Techniques - ClayPrint this page

Techniques - Clay

  1. The equipment needed
  2. The skills and technical knowledge needed
  3. The stages of the process
  4. References

The various types of clays

The terra-cotta, recovers all the objects, made from soil clay, which underwent an irreversible physico-chemical transformation during a cooking at high temperature.

The clay is one of older materials used by the man. Molded with some water, it gives a plastic dough which can be easily molded or shaped. After cooking, it gives a resistant and waterproof object. It should be noted that the clay without cooking does not allow to make so many details because it is much softer. These remarkable properties are at the origin of its very ancient use to realize ceramic, porcelain objects... Bricks and tiles are also made from a pressurized and hight temperature (1000 - 1 200°C) cooked mixture of clay and water.

Earthenware

The earthenware is a terra-cotta with clay. Most of the earthenware terra-cottas use a soil clay of ochre tint, mixture of potassium hydroxide, sand, feldspar and clay.

These clays contain enough iron and other mineral impurities to become solid, cooked from 950 to 1 100°C approximately. In the natural state, they are grey, greenish, red or brown because of oxides which they contain, oxidize of iron, oxide of titanium and others. Cooked, their color can go from the white or from the pink to the black via all the nuances of yellow, red, brown, according to the particular quality of every clay and the conditions of cooking.

The potter and the sculptor look for a soft and plastic earthenware, which they can possibly modify by adding a little of not plastic clay, chamotte, or fibers of cellulose.

Covered with their enamel with tin, white or colored, the pieces of cooked land become earthenwares. They stay however, because of their composition, porous ceramic, less sound, less hard, less dense than stonewares or chinas. The fragility of the earthenware object is caused by the double structure of the clay and the enamel which covers it, cooked both separately. Although the earthenware is more soft and more porous than the stoneware, its low cost and its ease of manufacturing compensate for these inadequacies. The earthenware pottery can be technically as delicate as porcelains, although it is not translucent and can be more easily crossed off.

Stoneware

Introduced in Western Europe in the XIVth century, the stoneware is a ceramic material characterized by a very strong hardness and an excellent resistance in the chemical or climatic attacks.

It is made with a silico-clayey clay giving a fine texture dough cooking at high-temperature, glazed and not porous. The waterproofness of stonewares intends them very early for the storage of liquids.

In Europe, the places of production are initially Rhein countries, regions from the east of France and Beauvaisis. In the XIXth century, the craze for the productions in stoneware leads to the creation of many factories. Stoneware ceramics appear in the center, in Burgundy and in the southeast.

Porcelain

The porcelain is a fine and translucent ceramic produced from the kaolin by cooking at more than 1200°C and mainly used in tableware.

It mainly consists of a mixture of quartz, feldspar and kaolin, added of ball clay to increase its plasticity. The quartz and the feldspar are reduced into powder under the effect of granite grindstones, then ground by a cylinder in rotation containing pebbles and water. The feldspar allows to lower the point of vitrification of the porcelain during the cooking.

The process techniques of the china reach their perfection in China in the XIIth century, and, in France, in Limoges in the XIXth century.

The chamotte

The chamotte, or crushed shard, is a raw clay cooked in a temperature of 1300 - 1400°C, crushed and sieved (to control the size of the obtained grains). It is actually an additive to mix with smooth modelling clay to give some consistency to it, it's then called chamotted clay of various size.

Pieces made with chamotted clay "hold" better, without collapsing during their preparation, as far as the degree of hygrometry and the plasticity of the clay are not too important.

The presence of the chamotte also decreases the retreat in the drying and in the cooking, which facilitates the drying and avoids the distortions and the cracks, that often appears after the cooking, when creating voluminous objects.

Furthermore, if the size of the chamotte is important, it becomes possible to modify the structure of the object by adding new parts when the object is almost dry.

Tuntables

Height adjustable turntable for modeling
Height adjustable turntable for modeling
Turntable for modeling to put on a table
Turntable for modeling to put on a table

To work his sculpture in clay, the modeller has at his disposal a whole choice working plans to choose according to his use and the volume of the sculpture he wishes to realize. There are diverse qualities of turntables, the advantage is that they are generally adjustable in height, what allows to adapt them to the size of the sculpture realized and so to work comfortably.

The turntables to put on a table, allow to realize small objects, but the inconvenience is that the height of the table does not often suit, as it is always preferable to work on an object placed at eye level.

