence of calcium chloride and magnesium chloride. These two chlorides act as deflocculants, preventing the formation of flocculation in the mixture and promoting particle separation, as otherwise, the particles tend to agglomerate. The particles of the resulting hemihydrate are even denser and smoother than the particles of autoclaved gypsum. Supergypsum is mixed in the ratio - 20 ml of water per 100 g of powder.

Application

Ordinary calcined or medical plaster is used as a material of general use, mainly as a base for models and models themselves, since it is cheap and easy to process. The expansion during solidification (see below) is not essential in the manufacture of such products. The same gypsum is used as an impression material and also in gypsum-bonded refractory molding formulations, although for such uses the working time and set time and set expansion are carefully controlled by the addition of various additives.

Autoclaved plaster is used to make models of oral tissues, while stronger supercast is used to make models of individual teeth, called dies. Various types of wax restorations are modeled on them, which are then used to obtain cast metal prostheses.

solidification process

When calcium sulfate hydrate is heated to remove some of the water, a largely dehydrated substance is formed. As a consequence, calcium sulfate hemihydrate is able to react with water and turn back into calcium sulfate dihydrate by the reaction:

It is believed that the process of gypsum hardening occurs in the following sequence:

1. Some calcium sulfate hemihydrate is soluble in water.

2. The dissolved calcium sulfate hemihydrate reacts with water again to form calcium sulfate dihydrate.

3. The solubility of calcium sulfate dihydrate is very low, so a supersaturated solution is formed.

4. Such a supersaturated solution is unstable and calcium sulfate dihydrate precipitates as insoluble crystals.

5. When the calcium sulfate dihydrate crystals precipitate out of solution, the next additional amount of calcium sulfate hemihydrate dissolves again, and this process continues until all the hemihydrate has dissolved. Working time and curing time

The material must be mixed and poured into the mold before the end of working hours. The working time for different products is different and is selected depending on the specific application.

For impression plaster, the working time is only 2-3 minutes, while for gypsum-bonded refractory molding materials, it reaches 8 minutes. The short working time is associated with a short setting time, since both of these processes depend on the reaction rate. Therefore, while the typical working time for impression plaster is in the range of 2-3 minutes, the setting time for refractory plaster molding materials may vary from 20 to 45 minutes.

Model materials have the same working time as impression plaster, but their curing time is somewhat longer. For impression plaster, the setting time is 5 minutes, while for autoclaved or model plaster, it can be up to 20 minutes.

Changing the handling properties or performance characteristics of gypsum can be obtained by introducing various additives. Additives that speed up the hardening process are the powder of the gypsum itself - calcium sulfate dihydrate (<20%), сульфат калия и хлорид натрия (<20%). Эти вещества действуют как центры кристаллизации, вызывая рост кристаллов дигидрата сульфата кальция. Вещества, которые замедляют процесс затвердевания, это хлорид натрия (>20%), potassium citrate and borax, which prevent the formation of dihydrate crystals. These additives also affect the dimensional changes upon solidification, as will be mentioned below.

Various manipulations when working with the powder-liquid system also affect the solidification characteristics. It is possible to change the powder-liquid ratio, and by adding more water, the solidification time will increase, since it will take more time to get a saturated solution, correspondingly more time will be needed for the dehydrate crystals to precipitate. Increasing the time of mixing the mixture with a spatula leads to a decrease in the solidification time, since this may cause the destruction of crystals as they form, therefore, more crystallization centers are formed.

Clinical Significance

Increasing the time of mixing gypsum with a spatula leads to a decrease in the hardening time and an increase in the expansion of the material during hardening.

Increasing the temperature has a minimal effect, since the acceleration of dissolution of the hemihydrate is balanced by the higher solubility of calcium sulfate dihydrate in water.

Fundamentals of Dental Materials Science
Richard van Noort