Kōrero: Rock, limestone and clay

Whārangi 7. Types of clay

Ngā whakaahua me ngā rauemi katoa o tēnei kōrero

Kaolinite

Kaolinite, New Zealand’s most common clay mineral, is formed by the deep weathering of many different rocks such as granite, schist and greywacke. It is widely quarried and manufactured into domestic brick, tile, pipe, ceramics and pottery. It is also used as a filler in rubber, bitumen and adhesives, and highly pure kaolinite is used for paper coating.

In the 2000s the largest kaolinite clay pits were in the wider Auckland area, while South Island clay pits were typically smaller operations. The biggest brick-making operation in the early 2000s was in Auckland. Small-scale pits also keep New Zealand’s potters supplied with raw material.

Making marbles

Len Castle, a New Zealand ceramic artist, first encountered clay in the 1930s as he sat under a pōhutukawa tree on Westmere beach, Auckland. He started his career early, making marbles out of clay for his classmates during the Second World War, when glass was in short supply. He would fire them in the kitchen oven, often with explosive results.

Halloysite – white clay

Under certain conditions the volcanic rock rhyolite weathers into a distinct type of clay. Known as halloysite, it has been altered by hydrothermal fluids passing through it and then weathered by rain and exposure to the atmosphere. The producers of halloysite clay from deposits in Matauri Bay, Northland, claim that it is the world’s whitest clay. The clay is indeed white and bright, and is very expensive.

The Matauri Bay operation is the largest single clay producer in New Zealand. Most of the clay is exported for the manufacture of high-quality ceramics including porcelain, bone china and technical ceramics. Silica sand, a by-product, is sold to the building industry and used in golf course bunkers. In the early 2000s there were sufficient resources for over 30 years’ production.

Bentonite

Bentonite is derived from the weathering of volcanic ash deposits. A small quarry digs bentonite from the Harper Hills, 65 kilometres west of Christchurch. This clay can absorb large amounts of water, making it useful as a lubricant in drilling operations, a medium for growing mushrooms, and a stock-feed additive.

Me pēnei te tohu i te whārangi:

Carl Walrond, 'Rock, limestone and clay - Types of clay', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/mi/rock-limestone-and-clay/page-7 (accessed 13 December 2019)

He kōrero nā Carl Walrond, i tāngia i te 12 Jun 2006