Working tools

There are numerous tools to work the clay, a lot of everyday objects can be also transformed to model or smooth the clay (knives, spoons, pieces of wood).

It is important to always work with clean tools because the clay hangs on the tools where it dries and then marks the object by creating irregularities.

Hands

The first tool of the modeller is the hands which allow to put the clay, to shape it and to model it. We speak moreover about the "relief" of the clay that we identify on the work of a sculptor and that makes it personal. Do not hesitate to rub or to wash your hands regularly because the dry earth whill hangs on (as on all the tools) and prevent your fingers from sliding correctly on the clay.

It's better to use your fingers to shape the clay rather than the palm of the hand to avoid to dry the clay. Also avoid playing with the clay in your hands before using it for the same reason.

Modelling tools

A set of modelling tools
A set of modelling tools

For the parts too small to be realized with fingers or if the sculptor wants to give a different aspect to his clay (creating hatchings or other tracks), the modelling tools allows to work the clay efficiently. They are wooden tools (often in box tree) of various forms, sizes and characteristics which are essential to the work of modelling and smoothing. The modelling tool is the basic tool for the sculptor to complete or replace his fingers, the finer continuation of which it is.

There are also small exotic wooden modelling tools which allow to work the smallest details and are very pleasant of use. It is interesting for the sculptor to constitute a range of modelling tools of various forms, that will allow him to choose the most adapted tool according to every project and every stage of realization.

Trimming tool

A set of wire trimming tools
A set of wire trimming tools
A set of wire cutting trimming tools
A set of wire cutting trimming tools

Trimming tools are tools with a metal end and a wooden handle. They are mainly used to remove material, in particular during the emptying of the object. There are two types of trimming tools, ones with a wire end and sharp ones. The sharp ones are the most effective to remove a big quantity of clay (during the emptying for example) or when the clay is hard, as the ones with a wire end allow to preserve the texture of the object.

Obviously, it is necessary to adapt the size of the trimmin tool to the size of the object to empty.

The weakness of the trimming tools is often the attach of the metal on the wooden handle which tends to get loose as soon as you force a little, thus prefer quality tools with a solid attach (which looks like a thread of rolled up metal), you will find these rolled up trimming tools in specialized stores as Peter Lavem.

Cutting wire

Cutting wire for clay
Cutting wire for clay

The cutting wires used in modelling are generally in metal with wooden handles. They allow to cut the clay carefully by protecting the shape of both pieces. They are used in particular to cut elements of the sculpture cleanly when proceding to the emptying, so both side will be less damaged than with a knife and thus easier to restick.

Knife

The knife is a tool essential to the modeller, whether it is to cut the clay, to smooth it or for the sharpness of its blade which allows to realize fine details. It also allows "to draw" on the clay. You can collect a large number of different knives, with or without teeth, round or sharp, with a thick or a fine blade, each of them can find its utility according to the part of the worked sculpture or the desired result.

Roller

Realisation of a clay plate
Realisation of a clay plate

The roller allows in particular to realize more or less fine plates of clay, but also to beat the clay to compact it. To realize plates of clay, place the clay on a plastic film to avoid it to stick on the working board, then spread it at the wished thickness with a roller, by pushing very hardly to avoid any air bubbles.

To make sure that the thickness of the plate is regular, you can use guides by placing wooden brackets of the wished thickness on both sides of the plate to drive the roller.

Texturizing tools

Metal ribbing tools
Metal ribbing tools

To give to the sculpture or to some of its parts a texture, you can use any tool as wire brushes, pieces of wood... Just collect and free your imagination.

To smooth a sculpture in clay, it is necessary to wait that the clay has a sufficient consistency to cross above a smooth, wooden, metal or plastic tool (as for example a ribbing tool), which can be beforehand dipped into the water. Attention if the clay is chamotted, it can cause the opposite effect by highlighting the grains of chamotte.

You can also, once the clay is dried, use sandpapers or fine steel wool to polish the sculpture.

The clay slip

Pot of clay slip
Pot of clay slip

The slip is made of dried, reduced and rehumidified clay powder. It forms a dough which can be used as a glue to restick parts of the sculpture which would break themselves, to repair cracks if the sculpture is not too dry or to restick the parts of the sculpture after emptying it. The slip is thus a major element for the work of a sculpture.

