Ceramic Firing Techniques

Salt glazed porcelain pottery fired at 1320 degrees centigrade in an LPG gas fired Kiln using iron and manganese slip
Salt glazed porcelain pottery fired at 1320 degrees centigrade in an LPG gas fired Kiln using iron and manganese slip

Methods of Ceramic Firing

Firing is the method for making ceramics that can endure a lot of pressure. Firing takes place in an oxygen-rich environment. When it comes to firing pottery, there are two primary methods. The use of kilns and open firing are the two methods. The crucial step in the ceramic process is firing. It’s the point at which clay transforms from clay to usable ceramic.

High-tech digitally controlled and monitored kilns are one type of firing technique. And sawdust and leaf-filled handmade brick structures are the most primitive firing method. By using an electronic controller I have the ability to program in a precise firing schedule. This is something a potter or artist can not do if doing a wood firing. Basically regardless of the firing method used a potter or ceramic artist must accomplish two firings, a biscuit firing and a glaze firing. Some folks call the biscuit firing, or bisque firing for short. The end result is the clay body is now called bisque.

Open Firing Techniques – Primary Method #1

Open Firing Techniques Infographic - Artabys

The most rudimentary of these processes are open firing, although producing pottery this way requires a great deal of skill. This approach does not need the construction or maintenance of any structures. The potter is to fire pottery without a kiln. The fuel and vessels are combined and burned in this way. This necessitates a very high temperature, therefore insulation is required. Choosing different types of fuel can also give you some control over the firing rate. Dung, for example, burns slowly and evenly, but straw burns fast and produces abrupt temperature changes. To make this approach work, the ceramics must be completely dry. Otherwise, they may explode.

1. Black ware Firing Overview

Black ware pottery may be made with an atmospheric control method. I lump this method under the open firing technique because it uses or can use an open pit to fire the clay. Pit firing (pit fired pottery) is a 20th to 21st-century pottery method developed by Puebloan Native American ceramic artists in Northern New Mexico.

Pueblo artisans and other artists all around the globe have been making traditional reduction-fired Black ware for ages. The patterns are applied by selective burnishing or the application of refractory slip. Another technique includes carving or incising patterns and polishing the elevated regions on the pots carefully.

This is the process of blackening ceramics by lowering the oxygen level in the kiln or an open pit fire. This happens when the temperature is high enough for sintering to take place. When at a low temperature sintering does not occur. Sintering occurs when the clay’s exterior surface area becomes soft and molten. Additional fuel is poured at this stage, and the fire is smothered with sand. This process creates smoke, which deposits carbon on the vessel’s surface and gives it its distinctive black shine.

2. Sawdust Firing Overview

An easy method for firing if you have a lot of sawdust. This is the simple approach and many potters choose to make their own sawdust kiln for firing clay. It requires minimal outdoor space and only a few simple supplies. Sawdust kilns are made from bricks or cinder blocks and are built up from the ground in a square pattern. I have even seen kilns made of a barrel. Barrel firing I am guessing would be an alternative to pit firing or having to build you own brick kiln.

The empty space within the brick kiln is packed with sawdust. The first layer of sawdust is loaded in the kiln followed by the placement of pots. After which more sawdust is layered in followed by more pots. This process is repeated until the kiln is packed full.

Sometimes chicken wire is used to separate different layers. The sawdust is set on fire and after the flames have subsided bricks can be placed on top of the kiln. I like to control the rate of burn by opening and closing the bricks on top of the kiln. To fire pots in a sawdust kiln takes a long time. Sometimes the kiln can burn for almost 2 days.

My opinion of sawdust firing is it great for creating artwork. But ware created by a sawdust kiln does not have any utility value due to its low fire technique. Pottery created by this method is very porous and therefore not good for food or water.

After the firing, the kiln is unloaded and the ones that are not broken are usually waxed and shined. The pot produced by this method comes out very dark and even black in color. Once burnished and given a shiny finish they can look very attractive.

