Why does clay crack and can it be avoided?

Why does clay crack?

The surface of the clay layer can crack for various reasons, and yet why this happens, consider the probable reasons and find out what to do to prevent the clay from cracking when drying:

• excessively thick layer. To avoid this, plaster with clay, applying a maximum two-centimeter layer. If there is a need to apply a second one, then the previous one should set well. In good weather, this takes at least a day. If it is intended to apply a plaster layer more than four centimeters thick, the surface must be reinforced with steel mesh;
• the plaster dries too quickly and begins to crack. Optimal conditions for this are 10 – 20 degrees Celsius. If the weather is hot, it is recommended to suspend work and moisten the walls more often. Perhaps the base of the wall quickly absorbs moisture. In this case, it is also moisturized abundantly;
• the solution turned out to be too greasy. Replace the clay or reduce its amount in the solution. The same applies to cement material.

Advantages and disadvantages

Plastering walls with clay has many advantages:

• Environmental friendliness; clay does not contain harmful impurities.
• Fire resistance: it is absolutely not afraid of fire.
• This material easily sticks not only to concrete or brick, but even to wood. In the latter case, shingles are used to secure it.
• Even a non-professional can work with plastic clay mortar.
• It is able to “breathe,” that is, absorb moisture from the air and release it when the relative humidity changes, which is very important for creating a comfortable microclimate.
• Adsorption. It has been noticed that allergy sufferers feel more comfortable in rooms decorated with clay plaster. After all, clay can absorb odors and bind harmful substances.
• It costs absolutely nothing: you can dig it up with your own hands in the nearest quarry or in your garden.
• Good sound insulation and heat conservation.

Despite the obvious advantages, clay plaster also has disadvantages:

• Duration of surface drying - a layer only 1 cm thick will dry for 5 days. Thicker 5-10 cm layers dry in 2-3 weeks.
• Moisture capacity. Due to their ability to absorb water, clay surfaces require additional protection.
• Low strength. Of course, it cannot be compared with cement mortar. Clay is easily scratched and chipped.

Types of clay-based plasters

Let's consider the main options for plasters and the technology for their preparation.

Fireclay clay

Difficulties arise in the work, the bricks hold weakly in the masonry, the plaster layer is covered with numerous cracks. This is due to the fact that during the firing process the clay almost completely loses its plasticity, which must be returned to it when preparing the solution. In these cases, craftsmen recommend adding quartz sand or a special adhesive composition. The procedure for preparing the solution is as follows:

• you will need a pack of fireclay powder, which is poured into a container;
• water is added gradually until it covers the entire mass;
• it is necessary to wait three days for the material to infuse;
• after this sand is added. If the solution turns out to be thick, you can add water, otherwise the clay layer will crack when it dries;
• The mass is thoroughly mixed.

A properly prepared solution should resemble thick sour cream in thickness, only in this case it will not drain and will adhere perfectly to the surface. To speed up the drying process and take measures to prevent the clay from cracking, it is necessary to add cement at the rate of two kilograms per package of fireclay.

It is possible to purchase ready-to-mix formulations that do not require steeping for three days.

Clay-sand mortar

How to properly make such a solution so that the clay does not crack? First you need to sift the sand, for which you will need a sieve with a hole size of 2 - 3 mm. Its amount will depend on the degree of fat content of the clay.

The clay is pre-soaked. It is placed in a tank and filled with water, leaving for one to two days. After this, the mass is mixed and passed through a three-millimeter sieve. The result should be a solution that is similar in thickness to sour cream.

We start adding sand. As a rule, one and a half times more sand is required per liter of solution. But when the clay is oily and the solution is prepared in such proportions, then cracks will definitely appear. And with low-fat clay, the layer of plaster will not be durable.

The ratio of sand and clay should be accurately determined. For this purpose, take a liter of solution, to which sand is added in doses, while stirring the mass. The end result should be a material that easily slides off a metal shovel, but does not spread over it.

Where is clay plaster used?

For many years, clay plaster was used, perhaps, only for laying partitions indoors, as well as for finishing and laying stoves and fireplaces.
But, if you look at old buildings plastered with clay compounds, you can be sure that it is still in good use. Red brick partitions are also very durable, in the masonry of which cement-clay mixtures are often used. Moreover, unlike fragile drywall, you can hang heavy racks on them without fear of them falling.

