Composing a Workable Parian Clay Body Antoinette Badenhorst

Appears in the November 2020 issue of Ceramics Monthly.

In the early 1840s, English potters developed a type of porcelain that became vitrified and marble-like without glaze, which allowed for the reproduction of otherwise large and heavy sculptures. This was developed to imitate bisque-fired porcelain from Sèvres, which was in essence fired but unglazed porcelain. Prior to this invention, all the manufactured modeling of ceramics was done by hand.

This new, warm-white clay body was self glazing. Claimed as a development by the English firm Copeland & Garrett, it was introduced as an affordable way for people to bring smaller-scale copies of the large marble statues and busts found in stately homes into their own homes. Minton named it parian ware, after the Greek island Paros, where fine white marble was used as sculptural material, while Wedgewood named it Carrara, referring to Italian marble. As with so many other controversies, more than one company in the UK claimed to discover parian ware. Although originally used for statues, parian clay was also used for dinnerware, especially jugs and casserole dishes, most forms heavily embellished with sculptural elements.

Early Development of Parian Ware

The early Chinese potters would keep their secret for translucent porcelain for 2000 years. The European search for porcelain first led to materials used to obtain translucency at lower temperatures. The common denominator in all these porcelains from those early days were the fluxes used.

The original quest to develop porcelain similar to the ancient Chinese porcelain led to European versions of hard-paste porcelain, bone china, soft-paste porcelain, and eventually also parian ware. Just as with all the other porcelains, various formulas were developed.

The main difference between the formerly popular bisque-fired porcelain and parian ware is that the latter is a self-glazing porcelain, while the bisque-fired porcelain stayed porous, matte, and non-translucent. In order to self glaze, a glass-like body needs to be developed with just enough clay to make it stand independently.

As with the many versions of hard-paste and soft-paste porcelains, parian ware researchers tried to include frit, various carbonates, ball clay, and silica into the clay bodies. All these tests led to a medium with 30–65% of feldspar added to kaolin with a small percentage of silica. These feldspar additions gave parian ware its distinctive hardness, smooth surface, and the semi-opaque shadows, similar to marble. Not possible with earlier porcelain bisque-fired clay recipes, the goal with these new clay bodies was to obtain a pale creamy surface, fine textural detail, and translucency. Parian ware’s advantages as a self-glazing material also included keeping production costs low.

Developing a Parian Clay Recipe

I wanted to develop a parian clay body based on materials I had in my studio. I started by researching fluxes to lower the vitrifying temperature of hard-paste porcelain to cone 6 and to obtain translucency. I was also looking into which kaolin to use. The goal was to find a combination of silica, flux, and kaolin that would enhance translucency. My focus was not necessarily on lowering the firing temperature, but rather adding enough flux to obtain a clay body that has a self-glazing effect once it is fired to vitrification and is translucent with a cream, gray, or white color. With so many fluxes to choose from, I used one from each category of fluxes and added it in different proportions to silica and various kinds of kaolin to see if I could obtain a shiny, glassy clay object.

The Parian Clay Body Tests

During my research about parian clay bodies I learned that although frit is sometimes used, feldspar is preferred to use since it normally contains a good percentage of alkali and melts well at midrange temperatures, and normally contains a percentage (15–25%) of alumina, which controls viscosity during the glassy phase of the melt.

A lower percentage of feldspar with higher clay and silica content makes the porcelain more refractory, which also moves the character of the clay body toward a regular hard-paste porcelain. When the refractory materials decrease and feldspar (especially sodium feldspar, which begins to melt at 1922°F (1050°C)) or frit increases, the firing temperature needed to achieve vitrification lowers. That said, when more refractory materials are used, the clay is more plastic.

The fluxes used were nepheline syenite, which is often used in cone 6 porcelains; Minspar 200 feldspar, which has a wide melting range from cone 6–9; and Custer feldspar, which is somewhat contaminated with iron, but a good flux source in general. I began testing with 40% feldspar and gradually increased the amount until it made up 70% of the recipe, which is the presumed composition found in a parian clay body. Although both slip-casting and hand-forming techniques are possible, the latter is harder to execute with high percentages of feldspar. It is possible with the addition of plasticizers; however, pieces will need to be supported during the firing, since the clay body goes into a complete liquid phase, resulting in a full vitrification and translucency at temperatures between 2102°F–2282°F (1150°C–1250°C).

Although only 10% pure silica is used in a clay body that normally contains 30% or more silica, the feldspar and clay supply additional silica. Feldspar contains anywhere from 60–70% silica in its composition, which contributes to a hard, durable surface when vitrified.

I decided against using a ball clay because of its high silica content, which would have exceeded the amount needed (that is already provided by the fluxes). In the test recipes where I used 40% silica, it may be useful to perform additional tests to lower the percentage of silica and add some ball clay; this may help to make a more workable body. On the other hand, it may negatively influence translucency and the amount of sheen from self glazing.

