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Clay is a fine-grained natural soil material that is rich in clay minerals. When wet, clays become flexible due to a molecular layer of water around the clay particles, but when dry or fired, they become hard, brittle, and non–plastic. The majority of pure clay minerals are white or light-colored, however impurities can give natural clays a range of colors, such as a reddish or brownish color from tiny quantities of iron oxide.

Clay is the earliest ceramic substance that has been discovered. Clay’s characteristics were discovered by prehistoric people, who utilized it to make pottery. Clay tablets were the oldest known writing medium, and some of the earliest pottery fragments have been dated to approximately 14,000 BC. Clay is utilized in a variety of modern industrial processes, including paper manufacture, cement manufacturing, and chemical filtration. Between half and two-thirds of the world’s population still lives or works in structures that use clay as a load-bearing framework, which is typically baked into brick.

Clay is a widely used material. The most common sedimentary rock is shale, which is primarily composed of clay. Clays are differentiated from other fine-grained soils by variations in size and mineralogy, despite the fact that many naturally occurring deposits contain both silts and clay. Silts have greater particle sizes than clays because they are fine-grained soils that do not include clay minerals.

Loam is a sand, silt, and clay mixture with less than 40% clay content. Soils containing a lot of swelling clays, which are clay minerals that expand a lot when they absorb water, are a big problem in civil engineering.

Clay’s fluidity while wet and capacity to harden when dried or fired are its distinguishing mechanical properties. Clays are extremely plastic throughout a wide range of water content, from a minimum water content (referred to as the plasticity limit) where the clay is just moist enough to mould to a maximum water content (referred to as the liquid limit) where the moulded clay is just dry enough to maintain its shape.

The plastic limit of kaolinite clay is around 36 percent to 40 percent, while the liquid limit is around 58 percent to 72 percent. The amount of mechanical labor required to roll a piece of clay flat demonstrates the toughness of high-quality clay. It has a high degree of internal cohesiveness, which explains its hardness.

Clay has a high clay mineral content, which gives it its plasticity. Clay minerals are hydrous aluminium phyllosilicate minerals made up of aluminium and silicon ions linked together by oxygen and hydroxyl ions form small, thin plates. These plates are strong but flexible, and they stick together in wet clay. The particles that emerge provide the cohesiveness that makes clay plastic. The connection between the plates in kaolinite clay is provided by a layer of water molecules that hydrogen bonds them together. The bindings are weak enough to enable the plates to slip past each other while the clay is shaped, but strong enough to keep the plates in place and allow the moulded clay to retain its shape once it has been sculpted.

Most of the water molecules are gone as the clay dries, and the plates hydrogen bind directly to each other, making the dried clay stiff but yet brittle. If the clay is wet again, it will return to its original plastic state. A dehydration process occurs when the clay is burned to the earthenware stage, removing more water from the clay and enabling clay plates to permanently cling to one another via stronger covalent bonding, strengthening the substance.

When the clay mineral kaolin is wet again, it transforms into metakaolin, a non-clay substance that stays stiff and hard. The metakaolin is recrystallized into even stronger minerals like mullite as it is fired through the stoneware and porcelain phases.

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