Category Art

Slip is a mixture of clay and water, forming a thick liquid. It can be used as a kind of glue to stick a handle onto a cup when both are “leather hard” (hard enough to handle but still soft enough to cut with a knife). Slip can also be poured into plaster moulds to form intricate shapes. The plaster absorbs water from the slip, causing it to dry on the outside first. If the rest of the slip is poured away, hollow vessels and ornaments can be made.

In pottery, pieces of clay sometimes need to be joined to each other.  A handle on a cup for example.  The way this is done is by scoring (scratching marks in) the edges that need to be joined, thereby creating a key.  Then liquefied clay is pasted onto the two edges before pressing them together.  This creates a bond.

The liquefied clay is called slip.  Sometimes the words slip and slurry are used interchangeably.  However, it is also argued that technically slip is thinner than slurry.  And that slurry is actually the particular kind of thick, gloopy slip that is used for bonding pieces of clay.

Generally, slurry that is used for joining pieces of pottery consists only of particles of the clay being used in the pot itself suspended in water.  This is important because different kinds of clay shrink and vitrify at different rates.  If you join pieces of clay together with slurry that has a different shrink or drying rate to the pot itself, the join may not be very strong.

There is no particular hard and fast ratio for clay to water for joining clay.  How liquid the slip needs to be depends on how wet your clay pieces are.  If the clay is still relatively wet the slip does not need to be too thick.  However, when the clay has begun to dry out, then the slip needs to be thicker in order for the pieces to key into one another.

Slip for joining clay is sometimes called ‘joining slip’.  It can be bought or made.  However, it is a good idea for the slip to be have the same clay body as the pot itself.  Therefore, it is often recommended that the potter makes the slip themselves.

Slip is also used to decorate items of pottery.  The term engobe is used to describe a clay slip coating that is applied to the body itself.  This can either improve the texture of the item or add color. Engobe is opaque and can be white or colored.  Colored engobe usually contains stains and metal oxides.

Engobe is different from slip in that it has lower clay content than slip.  It also contains more silica and flux than regular slip.  This means it shrinks less than slip when it dries.

Some people claim that engobe is half way between being a slip and a glaze.  Fired engobe surfaces have a bit of a sheen that fired slip surfaces do not have.  Others, state that engobe and glaze are quite different.  They point out that if an engobe creates too much of a glazed effect, it loses its opacity, which is its most important property.

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In the heat of the kiln, glaze would fuse with the shelf that the object stands on, so glaze is carefully wiped from the base of the object before it is fired.

In lower temperature firings, like cone 06–04, you can use a stilt, which is a small piece of ceramic material with pointed wires sticking out of it. You fire the pot sitting on the pointed wires. This leaves small marks in the glaze which sometimes have to be cleaned up a bit. In higher temperature firings such as cone 10, it’s not practical to glaze the bottom of a vase. The norm is to have what is called a dry foot where no glaze is applied to the bottom. A stain or a colored slip can be applied to the foot before firing if there’s too much contrast between the color of the bare ceramic and glazed areas. It’s also possible to construct a foot ring on the bottom of the vase in such a way that the glaze can at least be applied to the edge of the bottom without sticking the pot to the shelf. That does add a level of difficulty, though, since glaze can move during the firing, and you need to leave a little distance between the shelf and the glaze in case it does move. On my pots, I don’t attempt to do that, but accept the look of the dry foot.

The pot has to stand on something (or hang from something, with its own problems) during firing. It cannot float. During firing, the glaze melts. If you have glaze on the bottom of the pot, when the glaze cools it sets solid and fixes the pot to the kiln shelf or floor. Even if you can get the pot off the shelf, probably taking bits of shelf with it, you will have to grind down the rough bits. More likely the pot will break during cooling as the pot contracts more than the shelf, with (very sharp, beware) bits left stuck to the shelf and the pot ruined (also the shelf). If you can find a way to make pots float, then you can glaze the bottom.

You can hang them, but you need a suitable hanging point, and you may not want to have a hanging hole in your pot, and if you do, that part will have to be unglazed, so there will always be an unglazed part. And some plates appear to have completely glazed bases with no unglazed bits, but if you look carefully, you find breaks in the glaze between base and rim, where the plate has been supported during firing.

