TYPES OF PORCELAIN: Hard Paste, Soft Paste, and Bone China
The broad term “ceramics” usually refers to items made of fired clay. Ceramics are further divided in several categories, the primary ones being Earthenware, Stoneware and Porcelain. The main differences in these three are the temperatures at which they are fired in the kiln and the specific composition of their component materials.
Although the term Pottery is properly defined the same as the all-inclusive word Ceramics, nowadays it is used to refer mostly to Earthenware and Stoneware only (also known as Clayware), and leaves the term Porcelain as a separate category. This article will focus on the various types of Porcelain.
TRUE PORCELAIN (a.k.a. HARD-PASTE PORCELAIN): A high-fired Ceramic ware that exhibits translucent properties and is composed of White Clay (Kaolin) and a type of Feldspathic rock (Petunse). Kaolin is refractory and binds a piece together while in the kiln. Petunse fuses into a natural kind of glass that gives it its smoothness and brilliance. True or hard paste porcelain usually is fired at 1450 ° C. It allows for “tighter” modeling and more robust shapes, even in delicate or very thin designs. Most decorative porcelain figurines, urns, centerpieces, etc… are made using true or hard paste porcelain. This is the same type of porcelain invented at Meissen and was the first close recreation of porcelain coming from China at the time. In France, the term for hard paste is Pâte Dure, and in Sevres specifically Porcelaine Royale. Hard (or high) Fired translates to Grand Feu in French.
ARTIFICIAL PORCELAIN (a.k.a. SOFT-PASTE PORCELAIN): Termed ‘soft’ because of its ability to be cut with a file (hard paste porcelain cannot), this type of porcelain is composed of the same materials as hard paste, but is fired at a softer temperature, around 1200 ° C. Because of this lower temperature, soft paste porcelain tends to be more granular and porous since the component materials do not vitrify (fuse) as is the case of hard paste porcelain. Additionally, the surface is somewhat less white or brilliant and has an almost silky or marble-like feel to the touch. Some collectors prefer this look, especially on human figurines, where it allows for a softer texture and a more life-like appearance. Historically, soft paste porcelain was first made in Italy in late-16th century (Medici Porcelain), but it reached prominence in mid-17th century France. In the United Kingdom, the first factory to make soft paste porcelain was Chelsea (ca 1743). It is also known as Frit Porcelain in England, and Porcelaine de France or Pâte Tendre in France.
BONE CHINA: This is the same as hard paste porcelain, but with added bone ash (ashes from burned animal bones, mostly cattle). Bone ash contains lime and phosphoric acid, which helps fuse all of the ingredients and allows for a more stable final product, even at the lower temperatures required for soft paste porcelain. In a manufacturing setting, this means less damage due to cracking in the kiln and less waste or rejects, thus less cost. In Decorative Arts terms, it means that an object with at least 30 percent bone ash can achieve a brilliance and translucence typically seen on hard paste porcelain, but at the lower costs associated with producing soft paste. Bone China was introduced at the Bow factory in England in 1750 and was quickly imitated by Chelsea (1755) and Lowestoft and Derby (ca 1770). It rarely is produced in other European countries or the US and it is also is known as English China.
Other than a true scientific analysis performed at a Lab, a quick and practical way to confirm if an item is made of a particular type of porcelain is to examine an exposed or broken piece to determine how porous or granular its interior body appears. As a quick rule of thumb, granularity means soft paste, whereas a compact and fused interior means hard paste. Bone china is more brilliant than soft paste, but less so than hard paste. Also, hard paste porcelain is the most durable of the three types, with bone china coming second and soft paste third.
Finally, some other terms used for Porcelain include:
Soaprock, or soapstone porcelain, uses soaprock, a soft steatite mineral that feels like soap, also called French Chalk.
Biscuit (also called Bisque) is an unglazed porcelain (of all types) or an Earthenware which has been fired only once. It has a distinct marble-like appearance, also called Parian ware, and is used mostly for modeling or busts.
Alex and Elizabeth are WorthPoint Worthologists.