Some frequently asked questions about porcelain

What is porcelain?

Porcelain belongs to the large group of materials called "ceramics", which are products made mainly of clay or clay raw materials with mineral content. Ceramics range in texture from coarse, such as plain clay tile, to progressively finer products such as earthenware, majolica, faience, and stoneware, to porcelain - the finest textured of all ceramics. Porcelain represents the ultimate in ceramic quality and beauty. Porcelain consists of two main parts: 1) the compound, or body, and 2) the glaze. The white, glassy paste used to make the body accounts for porcelain's translucence and differentiates it from stoneware. The composition of the body's glaze is equivalent to glass and further enhances porcelain's translucence.

Are there different types of porcelain?

There are two types of porcelain: hard porcelain and soft porcelain. Hard porcelain is produced mainly on the European continent. Soft porcelain is produced predominantly in China, Japan, and England.

What is hard porcelain?

Most of the porcelain produced in Germany is hard porcelain. Its main features are its high kaolin content (50%) and its feldspat glaze, first fired at a temperature of 1400-1500o Celsius. At the same time, this heating process sinters the porcelain body's structured particles - that is, it brings the particles close together and links them. The heating process also fuses the glaze to the body underneath. The result is an extremely strong piece of porcelain, with an extremely hard surface.

What is soft porcelain, also known as bone china?

Soft porcelain, also known as soft-paste porcelain or bone china, consists of 50-60% bone ash or phosphates in the paste and a lower kaolin content than that used in hard porcelain. Because of the relatively low kaolin content, the firing temperature for soft porcelain is lower than for hard porcelain: soft porcelain is first fired at 1240-1280o C. Due to this lower firing temperature, soft porcelain does not have as high a tensile strength or resistance as hard porcelain.

What is bisque porcelain, also known as biscuit porcelain?

Bisque, or biscuit, porcelain is the name given to porcelain that is fired without glazing. The finished, fired piece is impermeable to water but has a rough surface finish.

How strong is porcelain?

Porcelain is pressure-resistant to 5000 kg per cm². Expressed in another way, a fully loaded 10,000 kg railroad car can be placed on a 2 cm² piece of porcelain without its breaking.

What does translucence mean?

Translucence is the distinguishing characteristic of genuine porcelain. That is, diffused light will shine through a piece of fine porcelain. The brighter and clearer the translucence of a piece, the better the quality of the porcelain.

Does porcelain age?

Despite diligent studies and rigorous tests, it has not yet been determined whether or not porcelain ages. It does retain its properties of hardness, density, tensile strength, brightness, and translucence - all of which are unaffected by time. Porcelain is likewise resistant to corrosion.

What does porcelain consist of?

Porcelain (hard paste porcelain) consists of 50 parts kaolin (porcelain earth), 25 parts quartz, and 25 parts feldspar. These components are bonded into a paste by grinding, mixing, and fusing with each other.

How are the different pieces of porcelain produced?

There are several basic techniques for producing porcelain pieces: molding, shaping, casting, and turning. These techniques may be used alone or in combination, depending on the size, shape, and complexity of the piece being created. Artisans use plaster molds to form the top of plates and other flat pieces; they use a template or rolling tool to fashion the bottom of these pieces.

They also use a template or rolling tool to shape the inside of cups and other hollow pieces, then rely on a plaster mould to form the outside of those pieces. Craftsmen use a hollow casting technique to fashion pots, bowls, and jugs. They use surface casting or solid casting to form oval or square platters and salad bowls. The technique of turning is reserved for pieces that are round. Casting is typically used to create porcelain figurines.

However, if a piece is highly complex, artisans may cast it in smaller component parts and later join the parts into a finished whole. For instance, Kaiser's white-headed eagle is composed of 25 separate parts. Plaster molds may be either cast or turned. For instance, patterns for relief or raised designs, such as Kaiser's Dubarry tableware or bisque vases, are inserted into molds.

The relief design is automatically embossed onto the plaster by turning, overturning, or casting. The plaster draws out the water from the moist compound in the mould, causing the porcelain body to shrink so that the body may be easily removed from the mould.

When and why is porcelain annealed?

