Burnishing pottery

Burnishing is a form of pottery treatment in which the surface of the pot is polished, using a hard smooth surface such as a wooden or bone spatula, smooth stones, plastic, or even glass bulbs, while it still is in a leathery ‘green’ state, i.e., before firing. After firing, the surface is extremely shiny. Often the whole outer surface of the pot is thus decorated, but in certain ceramic traditions there is ‘pattern burnishing’ where the outside and, in the case of open bowls, the inside, are decorated with burnished patterns in which some areas are left matte.

This technique can be applied to concrete masonry, creating a polished finish. Burnishing can also be applied to wood, by rubbing two pieces together along the grain. Hard woods take the treatment best. Burnishing does not protect the wood like a varnish does, but does impart a glossy sheen. If one wood has a dye in it or is colored in some way, it may rub off onto the other wood. Burnishing can also apply to relief printing.

Traditional method
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.

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.

Burnishing Tools
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.

Modern method
Burnishing pottery is a technique in which clay is polished to a beautiful sheen without the use of glaze (like this piece by Carol Molly Prier). Ancient potters used these techniques to produce their wares before glazes and kilns were developed. Today, modern potters are burnishing pottery to create works of great beauty.

A suitable burnishing stone can be found at any lapidary shop, and often in museum gift shops or flea markets. Any kind of stone that has been tumbled in a rock polisher may be a perfect burnishing stone. An ideal stone is large enough to grasp easily, and has at least one perfectly smooth, slightly rounded surface. Any nick, bump, or sharp edge is likely to scratch the pot as it is being burnished: you can check a stone for nicks by running your fingernail over it.

For a first attempt at burnishing pottery, start with a small rounded pot. Sharp angles, S-curves, flaring lips and grooves are difficult to burnish. To prepare the pot for burnishing, it should be sanded perfectly smooth when it is bone dry.

If the surface isn’t smooth enough, your stone will not be able to get into any little dips or depressions on the surface, and you will end up with dull, unburnished spots that the stone missed.

In addition to the pot and the stone, you need a towel or rag, a bowl of water and some vegetable oil. The first step is to wet the whole pot, inside and out, rubbing the water in quite thoroughly with your fingers. You need to rub the water into the clay before you start rubbing with the stone. This makes a bit of slip on the surface of the pot which helps to fill in any little scratches from the sandpaper, and dampens the pot so it doesn’t dry out too quickly while you are burnishing it. Then you re-wet small patches with a finger, starting at the rim, and rub with the stone until the clay becomes smooth and takes on a dull sheen.

The towel or rag is needed to wipe extra water off your finger and to wipe the stone clean as necessary. It is important to start at the rim, burnish the whole rim, then burnish all the way around just below the rim, continuing this way in a slightly overlapping spiral pattern from the rim towards the foot. Once a burnished patch has dried, you will scratch it if you rub it again with the stone. For this reason, once you begin you cannot stop until you’ve finished. Working in a spiral pattern ensures that you are never working on a patch adjacent to a section that has completely dried: by the time you are working on the bottom, the top – which may have been burnished an hour ago – is completely dry again, but the bottom ring of the spiral is still damp and can still be safely rubbed with the stone.

The second step is to cover the entire pot immediately with a light coating of vegetable oil and leave for 5–15 mins to soak in. After soaking in it leaves a whitish scum on the surface. Rubbed again with the stone, the clay takes on a high gloss. This step is much faster than the first, and it doesn’t matter what part of the pot is reburnished first. If there are patches of wet oil, you can work around them and reburnish the parts which are ready. During this step the pot will be easily marred by fingerprints and should be held with a hand inside. This will also help you avoid rubbing the oil off while you work. Lightly rubbing the pot with a chamois-leather after oiling and reburnishing will remove any extra oil and slightly improve the sheen.

The burnished pot can be decorated before firing by incising or by painting with terra sigillata (check out this article on decorating with terra sigillata) or slip, or it can be immediately bisque-fired to cone 018. Though this is a very low temperature, it is high enough to harden the clay and drive off the internal water, without sacrificing the shine you worked so hard to achieve. Because of the very nature of clay, firing to a higher temperature dulls the burnish.

Clay is made up of flat particles, called platelets. Burnishing works by pressing down the clay platelets on the surface of the pot, so they all face the same way and thus reflect light the same way. As clay is fired, it loses its platelet structure, so the higher the firing temperature, the more burnish you lose. With a suitable clay, cone 018 is hot enough to harden the clay without sacrificing too much sheen. Even if you are planning a subsequent firing such as a pit-firing, it is a good idea to bisque-fire your burnished pot to ensure that the finished pot is adequately durable, and to help the pot survive the fast temperature increase that is typical of many alternative firing methods.

Burnishing Pottery Steps
Wet the entire pot, inside and out, with your fingers dipped in water, then re-wet the inside of the rim. Work the water in well. With dry fingers, rub the stone on the dampened area, being sure to avoid leaving un-burnished streaks. If you have to rub hard to get the clay smooth, or if scratch-marks from sanding remain visible, you are not using enough water.

Once the inside rim has been burnished all the way around the pot, wet the top of the lip and burnish. The lip tends to be the most difficult area and should be carefully smoothed and rounded by sanding. As you burnish, bits of clay will collect on the surface you’re working on. Use the back of your hand to wipe these bits off so that they don’t cause scratches. Wipe clay off the stone with the towel or your thumb.

After burnishing all the way round just below the rim, continue in a spiral pattern towards the foot. When you dampen a new patch to burnish, slightly overlap the previously burnished area above. As long as an already- burnished spot has not changed colour (indicating it has dried completely) it is safe to go over it with the stone. As you progress down the body of the pot you can dampen fairly large areas and run the stone quickly over it horizontally and then more carefully vertically. Some people find it easier to burnish with a circular motion.

The last spot to be burnished is just above the foot (which is not burnished). Note the color difference between the just-burnished area near the foot and the dry area at the rim.

The burnished surface is covered lightly with vegetable oil. Holding the pot with a hand inside to avoid touching the surface, the pot is re-burnished wherever the oil has dried to a whitish scum. Burnish around patches of wet oil until they dry. If necessary, extra oil can be wiped off with a finger. If the stone scratches the oiled surface, add more oil and wait for it to soak in.