There has to be a break in the glaze where the ware is supported, it doesn’t have to be on the bottom, but if it isn’t it will be somewhere else, probably more visible.

Many items have an indented bottom which is glazed, leaving just a thin ring unglazed. Some potters like to do this as to them it looks more professional; others don’t, preferring the handmade look of a bare base. Many factories glaze the base this way, but there is still always an unglazed ring. It has to stand on something during firing!

Ceramics are baked to make them hard and waterproof. Until they are baked (fired) ceramics can be mixed with water again to form clay. Firing is done in a large oven called a kiln. In large ceramics factories, the kilns are heated all the time. They are like long tunnels, through which ceramics move slowly on trolleys in a never-ending process. The first firing that a clay article receives is called a biscuit firing. It makes the article hard and brittle, but it is still porous. Water can be absorbed by it.

Firing clay transforms it from its humble, soft beginnings into a new, durable substance: ceramic. Ceramics are tough and strong and similar in some ways to stone. Pieces of pottery have survived for thousands of years, all because clay met fire.

The temperature needed to transform soft clay into hard ceramic is extremely high and is usually provided by a kiln. You cannot fire pottery in a home oven because ovens do not get up to the high temperatures of more than 1,500 degrees Fahrenheit that you need for firing clay.

Firing is the process of bringing clay and glazes up to a high temperature. The final aim is to heat the object to the point that the clay and glazes are “mature”—that is, that they have reached their optimal level of melting. To the human eye, pots and other clay objects do not look melted; the melting that occurs is on the molecular level.

Bisque firing refers to the first time newly shaped clay pots, or greenware, go through high-temperature heating. It is done to vitrify, which means, “to turn it glasslike,” to a point that the pottery can have a glaze adhere to the surface.

Greenware is fragile. To start, it must be bone-dry. Then, it must be loaded into the kiln with a great deal of care. The kiln is closed and heating slowly begins.

Slow temperature rise is critical. During the beginning of the bisque firing, the last of the atmospheric water is driven out of the clay. If it is heated too quickly, the water turns into steam while inside the clay body, which can cause the clay to burst.

When a kiln reaches about 660 degrees Fahrenheit, the chemically bonded water will begin to be driven off. By the time the clay reaches 930 degrees Fahrenheit, the clay becomes completely dehydrated. At this point, the clay is changed forever; it is now a ceramic material.

The bisque firing continues until the kiln reaches about 1730 degrees Fahrenheit. At this temperature, the pot has sintered, which means it has been transformed to the point that it is less fragile while remaining porous enough to accept the application of glazes.

After the desired temperature has been reached, the kiln is turned off. The cooling is slow to avoid breaking the pots due to stress from the temperature change. After the kiln is completely cool, it is opened and the newly created “bisque ware” is removed.

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There are many ways of decorating pots. They can be dipped in a glaze, made of tiny particles of glass in a liquid, and fired for a second time. The glassy covering melts onto the pottery, making it completely waterproof. Pottery can also be decorated after glazing, with transfers, hand-painted designs, or by screen-printing. It may then be fired for a third time to fix the decoration.

Glazes are the most often used form of pottery decoration. They come in a huge variety, including nearly every color imaginable and many types of textures. Glazing can transform a simple pot into something really special and the techniques you can use are endless. On a practical level glazes are used to make a pot vitreous and both food and liquid safe. When a piece has been bisque fired and then put through a separate glaze firing, it makes the pot much more durable. Raw glazed work tends to flake off more easily and you won’t get this problem with a separate glaze firing.

Glazes can be laid on top of each other to create even more effects. This is called overglazing. Some “overglazes” are simply other glazes that are applied on top of another unfired glaze that will mature at the same temperature. True overglazes may also be applied after the base glaze has already been fired. These overglazes will require the ware to go through a third firing, at a lower temperature than the base glaze.

Underglazes are not glazes themselves but are colorants applied to unfired bisqueware (greenware) before an overglaze is applied (usually the overglaze is transparent to really let the colors of the underglaze shine through). Underglazes provide the flexibility for a huge range of creativity. Firstly, there’s the range of colors you can use, as you can mix the underglazes to get your perfect shade. With underglazes, you have the opportunity to paint your own detailed designs and patterns. You can even use the underglazes like watercolors, as when you’re painting straight onto a rough clay surface, there’s less chance of the glazes slipping.

