What Is The Process Of Firing Bone China?

Bone china is a type of soft-paste porcelain that is composed of bone ash, kaolin clay, and feldspar. It is known for its high levels of whiteness and translucency. Bone china is different from hard-paste porcelain because of its lower firing temperature (around 1260°C) and higher content of phosphate derived from animal bone. The history of bone china dates back to 18th century England.

In 1748, Thomas Frye began experimenting with bone ash to create a porcelain-like product that rivaled Chinese porcelain in fineness. He used bone ash, clay and alabaster and fired it at high temperatures, creating the first English porcelain. In 1749, Josiah Spode further refined the formula by adding feldspar, lowering the firing temperature and increasing the bone ash content. This resulted in the first formulation of modern bone china.

One of the key properties of bone china is its high level of whiteness and translucency, which is enabled by the high bone ash content (at least 30%). The bone ash adds phosphorus to the material, allowing it to become vitrified and translucent at lower firing temperatures. This gives bone china a delicate, refined quality not found in other types of ceramics. It is also lightweight and thin, enhancing its translucency. The kaolin clay provides plasticity for shaping, while the feldspar acts as a flux to lower the firing temperature. Together, these properties produce the distinctive translucent, white appearance of bone china.

Ingredients

Bone china contains three key ingredients – bone ash, china clay, and china stone. The proportions of these ingredients are what give bone china its trademark translucent white appearance and strength.

Bone ash, which comes from animal bone that has been calcined at extremely high temperatures, makes up approximately 50% of bone china’s composition. The high calcium phosphate content of bone ash enables the translucency and whiteness of the final product. Bone ash also gives bone china high strength and hardness.

China clay, also known as kaolin, makes up around 25% of bone china’s composition. China clay is a white clay that provides plasticity and gives shape to the bone china paste before firing. It also enhances whiteness and strength.

China stone, or petuntse, makes up the remaining 25% of bone china. China stone is a feldspathic rock that aids in vitrification during firing. This makes the bone china less porous and more resistant to staining.

The balance of these three ingredients at approximately 50/25/25 ratios provides bone china with its delicate, fine-grained, translucent qualities.

Preparing the Slip

The ingredients for bone china must be mixed into a liquid “slip” with the proper viscosity before shaping. This is a critical step, as the particle size and viscosity of the slip impacts the quality of the final product.

First, the bone ash, china clay, and china stone are milled down into a fine powder. Controlling the particle size is important – too coarse and the finished ware will have defects, but too fine can result in a lack of strength. The ingredients are then combined with water in a blunger to create the slip.

Getting the viscosity right is essential. The slip should flow smoothly but also be thick enough to hold a shape when pressed into a mold. Achieving the ideal thickness requires carefully controlling the water content and testing the viscosity frequently with a flow cup test during mixing.

Well-prepared slip with consistent particle size and viscosity allows the bone china to be smoothly shaped into delicate forms. Any imperfections in the slip will reduce the quality of the final fired ware.

Shaping Bone China

Once the bone china slip has been properly prepared, it is ready to be shaped into wares. There are several techniques used for shaping bone china:

Throwing – This is the process of shaping bone china on a potter’s wheel. It takes great skill to center the clay on the wheel and pull up the walls of an even, symmetrical piece. Throwing allows artisans to create one-of-a-kind shapes and pieces.

Slip casting – In this process, liquid slip is poured into a plaster mold. As the slip touches the plaster, it begins to solidify and forms a hollow clay shell that matches the mold. The excess slip is poured out, leaving the clay shell to be dried and fired. Slip casting allows for consistent, identical shapes to be produced.

Pressure casting – Pressure casting forces slip into a mold under high pressure. This allows for thinner, more delicate pieces to be formed without defects. The applied pressure packs the slip densely into the mold, creating smooth, even surfaces.

These various shaping techniques each have their advantages in creating bone china pieces with different characteristics and design aesthetics.

Drying

After the bone china pieces have been shaped, they must undergo a thorough drying process to remove any moisture before firing. This is a crucial step, as any remaining moisture can cause the pieces to crack or explode from rapid heating in the kiln. To prevent such defects, bone china must be dried slowly and evenly.

There are two main methods for drying bone china:

• Air drying – The shaped pieces are left on shelves in a warm, dry room for several days up to a week. The key is to allow gradual moisture evaporation. Keeping air circulation around the pieces with fans accelerates drying.

• Kiln drying – The ware is placed in an electric or gas kiln with the heat set very low, around 100-150°F. The slow, gentle heat continues evaporating any remaining moisture. Kiln drying may take 12-48 hours depending on size and thickness.

No matter the technique, monitoring progress and checking for dryness is critical. Moisture meters help indicate when bone china pieces are ready for bisque firing. Thicker items may require longer drying than thinner pieces.

Proper drying also prevents warping or deformation from uneven shrinkage. Maintaining consistency in air circulation and temperature is key. Rushing the drying risks uneven moisture loss and cracking.

