What Is Porcelain And How Is It Made
Porcelain is a ubiquitous material, known the world over for its durability, versatility and visual appeal. It is made from a selection of materials - but often including kaolin. Kaolin and a wide range of other minerals and materials are available from African Pegmatite, who combine the broadest selection, the furthest reach and the most experience to provide the right product at the right time.
Porcelain is the name for a series of ceramics that are made by heating their constituent materials in a kiln at temperatures between 1,200 and 1,400 °C, and often contain kaolin. Porcelain can be grouped into three distinct types: hard paste, soft paste and bone china. These three groupings are based on the composition of the paste used to make them and the temperature at which they were fired. Porcelain ceramics are notable for their strength and white colouration - though articles take glazes and pigments well. The hardness of the material stems from the vitrification process and the formation of mullite during the heating process within the kiln(1). Porcelain is often regarded as the superior ceramic, owing to its strength and delicacy, but is much more difficult to manufacture compared to earthenware or stoneware. Porcelain is impermeable, hard, highly resistant to thermal and chemical shocks, translucent (depending on thickness) and very strong.
Manufacture of Porcelain
Like all other ceramics, the process to make porcelain is simple. Clay is mixed with additives, shaped and then fired in a kiln. The only thing that differs for porcelain is the identities of the constituent materials, the temperature of the kiln and, crucially, the formation of mullite.
The formation of mullite
One of the key phases in the manufacture of porcelain is the in situ formation of mullite(2). Mullite is an aluminosilicate material and depending on stoichiometry, has the formula 3Al2O32SiO2 and 2Al2O3 SiO2. It is a very rare mineral in the ground, but it is understood to form at around 1,100 °C from feldspar and kaolin type materials during the kiln firing process.
Porcelain is made from a heated mixture of silica, clays, feldspar and flint materials of small particle sizes. These materials are combined in varying proportions – until they attain their fired and unfired properties - to form different types of porcelain. Let’s take a look at these materials one after the other.
Clay: In general, the exact composition of clay would usually depend on where it is extracted. Still, clay would often have closely related properties regardless of where it is gotten. For example, all clays vitrify only at high temperatures. The only exception is when the vitrification threshold has been lowered by the addition of some materials. Also, fired clays tend to add some refractory contribution to the finished product. Just as it is with glass, they hold their shape when heated. Porcelain, therefore, combines the low porosity of glass with the ability of clay to retain its shape. Hence, its popularity for domestic usage. Generally, the clay used for making porcelain are ball clay and china clay, and they are composed mostly of hydrous aluminum silicate and kaolinite.
Feldspar: This material is composed mostly of flint and aluminum silicate. Flint is a type of hard quartz used as a flux in a porcelain mixture. Fluxes can be used to reduce the temperature at which liquid glass is formed, usually at about 1000 to 1300ºC. In this liquid phase, the grains used in forming porcelain bond firmly together through vitrification.
Silica: Silica is a chemical compound formed by a combination of silicon and oxygen gas; two of the most abundant elements on earth. It exists naturally in crystalline, amorphous and impure forms as seen in quartz, opal, and sand respectively. Of all filler types, silica is the most commonly used for firing the porcelain body. Asides facilitating the firing and forming of the body, it also aids to improve the physical properties of the final product.
China stone: Used in the manufacture of bone china (see later), china stone is a partially kaolinised, feldspar rich form of granite that bears no iron. Often colocated with kaolin, its mineral composition includes mica, quartz and feldspar. China stone’s similarity to petuntse - which is used to make hard paste porcelain - is presumably the reason it became a key component of bone china.
Kaolin: Kaolin is a clay mineral bearing the general formula Al2Si2O5(OH)4. It is formed from the weathering of other aluminosilicate materials and is of a white colour. It is used in most modern ceramics.
Bone ash: Bone ash is the calcined, ground and finely powdered bones of animals, used in the manufacture of porcelain. Typical bone ash consists of approximately 56% by weight calcium oxide, 42% phosphorus pentoxide with the balance being water. The generally agreed formula for bone ash is Ca5(OH)(PO4)3. The calcination process takes place at around 1,000 °C. Bone ash is used in soft paste and bone china porcelains, it has the effect of replacing petuntse from hard pastes(3). Bone ash has other applications in the fields of metallurgy, machining and fertilisers.
In some cases, porcelain might contain low-alkali containing bodies like soapstone or alumina.