Before the annealing process, Kaiser artisans clean the porcelain pieces by removing any sharp protruding edges, such as casting seams. The artisans anneal the pieces by heating them to 900-1000o C and then gradually cooling them. This process frees the porcelain pieces from internal stress and removes their water content. The pieces remain porous but become strong and leather-hard; they cannot be reshaped. Next, workers use compressed air to remove dust from the porcelain pieces. At this point, craftsmen stamp the pieces with Kaiser's trademark and send the pieces on to the glazing shop for the next step in the manufacturing process.

How is glaze applied to porcelain?

The thin, liquid glaze is carefully mixed in a large trough. This milk-like glaze is continuously agitated to prevent separation and settlement of its ingredients. Craftsmen dip and wash each porcelain piece in the glaze, which remains on the surface of the piece like a coating of flour. Artisans wipe the pieces with moist foam rubber strips to remove glaze from those contact points which will rest on a porcelain "setter". This setter supports the glazed piece during the firing process and is then discarded.

How is porcelain fired?

Traditionally Porcelain is smooth-fired in tunnel kilns which are approximately 80 m long. The porcelain pieces ride on special cars through the firing zone and move slowly on the cooling zone. Nowadays Chamber Kilus are predominantly used the same effect in less than one third of the time.

Why does a piece of porcelain have rough patches?

Rough patches on porcelain pieces are inevitable because of the nature of the firing process. Under the high heat of firing, the porcelain glaze becomes highly viscous and sticks to the surface of the disposable porcelain setter on which the piece is resting. A fired piece cannot be lifted from the setter without leaving rough patches at the points of contact between the piece being fired and the setter on which it rests.

To minimize this problem, before firing, craftsmen remove as much wet glaze as possible from these contact points. At high quality porcelain factories, cups, for example, are fired with the opening facing downward on the setter. This method produces a cup with a smoothly glazed base but a slightly rough upper rim where the rim rested on the setter. This slight roughness on the tiny surface area of the rim is a comparatively minor flaw which would not likely be noticed by the casual observer.

However, craftsmen take the extra step of painstakingly polishing the cup's upper rim to remove any trace of roughness. This is an optional step, but worthwhile because it results in a piece that is perfectly smooth on all surfaces.

What is in-glaze?

In-glazing is a technique for permanently protecting decorated porcelain pieces from normal mechanical and chemical stresses. Artisans first decorate porcelain pieces by transferring or sliding screen-printed designs onto smooth-fired porcelain bodies. Next they add appropriate metal oxides and liquids to the glaze to achieve the desired colour effect. After they apply the glaze to the porcelain bodies, craftsmen fast-fire the pieces at 1200-1300o C for 60-120 minutes. During this firing, the decorative design is sealed under the glaze, or embedded within it; thus the term "in-glaze".

What is on-glaze?

On-glazing is a technique in which a smooth-fired porcelain body is coated with a colour enhanced glaze, the decoration is then applied onto the glaze, and lastly the piece is fired. The firing temperature for on-glazed pieces is determined by the melting point of the glaze's colours, which is 750-900o C, for quality control of the colours. In this process, the decoration is fused onto the glaze; thus the term "on-glaze".

How long does porcelain decoration last?

In-glazing permanently protects porcelain decoration from all kinds of external mechanical and chemical stresses because the decoration is sealed under or embedded within a hard, durable glaze. On-glazing fuses the decoration onto the glazed piece. Because an on-glazed decoration is more exposed to external stresses, it is more susceptible to damage.

What does "staffage" mean?

"Staffage", from the German word "staffieren", meaning to dress, trim, adorn, is the art of embellishing porcelain by the skillful addition of hand-painted colours and metallic trim (gold, silver, or platinum). "Staffage" can be used to accent knobs, handles, and rims of tableware and to enhance either smooth or relief-decorated period table services. The careful and tasteful application of "staffage" highlights a piece's shape and a variety of other design elements.

What makes porcelain the ideal tableware?The ideal tableware is hard and smooth, has an impermeable surface, with a finish that is highly resistant to mechanical and chemical stresses. Porcelain meets all these important requirements. Because porcelain is hard, it resists cracking, cutting, and scratching. It is impervious to the acids and alkalis found in the normal household. It has been proved that porcelain is even bactericidal. Porcelain has no odour or taste of its own and absorbs neither from food or drink. Fine porcelain is therefore extremely practical as well as beautiful.

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