Glazes - for the Self-reliant Potter (GTZ, 1993, 179 p.) 3. Temperature ranges and requirements 3.1. What is temperature? 3.2. Low temperature range 900-1100°C 3.3. High temperature range 1100-1300°C 3.4. Firing systems and glaze effects

Glazes - for the Self-reliant Potter (GTZ, 1993, 179 p.)

3. Temperature ranges and requirements

3.1. What is temperature?

Temperature means the amount of heat energy in a material. We raise the temperature of a material by providing it with heat energy, using a fire or electricity. What effect does this have on a material? We know that many familiar substances can exist in different states of solid, liquid and gas. For example, water can exist as ice, liquid water or steam. What is different about it? Only the temperature. All materials consist of atoms and molecules which are in constant motion. The amount of motion depends on the temperature. Cold materials have less motion and therefore appear solid to us (e.g. ice). When the temperature is increased, the motion of the molecules becomes greater and they can move more freely around each other (e.g. water). When the temperature is increased even more, the molecules become very active, as we can see when water boils. Then the molecules are even less bonded together and we see gas (e.g. steam).

Similarly, glazes are solid when they are cold (at room temperature), liquid when they are heated sufficiently (in the kiln), and become gas when they are heated too much.

It is also important to understand the relationship between clay and glaze. Most common red clay (such as brick clay) melts by 1100°C. This makes it useful for forming low temperature products. 1200°C, it can be used as a glaze.

3.2. Low temperature range 900-1100°C

Products called earthenware, whiteware, low-temperature ceramics, and terra cotta are all fired in the range of 900-1100°C. We will call these products generally "earthenware". What they have in common are clay bodies that develop their maximum strength in this range, and glazes that are based on low-melting compounds such as lead, sodium and potassium.

3.2.1. ADVANTAGES/DISADVANTAGES

Advantages: low temperature ceramics have the advantage of easy firing -it is much simpler to construct kilns and burner systems that have to reach no more than 1100°C, and fuel costs are lower. Bright colors are possible in this range. Most common clays cannot be fired higher than this.

Disadvantages: earthenware is often not as strong as high temperature ware, because the clay does not become vitreous. This means that it also has some porosity (the property of absorbing water) with the result that earthenware products often do not hold water unless the glaze is perfectly fitted to the body. Also, it is easier to chip the glaze away from the clay.

Historically, many earthenware glazes were based on poisonous lead because it is easy to melt: nowadays this is not a problem because lead can be replaced by non-poisonous materials.

Modern earthenware glazes are usually based on frits, which are expensive -the lower firing cost must be compared to the higher cost of the glaze.

3.2.2. APPROPRIATE PRODUCTS

Earthenware is used for all common household containers -cups, bowls, storage containers, oil lamps etc. Ordinary wall tiles, most low-cost tableware, sanitary ware, common unglazed containers, bricks, roof tiles etc. are all made in the low temperature range. Many countries have a long tradition of glazed red clay products, which are still useful in modern times. Most modern factories have changed their production to white clay products, which have become more feasible with recent developments of white bodies that become strong enough at low temperatures.

3.2.3. CLAY/GLAZE CHARACTERISTICS

Earthenware clay

Common red-burning clay is normally used, often mixed with talcum powder to increase its firing range. In many countries, red clay which contains lime is used because it makes it easier to formulate glazes that do not craze (crack). White firing clay bodies are often based on talc, ball clay and fluxes to make them harder.

Earthenware glaze

Earthenware glazes are based on low-melting materials, mainly lead oxide (white lead oxide, red lead oxide), sodium and boron compounds (soda ash, borax, boric acid) and potassium compounds (pearl ash, also known as potassium carbonate). Usually it is necessary to use these compounds in the form of frits (see chapter on frits).

3.2.4. RAW MATERIAL REQUIREMENTS

Most of the raw materials for low temperature glazes can be obtained from commonly available sources. They include: local clays, wood and rice husk ash, limestone, and even soap powder (based on sodium and boron compounds). Materials such as borax must be obtained from chemical suppliers. Ready-made frits can be obtained from glaze suppliers, but in many locations it is necessary to make them from raw materials.

3.3. High temperature range 1100-1300°C

Types of ware fired in this range are known as stoneware and porcelain.

3.3.1. ADVANTAGES/DISADVANTAGES

Advantages

High temperature products are generally stronger, more acid and abrasion-resistant. Raw materials do not require fritting. The clay is more vitreous and thus does not have problems of water seepage.

Disadvantages

Kilns for high temperatures require more sophisticated bricks and kiln furniture, and better burner systems. Fuel costs are higher.

3.3.2. APPROPRIATE PRODUCTS

High temperature products include stoneware utilitarian items, whiteware of various types, porcelain and electrical insulators.

3.3.3. CLAY/GLAZE CHARACTERISTICS

Clay

Clay body raw materials are limited to those clays which can withstand high temperatures without melting: fireclays, ball clays, china clays, "stoneware" clays. Most bodies also include feldspar to cause vitrification, which prevents water seepage through the body.

Glaze

High temperature glaze is easier to make than the low temperature sort, mainly because it is not necessary to frit the ingredients.

3.3.4. RAW MATERIAL REQUIREMENTS

Most stoneware and porcelain glazes are based on feldspar, quartz, limestone and clay, with other ingredients to provide specific properties of surface, color etc.

3.4. Firing systems and glaze effects

Different types of kilns and fuels have specific effects on glaze color and surface.

3.4.1. OIL, GAS, WOOD, COAL, ELECTRICITY, OTHER

These are the main options for fuel. Each fuel requires a different kiln design and burner system. You must first decide which fuel is most available and most economical. The choice of fuel will determine whether products can be open-fired on shelves, or whether it is necessary to use saggers to protect the glaze from ash and contamination from dirty fuel.

