Clay plaster

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Half-timbered infill with clay plaster in a weathered condition.

Clay plaster is used as wall and ceiling plaster indoors and outdoors. The use of clay plaster is one of the oldest construction techniques known to man, as the raw materials are widely used, readily available and easy to process. In Central Europe it is first recorded in houses of the Hungarian Körös culture .

As a natural building material with favorable properties, clay plaster has been the focus of attention from building owners , architects and interior designers since the end of the 20th century . The use of clay plaster in the design of interiors and exterior walls creates a decorative effect through special application forms , for example as colored decorative plaster.


Clay plasters can be differentiated according to their layer thickness, their processing, the tool with which they are applied or their area of ​​application.

  • Clay plaster layer thickness between 1 and 30 mm. Can be worked by hand (traditionally applied with a trowel) or by machine. Due to its very creamy consistency, the processing is similar to gypsum plaster. The only clay plaster that can be processed as a layer plaster.
  • When deposits plaster , the layer thickness is 7-10 mm, the plaster is applied by machine or hand plaster and stucco with tools such as the float or grapeshot warped. Areas of application are full plaster and clay plaster .
  • The base plaster serves as a base plaster and is applied 5 to 35 mm thick, the standard thickness is around 12–15 mm. Base plaster is also machine-compatible, so it can be applied with a plastering machine before warping. When using container goods, the use of silo mixing pumps is also possible.
  • Fine plaster is a solid finishing plaster in layers up to 5 mm thick, usually with additional binding agents such as B. cellulose, casein , methyl cellulose or gum arabic . The surface is rubbed or smoothed.
  • The filler plaster is applied by hand in layers of 1 to 2 mm. It is a decorative plaster. A so-called “Venetian smoother” is used to apply , a special, slightly conical trowel with rounded corners or a Japanese trowel . Decorative plasters can be colored, pure white or dark gray, with or without additives that create aesthetic effects.
  • Streichputz is a decorative plaster, may even be dark gray white, colored or and is applied in a thin layer of only 0.1 mm with the glaze brush. Often straw, plant fibers or stones are added to the decorative plaster, which set visual accents after wiping free and additional binders such as. B. casein , methyl cellulose or gum arabic.
  • Mineral plaster is a special formulation made from clay and special sands with good strength even without vegetable additives such as B. Chopped straw. It is machine-compatible and silo-compatible. The processing times are shorter than for clay plasters with vegetable additives. It is suitable as a base and top coat. It can be applied by cleaners without any special clay experience. The surface can be left natural, painted or wallpapered. It is important to ensure that the mineral plaster or also clay plaster or fine plaster made of clay remains uncoated so as not to restrict the properties of the clay.
  • Harl is a special clay plastering technique in which a layer of pebbles is applied to the fresh layer of clay.


The clay plaster offered by larger manufacturers is often available with or without fibers. The latter is also offered as purely mineral . The use of finely chopped barley straw with a length of around 5 to 30 mm has become established, as barley straw is less moldy than rye or wheat straw. Traditionally, dried animal manure such as cow dung and horse droppings were added to the clay, as these contain a large proportion of well-prepared, fine plant fibers. In the past, the use of animal hair was common in clay and lime plasters, especially in the finishing plaster. Today, among other things, (minced) calf hair and pig bristles are used.

