How Is Porcelain Made Step By Step?
Porcelain is a ceramic material made by heating clay and other materials to high temperatures. The word “porcelain” comes from the old Italian word “porcellana” meaning cowrie shell, due to its resemblance to the surface of polished shells.
Porcelain originated in China and dates back to the Tang Dynasty (618-907 CE), where it was known as “white gold.” According to the English.scio.gov.cn website, porcelain production flourished during the Song Dynasty (960-1279 CE), with kilns spreading across the country as techniques were refined [1]. The city of Jingdezhen became the porcelain capital of China and produced fine porcelain for the imperial court and export [2].
Chinese porcelain was highly prized in Europe once trading routes opened up. By the 18th century, European potters had discovered how to make their own porcelain, leading to factories sprouting up across Europe and eventually America.
Today porcelain is still valued for its strength, hardness, whiteness, translucency, and ability to be shaped into fine detailed forms. It continues to be used for tableware, sculptures, electrical insulators, dental prostheses, and more.
Clay Used for Porcelain
Porcelain is made from a specific type of clay that contains kaolin and other minerals that result in its white color and translucency once fired. The clays used for porcelain tend to be less plastic and more fine compared to other pottery clays, making them more difficult to work with when forming pieces.
The main properties needed in clays suitable for porcelain production include:
– Whiteness – Clays with high kaolin content and few impurities result in white fired porcelain.
– Refractoriness – Porcelain clays have high silica content, resulting in high melting temperatures and low shrinkage.
– Low plasticity – Porcelain clays are less sticky and elastic compared to other clays, making forming more difficult.
– Fine particle size – Finer particles compact well and result in low porosity and high strength after firing.
– Low content of fluxes – Low fluxes like iron result in pure white color and high vitrification temperatures.
Some examples of clays used for porcelain include kaolin, ball clay, feldspar, quartz, and bone ash. The ratios of these ingredients can be adjusted to optimize working and firing characteristics.
Preparing the Clay
Porcelain clay must be cleaned and fully prepared before it can be formed into pottery and ceramic pieces. This preparation process is crucial for proper plasticity and moisture levels.
First, raw clay straight from the bag or block must be cleaned. Any hard or dried bits of clay are removed. The clay is then cut into workable sizes (The Pottery Wheel). Next, the process of wedging occurs. Wedging involves kneading the clay by rolling it back and forth to distribute moisture evenly and remove air pockets (eHow). Proper wedging leads to good plasticity, allowing the clay to be flexible and moldable.
Moisture content is carefully controlled during preparation. Porcelain clay for pottery requires enough water for shaping but not so much that it becomes sticky. Getting the moisture just right takes practice. Tested techniques like the ribbon test help gauge if a clay’s moisture level is ideal for throwing on the pottery wheel or handbuilding.
With mindful clay preparation, potters can achieve smooth, consistent, workable porcelain ready for the next steps of production.
Forming the Porcelain
There are several techniques used for forming porcelain, including throwing, slipcasting, and pressing.
Throwing is a common technique where the potter uses a pottery wheel to shape the porcelain. The clay is centered on the wheel and the potter uses their hands to manipulate and form the clay into the desired shape as the wheel spins. This takes great skill to center the clay and achieve symmetry in the finished piece.
Slipcasting involves pouring liquid clay slip into a plaster mold. The porous plaster absorbs water from the slip, forming a layer of clay on the mold surface. Once the desired thickness is achieved, the excess slip is poured out. The remaining clay takes the shape of the mold as it dries and sets. Slipcasting allows for precise replication and is ideal for mass production.
Pressing is used for flatware and smaller detailed items. Porcelain powder is pressed into a metal mold at high pressure. The mold imparts the shape and design details to the clay. Multipart molds can be used to achieve undercuts and hollow forms. Pressing produces consistent results efficiently.
The choice of forming technique depends on the type of object being made, the skill of the potter or artisan, and the production volume. Each method lends itself better to certain applications.
Drying the Porcelain
After the porcelain has been formed, it needs to be dried slowly and evenly to prevent defects. Porcelain clay has high shrinkage, up to 20% from wet to fired state. If dried too quickly, it can crack and warp.1
During drying, moisture evaporates from the clay body producing shrinkage stresses. If moisture leaves the surface faster than the interior, the surface shrinks first causing cracks. Slow, even drying allows moisture to evaporate uniformly throughout the clay.2
Ideal drying conditions are cool (60-70°F), with moderate humidity (40-60%) and good air circulation. Thick pieces may require weeks of careful drying. Shelves should be spaced apart for airflow. Plastic tenting retains moisture while allowing ventilation. Avoid placing wetware in direct sun or near heat sources.3
Monitoring the drying rate and mass of the ware over time can indicate when it’s ready for bisque firing, fully dried with no more shrinkage.
Bisque Firing
The bisque firing is the first low temperature firing process used to harden the shaped clay before glazing. During bisque firing, the porcelain pieces are heated to approximately 1000°F to 1800°F depending on the clay body. This causes all moisture to evaporate from the clay and the materials to sinter, which fuses the clay particles together resulting in a strong, porous structure.
