What Is Grogged Clay?

What is Grog?

Grog is an important component of grogged clay. It consists of previously fired clay or other materials that have been ground up into a particulate form. The most common materials used for grog are:

• Fired clay – This could be leftover clay from previous projects that has been broken up and processed into grog.
• Chamotte – A commercial grog product made by firing clay, grinding it, and sizing the particles.
• Brick, tile, etc – Ceramic materials like old brick or floor tile can also be processed into grog.

The purpose of adding grog to clay is to alter its working properties and physical characteristics. Specifically, grog performs several important functions:

• It helps prevent shrinking and cracking during drying and firing of the clay.
• It allows moisture in the clay to escape more easily, reducing drying time.
• It lowers the plasticity of the clay, making it easier to work with for certain techniques like handbuilding.
• It lightens the weight of the finished piece without sacrificing strength.

The particle size and amount of grog can be adjusted to fine-tune the qualities of the clay body. Overall, grog gives the clay added stability for creating ceramic artwork and functional ware.

History of Grogged Clay

Grogged clay dates back thousands of years to some of the earliest human civilizations. The practice of adding grog, or crushed fired clay, first emerged in ancient cultures where functional pottery was an essential part of daily life.

One of the earliest uses of grogged clay was discovered in ancient China during the Neolithic period around 10,000 BCE. Chinese potters found that crushing previously fired pottery shards and mixing them into the clay body improved the performance and durability of vessels during firing. This innovation allowed their pottery to be thinner and stronger.

In the Americas, early pottery finds in places like the Olmec heartland along the Gulf Coast of Mexico revealed that Mesoamerican cultures were also creating grogged clay as early as 1500 BCE. Mayan potters frequently used crushed potsherds, volcanic ash, and even seashells as grog inclusions in their clay.

The addition of grog spread around the world along with the advancement of pottery techniques. Grog tempering became commonplace among ancient Greek, Roman, African, and Middle Eastern potters. Even as new materials were introduced over time, grog has remained an essential element in traditional and modern pottery production across cultures.

Making Grog

Grog is made by crushing or grinding fired clay or pottery shards into a granular powder. This process turns hard, solid clay material into a coarse aggregate that can be mixed with wet clay to improve its working properties and create grogged clay. To make grog, potters will take discarded or leftover pottery and break it into small pieces using a hammer or another heavy tool. The pottery shards are then further crushed using a mortar and pestle, rolling pin, or mechanical grinder until a sandy texture is achieved.

The aim is to grind the pottery down into particles typically ranging from pea-sized to fine sand grains. The coarseness of the grog can be tailored by how much the pottery is broken down – larger grog particles will create more porous clay while finer particles increase strength. Many potters will sift the crushed pottery through wire mesh screens to separate out particles of a consistent size. By making their own grog from recycled clayware and controlling the particle size, ceramic artists can customize the mix design for their specific needs.

Adding Grog to Clay

Adding grog to clay provides several important benefits for potters and ceramic artists. Grog helps prevent cracking and warping while drying and firing. It also lowers the overall shrinkage rate of clay and can help clay dry more evenly. The most common amount of grog added to clay is 10-30% by dry weight. However, some clays may incorporate up to 50% grog.

When grog is added to clay, it opens up the clay body, making it less dense and more porous. This porosity allows water to escape from the clay more easily while drying, reducing cracking and warping. The grog particles also help reinforce the clay, acting like rebar in concrete. This makes the clay stronger in both the wet and fired states.

In addition to physical properties, grog also affects the workability of the clay. As more grog is added, the clay becomes courser and stiffer to work with. The friction of the grog makes the clay less plastic. While too much grog can make clay difficult to wedge or throw on the wheel, the right amount makes clay much easier to handle and form without slumping.

Firing Grogged Clay

When firing grogged clay, it’s important to consider the effects of temperature on the clay body’s strength and other properties. Grogged clay requires slightly higher firing temperatures than non-grogged clay in order to fully vitrify the clay particles and grog inclusions. This is because the grog has a higher melting point than the clay body alone. Generally, grogged clay bodies should be fired between Cone 04 (2158°F / 1182°C) and Cone 2 (2185°F / 1196°C).

The main effect of firing temperature on grogged clay is its finished strength and porosity. Lower firing temperatures can result in a weaker clay body that is more porous and prone to breaking or leaking. As the firing temperature increases, the grog particles and clay matrix vitrify more completely, forming stronger bonds and decreasing porosity. However, firing too high can cause excess glass formation in the clay, leading to increased shrinkage, warping, and bloating defects.

It’s recommended to test-fire grogged clay samples at a variety of temperatures to determine the optimal cone for strength without defects. The amount of grog in the clay body will also affect the firing temperature – higher grog percentages require more heat to fully mature. Consulting the clay manufacturer’s specifications can provide a recommended starting point for determining the ideal firing range.

Uses of Grogged Clay

Grogged clay is a versatile material used for making many different types of pottery and ceramics. The addition of grog provides some key advantages over regular, non-grogged clay:

Types of Pottery and Ceramics

Grogged clay is commonly used for:

• Handbuilt pottery – The grog gives extra strength for handbuilding techniques like coil and slab building. Grogged clay is easier to join and attach.
• Sculptural pieces – The grog allows sculpting of more intricate shapes that would collapse under their own weight with regular clay.
• Large vessels – With increased sturdiness from the grog, very large pots, urns, and vases can be made without warping or cracking.
• Casserole dishes and roasting pans – Grog helps prevent thermal shock, making grogged clay suitable for high-temperature ovenware.
• Outdoor ceramics – Grogged clay’s durability makes it ideal for garden pots, decorative yard features, and outdoor ceramic art.
• Functional tableware – Grog provides added strength and durability for plates, bowls, mugs, and serving dishes used daily.