3. Pit Firing Technique Overview

And last but not least of the open firing techniques is pit firing. Pit firing is the oldest. And as you can imagine it’s the oldest because all you need is a pit and combustibles. How much more primitive can you get, right? Some folks also call this technique smoke firing.

It was developed between 800 and 1066 AD. Some potters, however, continue to utilize pit firing because of the beautiful results that may be achieved. Pit firing does not need the use of a kiln. Instead, a hole is excavated in the earth large enough to hold the ceramic pieces or ceramic sculpture. Flammable items such as wood, sawdust, and leaves are placed in the bottom of the pit. In my mind a potter who uses a primitive technique like this would also most likely use a potter’s wheel or some sort of wheel throwing technique.

The pottery is then put on top of this layer, with plenty of room between them. On top of and around the pottery, more flammable material is placed. Anything that burns slowly, such as coal or dung, can be used. Yep, even dung. That’s right. Actually, dung produces nice rich colors like Green, Black, Gray, Brown, and Yellow. Cow patties work well but color products will be highly dependent on what the cow ate. Sounds kind of crazy but it is true.

The pit firing technique and what is consumed in the fire produce all of the colors and patterns. Burning combustibles produce fumes, which whirl around the pots in the pit. The colored smoke vapor will enter and stay in the pores depending on the temperature. And if the pots achieve the proper temperature for the clay pores to open. Aside from that, the pot will be white or black to gray in hue.

The substance is then ignited and allowed to burn. The fire is left for a few hours after it has diminished to a smolder. The pit can then be filled with sand or soil. The amount of oxygen accessible to the fire will be reduced, resulting in a decrease atmosphere, (carbon monoxide is a deadly gas with no odor or flavor). A decrease in atmosphere is a fancy way of saying there is less oxygen, ha!

Before lighting the fire, some potters apply oxide stains to the pottery. Others use organic materials to wrap their ceramics in (organic matter) order to produce stunning designs on the fired pots. For example, some potters will wrap their pots or wares in copper wire. The copper wire depending on the composition of the wire, pots will have nice Red, Black, Blue, Green, and White colors transferred onto the pottery.

Due to their availability, leaves and grass clippings are a favorite. Green and Brown are the most common hues produced by these combustibles. Sawdust is also frequently used, and it produces lovely Blueish Gray, Black, and Gray hues. Finally, there are colorants that can be applied with power.

The pit fire must then be left, most commonly overnight, until it has burned itself out. After that, the objects are gently removed. Wax was used to clean and polish the pots. It’s not uncommon for a large number of pots to shatter during the pit firing. You might have anywhere from 30 percent to 50 percent breakage depending on your skill level and luck. And yes, you can get really lucky sometimes and not have a bunch come out broken.

Pit firing is considered a low-firing method, with temperatures typically hovering around 2000 degrees Fahrenheit. The temperature, is, however, surprisingly high enough to convert the greenware into ceramic due to the type of clay used.

Kiln Firing Techniques – Primary Method #2

Kiln Firing Techniques - Types Of Kilns - Infographic - Artabys

The use of kilns is not always less complicated than the open firing method. One significant advantage of using a kiln is the ability to regulate air conditions. Electric kilns are by far the most widely used because of their low cost of acquisition and operation.

When it comes to fuel-burning kilns, gas kilns are the most common. You can regulate the environment your work is exposed to using gas-burning, which has a direct impact on the final outcomes of your glazes and clay bodies.

Raku firing is a popular ceramic fire method that includes taking red-hot pots from the kiln and putting them in flammable materials containers. It not only involves smoke and flames, but it also produces stunning effects.

Electric Kilns Versus Gas Kilns

Kilns/Benefits Electric KilnGas Kiln
Lower firing temperature Y
Higher firing temperature Y
Low to no fumes Y
Soda firing Y
Reduction firing Y
Easy to set up Y
Requires a gas line Y
PortableY

1. Electric Kilns Overview

Electric kilns are basically boxes with doors and insulating bricks on the inside. The most complicated thing is going to be the electronics controlling the firing and not the box itself. These bricks are soft and porous. They also have a texture that reminds me of a soft cinder block. Top-loading or front-loading electric kilns are common.

