Ceramic chemistry

Ceramic chemistry is a branch of inorganic chemistry that studies the relationship between the physical properties of fired ceramic glazes and their chemistry. Although ceramic technicians have long understood many of these relationships, the advent of computer software to automate the conversion from batch to formula and analysis has brought this science within the reach of many more people. Physical properties of glazes in fired products (like thermal expansion, hardness, index of refraction, color and melting temperature or range) are directly (but not solely) related to the chemistry. Properties of glass melts like viscosity and surface tension are also principally products of chemistry.

Technicians in the ceramic tile, tableware, artware, pottery, sanitaryware, glass, fiberglass, bottle glass, optical and related industries all use this science^{[citation needed]}.

In ceramic chemistry, fired glazes are viewed as composed of oxides (examples are SiO₂, Al₂O₃, B₂O₃, Na₂O, K₂O, CaO, Li₂O, MgO, ZnO, MnO, Fe₂O₃, CoO). Each oxide is known to contribute specific properties to the fired glass. Materials suppliers publish chemical analyses of their products that cite percentages of these oxides as well as volatiles (oxides that burn away during firing like H₂O, CO₂, SO₃).

For example, in traditional ceramics here are some examples of what the application of ceramic chemistry can accomplish^{[citation needed]}:

• Fix glaze defects like crazing, blistering, pinholing, settling, clouding, leaching, crawling, marking, scratching, powdering.

• Substitute frits or incorporate better, cheaper materials, replace no-longer-available ones.

• Adjust glaze melting temperature, gloss, surface character and color (in ceramic color is a matter of chemistry).

• Identify weaknesses (e.g. proximity to volatile firing zones, use of unreliable materials) in glazes to avoid problems.

• Creating and optimizing base glazes to work with difficult colors or stains and for special effects dependent on opacification, crystallization or variegation.

• Create glazes from scratch and use native materials in the highest possible percentage.

In ceramic bodies the physical properties of the final fired product are more related to the firing curve, the physical properties (e.g. particle size and shape, decomposition history) of the ingredient materials and the mineralogy and interaction between the different particle types^{[citation needed]}.

Raku

Crackle glaze

The chemical make up of the Raku pottery glaze called “crackle” is very unique, and differs from most other glazes. Because it is a Raku pottery glaze, it melts and sticks to the clay piece at a much cooler temperature. The Raku kiln gets to about 1,900˚ Fahrenheit, much cooler than the regular stationary kiln can get up to 3,200˚F but usually stays around 2,200˚F. The Raku glazes melt at around 1800˚F, whereas under glazes, and specialty glazes melt anywhere from 1,200˚F-2000˚F^[1]. The specific “crackle” glaze has another variable having to do with the elements in the glaze that other glazes do not. After the firing in the kiln, and the reduction process, most glazes color is already “frozen” and simply needs the “quenching” process (emerging the piece into water) to avoid thermal shock, which can result in cracking of the clay itself^[2]. When the crackle glaze is taken out of the reduction barrel the color is not frozen and stuck on the piece entirely, it can have many different visual effects. The time the piece of pottery is in the air before it is “quenched” is a large variable, because the amount of time carbon has to get in the cracks of the glaze is what determines the definition of the black lines in the cracks of the white glaze. The fact that the glaze could still be smeared and altered shows there is room for variations in the look of the glaze. The “quenching” process cools the glazes, and once the glazes are not at their melting point the color and place of the glazes are final.

Reduction Process

One aspect of the Raku firing is the reduction process. The reduction process happens directly after the firing of the pieces in the trash can, one moves the piece to the second trash can filled with a combination of full pieces of newspaper, and small strands of newspaper. One then puts the top of the trashcan back on, blocking off the supply of O2 thus the combustible material uses all of the oxygen, and thus the combustion process begins. During this process the heat from the pot touching the newspaper and the paper immediately ignites ^[3]. With the lid on, the combustible material uses everything in the can and begins to pull Oxygen out from the glaze ^[4]. Incomplete combustion leads to the formation of carbon dioxide, which embeds itself in the pot creating a smoky black outer layer.

