Repsol YPF

The ceramics sector

The ceramics process

The ceramics process consists of transforming a mixture of raw materials into a solid element that can have many distinct uses- from rustic elements of little monetary value (bricks, slates) to more expensive decorative elements for use in construction (tiles, floor tiles) or decorative pieces (vases, artistic pieces).

It is therefore a large and diverse sector. However, the production process is very similar in all parts of the sector.

The traditional classification system divides the industry into two groups based on what raw materials are used:

• White ceramics: the main raw material is kaolin (china clay)
  
• Red ceramics: the main raw material is clay        

There are also products on the market made of combinations of the two raw materials.

For this study we prefer to classify this industry by how the final products will be used:

• Construction:
  - Flooring
  - Paving
  - Special pieces
  - Bricks, slates and similar products
  
• Utensils and decoration          

Generic production process

Below, the different stages of the ceramic production process will be discussed. Then we will focus on the steps that require heat (which are shaded on the diagram below).

Not all industries produce the product going through every step of the process. Normally, one industry handles only a few stages. Generally, the first three stages are handled by large specialized companies, due to the large investment expenses in fixed assets. For the next stages, companies buy clay that is already atomized. Now, the objectives and activities of each of the production steps will be discussed.   

A) MIXING
The first stage consists of mixing different raw materials in order to give the final product its necessary characteristics and stability. The basic raw material is clay (plastic materials) which makes it mouldable. Non-plastic materials are also added (such as silica, feldspar, ashes and other minerals) to confer special properties, such as lowering the required baking temperature and preventing the clay from breaking or changing shape during the drying process.

B) MILLING
Making particles the correct size needed for the process can be achieved by either dry milling (no water added) or wet milling (water added). Final milling is usually wet (for example in ball mills), since that makes it easier to obtain the correct grain size. After milling, the water must be removed. This is done in two ways: atomization or filtration.

C) ATOMIZATION AND FILTRATION
In atomization water is eliminated by evaporation. 

For filtering, filter presses are used. The filtered material undergoes a kneading and compressing process to eliminate air. The product obtained from this process is called "body" or "atomized earth."  

D) MOULDING
There are three moulding techniques:

• Moulding by extrusion and/ or compression: a portion of body or atomised earth is placed in a mould and undergoes compression or wax compression.
• Moulding with a potter's wheel: Can be used with pieces with cylindrical symmetry: a portion of the body is placed on the potter's wheel. As the wheel spins the clay is moulded. The potter's wheel is also used for manual processes when the master potter wants to give the piece a specific shape.
• Moulding by sizing: The product must be in liquid form- barbotine- which is a liquid obtained from wet milling, or from adding the water that was eliminated in the atomisation and pressing process to atomised earth. With moulding by sizing, barbotine is poured into a plaster mould. The mould is made of various parts so as to facilitate the extraction of the moulded piece. The plaster absorbs the water in the barbotine, and thus it becomes a solid, mouldable piece.          

E) DRYING
After moulding, the piece should lose the water it had when it was body or atomised earth. This is done by drying.

F) BAKING
The next step after drying is baking.  The first bake is called biscuit baking since the product obtained (cooked clay) is called biscuit or bisque.

Some ceramic products such as tiles, bricks and similar products (red ceramics) undergo only one bake, since they are not glazed or decorated. Other products need to be baked only twice, because they are not decorated or because the decoration is done jointly with the glazing. In the latter, a thin glazing is applied over the decoration and then there is a joint glazing-decoration bake. This produces pieces that are shinier and have more stable decoration. In general the most typical is the three-bake process with glazing and decoration (as shown in the chart above).
  
Lastly, some products are baked up to four times. The so-called fourth firing happens after a new final decoration to give the pieces special characteristics, such as a metallic shine. There is another ceramic process, called mono-baking, where the product is glazed and decorated after drying and there is just one bake for the body glazing and decoration.

G) GLAZING AND DECORATION
As mentioned in the previous section, glazing and decoration are processes to make the final product more attractive and shiny, and can take place between the different bakings. 

Frit and Ceramic Colour Production

Frits (a product that is similar to glass) are used to cover the glaze and used in decoration. The production process is different, as seen in the following chart:

In this case, the raw materials are basically dyes and glass products.

Cooling after the fusion can be done by pouring the melted liquid over water or over a cylinder cooled by water. The result is similar: small portions of a solid product are obtained. Milling will make these portions smaller and help obtain the correct grain size. Finally, the frit is suspended in water in order to obtain a new type of barbotine which will be used in the glazing and decoration.

During glazing, the entire piece is covered with frit barbotine (the glaze). The piece will absorb the water from the barbotine, leaving a layer of frit on the surface. The decoration consists of applying many ceramic colours in the form of barbotine with a paintbrush or spray gun or by silk screen printing. The water of the colours used is eliminated by drying.