The insulating bricks’ surface is carved with channels that run the length of the kiln. In the grooves, metal coils run. The metal coils that serve as heating components are subjected to high electrical currents.

In older model electric kilns I remember having to manually manage the firing. I had schedules written down on paper and would go by them when firing particular pieces. Now, kilns have sophisticated electronics and schedules are pre-programmed. This makes the firing process much simpler and easier. Electric kilns are very reliable.

2. Gas Kilns Overview

The insulated interior chamber of a gas kiln is heated by combustion. Combustion is defined as the act of burning natural gas or propane in the case of a gas kiln. A gas kiln, unlike an electric kiln, utilizes flames instead of heated electrical coils. In a kiln, the fuel-to-oxygen ratio helps to control and determines how the flame burns. This, in turn, has an impact on how the clay burns.

Gas kilns are no longer as widespread as electric kilns. They do, however, have a number of benefits over electric kilns. For reduction firing, you can utilize gas kilns, as well as salt firing or soda burning. These can produce stunning effects that are impossible to achieve with an electric kiln.

What happens inside a gas kiln? Carbon dioxide is the natural state of carbon. It absorbs oxygen from the glaze and clay to return to its natural state or condition. This is the reduction process, in which the clay and glaze textures are changed. It’s during this process that ceramic glaze colors grow more intense. Colors in a reduction environment are more rustic, richer, and organic in appearance.

For some potters, this feature in gas kilns is very important because it can create very beautiful and interesting colors in pottery. For me, this feature is not important because I am interested in producing an exact color that is predetermined in my mind thus not leaving anything to chance. I am not interested in a random or semi-random outcome of colors. Although beautiful it’s not my style.

There are other types of kilns but are much less popular and are probably reserved for the most enthusiastic of potters.

3. Wood Burning Kilns Overview

Talk about doing some work. When it comes to wood-burning kilns I am not sure if this is art or just work and more work. To tend to a wood-burning kiln you need more than one person to constantly stoke the fire. Constant attention is needed to keep the fire burning at the right temperature. In contrast to an electric kiln, all that is needed is to set the program and turn it on.

To operate a wood-burning kiln you have to be a hard-core dedicated potter. One who deeply values the primitive way of creating pottery and loves the rustic organic pottery outcomes. During a wood-burning kiln firing, is common for 40% or so of the wares to come out broken. This is due to the very hot wood ash that touches the clay pots. As the wood burns there is a lot of shifting going on inside the kiln. But it’s very thrilling when you have one beautiful rustic pot that makes it through a firing intact.

4. Soda Firing Kilns Overview

What is soda firing? Soda firing is injecting a combination of baking soda and sodium carbonate into the kiln at a temperature of about 2350 F (1287 C). Soda kilns are commonly made of brick. To spray the mixture, a brick will be removed from the wall and the nozzle will be inserted. Therefore the kiln has to be made so that at various locations on the kiln wall a brick can be removed so that soda can be injected. There are different ways of injecting the soda and spray is just one. another way is just to dump it in, no spray.

What happens in a soda kiln is that the vaporized mixture is transported throughout the kiln by the flame and sticks to the pottery. The soda mixture reacts with the clay’s surface and produces its own glaze surface. You now have glaze pottery or artwork.

For me, it’s too random of a process. I do have some control over where I can spray the mix and position it but still, it’s too unpredictable for me. however, I understand it’s not for everyone and the outcome can be spectacular. The end result is a rustic look in which can be quite intriguing.

During the firing in a soda kiln, The soda solution reacts with the clay’s silica, alumina, and minerals. This might result in vibrant hues. Yellow, red, brown, and gold are frequently produced.

