Published Sep 22, 2021
Historically, burnishing clay was a method used by early potters to make their pottery more watertight and sanitary. Nowadays, most potters turn to glaze for that purpose. But many choose to finish their work by burnishing because of the subtle, earthy beauty a burnished clay surface possesses. A burnished pot also has a soft, tactile quality all its own. It is hard to resist picking up and handling a piece of burnished pottery.
In this post, an excerpt from the book Low Firing and Burnishing, Sumi Von Dassow explains the basics of burnishing clay, from the tools to use, to a couple of basic techniques. - Jennifer Poellot Harnetty, editor.
Potters who burnish are often asked, "what glaze is that?" by curious admirers of their work. Non-potters naturally assume that all pottery is glazed, and the glossy surface of a burnished pot seems like a different and intriguing sort of glaze. Though glazed pottery can be brighter and more colorful, a burnished clay pot has a glow from within and a warmth that glazed pottery doesn't possess. The difference which non-potters sense without knowing it - and which fascinates potters - is that the surface of a burnished pot doesn't wear a coat hiding the clay itself from view. Glaze is glossy and reflective, but the reflecting surface consists of a millimeter or so of glass covering the clay. Underneath this layer of glaze the rough stony clay is always perceptible, even if not always visible. Burnished clay can have a surface just as glossy and reflective as any glaze, but behind this glorious surface there is no hidden roughness. Even the feel of a burnished pot is seductive: while a glazed pot feels hard and cold, burnished clay seems warm and almost soft to touch.
There are two methods of burnishing clay: rubbing the clay with a polished stone or other smooth object, and coating the pot with terra sigillata (check out this article about troubleshooting terra sigillata!) and rubbing it with a soft material such as a chamois leather. We'll discuss the former today.
Using a stone is more time consuming and takes a lot of practice, but can produce a higher degree of sheen. You also don't have to worry about the surface chipping or flaking off, and you can get a perfectly smooth surface with no brush-strokes or drip marks.
Any very smooth object can potentially be used as a burnishing stone. Many potters use rubber or plastic ribs for burnishing, particularly on leatherhard pots. The back of a spoon is a popular tool, though it may leave greyish marks on the clay. One of the more unusual burnishing tools I've heard of is used by Wally Asselberghs: he uses burnt-out lightbulbs of various sizes on leatherhard clay, because they are easier to grip. He does switch to a stone to finish the job once the pots are almost dry.
Burnishing Leatherhard or Black-hard Clay
Some potters find it easier to burnish a pot before it has dried completely. Timing can be tricky - you want the pot at the verge of dry, but with just enough moisture in the clay to allow your stone to glide across the clay without scratching it. Traditional leatherhard, the stage when a wheel-thrown pot can be easily trimmed, is a little too early. A leatherhard pot will show the marks of your burnishing tool as distinct little ridges, and in drying the rest of the way it will lose most of the shine you give it. Ideally, you want to catch the clay at black hard - when it is almost dry but has not yet changed color. If you burnish at this point, then cover the pot to slowly dry the rest of the way, perhaps even going over it once or twice more with the stone before it dries completely, you can achieve a good polish. One drawback to burnishing this way is that you won't be able to sand the pot before burnishing, so this technique works better with a wheel-thrown pot that can be smoothed with a rib when wet, or after trimming. If you want to burnish this way, you have to pay close attention to the pot as it dries, checking and rechecking it, and burnishing and re-burnishing it until it is too dry to burnish without scratching. In order to achieve the maximum level of burnish, Carol Molly Prier burnishes her pots four times, starting by first burnishing at leatherhard, immediately after trimming. If it is a handbuilt pot she scrapes it smooth at the leatherhard stage before burnishing. At this leatherhard stage she sometimes uses a flexible metal rib to burnish, instead of her stone. She burnishes twice more as the pot continues drying, before it becomes bone dry. Once the pot is bone dry she uses a soft facial tissue to apply a thin coat of salad oil over the entire surface of the pot. She lets it dry completely and then goes over the pot one last time with her stone.
Burnishing Clay on the Wheel
If you are burnishing a wheel-thrown pot, you may want to use the wheel to make the burnishing process easier. David Greenbaum burnishes wheel-thrown white earthenware pots on the wheel in a two-step process. The first burnish is when the pot is leatherhard, using a teflon plastic rib. When the pot is bone dry he rubs it all over with olive oil and allows it to soak into the clay. He then uses polished stones to burnish the pot again on the rotating wheel. Usually he makes three passes to eliminate any ridges the stone might have left the first and second time. With the pot still on the wheel he goes over the surface one last time with the Teflon rib to bring the surface to a glass-like gloss. He uses a Giffin Grip to hold the pot on the wheel for burnishing - the small rubber 'hands' that hold the pot don't mar the surface as wads of clay might.