|One of Janis Lang's soil paintings. http://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/edu/?cid=nrcs142p2_054282|
Specifically clay is the type of soil used for ceramics. Soil consists of several different sized particles: sand, silt, and clay. Clays are the smallest of these, but just because a particle is small (less than 2 microns) does not automatically make it a clay. Clay particles have a particular mineralogy or elemental make-up and structure that allows for various chemical and physical properties. One of these is that clays behave as colloids. Existing as a colloidal means when particles are suspended in water they do not settle out due to chemical characteristics. Furthermore not all clays are chemically the same in that they behave differently in regard to chemical reactions that can occur as well as how they interact with water.
|Dewatered (dried out) clay of a former salt marsh.|
We can see with our eyes the various chemical differences in clays from the color present. A red clay will have oxidized iron, while a grey clay may be reduced iron (Oxidation and reduction refers to the capacity of an atom to accept or donate an electron for those of you interested). A white clay is totally absent of iron. Likewise, the physical property differences can be seen and felt and are a reflection of the arrangement of layers of atoms in the clays. I won't go into specifics, but broadly there are two main groups, 1:1 clays and 2:1 clays. Clays from the 1:1 silicate clay group tend to have less water holding capacity (less shrinking and swelling when wet and dry), less plasticity (as in workability like play-dough), less stickiness and cohesion. This group is also known as the kaolinitic clays. The 2:1 clays have the ability to greatly shrink and swell causing major issues for buildings and agriculture (See Solid Ground post from May) and have greater plasticity and cohesiveness than the 1:1 clays. So why does having different physical and chemical properties matter in ceramics? The point of ceramics in the simplest sense is to mold a clay into a shape and keep it that way. Ceramics use kilns (ovens) to finalize this desired shape of the clay. Different properties of the clay will affect how these shapes will end up in terms of color (prior to glazing), and also affect what temperatures and the processes used to solidify the shape. Combinations of different types of clays as well as other soil particles (silt and sand) can create pieces that have a variety of different properties. The type of piece or use desired will dictate what type of clays (or non-clay materials are needed) to create. For example making a plate would require very different material than a brick.
|Mottles of oxidized iron (red) in clay soils.|
Clays will solidify when dried. However, only adobe clays are durable when air-dried. Therefore in ceramics, clays must be dried in a kiln. During this process the clay will shrink. The more water a clay holds to greater possibility for shrinking (possibly unevenly) and cracking. How the clay is dried and fired will effect the overall outcome of the piece. This process is typically slow to avoid the aforemenioned issues that can occur even in clays (even those with lower water holding capacity). As the temperature in the kiln increases water is lost through evaporation (100 degrees Celsius) and dehydration (350-500 °C). Above 500 °C the clay has lost water in the interior part of the clay layers and these chemical changes are not reversible. As the temperatures get higher carbon is lost (900 °C) and other compounds are oxidized and lost as well (e.g. sulfates). The final step is "vitrification" where the hardening step occurs. The original chemical structure (i.e. mineralogy) begins to disappear and the elements are fused together in a different pattern. With porcelain, this process occurs completely where the material is totally glass-like in that the original crystalline pattern is lost and in its place one that is not as "organized." You can think about it on a larger scale in terms of quartz crystals versus glass. Many clays begin as a crystal formation and through this firing process become more glass like.
To complete apiece in the ceramic process usually a glaze is applied to provide color and water-proofing. Glazes are a glass finish that is coated over the fired pottery. The glazed pottery undergoes a firing process in the ~600-1400 °C range, which will depend on the glaze used. Glazes are silica based, but contain other elements that provide color to the finished piece. Below are pieces that are from various clays and have several glazes used. Glazing temperatures for these four pieces ranged from 1223 °C to 1351 °C.
|An example of several ceramics that have various clays and glazes (the finishing color and waterproofing) from Brian Johnstone of Nehalem Clayworks.|
1) Wood ash glazed teapot with dark clay body and semi-porcelaneous handles
2) Salt-glazed two-piece iron-rich clay goblet with Rutile oxide slip (liquid clay w/ coloring oxides added) under
3) Dark Celadon-glazed porcelain lidded bottle.
4) Extreme high-temperatutre salt-glazed pitcher with rutile/cobalt underglaze slip.
The complexity of these processes unfortunately cannot be addressed fully in this short blog post. However, two excellent reads (thank you, Brian!) are Ceramic Glazes by Cullen Warner Parmelee and Clay and Glazes for the Potter by Daniel Rhodes (which much of the information presented here was gleaned from). Also, if you are traveling the Northern Oregon Coast be sure to stop by Nehalem Clayworks to see a ceramics studio in action and talk with Brian and Kate Johnstone about their work!
I hope you have enjoyed the the International Year of Soil themed blog posts from this past year. More posts to come in 2016, so stay tuned! As always requests for soil topics that interest you can be submitted in the comments.
Happy New Year from Talus Soil Consulting!