• 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.
• 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?
In Germany, mostly hard porcelain is made. Its main characteristic is a high percentage of kaolin (50%) and its feldspar icing, which only melts at a temperature of 1,400 – 1,500 degrees C. At the same time the body underneath “centers”, that is, the small particles of porcelain connect with each other. This leads to an extraordinary hardness of the glaze, as well strength of the body.
• What is soft porcelain (also called “bone china”)?
Soft porcelain means it contains 50-60% bone ash or phosphates in its body, 15-30% pegmatite, a mineral that consists of quartz and feldspar, and kaolin. The percentage of kaolin is clearly lower than in hard porcelain, therefore the firing temperature is lower. At the first firing (temperatures of 1,240 – 1,280 degrees C), bone china is immediately burned with a smooth surface. Because of this considerably lower firing temperature, soft porcelain does not have the same mechanical strength and resilience as hard porcelain.
• 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 primary characteristic of real porcelain. Translucency is the transmission of light through the porcelain when it is held up against light. The clearer and brighter the translucency, the better the porcelain.
• Does porcelain age?
Despite diligent studies and rigorous tests, and in contrast to other materials, 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 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 them with each other.
• How are the different pieces of porcelain produced?
Plates and cups are turned (plaster molded and cast). Plates and other flat pieces are plaster molded. A template or rolling tool is used to fashion the bottom of these pieces, while plaster molds are used to form the top of plates and other flat pieces. Cups and other hollow pieces use a template or rolling tool to shape the inside, then relies on a plaster mold to form the outside. A hollow casting technique is used 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. These techniques may be used alone or in combination, depending on the size, shape, and complexity of the piece being created. 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 mold, causing the porcelain body to shrink so that the body may be easily removed from the mold.
• 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 – 1,000 degrees 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 the 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 toward the cooling zone. These furnaces are called continuous furnaces because they are in constant motion.
• 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, which, after the firing process, is discarded. This method produces a cup with a smoothly glazed base and that is perfectly round. But, there remains a slightly rough upper rim where the rim rested on the setter, and which is removed by careful polishing of the cup's upper rim to remove any trace of roughness. The layman would hardly recognize the need for this expensive extra bit of finesse!
• 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 color 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". In-glazing therefore is a technique for permanently protecting decorated porcelain pieces from normal mechanical and chemical stresses.
• What is on-glaze?
On-glazing is a technique in which a smooth-fired porcelain body is coated with a color-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 colors, which is 750 - 900 degrees C, for quality control of the colors. 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, or adorn, is the art of embellishing porcelain by the skillful addition of hand-painted colors 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 proven that porcelain is even bactericidal. Porcelain has no odor or taste of its own and absorbs neither from food or drink. Fine porcelain is therefore extremely practical as well as beautiful.