Process of Manufacturing Porcelain
Having selected the required raw materials and taking out the required amounts by weight, they are made to undergo a number of preparatory steps. To begin with, they are crushed, then purified. After that, they are mixed together before being subjected to several forming processes. The forming process could be any of pressing, casting, stiff plastic forming or soft plastic forming. The choice of forming process would usually depend on the type of porcelain ware to be produced. After forming the porcelain body, it is then fired after which it may be glazed. Glazing is the process of firing onto a ceramic body a layer of decorative glass. Finally, the porcelain is then fired. Some porcelains are formed and fired in such a way that they are referred to as ‘bisque’, that is, they do not have the traditionally associated white colouration.
In summary, the manufacturing process of porcelain involves the crushing of materials to be used, cleaning and mixing of such materials, the forming process of the body, firing and then glazing and re-firing if needed.
Types of Porcelain
There are three broad classes of porcelain, based upon the paste type used to manufacture them. This in turn informs the firing temperature and therefore outcome and final application of the article.
Comparison of porcelain types(4,5)
|Type||Hard paste||Soft paste||Bone china|
|Origin||China, circa. 7th to 8th century CE||Continental Europe, 17th to 18th century CE||Britain, 18th century CE|
|Key materials||Petuntse, kaolin||Ball clay, bone ash, flint, quartz, soapstone||Kaolin, bone ash, china stone|
|Firing temperature||1,400 °C||1,100 to 1,250 °C||1,200 to 1,250 °C|
|Characteristic properties||Strong with an excellent crack resistance and heat tolerance||Softer and easier to work with - but more prone to breakage and collapse pre-firing||Highest levels of strength, whiteness and translucency|
Hard paste porcelain is the original porcelain ceramic and originated in China. A combination of petuntse and kaolin, fired at 1,400 °C making an incredibly strong material that would rapidly become the envy of the world. Chinese export porcelain was a highly valued commodity owing to its strength, durability and visual appeal. Petuntse is a broad term for a variety of feldspathic rocks that have undergone some kind of geological decomposition process. Typical formulations were equal quantities petuntse and feldspar for superior quality porcelain, with two thirds petuntse to one third feldspar being used for normal porcelain.
Originating in France, soft paste porcelain was produced in an attempt to mimic Chinese export porcelain. Whilst soft paste is mechanically weaker than hard paste (though their applications are largely the same), soft paste does not require the elevated temperatures in the kiln, requiring only 1,100 to 1,250 °C. Far from a clear cut set of ingredients, soft paste porcelain contains a wide mixture of components including bone ash, flint, ball clay, soapstone and quartz. The name comes from the French pâte tendre, referring to its easier workability compared to hard paste.
Originally produced in Britain from the 18th century onwards, bone china is a porcelain that contains in excess of 30% phosphate from animal bones and/or calcium phosphate from other sources. The key to bone china’s success is china stone (often mistakenly referred to as china clay, which is another name for kaolin/kaolinite, the material it is located alongside), which is a partially kaolinised, feldspar rich form of granite that has no iron. Its discovery as a viable mineral meant that a viable alternative to hard paste porcelain was now available - bone china and hard paste porcelain share many characteristics. Much of the use of bone china is in high end crockery and decorative materials. Traditional formulations were 50% bone ash, 25% kaolin and 25% china stone.
Subtypes Of Porcelain
There are various subtypes of porcelain, some of which include alumina porcelain, vitro porcelain and feldspar porcelain:
Alumina porcelain has a similar production method to feldspar porcelain. The major difference is that it is mixed with aluminum which is thought to improve its durability. However, with that increase in durability comes a decrease in translucency.
Its origin can be traced back to America. It has a warmer white tone than feldspar porcelain. It is very durable and a little transparent. In its production process, it is pre-fired at a temperature (800 ºC), a little lower than regular porcelain and another firing is done at 1,230 - 1,250 ºC.
Feldspar porcelain has been produced as far back as the 1700s when the Europeans tried to imitate Chinese porcelain. This porcelain type is white with a slightly blue tone than standard porcelain. It is also slightly translucent owing to the high percentage of glass phase in its structure. It is incredibly durable and is fired at a temperature of about 1,300 - 1,400 ºC. Feldspar porcelain is continuous with soft paste porcelain.
