Soil Types, Geology, Properties and Color Chart (GcCeramics)

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Lessons from Glendale College Ceramics
Original from GcCeramics: soil geology
For educational purposes only.

Soil types, geological origin and soil function properties

geological origin of soil

Clay is a mineral ‘stew’ that is the result of erosion of the earth’s crust over a vast period of time. What was originally the mineral feldspar in igneous rocks, primarily granite, breaks down over time and becomes the finer-grained clay that we form with our bare hands. How this change occurs is a matter of geology and time. Igneous rocks disintegrate due to the effects of erosion over a vast period of time, and the feldspar material turns into kaolinite, which is the recognizable substance in soils.

Soil deposits that remain at or near the site of the parent material (granite) are called residual or primary soils. These so-called residual soils are granular and lack the lubricity required for workability. These clays are called non-plastic because they do not take shape easily. Soils that have been carried by water, wind and ice and deposited in places away from the source material are called sedimentary or secondary soils. Compared to residual soil, sedimentary soil is more plastic, and the particles are smaller, more uniform, and more mixed with other materials. Under the microscope, the clay particles are similar in size to playing cards. They are flat, hexagonal and thin like a card. When wet, the particles can ‘slip’ over each other, as in a deck of cards. This ability to ‘slip’ gives a clay its workability, which is called plasticity. So, in short, potters need plastic clay to throw the wheel and make hands. Mining companies explore the world to find natural deposits of clay to mine and mix for sale to industry and studio potters. [Image (right) Red Terra Cotta Clay]

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In short:

Breaks I
sedimentary rock mixed with other mineralsI
– which are mixed with clay bodies –

earthen lumps

Rarely do potters use a single, sedimentary clay as the working soil. Experience has taught that even better results are obtained when many different soils are mixed together. Such mixed soil is called clay body. By blending, potters could also change the color and texture of their clay. There are two general categories of clay bodies:

earthenware clay
stoneware/porcelain clay
Firing temperatures rarely exceed 2500 °F. be more than Firing temperature 2050 – 2400 °F . happens between
Colors range from white to terra cotta (brick red) Clay color ranges from white (porcelain) to brown (stoneware)
texture varies from smooth to rough The texture varies from smooth (porcelain-no grog) to rough (stoneware-containing grog).

Note that the primary difference between the ranges is the maximum firing temperature possible. Earthenware clay will melt if stoneware and porcelain are fired at high temperatures. This is extremely important to know when buying soil for use. Wrong choice will result in all your work melting in the kiln, ruining what you have created. But this is only the beginning of the disaster. Your pieces will melt on our kiln shelves, fuse in them at extremely high temperatures. You will destroy the kiln shelf at a cost of $100.00 each! You will be responsible for this damage. Your pieces will also melt in the kiln on people’s work, ruining them too. It’s impossible to put a price tag on this disadvantage, but you won’t be popular in the classroom if it happens. Avoid the problem by only using clay sold by a bookstore.

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Soil Types, Geology, Properties and Color Chart (GcCeramics)
Soil Types, Geology, Properties and Color Chart (GcCeramics)
stoneware Porcelain terra cotta

Note that another difference in clay is color. Clay that is brown, gray or brick in color contains iron oxide (terra cotta and stoneware) as a coloring agent. Soils that lack iron oxide are brown to white in color (porcelain).

Note that another difference in clay is texture. Clay particles vary in size, and some are much coarser than others. Often coarse clay bodies contain a particulate additive called grog which gives roughness to the body. Porcelain clay contains little or no grog. Stoneware clay usually has a few. Earthenware soils may or may not contain grog, so this difference alone does not help us distinguish it from low-high-temperature soils. Grog is usually either sand or burning clay that has been crushed and shaped. Due to the lack of microscopic size and shape of clay particles, grog reduces the plasticity of the clay body, but has a beneficial effect on shrinkage. Since it is not clay, the grog does not shrink like clay. Therefore, its presence in the soil reduces the overall shrinkage rate of the soil; More grog = less shrinkage, less grog = more shrinkage. So, the presence of a small amount of grog in a clay body can be a good thing. This minimizes shrinkage, yet will not significantly reduce plasticity if not used in very high amounts. Porcelain clays lack grog, and consequently have the highest shrinkage rates, making them extremely difficult to use for inexperienced potters. Most of your work will break in drying. I don’t recommend starting with porcelain for this reason.

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DDifferences in clay ingots after firing

stoneware / porcelain
porous non porous
something delicate less fragile
brighter color range possible more muted colors possible
Color Chart: Some Examples of Burnt Clay

porosity Refers to the ability of a material to absorb water. Earthenware pieces, after being burned at low temperatures, do not fully mature, or vitrify, and as such, allow water to slowly pass through the wall of the pot. The higher the firing temperature, the less water can pass through, so ceramics stained near 2000°C will exhibit little porosity, especially if they are glazed. Conversely, less fired pieces, such as red clay planters, fired around 1200 degrees Fahrenheit, will exhibit clear pores. When stoneware and porcelain are fired at extremely high temperatures, little, if any, porosity is visible, even if the vessel is unglazed.

Just as higher temperatures lead to greater water retention, stoneware/ceramics that burn at temperatures are stronger and more durable for everyday use. Additionally, as the temperature increases, the dyes are driven by the glaze, so that fewer colors are possible at higher temperatures.

b-mix with sand A stoneware that is unusual in that it contains little iron oxide, which produces a stoneware that is unusually white in colour. This soil contains certain grogs, which are helpful in reducing cracks in light colored soils. B-mix is ​​a good clay for wheel throwing or hand making.

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long beach stoneware There is a dark stoneware, which contains iron oxide and grog. It is an excellent wheel and slab soil. ,


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