Short answers, pulled from the story.
Clay is a fine-grained natural soil material containing clay minerals, which are hydrous aluminium phyllosilicates such as kaolinite. The minerals form as tiny, thin plates of aluminium and silicon ions bonded by oxygen and hydroxide ions. Most pure clay minerals are white or light-coloured; natural clays pick up reddish or brownish hues from iron oxide impurities.
Some of the earliest known pottery shards have been dated to around 14,000 BCE. They were recovered in central Honshu, Japan, and are associated with the Jomon culture. Clay is the longest-known ceramic raw material.
Ancient peoples in Mesopotamia adopted clay tablets as the first known writing medium because local clay was easy to work with and widely available. Scribes wrote on them using a blunt reed stylus to produce the wedge-shaped marks of cuneiform script. Tablets could be reworked and reused, or fired permanently for long-term record-keeping.
Firing triggers a dehydration reaction that causes clay plates to bond irreversibly through covalent bonding, making the material hard and no longer re-wettable. At the earthenware stage, the clay mineral kaolinite converts to a non-clay material called metakaolin. Further firing through stoneware and porcelain stages recrystallises metakaolin into even stronger minerals, including mullite.
Clay soils have a high cation-exchange capacity, meaning they hold nutrient cations such as potassium and ammonium better than other soils, which supports fertility. However, clay retains water poorly for drainage, making crops harder to grow without artificial tillage and drainage systems. Because nutrients leach more slowly from clay, plants in clay soils may still require more fertiliser input.
Quick clay is a unique type of marine clay found in glaciated terrains in Norway, North America, Northern Ireland, and Sweden. It is highly sensitive to disturbance and prone to sudden liquefaction, meaning it can lose structural strength rapidly and flow like a liquid. Quick clay has been responsible for several deadly landslides.