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In the Studio: Underglaze Over Glaze

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A finished, fired vase showing the underglaze pattern over the clear glaze.

 

Traditional Zulu beer pots called ukhamba in the Zulu language are pots made from fine terra cotta and are produced by women throughout the KwaZulu-Natal region in South Africa. Pots of this kind are made for serving and drinking a beer that is brewed in larger, roughly made clay vessels. At the beginning of a meal or a visit, the beer pots would be passed around for everyone to drink from.

The pots were decorated with designs inscribed onto the surface, which was not only for beauty, but also to add grip. The smooth black finish was achieved by re-firing the pots in a dry grass fire, and then polishing them with animal fat (1).

Working with bone china, which gives the ultimate white translucent body, combined with my African roots, led me to create this series of vessels with flat and raised underglaze patterns. Having explored and experimented with various methods of surface decoration, the best result was achieved by decorating a glazed, fired, bone-china piece with a slip-trailed underglaze design.

Achieving a matte and sometimes textured relief design on the glossy white surface was the factor that bridges this work and the Zulu beer pots; however, they are the exact opposite of the traditional pots in color, material, and surface decoration. The painted design is a play of opacity against translucence with bone china being a body of great strength, yet delicate and fragile in appearance.


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Detail of a traditional Zulu beer pot showing raised texture.


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Casting a vase form in a two-part plaster mold using bone-china slip.


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Wet sanding the bisque-fired vase to remove seam lines from the mold.

Casting the Piece

A basic bone-china casting slip is poured into a plaster mold. A slightly thicker body, (3–4 mm in thickness) is cast (2) to prevent the shape from warping during the firing process, while maintaining translucency. It’s important to observe time accurately during the casting process to ensure even absorption and thickness, particularly because of the shape. Once the desired thickness has been achieved, typically after two minutes, the mold is tipped and drained. The rim is cut and then sponged smooth in the opposite direction of the cut to close up any possible cracks. The cast piece is left in the mold to reach leather hard, and once taken from the mold, allowed to fully dry before the first firing. To avoid breakage, no fettling, sanding, or sponging is done at this stage.

First Firing: Bisque

Standard bisque firing to 1832°F (1000°C) is done in an electric kiln. The bisque piece is sanded with 80-grit wet/dry sandpaper to remove the seam lines (3) and again with a finer grit wet/dry sandpaper to achieve a very smooth surface (4). Note: Always wear a properly fitted NIOSH respirator with the appropriate filters for the studio task at hand. Sanding a piece while wet minimizes the dust released into your studio.


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Final polishing of the bisque-fired pot to remove any texture and achieve a smooth surface.


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Applying an underglaze tree-branch pattern to the glaze-fired piece using a slip-trailing bottle.


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Developing the underglaze pattern further. It’s important not to touch the pattern, as the design smudges easily.

Second Firing: Glaze

The dried piece is dipped in a fritted glaze with a wide firing range of 2300–2336°F (1260–1280°C). Excess glaze is cleaned off of the base, the piece is signed, and when ready, fired to 2228°F (1220°C). A soak of 1½ hours is added, with the temperature rising to 2300°F (1260°C), then the kiln is left to cool at its own rate. The soaking period is necessary to obtain the white, translucent body.

Painting the Underglaze

The piece is washed and dried to prepare it for underglaze painting. Equal parts of jet black stain and a decorating medium (CMC gum) are mixed together. The gum adheres the stain to the glazed surface. To ensure even and intense color a little water is added, the mixture is ground with a spatula in a mortar bowl, and poured through a fine-mesh sieve into a slip-trailing bottle.

Using nature as my inspiration, my designs are very often based on bare tree branches (5 and 6), bamboo forests, leaves, and (more recently) vector swirls. To achieve a raised texture, a thicker mixture (heavy-cream consistency) of slip is applied, avoiding too thick a consistency that will cause a bubbling or crawling effect.

Third Firing: Fusing the Underglaze

I use one of two techniques to fuse the underglaze to the fired surface, depending on the desired results. For pots where I want the underglaze to be raised above the surface, a lower-temperature glaze firing is done at 1976°F (1080°C) to fuse the underglaze to the glaze. To achieve a bleeding effect in the underglaze decoration, the piece is fired to a higher glaze-firing temperature instead, similar to the second firing. Since the glaze fluxes and moves at this higher temperature, the underglaze melts into the surface, and moves with the glaze.

The pots are fired on an alumina-dusted kiln shelf. To prevent distortion, open bowls are glazed only on the inside and fired in a setter painted with alumina wash.

Rika Herbst is a studio potter living in Johannesburg, South Africa, where she operates Bohlokoa Studio. To see more of her work, visit https://bohlokoastudio.wordpress.com and www.rikaherbst.blogspot.com.

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