In my tests, I used three types of kaolin: halloysite, which is the purest available and the least plastic; china clay, which has a very small amount of impurities and is somewhat plastic; and EPK kaolin (Edgar Plastic Kaolin), which has more iron and opacifiers present in the body and is also the most plastic of the three. I kept the kaolin at 20% of the recipe every time.

I did not make tests with bentonite. Adding bentonite and other montmorillonite materials may help to improve plasticity as well keep the materials suspended in a slip casting. Testing would reveal any adverse effects on translucency or sheen.

The Clay-Forming Experience

Using a parian-ware clay body reminded me of making mud pies as a child. The clay has very limited workability due to its extremely open and short character. When I added a drop too much water, it almost instantly became muddy. In contrast, when I dried it out, it became instantly non-pliable.

The higher proportions of feldspar in the body composition left the clay with very limited workability in comparison with regular hard-paste porcelain and rendered the clay more suitable for casting.

My Process

I do not have plaster slip-casting molds in my studio, so I rolled the clay into a slab between two plastic sheets, partially to prevent to the wet clay from sticking to my roller and partially to keep it pliable enough to form a thin sheet. It did not allow for much smoothing out at all. I cut shapes from the thin slab and draped them over a small hump mold. When the cut slabs were almost dry, I carved leaf designs into some samples to obtain some texture. I also tested the clay in small sprig press molds that added texture similar to Wedgwood sprigs.

Fired Observations

I fired the parian ware tests on a bed of silica in my kiln to cone 9. The various color and translucency results were pleasing, but in contrast to all other porcelains, any unsupported areas slumped.

I obtained pure white to grayish-white colors. The most pleasant surprise was the beautiful, very fine, speckled gray-white with the EPK kaolin sample (Parian Test 1). Another interesting result was that in the areas where I had difficulty smoothing out the clay, the piece fluxed smoothly and also obtained a glassy appearance. The only test sample that could not handle being fired to cone 9 and began to bubble was the clay body with 70% Minspar 200 feldspar in combination with china clay (Parian Test 6). The area that sat on the silica was smooth, but had a dull appearance. Some sprigs lost their texture, which made me believe that I pushed the clay to its firing limits.

Although most samples obtained some sheen in the firing, they had a gritty feel, which is understandable, because of the large particle size of porcelain. It would be interesting to find the perfect balance where more clay/silica and flux may help completely eliminate the gritty feel.

Traditional parian ware was once fired and developed its own glassy sheen on the surface and therefore it should be fired in a similar way as bisqueware. That said, objects should not touch each other without a refractory barrier such as silica or alumina, otherwise objects may fuse together, similar to glazes.

Having exactly the same raw materials as other types of porcelain—namely kaolin for plasticity, feldspar for fluxing, and silica as filler for the structure—the dramatic increase in feldspar to make a parian-ware clay body raises the melting characteristics in the kiln. This results in formation of a complete liquid phase at the eutectic temperature, which can cause deformation. It also increases the degree of vitrification and translucency at a lower temperature. Therefore, I think tests at lower temperatures may result in more stable objects, which may still be translucent, but may also retain surface textures better.

Parian Ware by Christine Cherry and Joanna Świerczek

Christine Cherry (a potter, designer, and primary school teacher, who started her career as a student at Stoke-on-Trent over 30 years ago) found an old parian-ware recipe and altered it to develop the recipe she is currently using for slip casting. The process begins when she paints layers of color onto the surface of the mold, followed by casting the final layer. She carves through the layers at the leather-hard stage to reveal the embedded colors. After slow firing the piece, Cherry polishes the surface.

She considers her clay body to be more of a semi-porcelain like those used in the 19th century. When fired to temperature, her clay body is fully vitrified and resembles marble in texture. It has a fine, granular surface, is creamy in color, and is beautifully translucent. She has to fully finish her vessels in their greenware state as she once fires the pieces straight to vitrification. The delicate, leather-hard to bone-dry clay needs to be handled carefully since any distortion would worsen during firing.

I came across Joanna Świerczek’s (a Polish artist working part time in porcelain who creates sculptural and hand-painted porcelain objects and jewelry) work while she participated in one of my online workshops. It turned out that she was using a commercial parian clay body from Scarva, a UK-based pottery supplier (www.scarva.com/en/Clays/cc-29.aspx), and she asked me a few questions about firing it.

Świerczek creates jewelry with flowers. She attaches hand-formed petals to a slab also made from the same Scarva clay, often embellishing parts with more slip. She finishes the work with a thin layer of transparent glaze. Once fired, she refires the objects with Unidekor Botz ceramic paints, an overglaze or china-painting medium from a UK ceramic supplier (www.greatart.co.uk/botz-unidecor-ceramic-paints.html). She uses techniques to obtain watercolor effects. She also uses gold and silver lusters and glass beads to finish her work.

the author Antoinette Badenhorst is a ceramic artist specializing in porcelain. She is also ceramic instructor and a technical pottery writer. Her sculpted porcelain vessels are collected in museums and private collections across the world. She teaches hands-on ceramics workshops, as well as online through TeachinArt.com. Antoinette mentors artists of all skill levels and in all ceramic fields. To learn more, visit PorcelainbyAntoinette.com.