Why does clay become hard and waterproof when it’s heated? Why does temperature affect different clays in different ways?

Question asked by Volunteers at the Idaho Art Lab | Answered by Dr. Paul Link, ISU Department of Geosciences

The purest clay is made of the mineral kaolinite, KAlSi3O8. Ceramic clays also contain quartz and feldspar minerals. During the heating (firing) process to about 1400 degrees F, the feldspars melt, along with some of the quartz (silica), to form a glass phase that bonds the clay and silica into a durable ceramic. The quantity (percent by weight) and kind of aluminum and silicon in a particular clay determines the temperature at which that clay, when heated, will harden, and if heated enough, will become waterproof.

Clay is composed of the three most abundant elements in the earth’s crust, oxygen, aluminum and silicon. There are many clay minerals, and all are “layered”. Many are also hydrous, containing layers of water between the silicate layers. Impurities, small amounts of Iron, Copper, Calcium etc. give clay its colors (red, brown, tan, etc.) and affect the required firing temperature.

The making of clay pots first involves removal of the interstitial water in the clay minerals by heating. Before firing, clay pots can be soaked and the potter can start over. Kilns heat ceramic clay to about 1400 degrees F. At this temperature the feldspar and quartz start to melt. The hotter the clay is fired, the more dense and glasslike the pottery becomes.

With consultation from Dave Finkelnburg, Technical Editor, Ceramics Monthly, and Chris Picket, ISU Department of Art

Soil is the top part of the earth’s crust where plants grow and animals live. Soils normally contain air, water, minerals and humus (matter left over from animals and plants.)

Soil minerals are broken down rocks and volcanic ash. Soil minerals come in three types that differ in size: sand, silt, and clay.

  • Sand particles are 0.002 to 0.08 inches in diameter-and are visible to the naked eye. They are similar in size to table salt.
  • Silt particles are from 0.002 to 0.00008 inches in diameter and individual particles are too small to be seen with the naked eye. They are similar in diameter to an eyelash.
  • Clay particles are smaller than 0.00008 inches in diameter. Some clay particles are so small that ordinary microscopes do not show them. They are similar in size to flour particles.

To find out how much sand, silt and clay are present in a sample of soil, try this test.
Materials: about 1/4 cup of soil, paper, small bottle or glass, spoon, water

  1. Pour the soil sample on the piece of paper. Remove pieces of humus (plant or animal material) and gravel.
  2. Place the soil in the bottle.
  3. Fill the bottle to the top with water.
  4. Stir the water and soil. Place it on a flat surface and don’t touch it for an hour. At the end of an hour, the water will have cleared and you will see that the larger particles have settled. On the surface of the water there may be bits of organic matter floating. On the top is a layer of clay. If the water is still not clear, it’s because some of the finest clay is still mixed with water. In the middle is a layer of silt. At the bottom is a layer of sand.
  5. Measure the depth of the sand, silt, and clay and estimate the approximate proportion of each.

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