Burnishing is the technique of polishing clay to a beautiful sheen without the use of glaze. Ancient potters used these techniques to produce their wares before glazes and kilns were developed. Today, modern potters use burnishing to create works of great beauty.
Sumi von Dassow is one of those potters. She has been using low-tech pottery making techniques for more than thirty years. Though the burnishing technique is low tech, there are some secrets to really getting it right. In today’s post, Sumi shares her step-by-step method for burnishing pots.- Jennifer Harnetty, editor.
Many types of clay can be burnished, though it should be smooth and grog- free. A cone 6 (not 06) clay will be the strongest and most durable after firing to cone 018 or so. (Most low-fire clays are so under-fired at such a low temperature that they can be scratched with a finger- nail, like chalk.) Red clay seems to look more brilliant after polishing than white clay, and is often stronger both before and after firing. If a white clay is desired, choose a smooth cone 5 or 6 clay–not a porcelain or low-fire clay. White clays are best for bringing out the range of colour in a sawdust- or pit-firing. For the traditional all-black colour of southwest pottery a red clay may be preferable.
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, 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 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 – Step-by-Step
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 colour 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.
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