Slips and engobes are essentially the same thing. The difference in term is basically a difference in regional language preference. “Slip” is more common in Europe, and “engobe” is more common in North America. Both words refer to a liquid slurry consisting of clay or clay mixed with coloring agents. Slips and engobes are used to decorate wet greenware, adding color, texture, or two-dimensional design. The advantages of using an engobe are that you can use them for raw (or single) firing, meaning you can apply them to work when it is still slightly damp or even leather hard. Unlike, glassy glazes, engobes usually produce a matt surface rather than a glossy shine. The exception to engobes being matt in texture is terra sigillata, which can be buffed to have a higher shine to it.

Clay is a master chameleon. With skill, clay can successfully visually mimic all sorts of substances, from metal to old shoes. Clay is impressionable. Textures can readily be added to wet pots through impressing a variety of tools and objects into the surface.

Clay is also carve-able. Marks and designs can be incised into leather-hard greenware. By doing so at the leather-hard stage of drying, the cuts retain their crispness. Leather-hard greenware also allows for more ease when incising more intricate patterns.

Marbling with two different types of clay—say, a white clay body and a terracotta (or alternatively a colored clay)—is a wonderful way to create different effects on your pottery. One of the best ways to do it is to roll out the two different colors of clay into sheets, then stack them on top of each other. Then start gently rolling the whole block. The colors will mix together and make the most beautiful marbled patterns and from there you can hand build or use a mold to create your desired shape.

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Clay can be used to make a huge variety of ceramic articles, from tiny electronic components to bricks and baths. It is a good insulator and, when covered with a glaze, is completely waterproof. Unlike many metals, glazed clay is unreactive, so that acidic foods will not stain it, and exposure to water and the air will not tarnish or corrode it.

Clay is made from the slow chemical weathering of silicate bearing rocks like granite and feldspar and other igneous rock. Usually the weathering is from it is slightly acidic solution other times it is geothermal. It becomes a hydrolyzed aluminum phyllosillicate. Al2Si2O5 (OH)4) They form flat hexagonal sheets that are less than 2 micrometers. The sheets are made of tetrahedral silicate sheets and octahedral hydroxide sheets. There are about 30 types of clay. Natural clays are always a mixture of these many types. There are primary and secondary clays. Primary ones are found where they were formed. Secondary ones have moved by water and deposited somewhere else.

Clays sintered in fire were the first form of ceramic. Bricks, cooking pots, art objects, dishware, smoking pipes, and even musical instruments such as the ocarina can all be shaped from clay before being fired. Clay is also used in many industrial processes, such as paper making, cement production, and chemical filtering. Until the late 20th century, bentonite clay was widely used as a mold binder in the manufacture of sand castings.

Clay, being relatively impermeable to water, is also used where natural seals are needed, such as in the cores of dams, or as a barrier in landfills against toxic seepage (lining the landfill, preferably in combination with geotextiles). Studies in the early 21st century have investigated clay’s absorption capacities in various applications, such as the removal of heavy metals from waste water and air purification.

Medical use: A traditional use of clay as medicine goes back to prehistoric times. An example is Armenian bole, which is used to soothe an upset stomach. Some animals such as parrots and pigs ingest clay for similar reasons. Kaolin clay and attapulgite have been used as anti-diarrheal medicines.

As a building material: Clay building in South-Estonia Clay as the defining ingredient of loam is one of the oldest building materials on Earth, among other ancient, naturally-occurring geologic materials such as stone and organic materials like wood. Between one-half and two-thirds of the world’s population, in both traditional societies as well as developed countries, still live or work in buildings made with clay, often baked into brick, as an essential part of its load-bearing structure.

Also a primary ingredient in many natural building techniques, clay is used to create adobe, cob, cordwood, and rammed earth structures and building elements such as wattle and daub, clay plaster, clay render case, clay floors and clay paints and ceramic building material. Clay was used as a mortar in brick chimneys and stone walls where protected from water.

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