Advantages of fibrous clay mixtures
The shrinkage of the plaster is (apparently) reduced. The fibers act as reinforcement and prevent the formation of larger cracks, as they are able to absorb significantly greater tensile stresses than the plaster itself. The tensions that occur during shrinkage are distributed over the entire surface of the plaster by the fibers, so that instead of a few larger cracks, many smaller cracks tend to form. In the ideal case, the fine cracks are barely visible and can usually be easily closed by re-smoothing and simultaneously compressing the plaster while it is drying. Alternatively, they can be washed with the sponge board. The sand that usually loosens here has to be rubbed off in a further operation or pressed into the still damp plaster.
By distributing the tensions, the formation of clods can be somewhat reduced in strongly dwindling plasters, which in the worst case may detach from the substrate at the edges. However, reinforcing the plaster with a reinforcing fabric is more effective in this case.
The tensile fibers increase the abrasion resistance of the clay surface. Weather resistance can also be improved, since the fibers delay the washing out of the fine mineral particles.
When applying (thin) plaster to uneven substrates, added fibers can bridge cracks that are already present in the substrate or that arise due to swelling and shrinking of moisture in the substrate after the plaster has been applied.
Fine, swelling fibers, such as cellulose fibers , can have an influence on the workability of the plaster.
When adding fibers, the addition of heavy fillers such as sand can be reduced. This improves the thermal resistance of the plaster. Fine, swellable fibers leave voids behind after drying out, which also improve the thermal insulation.
When applying clay plaster to smooth and very weak or non-absorbent surfaces such as concrete and foam glass, the adhesion of the clay plaster is limited, as the fine clay particles cannot interlock with the substrate. The addition of very fine fibers can possibly improve the adhesion to the substrate, as these apparently cling to the substrate. Because the fibers bind moisture, the surface treatment can possibly be carried out earlier.
Disadvantages of fiber-containing mixtures
Fibers prevent the use of many cleaning machines.
If the drying of the plaster is delayed due to a lack of ventilation, low temperatures or thick layers, superficial mold often forms.
When using larger amounts of vegetable (swellable) fibers, the shrinkage usually increases . However, this is only noticeable in the case of solid components such as insulating cladding. In the case of wall and ceiling plaster, the adhesion of the relatively thin plaster layer to the supporting structure usually prevents the plaster surface as a whole (in the longitudinal direction) from shrinking and the fiber content prevents the formation of broad cracks.


Clay plaster consists of clay , sand and silt (fine sand). Due to the special structural properties of the clay, it hardens solely through evaporation of water and adheres to the substrate through mechanical claws. The fine, platelet-shaped clay components act as a composite or "glue".

To improve properties such as workability, (crack) resistance, adhesion, abrasion and moisture resistance and surface structure or to color finishing plasters, various materials are added to the clay plaster depending on the area of ​​application. These include, for example, pigments ; Rock flour such as marble powder; Fibers such as cellulose, chopped straw or hay, cow or horse manure, animal hair; Protein-containing substances such as whey, quark, animal blood; (hydraulic) binders such as silicates, lime or cement.

In some arid regions of the subtropics, the clay plaster is traditionally drawn over the entire outer skin of the house, including the roof. In order to increase the resistance against the rare but often heavy rainfall, various organic (fiber) substances or nowadays cement are added to the clay. In some regions the plaster is made more weather-resistant after application by slurry or barrier coats based on plants or by mixing with tar.

Structural properties

Clay plaster is soluble in water. Outside it is therefore subject to restrictions in regions with high levels of precipitation and should not be used unprotected on the weather side of buildings or in the base area. In indoor areas, on the other hand, clay has many advantages over conventional, cement-based plastering mortar, particularly due to its moisture-regulating properties.

The high binding power of the clay, which it already has as a raw material, can also be improved by adding plant starch and fiber material ( straw , reeds , horse manure, cellulose ). As with other building materials (e.g. lime or gypsum mortar, concrete ), a balanced grading curve ensures a resistant, hard surface of high strength. With suitable processing, clay plaster can even be used in bathrooms outside of the splash water area.

Clay adheres very well to a wide variety of materials. Very absorbent substrates may have to be pre-wetted. Application and adhesion can also be improved by applying a primer coat of clay slurry.

A clay top coat with the addition of barley straw, for example, has a flexural strength of 0.78 N / mm², a compressive strength of 2.1 N / mm², an adhesive strength of 0.30 N / mm² and the abrasion is determined to be 0.6 g .

After the old standards for earth building materials were suspended in the post-war period, the definitions in DIN 18942-1, earth wall mortar in DIN 18946 and earth plaster mortar in DIN 18947 have recently been dealt with.


The aggregate is classified by wet sieving according to DIN EN 1015-1 and designated according to DIN EN 12139.

