What Is Pottery Slip Made Of?

Pottery slip, sometimes referred to as just “slip”, is a liquid mixture of clay and water that has a creamy consistency similar to heavy cream or yogurt. It is used in various stages of the pottery making process to join pieces of clay together, coat the surface of pottery items, and make pottery products more impervious.

The main ingredients in pottery slip are clay and water, but other additives are often used as well to achieve certain characteristics in the finished slip mixture. The specific uses for pottery slip include joining unfired clay pieces together, “slip trailing” decorative designs, coloring or altering clay surfaces, and as a preliminary coating before glazing.

In the following sections we will take a closer look at the ingredients, mixing process, uses, and firing considerations for pottery slip.

Clay

The main ingredient in pottery slip is clay. Different types of clay can be used, such as stoneware, porcelain, earthenware or ball clays (Janson Pottery, 2020). Ball clays and kaolins are often preferred because they have very small particle sizes that help keep the slip smooth and fluid. The plasticity and fineness of the clay particles allow the slip to be easily applied to greenware surfaces.

Clays used in slip need to have certain properties that make them suitable for this application. The clay body must have good suspension and viscosity so that the slip does not settle or become too thick over time. The maturation temperature of the clay should match the intended firing temperature for the ware being decorated. The clay also needs to have minimal impurities that could cause defects. The ideal clay particles for slip are very small, between .5 to 2 microns in size (Soul Ceramics).

The specific clay content affects the properties of the slip. Lower clay content results in a thinner, more translucent slip. Higher clay content increases viscosity and opacity. Most pottery slips contain around 20-30% clay.

Water

Water is a critical ingredient in pottery slip. It serves several key functions:

First, water acts as a dispersing agent, separating clay particles from each other so they can move freely in the slip (Source). This gives the slip its fluidity and viscosity.

Second, water content directly impacts the properties of both wet slip and fired clay. More water makes a thinner, more pourable slip. But too much water can prevent the clay particles from packing closely together, resulting in excessive shrinkage and cracking when firing. The ideal water content creates a smooth, creamy slip that fires well (Source).

The optimal water amount depends on the specific clay body recipe. Most potters test different percentages to find the ideal balance for their desired use. Getting the water content right is crucial for a high quality casting slip.

Deflocculants

Deflocculants are added to pottery slip to prevent the clay particles from clumping together. They help keep the clay particles suspended uniformly in the water, resulting in a smooth, liquid slip ideal for pouring and casting. Deflocculants work by causing the clay platelets to repel each other electrostatically, overcoming the natural van der Waals forces that cause the particles to cling together.

Some common deflocculants used in ceramic slip include sodium silicate, sodium polyacrylate, Darvan 7, and vinegar. Sodium silicate is one of the most widely used deflocculants. It is alkaline and provides a negative charge to the clay particles. Polyacrylates are synthetic organic polymers that create anionic chains that push apart the clay particles. Vinegar contains acetic acid and works well for deflocculating slip at low concentrations. The amount of deflocculant required can vary based on the specific clay body.

Adding too much deflocculant can cause over-deflocculation, where the slip becomes too thin and watery. Insufficient deflocculant leads to a slip that is too thick, prone to gelling, and unsuitable for pouring smoothly. The proper amount of deflocculant results in a slip with a creamy consistency that pours evenly for slipcasting and resisting sagging when applied.

Other Additives

Additional ingredients are sometimes used in pottery slip to modify its properties and achieve desired effects. These ingredients, known as additives or flux, are added in small amounts.

Some common additives include barium carbonate, which increases thermal expansion, and sodium silicate, which lowers maturing temperatures. Using additives allows potters to control the shrinkage, texture, color, and surface quality of finished pieces.

For example, vinegar or Epsom salts can be added to slip to increase fluidity and make application easier. On the other hand, calcium carbonate thickens slip. Boric acid lowers melting temperature. Overall, additives in slip have significant effects on the end product.

Mixing the Ingredients

Mixing pottery slip involves combining the main ingredients – clay, water, and deflocculants – in the proper proportions to achieve the desired consistency. This is a crucial step that impacts the performance and workability of the slip.

Slip is mixed using various equipment depending on the quantity being produced. Small batches can be mixed by hand or using a blender. Larger amounts require a pug mill or mixer to efficiently incorporate the ingredients. Industrial slip production utilizes blungers – high speed mixers designed specifically for clay mixing.

