Ball Clay page

Ball clay is an extremely rare mineral found in very few places around the world. The blue clays of the Bagshot beds found around Wareham are of the finest in the world. The main concentration of ball clay in Dorset is to the north of the Purbeck Hills centered around Norden. The term "Ball Clay is applied to those plastic "transported" clays which when fired in an oxidising atmosphere to the temperature of certain pottery ovens- approx 1150°-1200°C have a white or nearly white colour. Its name dates back to the early methods of mining when specialised hand tools were used to extract the clay in rough cube shapes of about 30 cm. As the corners were knocked through handling and storage these cubes became rounded and ‘ball’ shaped. Like many other clays, Ball Clays are formed by the decomposition of felspathic rocks by natural agencies. In this decomposition silicates such as the felsphars break down, and the products ultimately undergo hydration with the formation of of the hydrated silicate of aluminum, kaolinite, and in many cases mixtures of hydrated oxides. Where these products are found resting in the parent rock, the clays are call residual; where they have been transported and deposited elsewhere, they are known as transported clays. The China clays of Cornwall are typical examples of residual clays and the ball clays of Devon and Dorset examples of transported clays. The two types differ in one important respect; the residual clay contains un-decomposed constituents of the parent rock; for example, in the case of china clay, the quartz and the mica; the transported clay, on the other hand has undergone a variable amount of natural 'sorting'. All this took place approximately 45 million years ago in tropical times. An ancient river Solent washed Kaolinite from its parent rock on Dartmoor. (At this time the Isle of Wight was still joined to Purbeck.) As the streams flowed from upland areas the mixed with other clay minerals, sands, gravels and vegetation before settling in low-lying basins to form overlaying seams of ball clay. Ball clays contain three dominant minerals: from 20-80% Kaolinite, 10-25% mica, and 6-65% quartz. In addition, there are other 'accessory' minerals and some carbonaceous material (derived from ancient plants) present. The wide variation both in mineral composition and in the size of the clay particles results in different characteristics for individual clay seams within a deposit.

Purbeck Ball clay has been used for thousands of years but large scale commercial extraction began in the middle of the 18th Century continuing to the present day with open cast workings at Povington, Arne and North of Wareham.

Before the rise of Poole, Wareham was the port for the area, and 32,000 tons of cargo were recorded shipped from the quay in the 1860's.

This view shows the old clay warehouse in Wareham from which ball clay was loaded on to ships. This building is now situated in the grounds of "The Priory" hotel.

The British Geological Survey describes ball clay as ‘relatively scarce globally’, and hence of importance to the UK’s economy. In 2002, UK ball clay was an essential ingredient of 50% of the world's production of sanitaryware.

Within the UK, ball clay occurs commercially only in the Wareham Basin of Purbeck and within two areas of Devon. Nowadays it is used primarily in the ceramics industry for the production of wall and floor tiles (about three quarters of output), tableware, sanitary ware and electrical insulators. It is also used as a filler in certain rubber and plastic products such as windscreen wipers, and garden hoses.

Over the four years prior to 2009, the production of ball clay in Dorset fluctuated around 250,000 tonnes per annum (tpa), although in 2008-2009 this fell to 150,000 tpa due largely to the global economic recession. Since then there are signs of recovery.

Click Here to read an account of life for the clay workers and others in Purbeck written in 1885 (year of the opening of the Railway) for the American Journal "Harpers New Monthly Magazine".

The company now extracting in Purbeck is Imerys who bought out English China Clays in 1999. Imerys has kindly made a substantial donation to Purbeck Mining & Minerals Museum group of all their redundant buildings at Norden.

Extraction methods have constantly evolved. From an early date a specially shaped spade - or tubal - was developed, which was eventually superseded by the pneumatic spade. The clay has been quarried and mined. When mined the use of shaft mines and adits was common place in Purbeck as conditions dictated.

Initially the extracted clay was weathered, a manual process that required it to be spread across large "beds" in the open. In later years it was found that this process could be speeded up artificially by shredding.

Individual raw clay selections are blended carefully according to pre-determined recipes to provide a product with a consistent and predictable range of characteristics and behaviour. The first stage in processing then is to shred (or ‘kibble’) the blended clay into smaller, more regular lumps about the size of a golf ball. Much blended clay is sold in this shredded form. Further processing through drying and grinding yields powdered plastic clays and ceramics manufacturers (particularly in the sanitaryware sector) also have benefited from the development of refined ball clays which offer improved performance and reduced manufacturing process costs. Refined clays are available in ‘noodled’ and slurried form. Ball clays also are sold in a partially dried form, which consist of golf ball size materials that are not powdered. .

A vital material in ceramics - Ball clays are used in many different industries, but in particular form a vital component in ceramic manufacturing. Kaolin (‘china clay’) produces a very white colour when it is fired, but used alone it is brittle and weak and must be mixed with ball clay to produce a workable, malleable raw material. As a result of their sedimentary origin, raw ball clays have a wide range of colours. However, many of them are valued by the ceramics industry for their white-firing properties, which are determined by the levels of iron and other colouring/fluxing oxides within the clay.

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