Clay-sand mortar is most often used to plaster wooden and clay (adobe) walls not only outside, but also inside the house. In principle, any surface can be plastered with clay. The main thing is to prepare them correctly, as well as select the fat content of the clay and maintain the proportions with additives (sand, sawdust, straw) in the solution.

This environmentally friendly material is able to absorb excess moisture and release it as needed. Clay plaster with the inclusion of flax or straw fibers looks original, so designers and craftsmen are increasingly using it as a decorative finish.

Nuances of testing clay plasticity

Different clays differ in their plasticity. There are:

• “fat” clays with high plasticity;
• material of medium ductility;
• “lean” clays with a low level of plasticity.

I determine the level of plasticity using a simple method. The clay is poured with water to turn it into a dough-like mass suitable for molding. The material is used to make cylinders seven centimeters long and 30 mm in diameter, the ends are pinched with fingers and slowly torn apart. Plasticity is judged by the shapes that the cylinder takes at the point of rupture, by its elongation. A plastic material is characterized by the formation of a long and thin neck, and if the clay is thin, the neck will be thick and short.

The level of plasticity is determined in another way. Flagella about twenty centimeters long and 1.5–2 mm in diameter are rolled out of the working material and bent into semicircles or pretzels. If cracks appear on the materials at this time, the clay is thin. Cracks that form during bending indicate that the clay is of medium plasticity, and fatty clay can be identified by the complete absence of cracks in the samples.

Advantages and disadvantages of clay compositions

The following features are highlighted as advantages:

• clay is a natural and environmentally friendly product;
• solutions perfectly absorb water and release it, which prolongs the service life of coatings;
• reasonable cost;
• the remains of the used solution are stored for a long time;
• the material perfectly protects the surface from mechanical impacts;
• the elasticity of the material allows it to be applied without any problems;
• Not only internal, but also external surfaces are finished with a clay mixture;
• using the solution, level and decorate the surfaces.

In addition to the positive aspects, clay-based plaster has a number of disadvantages, which explain most cases why clay cracks:

• There is no exact recipe for preparing the solution; craftsmen prepare the mixture using their own experience;
• there is a possibility that the surface will become covered with cracks;
• Such compositions are rarely used; there are few experienced craftsmen who know how to cover a stove with clay so that it does not crack.

Repairing cracks on a metal surface

If a metal stove begins to crack, welding is the best solution to repair it. If it is impossible to use this option, you can prepare a heat-resistant mixture for filling defects:

1. A solution based on clay with the addition of metal filings, manganese dioxide, salt and borax in proportions of 30 to 4: 2: 1: 1
2. A mixture of liquid glass and magnesite in a ratio of 3:10. Also, based on liquid glass, a mixture is made of two parts of manganese oxide, zinc white and one part of borax. Glass is added until the desired consistency is obtained. Whitewash is used in dry form.
3. Colloidal sulfur, metal filings and ammonium chloride are mixed in a ratio of 1.5:96:2.5.
4. Based on natural drying oil: you will need graphite, lead, manganese oxide in a ratio of 5 to 1 to 1. Lead is used in the form of calcined oxide. Another solution with graphite, lead and chalk is diluted in a ratio of 12: 4: 3. Eliminating cracks on a metal surface

Criteria for choosing a solution

Previously, classical solutions were used, taking into account combinations of heat resistance, increased heat capacity and thermal expansion equal to the brick material. To make such solutions, fatty clay was used.

Today you can purchase dry mixtures, certain types of clays and other compositions that simplify construction and repair work. It should be noted that not every such composition has the necessary characteristics - fire resistance, heat resistance, gas tightness.

The solution must be heat resistant, withstand high temperatures and exposure to chemically active substances. In addition, the gas density level must be quite high. The seams do not allow flue gases to pass through, but moisture must pass through the finished mixture for the stove to “breathe.”

Instructions for applying clay plaster

The work is performed in the following sequence:

• mortar cakes are applied along the wall at regular intervals, and the material is stretched over the entire surface with a wide spatula, a rule or a trowel;
• Evenness is checked using a level. If necessary, apply a second layer;
• the surface is given time to dry completely: from 1 to 3 months, depending on the season;
• finishing is being carried out. To do this, use a solution consisting of clay, cement and sand in a ratio of 1 to 1 to 3. The mixture is applied in a thin layer and leveled. After three to four weeks, the stove can be whitewashed with lime.

