Ceramic is an umbrella term for the hard and durable but relatively brittle substance that all traditional pottery and wares are made of, no matter the specific process or the combination of ingredients used.
Other metals, such as potassium oxide and iron oxide, may naturally be present depending on the type of clay in question. Manufacturers also may add these and other minerals or substances to the clay mixture to give it specific characteristics, such as a smoother texture or improved break resistance. These additions are commonly called inclusions. Everything from the size of the clay's individual grains to its water content will have an impact on its physical characteristics, including its color, plasticity, porosity, and reaction to being fired. The combination of ingredients that go into a specific type of clay is called the clay body.
Porosity of ceramics are traditionally classified into three main categories according to the pore size: macro-porosity (d > 50 nm), meso- porosity (2 nm < d < 50 nm), and micro-porosity (d < 2 nm)
My ceramic pieces are formed from the clay body by hand into the desired shape – be it a bonsai pot, vase, or any type of dinnerware. Each piece then undergoes a series of firings in which it's exposed to temperatures well above 1,000 degrees Fahrenheit in a kiln. Exposure to heat causes a physical transformation that makes the clay rigid, durable, and moisture resistant.
Depending on my desired results, each ceramic piece will likely receive a coating of glaze before or between firings. Because many types of clay retain some level of porosity even after they've been fired, this coating seals the surface of the piece, preventing it from absorbing moisture from food and drinks. Alternatively, I may use a transparent glaze if I want the natural color of the fired clay to show through. More often than not, glazes turn opaque when they're fired and are chosen for the decorative colored finish they give the final dinnerware.
What Does Vitrified Mean?
Many types of ceramic are fired to high temperatures and held there long enough for the clay to become partially or fully vitrified. Vitrification is a process by which the ceramic is brought to its melting point. At that extremely high temperature, the clay and minerals fuse together and form a crystalline structure. The term is derived from the Latin "vitreum," which means glass, because the process gives the clay properties very similar to glass. The transformation strengthens the material dramatically and makes it much less porous than non-vitrified ceramics, sometimes so much so that people may choose to forgo glazing ceramic pieces that have been fully vitrified.
There are many factors to consider when it comes to firing your pottery in a kiln. The terms oxidation and reduction refer to how much oxygen is in the kiln's atmosphere while the kiln is firing. An oxidation atmosphere has plenty of oxygen for the fuel to burn.
A reduction atmosphere occurs when the amount of available oxygen is reduced. This may not sound like things that will affect your pottery, but it can. The oxidation process, can alter the color of the glazes used. The reduction process, when oxygen is leeched out of your kiln atmosphere and pottery, can change the texture of your clay.
Basics of Oxidation
When heated sufficiently, many substances oxidize if there is free oxygen available. Volatile portions of compounds and molecules break free and the free oxygen then attaches to the remaining material, forming oxides. This process is called oxidation.
In firing a pottery kiln, the materials will normally convert to their oxide forms. For example, when copper carbonate is fired, the carbon will detach and burn off. As soon as the copper-carbon bond is broken, available oxygen will rush in and attach to the copper, forming copper oxide.
When a kiln is not in reduction, and there is enough oxygen for efficient fuel consumption but not an abundance of oxygen, the kiln can be considered to be in a neutral atmosphere. Electric kiln firings are generally considered to have either a neutral or slightly oxidizing atmosphere.
Many potters question if there is any such thing as a truly neutral atmosphere. Their main point is that there is enough oxygen in the kiln so that the glaze and clay body materials oxidize.
Basics of Reduction
Fire requires oxygen to burn. When there is a lack of oxygen, the fuel does not burn completely and the kiln atmosphere becomes filled with free carbon. The free carbon atoms will aggressively grab up any oxygen atoms they can find.
Carbon atoms are so oxygen-hungry that they are able to break molecular bonds. The carbon literally robs the clay and glaze materials of their oxygen.
When the carbon reduces the amount of oxygen in the clay and glaze molecules, the colors and textures of the clays and glazes can change. These changes can sometimes be quite dramatic.
Raku is a type of firing of low firing process that was inspired by traditional Japanese Raku firing. It dates back to the early 1550s especially for the Zen Buddhist Masters. Raku firing really is one of the most natural techniques that you can encounter in pottery. So while the pieces look incredible, they are not really used for functional pieces. But they can be used for indoor bonsai. A lovely fact about Raku is that is name literally translate as "happiness in the accident".
