Porcelain can be worked like other clays, but when fired can reach a state of extreme whiteness, becoming vitreous and often translucent, similar to glass. When tapped on, it has a ringing sound like a bell. The whiteness of porcelain allows for coloring the clay itself, painting stains and oxides onto its surface, or glazing it with an outcome of brilliant and often dramatic colors.
Translucency is obtained under specific circumstances. High percentages of glass-forming ingredients like silica and feldspar in porcelain—in combination with thin walls and efficient firing—enhance translucency, but might also increase the difficulty of forming and shaping it. To some potters, translucency can add to the decorating process, but many choose a more plastic porcelain clay body with easier working characteristics.
Tips for Working With Porcelain
- Always wedge clay from a few hours to up to a day before using it to make sure that the water content is evenly distributed throughout the clay ball. This also helps to align the clay particles into a circle or spiral. Allowing the porcelain to rest after it’s wedged is important, because it tends to fatigue easily.
- Handle the clay as little as possible to limit it from getting fatigued. I manipulate the wedged ball into a pear shape and place it on the wheel head with the small end facing downward to take advantage of the circular movement that started forming during wedging.
- Pay special attention to centering and always cone the clay to get all the clay particles aligned. Many potters consider coning as just another way of wedging, but in many instances porcelain reminds me of the fairy tale of the princess that could not sleep with a pea under her mattress. The slightest little lump or unevenness can force you back to the beginning.
- Porcelain is normally thirsty, absorbing water quickly, and collapses easily when too much water is used. Even the more plastic porcelain clay bodies function better with less water. Adding a spoonful of vinegar in the throwing water gently deflocculates the clay and helps in lubricating it. Since porcelain shrinks more than other clay bodies, using less water minimizes problems related to shrinkage.
- Porcelain cracks easily for different reasons. Having uneven wall thickness and attaching pieces that have different moisture content will result in cracking. Cracks in the bottom of a form are usually caused by uneven thickness throughout and/or improper compression. Some cracks in the bottoms are caused by water left inside a form, which weakens the bottom. Cracks on rims are usually caused by too much pressure applied when trimming the foot. Using a foam bat on the wheel head while trimming absorbs the shock and eliminates most rim cracks. You can also prevent excessive pressure on fragile pieces by using sharp tools.
- Fill a spray bottle with water and use it to keep the pieces damp as long as is needed while you’re working on them. Be careful, as it takes some training of the hand and eye to prevent delamination of walls when spraying semi-dry pots to rehydrate them. Every porcelain body is different and needs to be evaluated separately.
- To be safe, never leave freshly thrown work in the open air longer than 15–30 minutes, no matter if you are working in a humid or arid environment.
- To keep unfinished pieces leather hard for weeks while you work on them, make a damp box by taking a plastic storage box and pouring an inch or so of plaster into the bottom. After it cures, dampen the plaster slab, and it will slowly release moisture into the air within the closed container.
When working with porcelain, there are specific things to bear in mind in the design stage that have a direct effect when firing a piece. Porcelain slumps easily, so avoid large horizontal areas that are not supported. Wide-domed lids, wide-rimmed bowls and plates, handles, and spouts should have an angle of at least 45° built into the design. Some pieces will even split or separate during the final firing if unsupported. I use different systems in the kiln to support my work. It’s an ongoing process of planning and improvisation, since my work is one of a kind and using supports only works if the area to be propped up is unglazed.
Porcelain utilitarian work is normally the same thickness or slightly thinner than stoneware, but it’s still important to be aware of possible slumping and to design works accordingly.
Because porcelain fluxes and starts to melt somewhat at its peak temperature, any supportive materials should be dusted with a refractory material such as silica or calcined alumina to prevent them from sticking to kiln shelves or stilts. The same refractory materials are necessary to prevent lids from sticking to pots. I found that regular kiln wash is not enough to prevent my pots from sticking, so I wet each piece and dip it in a thin layer of silica that I can wipe off after the piece is safely fired.
Dimples in fired porcelain may be caused by a very open, less plastic clay body or by gases that are either created by burn-off from plasticizers or other organic materials that might be trapped in the clay. Slow firing, soaking bisqueware for 30 minutes, and a soak hold when the final glaze firing temperature is reached are all precautions you can take to allow these gases to escape. For a very open clay body, it’s sometimes useful to dampen the pieces slightly before glazing. Be aware that if the piece is too damp (which happens quickly with thin work), it can’t absorb as much water from the glaze solution, and so the glaze coating will be too thin.
If you’re having problems with cracks forming during the firing, they can be prevented by down firing the kiln, which helps to cool pieces (especially thin ones) slowly.
This article was excerpted from its original publication in the May/June 2009 issue of Pottery Making Illustrated. You can view the entire article in the PMI archives, visit /pottery-making-illustrated/search-pottery-making-illustrated?Keywords=&DateFacet=&SortOrder=DESC&TypeFacet=PotteryMakingIllustrated.
Antoinette Badenhorst has worked with translucent porcelain since the early 1990s. Learn more on her website at www.porcelainbyantoinette.com.