We now know how to coat a stove with clay so that it does not crack. But you should know that half the success depends on the correct preparation of the solution.

To make the clay more plastic, you can add glycerin to it.

How can I cover the stove to prevent it from cracking?

If the damage is small and superficial, you can remove the old layer of finishing from the bricks to re-plaster it. If the damage is through, you will have to dismantle the damaged area and restore it.

Several options for covering the stove to prevent it from cracking:

• fireclay clay composition

This is a widely used material for restoring stove surfaces. Before use, fireclay clay should be kept in water for about two days.

• composition of clay and sand

The cheapest version of the material, often used due to the plasticity and viscosity of the mixture, as well as durability after firing. For a high-quality solution you will need any clay, the main thing is that it does not contain any impurities. You can prepare it as follows:

1. The clay is filled with water in a ratio of 1 to 3, after which it is infused for at least a day.
2. After it has infused, it needs to be mixed with the addition of water. Then strain through a fine sieve and let it brew again.
3. A properly prepared mixture should have a thick consistency.
4. Mix clay and sand in proportions of 1 to 2.5.

After mixing the solution, you can check its readiness by rolling a small ball. If it cracks, it means there is not enough liquid in the solution. To increase the strength of the clay mortar, you can add asbestos, cement or gypsum to it.

Stove mixture

• oven glue

This material can be purchased at the store, it is durable and fire resistant. The composition of oven glue includes fireclay powder and refractory cement. It comes in two types: solid and plastic mixture. The first type is used to wipe down the entire surface of the stove, the second type is used to fill the seams and cracks. To prepare the adhesive solution, you need to dilute it in small portions, because... it dries out quickly. For 1 liter of plastic mixture, you need to knead 5 liters of solid mixture. A plastic mixture is applied to the wetted bricks and all defects are repaired. A final coating in the form of hard oven adhesive is then applied.

• special compositions for grouting ovens (you can purchase or make your own).

If it is not possible to purchase ready-made oven grout, you can prepare it yourself. To do this, as in the first two cases, you need to soak the clay and let it brew for 24 hours. Then add sand in a ratio of 1 to 4 and mix thoroughly. During the stirring process, you need to gradually add finely chopped straw, and at the end add a pack of salt.

How to cover a stove with clay so it doesn't crack?

All existing cracks must be knocked out with a chisel and the brick must be cleaned. If the surface is completely plastered, the seams need to be cleared to a depth of about 1 cm. Before covering the stove so that it does not crack, all cracks, seams and cracks must be moistened generously with water or primer.

Stove

Before coating the stove with a clay solution, it is necessary to heat it to such an extent that its surface becomes slightly warmer. Then the stove is covered with a thin layer of liquid clay mortar and left to dry for a while. Then the surface is reinforced: burlap or a thin metal mesh is applied, this will strengthen the plaster. The burlap is glued overlapping, starting from top to bottom, and the mesh is attached with nails in the seams between the bricks. This sequence of actions is the same both for plastering the entire surface area and when it is necessary to cover individual areas.

Next, two layers of plaster, each 5 mm thick, are applied. A layer that is too thick will be uneven and uneven. When the surface is dry, it is rubbed, and after complete drying it is covered with lime mortar. If you do not plan to whiten the stove, you can add a little lime at the stage of mixing the clay solution, the clay will become lighter.

After the coating has completely dried (this will take at least 7 days), you can start heating it, but not before, otherwise it will continue to crack.

Reasons for furnace failure

Before looking for the best way to fix the problem, it is necessary to determine the causes of cracks. It could be:

• low quality of the solution used;

• violation of finishing technology;

• shrinkage of the brick structure;

• non-compliance with the rules for operating the stove.

If the stove sank due to an improperly poured foundation, smoke will pour out of all the cracks during the fire, the doors will no longer close tightly, and the brick will gradually begin to collapse. If such problems are discovered, you should immediately dismantle the entire structure, strengthen the foundation and lay it out again. However, in some cases you can do without radical measures.

All other reasons can be easily eliminated; it is only important to figure out how to cover the stove so that it does not crack.