Different clay bodies and firing procedures yield ceramics with different properties. Those physical characteristics determine the suitable applications, perceived quality, and value of each dinnerware type that's produced. Despite these differences, however, all these pieces have one thing in common: They started out as clay, meaning they're all made of ceramic. The terms "earthenware" and "stoneware," as well as "porcelain," "china," and "bone china" each indicate a specific type of ceramic with specific physical properties and appearance.
One of the most important distinctions between different ceramic types is porosity. ASTM International standards define the test that measures how porous a certain piece of ceramic is. The procedure generally involves immersing the ware in boiling water for several minutes and then soaking the piece in room-temperature water for several hours. The difference in the piece's dry and wet weights will help testers determine how much water it absorbed. A piece of ceramic's porosity will partially determine whether it's classified as earthenware, stoneware, or porcelain. These definitions are based on the amount of water in proportion to a piece's weight that it absorbs when tested using the ASTM method described here.
Both porcelain and fine china are defined as ceramicware that won't absorb more than half a percent of its weight in water. Stoneware is defined as ceramic that won't absorb more than three percent of its weight in water.
Earthenware will absorb more than three percent of its weight in water.
These measurements are considered significant because the amount of water that a piece of crockery absorbs is one of the strongest indicators of its durability, as well as a reliable standard to ensure lower grades of ceramic aren't labeled in a way that's misleading to consumers.
Here's a rundown of two types of ceramic you might see in the kitchen or dining room.
Earthenware is the simplest type of ceramic available. Compared to other types, it's generally made with the least refined clay body and fired to the lowest temperatures in a process that's very similar to the ones employed by prehistoric humans to make rudimentary ceramics. This simple process yields wares that must be made relatively thick in order to remain durable. This type of ceramic also is very porous, so manufacturers must cover it with a full coat of glaze if it's intended to hold food or liquid.
Terracotta pottery is a common example of earthenware ceramic. If you've ever dropped a terracotta pot, you know how fragile it can be and why it's not often used to present food. Additionally, earthenware's porous nature means that a cracked or chipped piece can quickly absorb water and food residue, enabling microbial buildup. You're unlikely to find earthenware at any commercial foodservice establishment unless it's being used to decorate the dining room, although residential dinnerware is sometimes made of glazed earthenware.
Stoneware is a step up from earthenware in terms of durability and is considerably less porous. Pieces made of this material can be made thinner and lighter but still must be glazed to prevent the material from soaking up water. Nearly all restaurant dinnerware that isn't classified as porcelain is made of stoneware. Stoneware is fired into a vitreous or semi-vitreous state and tends to be a creamy or off-white color. Manufacturers may apply a transparent glaze that lets the natural color shine through. Alternatively, they may apply a colored glaze and any number of decorative decals. Stoneware can usually be purchased at a lower price than porcelain or bone china, making it a common choice for restaurant dinnerware at semi-casual, sit-down restaurants. For the most durable stoneware, look for options marked as "fully vitrified."
Porcelain making is an old art. The earliest porcelain, commonly called "primitive porcelain," appeared during the Shang Dynasty (1600 to 1046 B.C.), but the first true porcelain was produced over a thousand years later during the Eastern Han Dynasty, around 202 B.C. – 220 A.D. It was brought to Europe by Marco Polo who picked a small gray-green jar during his travels in the 14th century, European craftsmen set about trying to emulate the Chinese artform, but it took them until the late 18th century to figure out how to make porcelain. That was when a book called L'art de la Porcelaine was published, and only when the recipe for making porcelain was written in a book were Europeans truly able to crack the code and open their own porcelain factories.
There are six types of porcelain: dental, bone, hard and soft porcelains, vitreous or glass-like porcelain, and something called steatite porcelain, designed for use in thermal insulators, car parts, and porcelain dishes that can be used directly on a gas burner. Most dishes are made from soft porcelain.
The exact composition of porcelain varies depending on its use and the manufacturer, although one common ingredient is kaolin, a soft white clay that is combined with other ingredients like bone ash, sand, magnesium, quartz, and feldspar. The mixture is then shaped and fired in high heat to become hard. If that sounds a lot like how ceramics are made, you're correct. Porcelain is technically a specialized subset of ceramics, both are made of clay and kiln-fired, but porcelain uses different raw materials, glazes, and has higher density and is fired at a higher temperature of around 2,250 degrees Fahrenheit. That makes porcelain more durable and more water resistant than ceramics,