The grain group designates the lower (d) and upper (D) sieve size and is given as d / D. Few grains are allowed to remain on the upper sieve and are called oversized grains . Also, some grains are allowed to fall through the lower sieve and are called undersize . Largest grain denotes the opening width of the upper test sieve D of the grain group in which no or only individual oversize grains remain.

In the product data sheet of a factory mortar, in addition to the grain group, the oversize grain size must also be specified, which indicates the opening width of the test sieve in which no residue remains. The limit values ​​for clay mortar are contained in DIN 18946 and 18947.

Influence on the indoor climate

Basically, all wall coatings have an effect on the room climate : diffusion-open , ie "vapor-permeable" coatings, designed to be capillary-conductive, enable the wall layers behind them to absorb moisture and release it again. In connection with the property of the clay plaster to absorb a lot of moisture (up to nine times more than plaster of paris), a climate buffer is formed on the wall, which absorbs moisture and releases it again when the humidity is low. With these properties, clay plasters stand in contrast to so-called "film-forming" surfaces such as emulsion paint and latex paint , which allow little or no moisture to penetrate into the layers behind.

The area of ​​the plastered wall has the greatest influence on the ability of the clay plaster to act as a climate buffer. The layer thickness of the clay is of secondary importance when rooms are used, because more than 80% of the moisture is initially bound in the top two millimeters of the clay wall. Only 10 mm are relevant for the “climate buffer effect” in the case of “normal living behavior”, since the reactivity of thick plaster layers (> 20 mm) is too slow to be able to react to the constantly changing room air humidity.

Clay stores heat (depending on the built-in amount) and due to the high specific heat capacity, clay walls are able to compensate for temperature differences. The thermal conductivity is 0.47… 0.93 W / (m · K).


  • Gernot Minke : Handbook of earth building - building materials science, techniques, earth architecture, 7th edition, eco book, Staufen near Freiburg 2009, ISBN 978-3-936896-41-1
  • Wolfgang Lenze: Half-timbered houses, restore - renovate - modernize. Materials and procedures for permanent repair. 3rd expanded edition. Fraunhofer-IRB, Stuttgart 2004, ISBN 3-8167-6431-2 .
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  • Ulrich Röhlen, Christof Ziegert: Earth building practice, planning and execution. 1st edition. Bauwerk, Berlin 2010, ISBN 978-3-89932-125-8

Web links

Individual evidence

  1. ^ Alasdair Whittle (Ed.): The Early Neolithic on the Great Hungarian Plain: investigations of the Körös culture site of Ecsegfalva 23, County Bekes. Varia Archaeologica Hungarica XXI, Budapest, 2007. 810 pp. ISBN 978-963-7391-90-3
  2. a b Annelie Meisinger: clay plaster in the interior , In:; accessed in May 2019
  3. Animal hair and vegetable fibers from the manufacturer Conluto; In:; accessed in May 2019
  4. The clay plaster SanReMo by the manufacturer Claytec contains fine fibers. It is intended for (thin) reworking of uneven substrates and can also be applied to non-absorbent surfaces; In:; accessed in May 2019
  5. ^ Backe, Hiese: building materials science . Werner Verlag, 2004, ISBN 3-8041-4459-4
  6. Information from the Agency for Renewable Raw Materials e. V. (FNR)
  7. Zawde Berhane: Rigours of the humid tropics . Weather effects on buildings in a humid tropical climate. In: Batiment international magazine. Building research and practice, ISSN  0182-3329 , No. 1/2, 1985, pages 52-54, fig., Lit.
  8. Franz Kröger: Material culture and traditional handicraft among the Bulsa (Northern Ghana)
  9. Earth-moist earth-top plaster - 05.010.1 / 05.010.2 Earth plaster mortar DIN 18947 - LPM 04 f - S II - 1.8 , In:; accessed in August 2019
  10. Horst Schroeder: [1] , Springer Verlag
  11. Ulrich Röhlen, Christof Ziegert: earth building practice: design and execution Beuth Verlag, April 25, 2014
  12. Gernot Minke: Lehmbau Handbuch . eco book, 1st edition 1994, ISBN 978-3-936896-41-1 .