5. Raku Firing Method

Raku ware in the kiln.The aluminum container serves as a reduction chamber in the western form of Raku firing. Closing the container creates a reduced atmosphere. The color is affected by an interaction between oxygen and clay minerals in a reduction atmosphere. The term "reduction" refers to a decrease in oxidation. After flammable items like sawdust catch fire, closing the container decreases the oxygen content, forcing the process to draw oxygen from the glazes and clay minerals.
Raku ware in the kiln. The aluminum container serves as a reduction chamber in the western form of Raku firing. Closing the container creates a reduced atmosphere. The color is affected by an interaction between oxygen and clay minerals in a reduction atmosphere. The term “reduction” refers to a decrease in oxidation. After flammable items like sawdust catch fire, closing the container decreases the oxygen content, forcing the process to draw oxygen from the glazes and clay minerals.

I saved Raku firing for last when talking about firing techniques using a kiln because you really do not need a special kiln to fire Raku pottery. It is a process rather than needing a special kiln. In the 16th century, Japan developed a quick spontaneous process of glazing and firing called, yes, you guessed it, Raku firing. Raku firing basically entails taking the pottery out of the kiln when it is still red hot. The main requirement for a Raku kiln is it should have a door that can be opened and closed. Most kilns have a door in which can be opened and closed. Some folks refer to naked Raku as being a pot with no glaze.

Now there is one problem with most kilns, which is that if it opens from the top it’s not going to be ideal for Raku firing. Having to open the door and stand above the intense heat to pull out a pot is not going to be ideal. In fact, it would be very dangerous. Therefore front-loading kilns are more practical for Raku firing. Most often Raku kilns are gas kilns.

Tips: In my opinion, Raku firing is limited in its value other than for its artistic value. Because of the porosity of the clay and the rigors of Raku firing, the pottery is delicate and the glazes flake readily. As a result, the pottery is more ornamental than utilitarian, and it cannot be used as tableware.

How does Raku firing work? The glaze is still molten while the pottery is in its red-hot condition. The glaze fractures (in the glazed surface) and provides a crazy appearance when it cools quickly. The term “thermal shock” refers to the fast cooling process. When pottery is taken from the kiln and quickly put into a can of combustible material like newspaper, sawdust or even cold water (that would certainly get earthenware or stoneware cool enough), (if using newspaper) the carbon in the paper gets embedded in cracks. This creates dark lines where the glaze has cracked. Make for a very cool-looking design.

Summary Of Ceramic Firing Techniques

Summary Of Ceramic Firing Techniques
Firing techniques for pottery

The two types of ceramic firing techniques used are open firing and kiln firing.

The three open fire techniques are Black ware Firing, Sawdust Firing, and Pit Firing.

The crucial step in the ceramic process is firing. It’s the point at which clay transforms from clay to usable ceramic. Firing takes place in an oxygen-rich environment. The use of kilns and open firing are the two primary methods for firing pottery or firing clay.

Open firing is the most basic of these methods, however, it needs a considerable wealth of skills to produce ceramics this way. This method does not necessitate the building or upkeep of any structures. In this method, the fuel and vessels are mixed and fired. This demands an extremely high temperature, necessitating the use of insulation. You may also have some control over the firing rate by using different types of fuel. Dung, for example, burns slowly and evenly, but straw burns quickly and causes temperature fluctuations. The ceramics must be totally dried for this method to function. Otherwise, they may blow up.

The five kiln firing techniques are firing using electric kilns, gas kilns, wood-burning kilns, soda firing, and Raku firing.

On the inside, electric kilns are essentially boxes with doors and insulating bricks. The electronics that regulate the firing. Electric kilns with top-loading or front-loading doors are typical.

Combustion heats the insulated inner chamber of a gas kiln. In the case of a gas kiln, combustion is defined as the act of burning natural gas or propane. Unlike an electric kiln, a gas kiln uses flames rather than heated electrical coils.

A hard-core dedicated potter is required to operate a wood-burning kiln. One who treasures the traditional method of making ceramics and enjoys the rustic, organic results. It’s not uncommon for 40 percent or more of the goods to be shattered during a wood-burning kiln fire.

