Trancriber's Note Text emphasis is displayed as _Italic_ and =Bold=. Whole and fractional parts are displayed as 8-1/2. Although in general, subscripts are denoted _{#}. The formulae for the minerals are presented as K2OAl2O3·6SiO2 (Orthoclase) where the number following the element would normally be subscripted. The Cambridge Manuals of Science and Literature THE NATURAL HISTORY OF CLAY CAMBRIDGE UNIVERSITY PRESS London: FETTER LANE, E.C. C. F. CLAY, Manager [Illustration] Edinburgh: 100, PRINCES STREET London: WILLIAM WESLEY AND SON, 28, ESSEX STREET, STRAND Berlin: A. ASHER AND CO. Leipzig: F. A. BROCKHAUS New York: G. P. PUTNAM'S SONS Bombay and Calcutta: MACMILLAN AND CO., Ltd. _All rights reserved_ [Illustration] THE NATURAL HISTORY OF CLAY BY ALFRED B. SEARLE Cantor Lecturer on Brickmaking, Author of _British Clays, Shales and Sands_; _The Clayworker's Handbook_, etc., etc. Cambridge: at the University Press New York: G. P. Putnam's Sons 1912 Cambridge: PRINTED BY JOHN CLAY, M.A. AT THE UNIVERSITY PRESS _With the exception of the coat of arms at the foot, the design on the title page is a reproduction of one used by the earliest known Cambridge printer, John Siberch, 1521_ PREFACE Both as raw materials and in the form of pottery, bricks, tiles, terra-cotta and many other articles of use and ornament, clays are amongst the most important rock products. Yet the origin of the substances we know as 'clay,' the processes occurring in its formation and the causes of some of the most important of its characteristics are of such a nature that it is remarkable that its use should have become so extended in the arts and sciences, while we know so little of its properties when in a pure state. In the following pages an attempt has been made to state in a simple form an outline of our present knowledge of the subject and to indicate the problems which still lie before us. The experimental solution of these problems is rendered peculiarly difficult by the inertness of the materials at ordinary temperatures and the ease with which the clay molecule appears to break down into its constituent oxides at temperatures approaching red heat or as soon as it begins to react with alkaline or basic materials. Another serious difficulty is the highly complex nature of that property known as 'plasticity' to which many clays owe their chief value. For many years this has been regarded as an elementary property such as hardness, cohesion or colour, but it is now known to be of so elusive a nature as almost to defy measurement with any degree of accuracy.