The cost of fuel should be thought about very carefully. One kg of fuel produces a certain amount of heat. Heat is usually measured in calories or in British Thermal Units (BTU). One calorie is the amount of heat required to raise the temperature of one cubic centimeter of water 1°C. The table at page 170 shows the heat value of different fuels. Because a calorie is very small, the usual unit of heat is expressed as kilocalories (kilo = 1000, so 1 kilocalorie = 1000 calories).

A particular kiln, loaded with an average number of products and fired to a specific temperature, will usually require the same amount of fuel each time, since it requires a specific number of calories to convert raw clay and glaze into finished ceramics. When you know the total kg of products and the total cost of one firing, it is easy to calculate the cost per kg of product:

Total cost/Total kg = cost per kg

You can also calculate the total number of calories required to do one firing. If you are using kerosene, you can find from the table that one lifer of kerosene supplies about 12,000 kilocalories of heat. So, if you use 80 lifers to do a firing, the calculation is:

(Total fuel) X (kilocalories per unit) = total kilocalories required

80 X 12,000 = 960,000 kilocalories

When deciding on the type of fuel to use, you should find out the cost per kilocalorie for different fuels in your area.

Oil

Oil is available in many different forms, all of which can be used by the potter, including kerosene, diesel, furnace oil, and waste crankcase oil. Kerosene is the most clean-burning (without too much smoke or impurities), and waste crankcase oil is the dirtiest to use. Normally, products can be open-fired, but oil will produce some discoloration. For high quality whiteware, saggars may be necessary. Oil is suitable for high or low temperatures.
Oil provides between 9000 and 11000 kilocalories per kg.

Gas

Gas is available as natural gas, producer gas or liquid propane gas. Where gas is available at a reasonable cost (compared to other fuels), it is the easiest fuel to use. Gas is very clean-burning, does not require saggars, and the burners are also simple to manufacture locally. It is suitable for any temperature.

Wood

Almost any kind of wood can be used for firing kilns. Nowadays, wood is a scarce resource in most countries and more and more it is being replaced by other fuels. Firing with wood is labor-intensive. Because it produces a large volume of ash, it is usually necessary to fire the ware in saggers. It is suitable for any temperature.

On the other hand, wood is a renewable resource and in many areas of the world it is produced as a cash crop, which makes it appropriate to use.

The calorific value of wood is difficult to calculate, because it depends on the type of wood, whether it is wet or dry, and the efficiency of burning. Dry wood can supply between 3000 and 4500 kilocalories per kg, whereas the same wood when wet may produce only half the calories.

Coal

Coal comes in many different grades, all of which are suitable for firing kilns. Firing with coal is labor-intensive, but in many countries it is the cheapest fuel available. Coal also produces ash and impurities, so it is usually necessary to fire the ware in saggars. It is best for high temperatures, but can be used at any temperature.

Coal can provide between 4500 and 7700 kilocalories per kg.

Electricity

Electric kilns are practical for the small producer where there is a reliable source of electricity. Because there is no combustion, electricity is the cleanest fuel of all. Electric kilns fire very evenly and do not require saggers. Electricity is best for temperatures up to
1100°C.

Other fuels

These include tires, which burn very well but produce a lot of smoke, and also produce poisonous gases. They can be used in kilns designed to burn wood or coal. Some brick industries use scrap asphalt from roads as fuel. Also in this category are such fuels as brushwood, sawdust and rice husk. Most of these are dirty-burning, so require the use of saggers. They are best for low temperatures.

3.4.2. OXIDATION/REDUCTION

To understand oxidation and reduction, it is necessary to know how fuel burns. All fuel produces heat when it combines with the oxygen in the air. As anyone knows who has made a wood fire, if there is plenty of air the fire burns hot and clean, with little smoke. This is called an oxidation fire. If the air is reduced, there will be less heat and more smoke. This is called a reduction (or reducing) fire, which simply means reducing the amount of oxygen. So:

- Oxidation firing means there is plenty of air and no smoke.
- Reduction firing means there is little air and more smoke.

Glazes will have different colors and surfaces depending on whether they are fired in oxidation or reduction conditions. Oxidation has its greatest effect on the metallic oxides that are used to create color in glazes. For example:

OXIDE	OXIDATION	REDUCTION
red iron oxide	brown	red-brown, black
copper oxide	green, blue	red

Iron also changes from a grey color to a red color when it rusts. This is because oxide from the air-combines with the metal and forms iron oxide.

In firing, it is difficult to exactly control the amount of oxidation or reduction. Many beautiful glazes can be obtained in reduction firing, so it is widely used for decorative stoneware, and for lusterware. However, the results are variable and difficult to reproduce every time, and even in one kiln-load there will be differences. For that reason, most producers who need to supply a uniform product use oxidation firing.

3.4.3. VAPOR GLAZING

In vapor glazing techniques, the glaze is not applied to the product before firing in the usual manner. Instead, glaze is introduced into the kiln through the firebox at the end of the firing, when there is enough heat to change the glaze into vapor form. The most common material for vapor glazing is ordinary salt. At temperatures above 1100°C, salt breaks down into sodium and chlorine vapor, which circulates through the kiln. The sodium is attracted to silica in the clay and forms a strong, durable glaze. Salt glazing is used mainly for sewage pipes, because it is cheap and a perfectly glazed surface is not necessary. In Europe, it was once used widely for household items, even including beer bottles. Nowadays, salt glazing is less popular because it produces toxic smoke that harms the environment.

Salt is sometimes replaced by soda ash and sodium bicarbonate, which produce a similar vapor glaze without the poisonous side effects. Vapor glazing is not recommended for the small producer, except for making specialized art ceramics.