However, it's important to make a distinction. There is a difference between top soil and mud. Top soil is mostly organic matter, which would burn off in in a kiln firing.

Clay is not a single mineral, but a number of minerals. Clay has a high Alumina (AL) and Slicia content. Clays can also contain other materials such as iron oxide (rust) and rock fragments. These impurities can change the characteristics of the clay. For example, iron oxide colors clay red. The presence of silica increases the plasticity of the clay (that is, makes it easier to mold and form into shapes).

Commonly, Alumina content of fire clay ranges between 24% – 34% and Silica from 50% to 60%.

Where to find & collect clay in nature?

Mud can be found below top soil. Think of a place outside where you saw mud of a lighter color. This place would most likely be a location where there has been some water erosion or excavation work going on. (Remember, you have to get under the top soil.) Clay is everywhere around us but can not be seen until it’s uncovered. Also, you have to find a spot where it is not mixed with sand or rocks, and remember the light color. You need to uncover a mud which has a similar structure to play dough.

1. Stretch and flatten piece of the clay into 13cm or 5 inches long strip
2. With a sharp object make 2 marks in it - EXACT 10cm or 4″ between the marks
3. Leave the clay to dry
4. Remeasure when dry to see the shrinkage difference

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