The clay and water are combined first. Enough water needs to be added to fully hydrate the clay particles. Deflocculants are then measured and poured into the mixture. Additional water may be added to achieve a smooth, creamy consistency. The ingredients must be thoroughly blended to ensure the deflocculants are evenly distributed throughout the slip.

Well-mixed slip should flow easily and have no lumps or dry pockets of clay. The ideal consistency is like heavy cream or melted ice cream. Adjustments to the amount of deflocculant or water can fine tune the viscosity. Careful mixing and testing produces a smooth, lump-free slip ready for use.

Source: https://www.youtube.com/playlist?list=UUG7kd_tehx9E7CN5OIQMCXg

Using Slip

Slip has several main uses in pottery, including joining, coating, and casting:

Joining: Slip is commonly used to join two pieces of unfired clay by applying it between the two surfaces and smoothing them together. The water in the slip helps the clay particles bond with each other to create a strong join after drying and firing.

Coating: Slip can be used to coat the surface of leather-hard clay pieces to create a smooth finish, hide surface defects, or prepare the piece for decorating. Multiple layers of slip are often applied to build up a layered surface.

Casting: Liquid slip can be poured into plaster molds to create pottery pieces with very thin walls. The plaster absorbs moisture from the slip, forming the clay layer against the mold surface. Excess slip can be poured out after the layer has formed.

For joining slip, a fairly thick consistency is desired to bridge gaps between surfaces. Slip for coating is thinner to spread smoothly and build up layers gradually. Casting slips must be thin enough to pour easily but thick enough to hold their shape.

Some clay bodies are more suitable for specific slip applications than others. For example, a slip high in ball clay will be very smooth and flexible when dry, making it good for joining but more prone to cracking if cast too thinly.

Firing

Firing is a critical step in working with pottery slips. When clay and slip are fired, several important changes occur that affect the properties and performance of the slip:

During bisque firing (the first firing cycle), the chemically combined water in the clay and slip is driven off. This causes the clay particles to fuse together, making the bisque stronger and less porous. As the clay vitrifies, the bisque becomes denser, less permeable to water, and unable to be returned to a plastic state with the addition of water. For slip, bisque firing results in a permanent, non-reconstitutable bonding to the clay surface. The bisque fired slip is also less permeable and adheres tightly to the clay body.[1]

Bisque firing also causes subtle chemical and physical changes in the slip that affect its working properties. The viscosity increases as water is driven off, and the surface tension decreases. These changes make the slip thicker and less likely to settle into fine details and textures. Proper bisque firing helps create a durable slip coating that is integrally bonded to the clay body.

Troubleshooting Issues

Here are some common problems that can occur with pottery slip, and tips for adjusting the ingredients to fix them:

Cracking

If cracking occurs during drying or firing, the slip likely has too much clay and not enough deflocculant or water. Try adding 2-5% more deflocculant and/or water to improve the slip’s workability. Slow down the drying process as well. According to ceramicartsnetwork.org, cracking is often caused by the slip drying too quickly.^[1]

Peeling/Flaking

If the slip peels or flakes off the piece after firing, there may be too much water and not enough clay or binder. Try reducing the water by 5-10% and add a small amount of CMC gum or binder like VeeGum T. Thorough mixing and careful application of the slip in thin layers can also prevent peeling issues.

Pinholes

Pinholes can occur from trapped air bubbles in the slip. Adding a deflocculant like sodium silicate helps release air bubbles. Slowly mixing the slip and letting it sit before use allows bubbles to rise. Applying multiple thin coats of slip rather than one thick coat can prevent pinholes too.

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^[1] https://ceramicartsnetwork.org/daily/article/10-slip-casting-problems-and-how-to-solve-them

Conclusion

In summary, pottery slip is an essential part of the ceramics process. It is composed primarily of clay and water, with small amounts of deflocculants and other additives mixed in. The ingredients must be combined in the proper ratios and mixed thoroughly to achieve the desired viscosity and workability.

Applying an appropriate slip is key for joining pieces of clay together securely and minimizing flaws in the final fired ceramic. Slip also allows decorative techniques like sgraffito and mishima. After applying slip, pottery must be fired properly to harden and stabilize the clay material.

With some care and practice using slip, ceramic artists can produce high quality pottery with clean smooth surfaces, crisp carved designs, and sturdy bonds between attached pieces. Mastering the use of slip allows for more creative freedom and artistic expression in pottery and ceramics.