Uses of Porcelain
Porcelain is widely used as a form of building material, especially for floor finishes. Basically, there are two types of porcelain tiles used in buildings. They are, namely glazed and thorough-bodied porcelain tiles. Glazed porcelain tiles are tiles with a glaze finish over the top. The glaze is usually added to add color and finish to the tile. These types of tiles are generally used for decorative purposes because of their fine finish and decoration rather than for more practical purposes.
In contrast, thorough-bodied porcelain tiles do not have any form of glaze over their top. They are mostly used for floors, walls, and countertops since they do not have glazes that could be easily eroded from abrasion and constant use.
Porcelain tiles can be used for floors in high-traffic areas such as in malls due to the hardness and toughness of the materials. They are generally non-porous and non-absorbent. Hence, they can be used for floors and surfaces that are regularly exposed to bacteria.
This is one of the most common usages of porcelain. In addition to being thermal shock-resistant, they are also good insulators and are used in areas of high voltages. They are used in the insulation of antennas, as well, as in high-voltage cable terminals. The American Ceramic society even suggests that they are an essential part of electronic components that ensure the proper functioning of telecommunication devices like smartphones and computers. Porcelain can behave in a superior fashion to polymer type electrical insulations - but does come at a higher cost(6).
This might come as a surprise to a few; however, porcelain is also used in medicine. It is used in dentistry for making porcelain jackets otherwise known as caps and crowns. A porcelain jacket is used in dentistry for protecting the surface of a weak or broken tooth. It is usually recommended for rebuilding teeth that have either been ruined by large holes and decay or have previously been broken. In some cases, they are used to help reshape, whiten or align better the existing teeth. It is imperative to note that they are similar to a natural tooth and for this reason are used for anterior teeth. Porcelain in dental settings is cited for its hardness but also relative flexural strength when glazed(7). A broader review on the use of porcelain on medical settings is here(8).
Despite its use as a utilitarian material, the elegance and beauty of porcelain make it an interesting choice of art material. In ancient times, people discovered that asides being exquisitely beautiful and elegant in design, porcelain objects were also durable. Consequently, porcelain was used for collectibles. In modern times, they are collected as treasures and sold for huge sums.
Ancient treasures like incense burners used in sacrificial ceremonies or tomb figures from burial grounds are now collected as treasures and stored away at museums or sold for huge prices.
Porcelain can also be used for making decorative objects. Some of these objects are used around the home while some others can serve as souvenirs and gift items. Many trinkets and statue exchanged between nations were made from porcelain.
This is one of the earliest uses of porcelain. This ceramic material was used in times for making kitchen wares used for preparing and dishing out food. It was discovered that utensils and dishes made from porcelain were impervious, easy to clean and reuse and were durable when heated. Their use has now gone beyond kitchen wares to the making of other household items like paperweights and penholders.
Because of the durability of porcelain and its electrical resistivity, porcelain is used for making laboratory materials and equipment. For instance, they are used for making wares in a chemical laboratory because of its strength, durability, and ability to resist chemical attack.
- Porcelain is a ceramic material made by heating its constituent materials - often including clay and kaolin - to between 1,200 and 1,400 °C in a kiln
- Porcelain materials are known for their strength, harness and impermeability amongst many other characteristics
- There are many types of porcelain - these depend on the type of paste used to make the ceramic and what other additives it may contain
- A thoroughly ubiquitous material in the modern world, porcelain finds uses from in building materials and electrical insulation through to dinner plates and objets d’art
Porcelain is a name for a group of strongly related and widely used ceramic materials that often contain kaolin. As the preferred industrial partner, African Pegmatite mills, processes and supplies the widest range of minerals for the ceramics industry - including superior quality kaolin for the most demanding applications.
1 W. E. Lee et al., J. Eur. Ceram. Soc., 2008, 465
2 G. V. White et al., J. Eur. Ceram. Soc., 1996, 16, 115
3 Industrial Ceramics, 1989, 843, 767
4 F. Singer and S. S. Singer, Industrial Ceramics, Chapman Hall, London, 1963
5 W. Burton, Porcelain: its nature, art and manufacture, Batsford, London, 1906
6 M. Shah et al., IEEE Elect. Insulat. Mag., 1997, 13, 5
7 S. Prosad et al., J. Prosth. Dentist., 2009, 101, 20
8 A. S. Vlasov and T. A. Karabanova, Glass and Ceramics, 1993, 50, 398