Advantages over Non-Grogged Clay

The main advantages grog provides include:

• Increased strength – Grog particles reinforce the clay matrix, leading to less warping and cracking.
• Improved thermal properties – By reducing shrinkage, grog helps prevent cracking and breakage in firing.
• Greater durability – Fired grogged clay is more resistant to chipping, wear, and tear during everyday use.
• Easier joining – The grogged surface provides more tooth for attaching clay components.
• Extra workability – Grogged clay can be sculpted into more intricate shapes before firing.

Making Coil Pots with Grog

Working with grogged clay offers many benefits when constructing pottery using the coil method, which involves rolling and stacking coils or ropes of clay to build a vessel from the ground up. The addition of grog gives greater strength and structure to the clay coils.

There are a few key steps to properly making coil pots with grogged clay:

1. Prepare the clay by wedging and kneading it thoroughly to achieve an even distribution of grog particles throughout the clay. This helps prevent weak points or cracks from developing.
2. Roll the grogged clay into long, uniform coils or ropes. The clay should be soft enough to shape but firm enough to hold its form without sagging or slumping.
3. Begin coiling from the base, gradually building upwards. Apply slurry between coils to help bond them together. Use fingers, a rounded rib, or a paddle to blend the coils together.
4. Allow the vessel walls to firm and thicken slightly before continuing upwards. Support the interior walls with a rounded form or shape as needed.
5. When the desired height is reached, refine the rim and smooth the exterior using a metal rib or wooden knife. Let the pot dry completely before firing.

The grog particles create a rougher, sandier surface texture that aids in bonding the coils together. Grogged clay coils stack securely while leather hard, maintaining shape without warping or cracking. The end result is sturdier coil-built vessels that can withstand daily use.

Sourcing Grogged Clay

If you don’t want to go through the process of making your own grog and mixing it into clay, you can purchase pre-made grogged clay from ceramic suppliers. Many clay manufacturers offer grogged versions of their clays in different proportions of grog to clay. For example, you can buy cone 04 grogged stoneware that contains 20% grog particles. The more grog that is added, the more porous and coarse the final fired clay will be. Suppliers may classify their grogged clays as low-fire or high-fire depending on the type of base clay used.

When buying pre-made grogged clay, look for a reputable brand from a ceramic supply retailer. Ask the retailer for guidance on choosing the right type and grog percentage based on your project needs. It’s generally best to use a grogged clay body that has been optimized by the manufacturer rather than trying to add grog to clay yourself.

If you do want to create your own grogged clay mixture at home, start with a base clay and add 10-30% grog particles by volume. Make test tiles with different grog percentages to determine the ideal amount for your purposes. Consider using fire clay, stoneware, or earthenware as your base clay. Always wedge and knead the grogged clay thoroughly to evenly distribute the grog particles before using.

Grog Alternatives

While grog (crushed fired clay) is the most common additive used to temper and strengthen clay bodies, there are other natural materials that can be used instead:

Sand – Adding sand is an affordable and accessible alternative to grog for many potters. It performs similarly to grog in that it helps prevent cracking and makes the dried clay easier to handle. However, sand can make the clay body more prone to warping. The smooth rounded particles of sand also do not bond with the clay as well as the porous, jagged edges of grog.

Crushed shells – Seashells like oyster or clam can be washed, crushed and added to clay as a substitute for grog. Shell temper gives the clay added strength and creates an interesting surface texture. However the availability and sourcing of seashells depends greatly on one’s proximity to the ocean. Shells also burn out at lower temperatures than grog.

Other materials – Some potters also experiment with alternative temper materials like mica flakes, crushed bricks/tiles, glass cullet or fiber like straw. However, not all additives are ideal, and they can create technical issues with drying, firing and longevity. Test batches are recommended.

While viable alternatives exist, grog remains the preferred choice for many ceramic artists due to its ideal properties and performance in clay bodies. Grog strikes the best balance between strengthening clay and allowing it to retain plasticity and workability.

Notable Grogged Clay Examples

Grogged clay has been used for millennia to create functional and artistic works. Here are some notable examples of grogged clay artifacts and artworks from history and modern times:

Famous Archaeological Artifacts

Archaeologists have discovered many examples of ancient pottery made with grogged clay. This includes:

• The Venus of Dolní Věstonice – A ceramic Venus figurine found in the Czech Republic dating back 29,000-25,000 BCE. It is one of the oldest known ceramic artifacts.
• Native American pottery – Indigenous tribes across North America created grogged clay pottery for utilitarian and ceremonial purposes. These include the Catawba pottery tradition which dates back 4,500 years.
• Ancient Greek pottery – Much of the famous painted Greek pottery from the 8th to 3rd centuries BCE was made from grog-tempered clay.

Modern Grogged Clay Artworks

Many modern ceramic artists also work with grogged clay to create one-of-a-kind sculptures and functional ware, such as:

• Peter Voulkos – An American ceramic artist known for his abstract expressionist stacks and sculptural works made from grogged stoneware clay.
• Patti Warashina – A contemporary ceramic artist who creates narrative figurative sculptures from grogged porcelain.
• Lisa Orr – Crafts intricate porcelain vessels with grog additives that create earthy textures and patterns when fired.

The practice of grogging clay continues today both for utilitarian and artistic purposes across cultures worldwide.