Bone china is a very seductive material to work with, possessing qualities of intense whiteness, translucency, and strength. I think of it as a very single-minded clay, which forces the maker to work with clarity and precision. Its technical inflexibility and idiosyncratic making and firing characteristics might easily be a deterrent to investigation, however I consider these restrictions and limitations a challenge to creativity and working methods.
I have adapted the industrial techniques of mold making and slip casting associated with bone china for my studio production. My models, or positives, are made by turning plaster on a lathe (figures 1 and 2). Any twists and facets are introduced by hand carving using a Surform and a series of hacksaw blades. After the model is finished a plaster mold (negative form) is made for slip casting the bone china (see figure 5).
My particular interests lie in the translucent properties of this material and the differing degrees of luminosity possible. In order to enhance these qualities in my work, I have developed three variations of decoration that aim to exploit this characteristic.
The first technique focuses on specific points of translucency by employing a layering and slicing technique. I use commercial body stains to color the bone china casting slip (I add 2–20% body stain per pint of prepared liquid slip) and use a technique of multiple-layered casting with the first layer being poured into a mold then emptied immediately (figure 3).
As soon as the sheen has disappeared, a different colored layer is added (figure 4) and so on until the desired number of layers has been achieved (normally three or four but more recently I have managed five—any more than that and the first layer can pull away from the mold wall resulting in no porosity for the subsequent layers).
The cast is removed from the mold (figure 5) and, when bone dry, subtle facets are made, using a sharp razor-blade edge, (figure 6) on the curved outer surface of the pieces. This involves paring back through three or four coats of different colored slips to reveal the underlying and increasingly transparent layers.
The piece is then fired to a low temperature of 1832°F (1000°C) after which it’s wet-sanded using 320-grit wet/dry paper. This gives the work a smooth cut-glass appearance. The ware is fired a second time to a higher maturing temperature of 2300°F (1260°C), with a 1½ hour soak. The soak ensures even heat throughout, resulting in optimum translucency. After the second firing, it’s polished again with wet/dry paper to achieve a satin-matte appearance.
Layering and Incising
The second technique uses the same layered casting process as above, however, the cast is removed from the mold while still quite soft. A loop tool is then used to incise or gouge through the layers (figure 7). This has the same effect of revealing the underlying colors until the last, or interior, layer is exposed. Great care is needed to avoid piercing through the piece as it’s particularly vulnerable at this stage and it’s also important to complete the decoration in one sitting as a rhythm needs to be established with pattern and depth.
Water Erosion (Etching)
The third process, the decorating process of water erosion (sometimes called water etching), focuses on the entire surface of the piece using a single cast and no color. The variations of translucency are achieved by painting an acrylic medium, I use Liquitex, onto the raw cast (figure 8). This acts as a resist when the piece is wiped down with a damp sponge until a relief pattern is revealed (figure 9).
Special Casting Concerns
Distortion and warping are the main disadvantages when casting with bone china. This is due to the fact that the memory present in this high-firing clay exaggerates any fault or imperfection. An inadvertent knock, or nudge, at the damp stage will reappear after the firing as will any seam line on a multi-piece mold, even if the piece has been sanded to remove it. This phenomenon is caused by the mica particles in the clay aligning themselves wherever there is an irregularity or gap, however small, in the mold. This alignment creates a thicker, or raised area, which cannot be avoided in high-firing clays.
Ways to avoid the seam lines showing is to either, eliminate them altogether by designing a piece that will release from the mold directly, as in the case of a drop-out mold. Or, for a piece requiring a multi-piece mold, introduce twists and facets onto the model where a seam line could be hidden.
Another method is to use decoration as a decoy, for example, using a transfer or painted line to hide a seam.
In cases where the bisque firing is much higher than the glaze firing, as is normally when working with the case with bone china (between 2246–2300°F (1230–1260°C)), it’s essential that the clay ware receives the maximum support during the firing process, particularly as most of the movement occurs when the kiln reaches its highest temperature. Due to the high distortion factor encountered with bone china, if the shape is to be controlled, any open form requires a setter of some kind.
If the form is circular at the top, it can be placed upside-down on a refractory clay ring, or setter, with an alumina wash to prevent sticking in the firing.
If the form is irregular at the top edge an individual setter needs to be made. In simple terms, this is an item that resembles a lid and is cast separately (the preferred making method for bone china). Its function is to contain the top edge, or rim, of a form during firing (figure 10) and is discarded afterward. If the piece has been successfully fired with no distortion, the ware can be glazed and fired at a lower temperature of 1832–1976°F (1000–1080°C), if necessary.
As previously mentioned, the historical development of bone china’s firing cycle was due, to economic and technical constraints. This resulted in the norm being a high bisque firing between 2228–2300°F (1220–1260°C) with a low glaze firing between 1832–1976°F (1000–1080°C).
Given that there is little silica present in bone china, and it’s usually made thin, it’s unusual in that it can be fired quickly up to and over the quartz inversion temperature of 1063°F (573°C). It’s possible, in fact, to fire it to maturity in two hours (as is sometimes done in industry). However, the deterioration to the kiln’s elements would discourage the studio ceramic artist to do so.
After several firing experiments (in an electric kiln), I have found a reliable high bisque-firing cycle for Valentine’s bone china casting slip (produced by Valentines Clay Products, Stoke-on-Trent) as follows:
- Controlled climbing of 212°F (100°C) per hour to 1022°F (550°C).
- Spyplugs in at 1022°F (550°C) then kiln climbs at own rate to 2246°F (1230°C).
- Soaked at this temperature for 1½ hours (where temperature rises to 2300°F (1260°C).
- Cooling at own rate and kiln opened at approximately 302°F (150°C).
Experiments with the firing cycles proved the following:
Fired to the lower end of the range 2280°F (1220°C) meant the forms retained their shape very well, due to very little or no movement at the top temperature. However, there was also very little translucency, if any. The body had a slight pinkish hue, which is a reliable indication of underfiring and one to be avoided particularly if glaze is required, as crazing will occur.
Fired to the higher end of the range 2300°F (1260°C) and above distortion was dramatically increased (even while using setters). Translucency, however, was extreme. Small, flatter pieces could be successfully fired if packed in alumina to prevent distortion.
Soaking, or holding, the temperature at a given point for a given length of time, is the key to obtaining a mature, white, translucent body.
Bone china has a low-glaze cycle following low-fire clay temperature ranges. The pieces need to be warmed up, usually to about 212°F (100°C) prior to glazing. This helps to evaporate water from the glaze slurry as it’s applied to the piece. When glazing, I pour glazes on the insides of the ware to line them and if I glaze the outsides, I spray them. Use a thicker application of glaze with a drop or two of gum Arabic added to the glaze to help with adhesion, as the body will have vitrified during the high firing.
Sasha Wardell is a ceramic artist, author, and workshop instructor. Wardell is the author of Slipcasting, co-published by A&C Black, London, England, and University of Pennsylvania Press, Philadelphia, Pennsylvania, 2007. To see more check out www.sashawardell.com.
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