Determining the extent of damage

Before getting started, you should consider the damage and determine the scope of work. If small cracks appear on the surface of the stove (even if in large numbers), to eliminate them you will need to remove the lining and clean the brick from the plaster.

For these purposes, you can use one of the following mixtures:

• made of clay and sand;

• from special grouts for ovens;

• made of fireclay clay;

• from oven glue.

Can’t decide what to cover the stove with to prevent it from cracking? Consider each option individually.

Popular brands

When purchasing a plaster composition, it is important to pay attention to the following:

• permissible operating temperature limit;
• whether the composition is intended for indoor or outdoor work;
• environmental friendliness of components;
• hardening time;
• resistance to moisture, sudden heating or cooling.

The integrity of the packaging, date of manufacture, and compliance with storage conditions are important.

Heat-resistant plaster for stoves and fireplaces

The following types of brands are the most popular.

PECHFORM PC400 W

Heat-resistant variety. Suitable for lining the flood zone, areas near the hob, side walls of the stove, fireplace. The maximum permissible operating temperature is 400 degrees. There is no need to additionally putty the surface on top. Can be used for indoor and outdoor work. Surface material for application:

• brickwork;
• concrete;
• limestone;
• composite slabs.

5 minutes after mixing the solution it is ready for use and remains suitable for application after 2 hours. Do not apply on top of metal parts. During operation and drying of the composition, the temperature should be from +5 to +30 degrees. Painting is allowed after 2 weeks.

Typhoon Master

Products from a domestic manufacturer. Heat-resistant plaster is used for finishing walls, fireplace chimneys, barbecues and barbecues made of brick. It has the following properties:

• heat resistance;
• tolerates moisture well;
• plastic;
• convenient for vertical application.

The average mixture consumption per 1 square meter is from 1.6 to 1.8 kg.

Kalina Master

Fireproof plaster. Contains Portland cement. It is characterized by increased strength. Excellent resistance to high temperatures. Quickly sets to the surface. Resistant to mechanical damage.

Pechnik

The base is fireclay clay. Suitable for masonry and finishing of stoves. Hardens quickly. It is used in working with old and new furnaces. Convenient to apply.

It does not collapse when the product shrinks, it is plastic.

Heat-resistant adhesives

They act as a connecting substance for fastening tiles to the surface of a stove or fireplace. Two types are in greatest demand.

Terracotta

Sold dry. The main raw material is refractory kaolin clay. Does not lose properties at +400 degrees (permissible limit).

It is distinguished by its plasticity, ease of installation, and good adhesion. Used for leveling walls and plastering. All components of the composition are environmentally friendly. It hardens quickly, within an hour after mixing. The recommended layer thickness is up to 1 cm.

Hercules

Heat-resistant glue. Used for laying fireplaces, stoves, barbecue complexes made of refractory bricks. Suitable for fixing tiles.

Packaged in bags of 5, 12 and 25 kg. Operating temperature range from –50 to +1200 degrees.

• from oven glue.

A mixture of clay and sand

Sealing cracks with clay mortar is the simplest and most economical way to restore a stove. At the same time, the quality of repairs will be no worse than using newfangled and expensive materials.

For these purposes, you can take absolutely any clay, the main thing is that it contains as few impurities as possible. After soaking, high-quality raw materials should become oily, homogeneous and plastic. Before covering the stove with clay (to prevent the surface of the fireplace from cracking in the future), pay attention to the specifics of preparing this solution.

Preparing walls before plastering

Clay mortar, due to the lack of chemical bonds with the plastered surface, does not adhere well to a smooth base. Therefore, when facing bricks, the joints must be jointed before plastering.

To reinforce wooden surfaces finished with clay plaster, shingles are used . The use of plaster mesh in this case is not very convenient. Since clay is not as flexible as gypsum, it will not bond well to the mesh.

Shingles are narrow boards 3-5 mm thick, which are nailed onto the wall. The wooden planks are positioned at an angle of 45°. Moreover, the nails should not go into the wall completely, but only halfway. The rest of them are bent and recessed into the wall. In this case, the shingles will hold up better.

How to make your own clay putty

Mixing the mixture is carried out in several stages.

1. Before mixing clay with sand, it should be soaked in warm water for at least 12 hours.

2. The wet solution should be thoroughly mixed and all lumps formed should be broken up. In this state, the clay combines with sand much better.