We become what we think about

Marcus Aurelius

Soda firing is the process of putting a mixture of baking soda and sodium carbonate into a kiln at a temperature of around 2350 degrees Fahrenheit. Brick is often used in soda kilns. A brick will be removed from the wall and the nozzle will be inserted to spray the mixture. As a result, the kiln must be built in such a way that brick may be removed from various points on the kiln wall so that soda can be injected.

To fire Raku pottery, you don’t need a specialized kiln. Rather than requiring a specialized kiln, it is a process. Raku firing is the process of removing the pottery from the kiln while it is still red hot. The most important feature of a Raku kiln is that it has a door that can be opened and closed. There are many good books on this topic. I would look at getting a hardcover book, then if I needed, I would have more paper to fire my pots if the book was no good. Or make just use a page or tow to start the firing process. Just joking of course.

My Favorite Firing Technique

I love to use the good old electric kiln. The reason why is because I do not have to have a gas line installed. I am not operating out of a studio. All I need is an electric plug installed by an electrician in which is much less costly than having a gas line installed. Plus, depending on where you live most folks usually have electricity. Obtaining gas or propane may be more difficult.

I love the portability of an electric kiln. My kiln is set up on rolling lockable wheels. It’s simple to move the kiln out of the way or store it when not in use if you do not have a dedicated space for it. Simply unplug the kiln from the wall, unlock the wheels and roll it to its new location.

Electric kilns are relatively inexpensive. The costly part is the electronics. And depending on the brand of your kiln many manufactures can sell you electronic replacement parts and or entire controllers. This can save you from having to buy an entire kiln if something breaks.

So as you can see there are many advantages to an electric kiln. I am sure I missed some. This is why they are the most popular kilns. They have many advantages over other kilns. However, if you are a hard-core potter or wish to explore other types of firing techniques an electric kiln has its limitations and is not as versatile.

Definitions

Leather Hard

Leather-hard refers to a clay or clay body that has been partially dried to the point where all shrinkage has occurred and has a consistency akin to leather of the same thickness as the clay. At this point, the clay item has a moisture level of around 15%.

Clay Particles

Clay-sized particles are rock and mineral pieces with a diameter of less than 2 micrometers, or roughly the same size as most bacteria. Physical weathering of bigger rocks and minerals produces clay-sized particles. In soils, some clay minerals, such as vermiculite, may be seen and resemble gold flakes.

Crystalline Glazes

In comparison to conventional glazes, crystalline glazes have a high concentration of zinc oxide, which can make up to 30% of the glaze.

Slip Trailing

Squeezing lines of thick, liquid clay or slurry onto a wet or leather-hard clay surface using a squeeze-bulb or tube applicator is known as slip trailing in ceramics. The method of slip trailing is not new.

Oxidation Firing

An electric kiln is used for oxidation firing, however a gas kiln can also be used. When the glazes are fired, oxygen is free to interact with them. Colors that are exceedingly brilliant and rich are possible with oxidation firing. This can be accomplished by changing the fuel in a gas kiln or by adding organic material to a Raku kiln.

Terra Sigillata

Modern researchers use the Latin word terra sigillata to describe a type of painted red-gloss ceramics. Not every piece of red-gloss china was embellished.

Metal Oxides

Metal oxides are crystalline solids with a metal cation and an oxide anion in them. They usually create bases or salts when they react with water or acids.

Lustre Fuming

Luster is an over-glaze, which means it is placed over a vitrified, glaze-fired item and then fired again. Luster is made up of actual gold particles suspended in a liquid media, most often pine oil resin. It’s burned after it’s been put on your work to burn off the organic binder. this technique works on a wide variety of pre-glazed surfaces.

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References
https://en.wikipedia.org/wiki/Salt_glaze_pottery#/media/File:Salt_glazed_porcelain_fired_1320_deg_centigrade_lpg_gas_fired_iron_and_manganese_slip.jpg
https://en.wikipedia.org/wiki/Raku_ware#/media/File:Raku_15.06.2014_13-45-11.jpg