Of course the slip used to restick the sculpture must be realized with the same clay. It is thus interesting to let dry little bits of clay during the work of modelling to be able to realize the slip when needed.

The realization of the slip begins with the drying of pieces of clay. Once these very dry, they can be broken and reduced into fine powder. This powder is poured in a jar (with lid) with some water inside until obtaining a pasty texture. Avoid stirring immediately because the clay has to absorb the water first otherwise it would stick in packages and would become waterproof.

Working with slid is explained in the part about technical knowledges.

The patina

Oxides

Certain oxides of metals are used in modelling and ceramic to give of the color to enamels and to engobes.

Here are some examples of the most common oxides and the colors which they give to a cooked enamel in oxidation: cobalt (blue), copper (green, blue), manganese (brown, purple), iron (yellowish, reddish, greenish), chrome-plates (green, yellow, red), nickel (brown, greenish, grey), and there are others (uranium, titanium, vanadium).

The intensity of the color depends of the rate of coloring and the power of covering of the oxide (for example, 0,1 % of oxide of cobalt is going to give a very powerful blue, while the same rate of iron oxide will give a very pale tint).

The temperature and the atmosphere (reduction or oxidation) of cooking have an influence on the color given by an oxide. This complex and little predictable behavior explains the necessity of accurate tries but also the multitude of possibilities.

This article resumes in a very complete way the colouring agents of the main metal oxides: http://terremjcaubagne.over-blog.com/article-le-pouvoir-colorant-des-oxydes-metalliques-39109557.html

Coloured slips

The coloured slip is a clay slip to which are added some oxides to color it. The slip applies by means of a brush on the sculpture before cooking when this one has a leather consistency. The sculpture can then be cooked what will fix the slip to the sculpture.

You can find prepared slips in shops (be careful to choose one with the same temperature of cooking range as your clay), but it is also easy to mix some oxide with clay. Attention however on the color of the used clay, mixing a black oxide with white clay will create a black dough which will turn to grey during the drying and the cooking (because the clay called white is grey before cooking), it is then better to use a black clay of the same type (same temperature of cooking and retractation).

Coloured slips are very pleasant to use because they apply before cooking and do not thus require a second cooking. Furthermore it is a coloured clay what allows to apply them more or less thick and more or less textured with the same pleasure and the same ease as during the work of the clay. It is also easy to remove a badly applied engobe by scratching it.

Enamels

The enamel is a glassy substance consisted, in particular, of silica, feldspar, kaolin and metallic oxides. This mixture, close to a clay or to a dough, glazes under the effect of the temperature, during the cooking. The enamel can be applied by soaking of the object in a bath, with a brush or with a pistol.

The soaking allows a very uniform and easy result but require a big quantity of enamel to realize a bath into which the object can be completely plunged. It is adapted thus rather to small objects and when there are a lot of objects to cover with the same enamel (reasons why this technique is generally used in the ceramic workshops). It is necessary to use clamps to hold the object during the soaking and pay attention on the part which they enclose because the enamel will not apply on this part.

The brush allows to apply the enamel to an object and to create decorations. It is however difficult to master because it can leave tracks and the application be irregular.

The pistol allows a fine and uniform application of the enamel but require an adapted equipment (cabin of enamelling, ventilation...).

Pigments

Numerous pigments can be used to realize patina on cooked sculptures: ochre, natural earth; without forgetting the pigments of ingredients of the everyday life as tea, coffee, some fruits, etc....

These pigments can be applied with water (successive coats with progressive addition of caparol to fix pigments) what gives a patina full of nuances and lightness (but also makes more visible the defects of the sculpture) or with some shellac for a more covering result (for example to obtain a bronze effect). There are also techniques of patina with milk (with talc) which give a powdered effect.

Do not forget colorant products with fixative spray integrated as shoe polish or some patina for metal. They can be used directly without mixture.

The subject of patina could be the object of a long explanation and has no place in this article. A lot of information, recipes and tricks exist on the Internet to answer the questions of each (do not in particular hesitate to look on the very interesting forum dedicated to the sculpture: http://sculpture.forumactif.com). You can then test different recipes to find the patina which corresponds to your creations. I encourage you to preserve even broken, cooked sculptures, various tints of clay to be able to realize experiences before beginning to work on your definitive sculpture and to always take notes about successive stages and proportions.

 

 

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Read more: The skills and technical knowledge needed The skills and technical knowledge needed

 

 

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