Do You Have To Bake Porcelain Clay?

What is Porcelain Clay?

Porcelain clay is a type of clay that is white and fine-grained, consisting primarily of kaolin (also called china clay). It is formed by the weathering of aluminum silicate minerals like feldspar. Porcelain clay is highly heat-resistant and, when fired at high temperatures, it becomes dense, hard, and very strong.

Some key properties and characteristics of porcelain clay include:

• Translucent – When thin, porcelain has some translucency.
• White and fine texture – Porcelain clay has a very smooth, fine texture and is white or off-white in color even when unglazed.
• Non-porous – When fired at high temperatures, porcelain becomes vitrified and non-porous.
• Hard and durable – Porcelain is very hard, dense, and durable compared to other types of pottery.
• High mechanical strength – Due to minimal flaws and high density, porcelain has excellent mechanical strength.
• Low water absorption – Porcelain has very low water absorption, less than 0.5%.

There are three main types of porcelain clay:

• Soft paste porcelain – Lower firing temperature, more porous and prone to chipping/cracking.
• Hard paste porcelain – Higher firing temperature, less porous and more durable.
• Bone china – Made from bone ash, kaolin clay and feldspar. Extremely white, translucent and delicate.

Porcelain clay is valued for its whiteness, strength, translucency and resistance to water compared to other types of ceramics and clay (Sources: https://www.collinsdictionary.com/us/dictionary/english/porcelain-clay, https://www.thefreedictionary.com/porcelain+clay)

Why Bake Porcelain Clay?

Baking porcelain clay is an essential step in the process of working with this material. There are several key reasons why porcelain clay must be baked:

Baking hardens the clay through a process called sintering. During sintering, the clay particles fuse together when heated, forming stronger bonds and decreasing porosity. This transforms the soft, malleable clay into a hard, durable material.

Baking also strengthens the structural integrity of porcelain pieces. In its unbaked state, porcelain is quite fragile and delicate. The baking process reinforces the clay body, making it much more resistant to damage from impact or stress.

Additionally, baking makes porcelain clay far more durable over time. Unbaked clay remains porous and prone to damage from moisture and contaminants. Baking vitrifies the clay, sealing the surface and preventing water absorption.

Finally, baking seals colorants like stains, underglazes, and glazes into the body of the porcelain. Without baking, these decorative elements would remain fragile and tend to wear off or fade over time. The heat of baking fuses them to the clay permanently.

In summary, baking is mandatory for porcelain clay in order to achieve finished pieces that are hard, strong, and durable works of art. The sintering process transforms the raw clay into a robust ceramic material.

What Temperature To Bake Porcelain Clay?

Different types of porcelain clay require slightly different temperatures for baking, but most fall within a similar range. According to Soul Ceramics, the most common temperature range for baking porcelain is between 1200-1400°F (649-760°C) [1]. Firing at higher temperatures within this range can make porcelain stronger and more durable. However, exceeding 1400°F runs the risk of warping or bubbling the clay [2].

It’s important not to under-fire porcelain either. Temperatures below 1200°F may result in porcelain that is weaker, more porous, and prone to breaking. The optimal temperature allows the clay to vitrify completely without deforming the piece.

Most commercially available porcelain clays will provide a recommended firing range on the packaging or product description. When in doubt, aim for 1260-1300°F to achieve strong, durable porcelain results.

How Long to Bake Porcelain Clay?

The baking time depends on the thickness of the porcelain clay pieces. Thinner pieces require less time, while larger or thicker pieces take longer to fully bake through.

The typical baking time range for porcelain clay is 8-12 hours. Pieces that are less than 1/4 inch thick may only need 1-2 hours, while a 1 inch thick piece could require 12-24 hours.

Larger or thicker porcelain clay pieces that would require very long single firings can be baked in multiple shorter sessions. Each session should bake the piece for at least 8 hours. Allow the piece to completely cool between sessions before returning to the oven or kiln.

Test pieces can determine the minimum baking times needed for a particular porcelain clay recipe and thickness. Under-baked pieces will not reach their full hardened strength.¹

Baking Methods for Porcelain Clay

There are three main methods for baking porcelain clay:

Kiln Firing

Kiln firing is the most common method for firing porcelain clay. Kilns allow firing at very high temperatures (over 2000°F) needed for porcelain. Kilns provide an enclosed, controlled environment with adjustable temperature settings. The benefits of kiln firing include:

• Achieving the high temperatures needed for porcelain
• Precise temperature control
• Consistent results
• Faster firing times

The drawbacks are that kilns can be expensive to purchase and operate. Kiln firing also requires knowledge of firing schedules and kiln maintenance.

Oven Firing

Firing porcelain clay in a standard kitchen oven is possible but has limitations. Oven temperatures only reach around 500°F maximum, which is too low to vitrify porcelain. However, oven firing can be used for bisque firing before a final kiln firing. Benefits include:

• Uses existing oven in homes
• Good for bisque firing at lower temps

Drawbacks are the limited temperature, uneven heating, and energy costs of prolonged firing. Extra care is needed to prevent cracking.

