How Is A Slip Made In Pottery?

A slip is a liquid clay mixture used in the production of pottery and ceramics. The clay mixture is thinned with water to create a pourable consistency that can be used to coat pottery items before firing.

Slips serve several important purposes in pottery making. They provide an impermeable surface coating to the porous clay body, which makes the ware less permeable and more resistant to staining. Slips also even out the colors on multi-colored clays and help join pieces of unfired pottery together through “slip and score” techniques. After firing, colored slips can create decorative effects on the pottery surface.

Clay

The type of clay used is an important factor in pottery slip making. Clays can be categorized into three main types: earthenware clays, stoneware clays, and porcelain clays. Earthenware clays are lower fired clays that mature at lower temperatures, typically between 1500-2200°F. They tend to be quite porous. Stoneware clays are higher fired clays, maturing between 2200-2400°F. They are less porous than earthenware clays. Porcelain clays are the highest fired clays, maturing around 2300-2600°F. They become vitrified and non-porous at high temperatures.

For pottery slips, stoneware and porcelain clays are most commonly used as they become denser and less porous at high temperatures. Key properties needed in the clay for good slip making are plasticity, a fine particle size, and low shrinkage. Plasticity allows the clay particles to remain suspended in the water medium of the slip. A fine particle size leads to a smooth, high quality surface when applied. Low shrinkage minimizes cracking and defects. Many potters use mixing different types of clay to achieve the desired properties for their slips.

Other Ingredients for Making Slips

In addition to clay, there are other important ingredients that go into making slips for pottery:

Water – Water is crucial for mixing slips and allowing the clay particles to flow smoothly. The amount of water controls the viscosity and thickness of the slip. Too much water will make it runny, while too little can make it thick and hard to pour.

Deflocculants – Deflocculants are chemicals that prevent clay particles from clumping together. This allows water to fully penetrate and separate the particles so the slip remains fluid. Common deflocculants used are sodium silicate and Darvan.

Coloring Oxides – Small amounts of metal oxides can be added to tint the slip different colors. Common coloring oxides include cobalt for blue, chromium for green, iron for reddish hues, and copper for greenish blues. The oxides must be measured carefully to achieve the desired shade.

Mixing

The process for mixing the clay and other ingredients to make slip is crucial for achieving the right consistency. Most potters will mix the ingredients by hand to get a feel for the texture and make adjustments as needed. It’s important not to overmix or undermix.

Start by gradually combining the dry clay powder with water in a large bucket or container. Use your hands to knead the mixture, while slowly adding more water until it forms a smooth, liquidy paste. The ideal consistency is like heavy cream. Make sure there are no lumps or dry pockets of clay.

You may need to strain the slip through a fine mesh sieve to catch any remaining gritty bits. Check the viscosity by dripping a small amount from your fingers. If it’s too thick, stir in more water. If it’s too thin and runny, carefully mix in more clay powder.

Getting the perfect creamy, liquid texture takes practice and a delicate touch. Test batches on a small scale before mixing large amounts. Well-mixed slip should flow smoothly for pouring and hold its shape when drying.

Casting and Pouring the Slip

Once the slip has been properly mixed and strained, it is ready to be cast or poured into molds. There are a few techniques potters use when casting slip:

Pouring into Molds

Potters often pour liquid slip into plaster molds to form the basic shape of a piece. The porous plaster absorbs water from the slip, forming a layer of stiff clay on the interior walls of the mold. Once the desired thickness is achieved, the excess slip can be poured out of the mold. The moist clay will hold the shape of the interior as it continues to dry and stiffen.

Brushwork Techniques

Another option is to use soft brushes to paint layers of slip onto the interior walls of a mold. This allows the potter to build up thickness slowly and evenly. Different brush shapes and sizes can create unique textural effects on the surface. The brushwork method provides the artist with more control over the form and decoration.

Drying

After the slip has been cast and poured into molds, the next step is drying. Proper drying is crucial for avoiding cracks and ensuring the structural integrity of the finished ceramic piece.

Drying time can vary substantially based on factors like the size and thickness of the piece, the clay body ingredients, and the drying environment. Thicker pieces require more extended drying times, sometimes upwards of several weeks. The environment should be kept at a consistent warm temperature with some airflow to facilitate even drying.