The bisque firing cycle typically follows a carefully controlled schedule to avoid defects. The temperature is slowly increased to allow water and carbon to escape from the clay. It’s held at around 1000°F before continuing to rise steadily to the peak temperature. The firing is then cooled slowly over 12-24 hours. Firing too quickly can cause cracks, warping, or bloating.
Proper bisque temperatures provide porcelain with enough strength, porosity, and hardness to accept glazes without deforming. Higher temperatures up to cone 04 (1945°F) lead to lower porosity with increased strength. Lower bisque temperatures around cone 08 (1693°F) allow better absorption of glazes. The ideal bisque temperature depends on the clay body and glaze to be used.
Glazing
Glazing is an important step in porcelain making that adds color, texture, and protection to the final piece. Glazes are made from a mixture of silica, fluxes, and stabilizers that form a glass-like coating when fired at high temperatures.
Common glaze ingredients include feldspar, silica, whiting, kaolin, quartz, dolomite, and colorants like metal oxides. These raw materials are combined and ground into a fine powder to produce the glaze. Typical glaze recipes require balancing the fluxes, silicas, and stabilizers to achieve the desired effects.
Glazes can be applied through dipping, pouring, brushing, or spraying. Each technique requires proper viscosity and thickness to evenly coat the bisqueware. Too thin and the glaze will drip and pool. Too thick and it will dry with an uneven texture.
Some common glaze defects include pinholing, crawling, blistering, and crazing. These are often caused by issues with glaze chemistry, thickness, firing temperature, and clay body properties. Careful testing and adjusting of glaze recipes helps avoid defects.
The glazed porcelain must be fired again at a high temperature around 2,300°F to mature the glaze and create a smooth, glassy surface. This firing is called glost firing. The glaze melts and fuses with the clay body during this process.
Glost Firing
The Glost firing, also known as the gloss firing, is the second high-temperature firing done in porcelain manufacturing. It is typically performed between 1,220-1,350°C (2,230-2,460°F), which is a lower temperature than the bisque firing.1 The purpose of the Glost firing is to melt the glaze onto the bisque ware to give an impervious, glassy coating. The high temperatures fuse the glaze with the body to produce the finished vitreous surfaces associated with porcelain.
During Glost firing, the firing temperature is carefully controlled to ensure the glazes melt completely and fuse with the porcelain body. However, the temperature must be kept below the melting point of the body itself to avoid deformations. Slow firing rates are used to minimize thermal shock and prevent cracking or crazing of the ware. The firing time depends on the size and thickness of the pieces, often taking 12-36 hours to complete.
Correct Glost firing is critical for developing the translucency and delicate beauty typical of fine porcelain wares. It requires precise control over temperatures and firing cycles in the kiln. Tunnel kilns enable rapid, consistent Glost firings to efficiently produce high-quality porcelain at scale.2 The resulting vitrified glossy coating and fusion to the porcelain body is essential to create durable, functional, and aesthetically pleasing porcelain products.
Quality Control
Ensuring consistent quality is a critical step in porcelain manufacturing. There are several key tests conducted:
Thermal shock testing involves rapidly heating and cooling samples to induce thermal stress and check for cracks or defects. According to Intouch Quality, thermal shock test is one of the top on-site tests for porcelain dinnerware quality.
Stain resistance testing evaluates how well the glazed porcelain withstands staining from substances like coffee, tea, and ketchup. Samples are exposed to various staining agents and then evaluated. Intouch Quality also lists stain resistance as a critical test.
Visual inspection of the unglazed portions can help determine if the clay body has sufficient vitrification. According to Holst Porzellan, porcelain should have a smooth, glass-like surface when polished on an unglazed section, indicating full vitrification.
Testing key optical properties like translucency, whiteness, and gloss of the glaze is also standard. The final porcelain products should meet specifications for these visual parameters.
Summary
Porcelain making is an intricate multi-step process that requires expertise and precision. Here is a recap of the porcelain production process step-by-step:
First, the specific porcelain clay is prepared by mixing various minerals like kaolin, quartz, and feldspar. This clay mixture is refined through filtering and magnetic separation to remove impurities.
Next, the purified porcelain clay is formed into the desired shapes through methods like throwing, slip casting, pressing, or hand building. Complex pieces may require multiple forming techniques.
The unfinished porcelain pieces then undergo an important drying process to carefully remove the moisture content. This is done at controlled temperatures to prevent cracking and warping.
After drying, the first firing known as bisque firing vitrifies the porcelain at around 2,200°F. This further hardens the material in preparation for glazing.
Glazes are applied through methods like spraying, brushing, or dipping to coat the bisque ware. The glaze melts and fuses to the porcelain body during firing.
The final glost firing at around 2,300-2,400°F melts the glaze into a glass, sealing and hardening the porcelain into its finished form.
Throughout the process, quality control checks are conducted, like testing clay plasticity, drying rates, and fired strength. The result is high-quality porcelain prized for its hardness, whiteness, translucency, and resonance.
