Forming a Foundation

Bowl, 7 in. (18 cm) in diameter, slip-cast fritware, slip resist, sgraffito, alkaline glaze, terra sigillata, fired to cone 04 in an electric kiln, 2016.

Our life and values are strongly rooted in self-reliance, so when Kevin and I began working in clay we resisted investing in studio equipment. Instead, we invested time and spent a year and a half developing a clay body, slips, and glazes that would later form the foundation of our work.

Looking to focus on creating highly functional tableware, finding a dense, non-porous clay body was a priority. Low-fire ceramics appealed to us from an economic perspective but unfortunately didn’t offer the physical properties we desired. This prompted research into fritware and its ability to achieve high vitrification at low temperatures. Discovering Abu’l-Qasim’s 13th-century treatise on ceramics formed a base to start experimenting with formulas.

Fritware originated in Persia in the 9th century as an attempt to emulate porcelain. It contains high levels of silica and fritted glass, making a clay body with very low plasticity. This property lends itself well to slip casting, which relies on sedimentation, whereas techniques using plastic clays rely on compression.

Drawings transferred to hardboard to be cut out with a jigsaw.

Armature assembled from hardboard and wooden spacers attached using hot glue.

Armature packed with clay and modeled with hardboard template.

Another property of fritware is its high shrinkage rate upon maturing, which can result in glaze faults, such as shivering. To overcome these challenges, the use of a glaze calculator helped us to match the coefficient of expansion between our clay body, decorating slip, and glazes.

Working true to his nature, Kevin builds pots. His background as a journeyman steel fabricator informs his approach and although he now works at a much smaller scale, each form is designed to the millimeter and begins with a detailed set of blueprints.

Pouring casting slip into the finished mold.

Attaching the foot onto the bowl using casting slip as a joining slip.

Cutting paper resist illustrations with a jeweler’s saw.

Modeling

Technical drawings of the bowl are transferred to hardboard and cut out with a jigsaw (1). Hardboard, which is made from compressed sawdust, tends to swell when it becomes moist. To minimize any swelling, we seal the hardboard with polyurethane. Once dry, an armature of the model is constructed by centering the three pieces of hardboard and spacing them apart with wood (2). After the pieces are aligned, they are attached using hot glue. The armature is then packed with clay and the clay is shaped with a hardboard template to achieve the desired profile (3). Once the model is complete, a one-part plaster mold is cast.

 

Brushing white decorating slip over the paper resists.

Removing paper resists.

Punching holes through tracing paper to create an image transfer.

Slip Casting

Plaster molds work by absorbing moisture from casting slip, resulting in a layer of clay forming on the inside wall of the mold. Casting slip is liquid clay that is held in suspension by a dispersant, such as sodium silicate, which acts as a deflocculant. Deflocculation is a state of dispersion of a solid in a liquid in which each solid particle remains independent of adjacent particles. It is beneficial to have a clear understanding of rheology—the branch of physics that deals with the flow of matter—when working with slips, glazes, and terra sigillata.

Casting slip is poured into the mold and allowed to sit until it has reached a desired thickness, then poured out (4). This process is repeated with white slip, resulting in an even coat on the inside of the bowl. Any excess casting slip is trimmed away from the top of the mold using a utility knife. The piece remains in the mold until it has reached leather-hard consistency and can be handled without distorting. It is then turned out of the mold onto a bat. At this point we attach the foot to the body of the bowl using casting slip as a joining slip (5). We then tent the piece under plastic and it is left to continue drying overnight.

Pouncing powdered graphite through an image transfer.

Carving through the white slip using the graphite transfer as a guide to create a sgraffito pattern.

Bowls with burrs from the sgraffito process removed, sanded, and bisque fired to cone 09.

Brushing on terra sigillata.

Surface Decoration

Although fritware was traditionally made using a white clay body, we opted for a red body that, when combined with white slip resist and sgraffito, would result in a rich, multi-layered surface. While designing our surface patterns, we were thinking about how slip resist could be used to suggest layers in an illustrated narrative. The graphic contrast of the silhouettes creates an interesting surface that can be seen from a distance, while the intricate sgraffito lines must be explored up close. The composition of elements is approached intuitively for each piece at the time of application. This creates a constantly changing and evolving surface, which is rewarding both in practice and result.

To create our paper resists, we sandwich and staple approximately 24 pieces of newsprint between card stock and stack cut them using a jeweler’s saw (6). Once the bowl is leather hard, the paper resists are dipped in water and gently applied to the surface using a moist sponge to smooth over any wrinkles. White decorating slip is then brushed over the entire surface (7). Once the wet sheen has disappeared from the slip, the resists are removed (8). Paying close attention to composition while applying the resists, our goal is to create interesting negative space or movement over the surface. Keeping the bowl turned upside down supports the rim and prevents distortion while being re-wet and handled.

The piece is set aside to dry until it has reached a point where, when sgraffito lines are carved into the clay, the burrs curl and fall away on their own. Carving when the piece is still too wet will result in lines that are deep with edges that flare and are difficult to clean. Alternately, waiting too long and dry carving results in a poor, chattered line quality.

To help compose the sgraffito illustrations on our surfaces we create pierced image transfers out of tracing paper (9). This is especially useful with the bowls as it can be a challenge to visualize and plan a narrative upside down. Placing the transfer gently against the surface of the bowl, we pounce powdered graphite over the image transfer (10). The graphite dots left behind create a guide to follow while carving (11). The tool we use most often for sgraffito is a dental pick that has been straightened into a stylus. Once the bowl is bone dry, the burrs of clay left from the sgraffito are removed and the rims are gently sanded to round any sharp edges.

Bowl, 7 in. (18 cm) in diameter, slip-cast fritware, slip resist, sgraffito, alkaline glaze, terra sigillata, fired to cone 04 in an electric kiln, 2016.

Glazing and Firing

Due to the early maturing temperature of fritware, we soft bisque fire our pieces to cone 09 (12). After the bisque firing, we brush terra sigillata thinly over the bottom section of the bisqued bowl to add rich color and to seal the bare clay (13). A clear glaze is applied to the inside of the bowl and, once dry, it is dipped in an alkaline polychrome glaze. It is then fired to cone 04 and held for an hour and a half soak to give the glaze opportunity to flow and heal imperfections.

the author Anna and Kevin Ramsay work from their home studio in Nova Scotia, Canada. Learn more about their work at www.ramsayceramics.ca and follow them on Instagram for more process photos @ramsayceramics.


 

Subscriber Archive Article

You can read Zygote Blum’s article “Paper Stencils” from the November 2012 issue, here: https://ceramicartsnetwork.org/wp-content/uploads/sites/6/2017/01/Techno-File-Chrome.pdf.

Comments

Comments are closed.

Enter Your Log In Credentials
This setting should only be used on your home or work computer.

Larger version of the image

Send this to a friend