Bone China
Introduction
Bone china is a type of porcelain that is composed of bone ash, feldspathic material, and kaolin. It is known for its high levels of whiteness and translucency, and its high mechanical strength and chip resistance. Bone china is the strongest of the porcelain or china ceramics, having very high mechanical and physical strength and chip resistance, and is known for its high levels of whiteness and translucency. The high calcium phosphate content of bone ash gives bone china its unique properties.
History
The development of bone china dates back to the mid-18th century in England. The first commercially viable bone china was developed by Josiah Spode in around 1794, and his formulation has remained essentially unchanged to this day. Spode's formula contained six parts bone ash, four parts china stone, and three and a half parts kaolin. This combination provided a material that was both strong and aesthetically pleasing, which quickly gained popularity.
Composition and Manufacturing Process
Bone china is composed of approximately 50% bone ash, 25% kaolin, and 25% feldspar. The bone ash used in bone china is derived from animal bones that have been calcined (heated to high temperatures) to remove organic materials and then ground into a fine powder. The kaolin and feldspar are mixed with the bone ash to form a paste, which is then shaped into the desired form.
The manufacturing process of bone china involves several stages, including:
Forming
The paste is shaped into the desired form using various techniques such as slip casting, jiggering, or pressing. Slip casting involves pouring the liquid paste into a plaster mold, which absorbs the water and leaves a solid shell. Jiggering is a process where the paste is shaped on a rotating mold, while pressing involves pressing the paste into a mold under high pressure.
Firing
The shaped pieces are then fired in a kiln at a temperature of around 1200°C (2192°F). This initial firing, known as the bisque firing, removes any remaining water and organic materials and hardens the pieces. The bisque-fired pieces are then glazed and fired again at a slightly lower temperature (around 1100°C or 2012°F) to fuse the glaze to the surface and give the pieces their final appearance.
Glazing
The glazing process involves applying a layer of liquid glaze to the bisque-fired pieces. The glaze can be applied by dipping, spraying, or brushing. The pieces are then fired again to fuse the glaze to the surface, creating a smooth, glass-like finish.
Properties and Characteristics
Bone china is known for its unique combination of properties, including:
Whiteness and Translucency
The high calcium phosphate content of bone ash gives bone china its characteristic whiteness and translucency. When held up to the light, bone china exhibits a delicate, almost ethereal translucency that is not found in other types of porcelain.
Strength and Durability
Bone china is the strongest of the porcelain or china ceramics, with high mechanical and physical strength and chip resistance. This makes it ideal for use in fine dinnerware and other items that require both beauty and durability.
Thermal Shock Resistance
Bone china has good thermal shock resistance, meaning it can withstand sudden changes in temperature without cracking or breaking. This makes it suitable for use in both hot and cold applications.
Uses and Applications
Bone china is primarily used for high-quality tableware, including plates, bowls, cups, and saucers. It is also used for decorative items such as vases, figurines, and ornaments. The unique properties of bone china make it ideal for these applications, as it combines beauty with practicality.
Care and Maintenance
Bone china is relatively easy to care for, but it does require some special handling to maintain its beauty and durability. It should be washed by hand using a mild detergent and a soft cloth or sponge. Avoid using abrasive cleaners or scouring pads, as these can scratch the surface. Bone china should also be stored carefully to prevent chipping or breaking.