Bisque Firing

Once the clay articles have been formed and dried, they undergo a first firing called bisque firing. This firing process hardens the clay body and prepares it for applying glaze and subsequent firing. Bisque firing is done at a relatively low temperature, usually around 1000°C (1830°F).

During bisque firing, the physical water and chemically combined water are slowly driven off as the temperature rises. This process happens gradually to prevent the clay from cracking. At around 100°C (212°F), free water evaporates from the clay. Between 500-600°C (930-1110°F), hydroxyl units and combined water begin to break down. This causes the clay to contract and shrink slightly. By the time the temperature reaches 1000°C (1830°F), the clay has become hard, durable, and porous.

The bisque firing changes the molecular structure of the clay so that it can withstand the higher temperatures required for glaze firing. It also provides an absorbent surface for the glaze to adhere to. Proper bisque firing is essential for producing high quality bone china that has sufficient strength and beauty for daily use.

Glazing

Glazing is a critical step in producing high-quality bone china that is both beautiful and functional. The purpose of the glaze is to provide an impermeable and decorative glass coating on the surface of the bisque-fired clay.

Applying glaze layer for appearance and impermeability: Bone china producers use a variety of techniques to apply the glaze in order to achieve the desired appearance and surface quality. The glaze slurry is usually applied by spraying or dipping, which allows an even coat on the bisqueware. Spraying produces a finer glaze layer, while dipping leads to a thicker coat.

The specific glaze recipe and application technique affect the final look. For a smooth, glass-like surface, the glaze is applied in multiple thin layers that fuse together in the kiln. Using a thicker glaze layer can create an orange peel effect or make the final surface more matte. Complex multi-step glazing produces decorative patterns and textures like crackle glazes.

In addition to appearance, the glaze must fully vitrify during firing to create an impermeable seal. Insufficient glaze can allow liquid and bacteria to penetrate the porous clay body, making the ware unsuitable for serving food. Applying an even coat is essential for functional bone china tableware and permeable decorative pieces are usually glaze fired a second time.

Glost Firing

The glost firing, also known as glaze firing, is the high temperature firing process that melts the glaze onto the bisque ware. This firing is done in the glaze kiln and typically reaches temperatures between 1200-1400°C (2190-2550°F). The purpose of glost firing is to mature the glaze and bond it to the body, as well as to harden the body and glaze together.

During glost firing, the pieces are carefully stacked in saggars and loaded into the glaze kiln. Slow firing is essential to allow the glaze to melt and flow gradually without defects. The kiln is heated up at a rate of around 100-150°C per hour until approximately 1000°C. After this point, the heating rate is slowed to 50-80°C per hour until the peak temperature is reached.

Firing typically takes 8-12 hours to complete, sometimes longer for larger pieces. The firing schedule has built-in holds at certain temperatures to allow the glaze to even out as it melts and flows. Near the peak temperature, the glaze will become molten and fuse to the clay body, resulting in a continuous glaze surface. The pieces are then slowly cooled to avoid stresses and cracks.

Throughout glost firing, expert monitoring is required to check the glaze melt, viscosity and surface quality. Small test rings are often fired along with the pieces to examine the glaze characteristics. The results of the glost firing largely determine the quality and appearance of the final bone china product.

Quality Control

Once the bone china pieces are glow fired, they undergo a rigorous quality control process to check for any defects before being approved for sale or use. Some of the key aspects examined during quality control include:

Warping – Bone china pieces are checked to ensure they have maintained their proper shape and have not become distorted or warped during firing. Severe warping renders the piece unusable.

Crazing – The glaze is inspected for fine craze lines, which are spiderweb-like cracks that can form in the glaze. Minor crazing may be considered acceptable, but severe crazing weakens the piece.

Chipping/Cracking – Edges and decorations are checked for any chips, cracks, or other signs of damage that may have occurred during manufacturing or firing.

Pinholes – The glaze is examined for pinholes, which are tiny holes formed by trapped air bubbles. While barely visible, numerous pinholes detract from appearance.

Color/Decoration flaws – The colors and painted decorations are inspected to ensure they have set properly and there are no smears, fading, or bleeding.

Production defects – Other production defects like mold lines, seam lines, or debris can also be identified and rejected.

Pieces that pass the stringent quality control checks are approved for sale or use. Those that don’t meet quality standards are rejected and never make it to the customer.

Conclusion

The firing process is a crucial step in creating high quality bone china that lives up to its reputation for durability and beauty. Proper firing transforms the unfired greenware into a final product that is translucent, hard, and able to withstand repeated use. Maintaining the correct kiln temperatures and firing schedule ensures the bone ash fully vitrifies with the clay and flux minerals. This results in bone china’s characteristic milky whiteness and refined surface. When done correctly, firing gives bone china its delicate yet strong physical properties. The bisque and glaze firings require careful control and monitoring to achieve the optimal effects. While firing may seem like a simple process of applying heat, it actually involves complex chemical and physical changes at the molecular level. Mastering the firing process is an artform that allows potters to create stunning bone china worthy of gracing the tabletops of discerning clientele.