As slip dries, the moisture content is reduced. This causes shrinkage, usually around 8-12%. Care should be taken during drying to avoid uneven shrinkage which leads to warping. Sometimes pieces are dried slowly with periodic rewetting to control the shrinkage rate. Once bone dry, the pieces are ready to be fired.

Firing

Firing is a critical step in creating pottery that hardens the clay and sets the color. When firing pottery, most clays reach temperatures between 1,800-2,400°F. However, different types of clay require different optimal temperatures:

• Earthenware clays are fired between 1,800-2,000°F.
• Stoneware clays are fired between 2,200-2,400°F.
• Porcelain clays are fired between 2,400-2,600°F.

Firing has a significant impact on the color of the finished piece. Earthenware fires to a natural red or brown. Stoneware emerges gray, beige, or ivory from the kiln. Porcelain becomes pure white after firing. The clay can also be stained with mineral pigments before firing to create bold colors. Firing pottery is an intricate art that requires precise temperatures and timing to achieve the desired hardness, color, and finish.

Glazing

Glazing is an important step in creating pottery that gives the finished piece a smooth, glassy finish and can add color and decoration. There are many options when it comes to choosing a glaze.

Most glazes consist of a mixture of silica, fluxes, and alumina. Silica, often in the form of silica sand, quartz, or flint, forms the glassy matrix of the glaze. Fluxes like soda ash or potash lower the melting point of the glaze so it melts and fuses with the clay body during firing. Alumina, usually in the form of clay, helps prevent running and promotes an even finish. There are natural glaze ingredients like wood ash and feldspar, but many glazes today use refined chemicals.

Glazes come in various colors from bold primaries to neutrals like white, brown, and black. Special effects glazes feature crackles, crystals, mattes, and more. Luster glazes have metallic finishes like gold, silver, copper, and bronze. Underglazes are applied before firing and overglazes are applied after firing to a finished piece.

There are a variety of techniques to apply glaze. Dipping coats the entire piece evenly. Brushing allows control for covering specific areas. Spraying through an airbrush or spray gun gives an even, thin coat. Sponging or dabbing can create textured effects. Glazes fire and mature at different temperatures, so it’s important to use glazes meant for the clay body and kiln firing temperature.

Final Firing

The final firing of the pottery is one of the most critical steps in achieving a beautiful finished product. During this firing, the glaze will melt and fuse with the clay body to create a glossy, decorative surface. There are several high fire glaze techniques that can be used to create different visual effects:

Crystallization – Certain ingredients like zircon and tin oxide will crystallize as the glaze melts, creating speckles and stripes. Crystalline glazes require fast firing up to around 2,300°F.

Phase Separation – Some glazes will separate into two distinct phases during firing, one silica-rich and one flux-rich. This can create visible layers or even crawling effects. Phase separation is encouraged by wide temperature fluctuations during the firing cycle.

Flashing – Applying multiple layers of contrasting colored glazes can result in flashing where the lower layer breaks through the outer layer in places. Flashing is achieved by firing rapidly to a peak temperature.

Texturing – Certain materials can be added to high fire glazes to create visual or tactile surface textures. Options include grog, silicon carbide, wood ash, sawdust, fiber, or even salt.

Special Effects Glazes – There are many commercially available high fire glazes designed to produce dazzling effects like crackling, crawling, spotting, blushing, bronzing, and more. These glazes are formulated to react in specific ways during firing.

Conclusion

Making slips for pottery involves several key steps. First, an appropriate clay body must be chosen and combined with water and other ingredients like fluxes or clays to achieve the desired consistency and working properties. The slip is then mixed thoroughly before being cast into plaster molds or poured directly onto leather-hard clay or bisqueware. Newly poured slip requires time to dry slowly and evenly to prevent cracks and warp. Once dry, the slip-decorated ware undergoes a first firing to harden and stabilize the clay body. At this stage, decorative glazes may be applied for color and effects before a final high-temperature firing fuses the glaze permanantly to the clay.

The slip process allows for immense creativity in pottery. Slips can be colored with stains, textured with additives, layered, carved, resist-decorated, and more. A vast array of surfaces and patterns can be achieved by varying the slip ingredients, application techniques, and firing methods. With so many possibilities, potters can create truly one-of-a-kind pieces using time-honored slip decoration.