Ceramic plates, ubiquitous in our daily lives, are more than just simple tableware; they are the result of a fascinating interplay of materials and processes honed over centuries. The journey from raw earth to a functional and aesthetically pleasing plate involves selecting the right clay, shaping it with skill, and transforming it through the alchemy of fire. Understanding what ceramic plates are made of involves delving into the world of different clay types and the intricate steps of their creation.
Types of Clay and Their Properties
The foundation of any ceramic plate lies in the type of clay used. Each type brings its unique characteristics to the final product, influencing its durability, appearance, and suitability for different uses. Earthenware, stoneware, porcelain, and bone china are among the most common types, each boasting a distinct composition and firing temperature. Earthenware clay, known for its porous and textured nature, stands out as a forgiving medium for beginners. Fired at lower temperatures, it yields vibrant pieces and its smooth texture makes it suitable for hand-building and wheel-throwing projects. Stoneware clay, celebrated for its robustness and versatility, is a favorite for crafting everyday items like dinnerware and mugs. Fired at higher temperatures, it results in strong and resilient pieces, providing a broad canvas for various glazes and surface treatments. Many colored stoneware clays produce durable ware and react favorably to the manipulation of air, gas, and smoke in reduction firing. These clays have good plasticity and will perform well in both wheel throwing and hand building methods. Ball clay, another type of clay, contains kaolinite, mica, quartz, and other organic matter, and is also known as “plastic clay”. It contains a large percentage of kaolinite and quartz, with around 10-25% mica. Porcelain clay, often hailed as the epitome of ceramic artistry, is known for its delicate appearance and translucent finish when fired at high temperatures. Hard-paste or “true” porcelain is the most common type. “True” porcelain is fired at very high temperatures such as cone 10 (2345℉ or 1285℃) and yields sturdier objects. Bone china, a type of porcelain, has mostly replaced true porcelain in modern times. It contains kaolin, bone ash, feldspar, and phosphates, making it the strongest kind of porcelain. For more information on clay types, see this article: clay types: earthenware, stoneware, and porcelain.
The Manufacturing Process
The journey from raw materials to a finished ceramic plate is a multi-stage process that demands precision and care. It begins with the meticulous selection and preparation of raw materials, including clay, feldspar, and kaolin, ensuring uniformity in composition. Impurities are carefully removed to guarantee the quality of the final product. These materials are then mixed thoroughly in a large drum mixer, with water added to create a slurry that can be easily molded. The mixed slurry is poured into molds, which give the tableware its final shape and the molds can be made of plaster, metal, or other materials. The slurry is left to dry in the mold for a certain period of time, depending on the size and shape of the item. After the ceramic tableware has dried, it is removed from the molds and trimmed to remove any excess material. The edges are smoothed and refined using sandpaper or other tools. The shaped and finished ceramic tableware is then fired in a kiln to achieve its final hardness and durability and the firing process can take several hours or even days, depending on the size and thickness of the items. Following the firing process, the ceramic tableware is glazed to give it a smooth and glossy surface. The glaze adds an extra layer of protection against scratches, stains, and other damage and can be applied by spraying, dipping, or brushing on the surface. After glazing, the ceramic tableware must be fired again to melt the glaze and create a smooth, even finish. The ceramic tableware is then inspected thoroughly for any defects, cracks, or roughness and is also tested for durability, strength, and resistance to heat and stains.
Glazes and Potential Defects
Glazes play a crucial role in the final appearance and functionality of ceramic plates. Applied as liquid coatings before firing, pottery glazes create a smooth, ornamental, and protective finish. These glazes enhance visual appeal, water resistance, and durability, transforming pottery from artistic to practical. Glazes are made from silica, fluxes, and colorants, ceramic glazes bind to the surface in a kiln under high temperatures. Clear glazes are transparent and are often used to highlight underlying clay or decoration. Glazes rich in iron oxide produce colors ranging from blue, green, yellow, amber, brown, and black. There are also matt glazes, which have a smooth matte surface and crystalline glazes, which are produced by the crystallization of some substances that are separated and become visible. Despite the care taken in manufacturing, defects can occur in ceramic plates. Cracking refers to cracks of different sizes on the product and the reason is that the moisture of the green body entering the kiln is too high. Pinholes refers to tiny dents or small holes on the glaze of the product. Another common glaze defect is crazing, which is the most common glaze defect, and normally the easiest to correct. Shivering is the most problematic of glaze defects, particularly in any functional ware, since slivers of glaze from the edges can drop into food or drink. Crawling is caused by a high index of surface tension in the melting glaze and is triggered by adhesion problems, often caused by bad application. To learn more about glazes, you can read this article: unlock the magic: mastering crystalline glazes.
Health and Safety: Lead Content
The health and safety considerations surrounding ceramic plates, particularly concerning lead content and leaching, are of utmost importance. Lead has been used to make ceramic dishes for centuries, often being added to a glaze for the bright colors and for the smooth, transparent glaze. But in some cases, lead contained in old dishes, ceramics, and other types of tableware or cookware may leach into food or drink, leading to potential health problems associated with lead poisoning. In the past, lead was used as part of the glazes or decorations covering dishes and other ceramic ware and it was favored because lead provides tableware with a smooth, durable, glass-like finish. It also intensifies bright colors, especially reds, oranges, and yellows, and makes them last longer. If the pottery is not manufactured properly, this lead can leach into food and drink that is prepared, stored, or served in the dishes. While lead can affect nearly every bodily system, its effects depend upon the amount and duration of lead exposure and age and infants, young children and the developing fetus can be affected by chronic exposure to amounts of lead that may not result in obvious symptoms of lead poisoning. Lead poisoning in children has been linked to learning disabilities, developmental delays, lower IQ scores. The safest practice is not to use tableware that you are unsure of with food or drink and this is especially true for tableware used by children, pregnant women, or nursing mothers.