Pit Firing

Pit firing involves building an outdoor fire pit to fire clay pieces. Temperatures can reach over 2000°F by burning wood over several hours. Benefits include:

• Historic firing method
• Unique natural effects from wood ash
• Lower equipment costs

Drawbacks are less control over results, smaller scale, and labor-intensive process.

In summary, kiln firing is the most reliable method for achieving the high temperatures and consistent results needed for porcelain clay firing. Oven and pit firing have limitations but can be useful in certain circumstances.

Preparing Pieces for Baking

Proper preparation of porcelain pieces before baking is crucial for achieving the best results. Here are some key tips for getting pieces ready for the kiln:

Allow proper drying time – Let pieces air dry completely before baking. Moisture in the clay can cause cracking or exploding in the kiln. Let pieces dry for 24-48 hours or until no moisture remains.

Apply glazes/colorants before baking – Any glazes, slips, stains, or other colorants must be applied before baking. These will fuse to the clay body during firing. Make sure glazes are kiln-safe for the intended temperature.

Use kiln wash on shelves – Coat kiln shelves with a thin layer of kiln wash before placing pieces to prevent them sticking. Reapply wash each time shelves are used.

Place pieces with space between – Arrange pieces in the kiln allowing 1-2 inches between them. This prevents glazes from sticking pieces together and allows heat circulation.

Firing Schedule for Porcelain Clay

The firing schedule for porcelain clay is critical for achieving the desired results. This includes slowly ramping up the temperature, holding at maximum heat, and then cooling. Controlling the temperature and duration at each stage is important.

A sample bisque firing schedule for porcelain clay is:

• Raise temperature at 200°F/110°C per hour from room temperature to 1000°F/538°C
• Hold at 1000°F/538°C for 30 minutes to harden the bisque
• Slow cooling at 400°F/205°C per hour down to room temperature

For glaze firing [1]:

• Increase temperature at 300°F/165°C per hour from room temperature up to 1900-2200°F/1038-1204°C
• Hold at max temperature for 10-20 minutes to mature glazes
• Cool slowly at 100°F/55°C per hour down to 1000°F/538°C
• More rapid cooling to room temperature

This gradual heating and cooling allows moisture to escape, prevents cracks and shattering, and provides energy for chemical transformations to occur in the clay and glaze.

Tips for Successful Baking

Some useful tips to ensure your porcelain clay pieces come out of the kiln in good condition:

Use witness cones to test unknown glaze and clay combinations. With new pieces, be sure to include witness cones in the kiln to indicate when cone temperatures have been reached. This will prevent under or over-firing of your ware. Refer to this guide on using witness cones.

Allow for even airflow inside the kiln. Make sure pieces are spaced properly and do not touch each other. Having adequate airflow ensures the entire kiln heats evenly.

Go slow and do test firings for the first few times. When firing a new kiln or firing unfamiliar clay bodies, ramp up the temperature slowly and do test firings before firing your finished work. This helps identify any hotspots or inconsistencies in the kiln.

Note firing behavior carefully. Pay close attention to how your clay pieces react at various temperatures. Look for signs of bloating, scorching, or cracking and adjust temperature accordingly on subsequent firings. Taking detailed notes will help dial in the optimal firing schedule.

Common Baking Issues

There are a few common issues that can occur when baking porcelain clay:

Bubbling or Cracking From Too High Temperature

If the baking temperature is too high, the clay can bubble or crack. Porcelain clay should be fired at lower temperatures, usually between 175°F and 250°F. Going above this range can cause the material to expand rapidly and create cracks or bubbles in the surface as gases try to escape ¹.

Warping from Uneven Heating

If the clay heats unevenly, with some areas getting much hotter than others, it can cause the piece to warp or bend during firing. Using an oven thermometer and rotating pieces can help ensure even heating. Placing pieces on a level baking surface rather than directly on the oven rack also helps ².

Discoloration from Impurities

Impurities in the clay can create dark or mottled discolorations after firing. Using high purity porcelain clay and proper preparation helps avoid this. But if discolorations do occur, careful sanding or grinding after baking may remove superficial stains ³.

Caring for Baked Pieces

Proper care and handling of baked porcelain clay pieces is important to preserve their beauty and integrity over time. Here are some tips for caring for finished porcelain clay crafts:

For storage, avoid exposing baked clay to direct sunlight, excessive heat, or high humidity for prolonged periods, as this can cause discoloration or damage over time. Store pieces in a cool, dry place. When displaying porcelain clay crafts, use felt pads under any items that could potentially scratch the surface.

Clean finished porcelain clay gently using a soft cloth or sponge with mild soap and warm water. Avoid abrasive cleaners or scrubbing that could scratch the surface. Rinse thoroughly and dry completely. For stubborn stains, try using toothpaste or baking soda as a gentle abrasive applied with a soft cloth.

If a piece chips or cracks, it may be possible to repair it by gluing fragments back together using superglue or epoxy resin adhesives. Be sure to smooth seam lines after the adhesive dries. Small chips can be filled in using liquid polymer clay or acrylic paint matched to the piece. Use paint pens to touch up paint details.

With proper care and handling, beautiful porcelain clay crafts can last for many years. Avoid exposing pieces to excess moisture, heat, sunlight or physical impact. Gently wash, store appropriately, and make repairs as needed.