Factors and Process

Raku ware, the ancient Japanese pottery method ^[5]. There are many factors that go into the raku process. After an initial firing, step one of this process is to put the piece into the raku kiln ^[6]. This kiln will heat the piece up to 1800 degrees fahrenheit which melts the glazes ^[7]. After it is at its maximum temperature the piece is removed with metal tongs and placed in an reduction chamber ^[8] The piece immediately ignites because of the chemistry. When the chamber is capped, the fire slowly burns out due to the lack of oxygen, creating a combustion reaction. A combustion is a chemical reaction between fuel and oxygen ^[9]. In the combustion reaction, the lack of oxygen causes a reaction in the glazes, making the glazes crack ^[10]. The amount of oxygen in the glaze, and the chemical makeup of the glaze depends on what color process it goes through. This process of reduction takes ten to thirty minutes ^[7]. The last step in the raku process is taking the pieces out of the can, or reduction chamber, and putting them into a bucket of cool water, creating the cooling process.^[11]. The factors that go into the results of a raku piece are the temperature you pull at, the temperature outside that day, how fast you go into the can, how fast the paper ignites, how long you hold the piece in the flames before you set it down, and how quickly you get the flames out when you put the lid on the can ^[12]. The process of raku is what determines the factors, and these factors determine how the piece looks in the end.

Combustible Material

An important part of Raku firing is the combustion reaction that occurs in the trash can as a result of various combustible materials reacting with oxygen once ignited. Every combustion reaction needs fuel and oxygen to occur ^[13]
In these reactions, fuel is a variable because there are so many different options for fuel. It could be anything that is combustible as long as carbon will be produced ^[13]. Some examples of these combustible materials are saw dust, newspaper, and dried leaves. The most common combustible material to use in the Raku process is any form of wood, for example sawdust or newspaper. This is because wood is made of 48.5% carbon, an extremely important element in Raku firing.^[14]. In Raku, it is important for carbon that is produced in the combustion reaction to absorb in the clay piece. This carbon will also give the piece a black color ^[15].

The Kiln

The materials used to make kilns are specially designed to hold heat and quickly rise to high temperatures. The cylinder shaped kilns are often lined with several materials that allow them to concentrate and confine the intense heat.^[16]
These materials have hight concentrations of alumina and, or silicon, metalloids which are able to reflect the heat from the pottery so that the Kiln itself dos not melt.^[17].
The high specific heat of these materials is what prevents the kiln from melting. Specific types of insulation that are used are refractory bricks, ceramic fiber, and aluminous cement. Gas burners are more commonly used for Raku firing because they allow the kilns to reach the neceassary temperature faster than a firebox using wood. The variations of kilns serve different purposes. An example is the downdraft kiln. It draws hot air into a side chamber where the pottery is located, this allows for a more even temperature throughout the kiln.* These special kilns are the main reason that this process is able to occur so quicly and while reaching such high temperatures at 990-1000°C, or 1652-1832°F

References

1. ^ www.theclaycellar.com/raku.html
2. ^ http://pottery.about.com/od/temperatureandmaturatio1/tp/glazerange.htm
3. ^ Rhodes, Daniel. "Clay and Glazes for the Potter"
4. ^ "THE LANGUAGE OF CLAY'; WORKS BY ARTISTS IN EVERSON EXHIBIT SHOW UP AT STONE QUARRY HILL POTTERY FAIR
5. ^ www.ceramicstoday.com
6. ^ www.ellenspottery.com
7. ^ ^{a b} www.ceramics-pottery.suite101.com
8. ^ www.ceramicspottery.suite101.com
9. ^ www.grc.nasa.gov
10. ^ www.nitaclaise.com
11. ^ www.ellenspottery.co
12. ^ www.raku-art.com
13. ^ ^{a b} Walker, Denise. Chemical Reactions. North Mankato, MN: Smart Apple Media, 2008. Print.
14. ^ Allaby, Michael. Fire: The Vital Source of Energy. New York, NY; Facts on file, 1993. Print.
15. ^ Mitchell, John. “Raku”. Personal interview. 30 Apr. 2010.
16. ^ Warshaw, Josie, Richard, and Stephen Brayne. "Kilns and Firing." The practical Potter, a Step-by-step Handbook: a Comprehensive Guide to Ceramics with Step-by-step projects and Techniques. New York: Hermes House, 2001.Book.
17. ^ http://robertcomptonpottery.com/Method%20of%20Raku-Firing-Pottery.htm

www.ceramicspottery.suite101.com
THE LANGUAGE OF CLAY; WORKS BY ARTISTS IN EVERSON EXHIBIT SHOW UP AT STONE QUARRY HILL POTTERY FAIR
Allaby, Michael. Fire: The Vital Source of Energy. New York, NY; Facts on file, 1993. Print.
Mitchell, John. “Raku”. Personal interview. 30 Apr. 2010.
Walker, Denise. Chemical Reactions. North Mankato, MN: Smart Apple Media, 2008. Print.
Warshaw, Josie, Richard, and Stephen Brayne. "Kilns and Firing." The practical Potter, a Step-by-step Handbook: a Comprehensive Guide to Ceramics with Step-by-step projects and Techniques. New York: Hermes House, 2001.Print.
http://robertcomptonpottery.com/Method%20of%20Raku-Firing-Pottery.htm
www.corvusmoon.com/kiln.htm