3. To prepare the working mixture, it is recommended to take 4 parts clay and 2 parts sand. The last component should be as clean as possible and pre-sifted. The volume of liquid depends on the degree of humidity of all components.

4. If you do not plan to whiten the stove, add lime to it at the stage of mixing the solution. All components are mixed until the consistency of sour cream. The finished mixture is covered with plastic wrap and left for 12 hours.

5. The settled solution is kneaded again. To check if there is enough water in it, take the putty in your palm and roll it into a ball. If it starts to crack when you squeeze it, add more water.

6. The oven should be lightly heated, water should be poured over the cracks, after which you can begin sealing.

7. To prevent the appearance of new cracks in the clay, you can add straw, but a more reliable way is to use a plaster mesh. If you apply a clay solution to a stove lined with such material, you can forget about the cracks for a long time.

Fireclay clay

Fireclay clay is often used to restore old stoves. This material is characterized by fire resistance, so stoves finished with it are reliable and durable.

You can purchase this type of clay at any construction market. It is sold in 20 kg bags and is not expensive at all. The working solution is prepared strictly following the instructions on the packaging.

Grout

To seal a brick oven, you can purchase a special grout. In stores there are various formulations that are produced specifically for these purposes. It’s worth noting right away that this option will cost you quite a lot, however, it is optimal for lining stoves and fireplaces.

If you cannot afford to purchase a specialized composition, prepare it at home. To do this you will need the following components:

The clay needs to be thoroughly kneaded, filled with water and set aside for 12 hours. After the specified time has passed, the required amount of sand should be added to the mixture. In the process of kneading the two components, finely chopped straw and salt are gradually added to them. Clay and sand are taken in a ratio of 4:1, with about 50 kg of straw and a pack of salt added for every 4 buckets of clay.

The resulting composition can withstand temperatures of more than 1000 degrees.

Oven glue

To prevent the stove from cracking in the future, you can use heat-resistant glue to repair it. It belongs to the category of ready-made materials for finishing fireplaces, which are sold on construction sites. Its main qualities include resistance to high temperatures and durability.

This adhesive mixture contains fireclay powder and fire-resistant types of cement. Today, there are two types of glue - plastic and hard.

The first option is used for sealing cracks, and the second is intended for plastering the entire surface of the stove.

The main advantage of oven glue is its very fast drying, so you should not mix the solution in large quantities.

Why does clay crack on the stove when it dries?

Sooner or later, owners of country houses and private houses will have to solve the problem - how to cover the stove? A failed heating device poses a threat to the life and health of the inhabitants of the house. It is no coincidence that fire safety regulations prohibit the operation of a faulty stove. The work of a master stove maker is not cheap. However, if necessary, you can repair the “heart of the house” yourself.

The oven can fail for various reasons. Sometimes it’s easier not to cover up any defects that appear, but to rebuild the stove. However, such drastic measures are rarely required. As a rule, the main reasons for the emergency condition are shrinkage, poor-quality materials and errors made when laying out the fireplace.

Heating systems require regular maintenance. According to SNiP standards, heating devices in a dacha must be inspected twice a year. And in a private house at least once a quarter. Unfortunately, most homeowners neglect these requirements.

Only a complete oven can be used

The main danger for the residents of the house is not a collapsed fireplace, but the cracks that appear in it. The explanation is simple - no one will heat a stove that is obviously in emergency condition, but cracks cannot always be noticed in time. Carbon smoke leaking through them is a serious danger for the inhabitants of the house. According to statistics provided by the Ministry of Emergency Situations, in a fire, 70% of people die from smoke, and of those who survive, 42% end up in hospital with carbon monoxide poisoning.

Therefore, timely repairs are the key to the safety of your family. Most stove makers, when asked: “how to cover the stove so that it does not crack from the heat?”, will advise using clay. However, there is a nuance here. There are several types of mixtures and almost all of them use clay.

Clay is the main component in the manufacture of masonry stove mixtures. This is explained by its unique quality - when heated, it turns into stone. The clay solution can withstand high temperatures and ensures the reliability of the entire structure. But other ingredients are also used for production. Clay is divided into two types: dry and oily. When heated, the first one crumbles, and the second one cracks and dries out.