36 CONTEXT 182 : DECEMBER 2024 of the equator to where it is now, it passed through climate zones which form deserts south of the equator, swamps at the equator and more deserts to the north. Changes in climate, atmosphere, land mass, sea level, and the amount and form of iron present in the environment, have all influenced the appearance of the stone we see today. Some sandstones were formed in a dry, desert-like environment. In the absence of water, they can be coloured with iron(III) oxide (Fe₂O₃, ferric oxide), also found as the pigment Venetian red. As an example, 250 million years ago or so, what is now England was at a similar latitude to where the Sahara desert is today, and the sandstone that formed (New Red Sandstone) is coloured with iron(III) oxide. Sandstone that is formed in a wet environment with iron present can be coloured with iron(III) hydroxide, which brings yellow to brown colours. Iron(III) hydroxide is found as the mineral limonite and as the main component of the pigment raw sienna. Examples are the sandstones of the Carboniferous period 360 to 300 million years ago, when Britain was in the vicinity of the equator. Iron hydroxide breaks down when heated above 200–300 centigrade to form ferric oxide, which explains why some yellow or buff stone acquires a pinkish blush when exposed to fire. Initially I thought that this was a fairly conclusive and straightforward explanation for Ferrous oxide to the fore at Holy Trinity Church, Newcastle under Lyme (not a church covered in The Pattern of English Building), described by Nikolaus Pevsner as ‘a crazy effort in blue brick’. But the bricks are not a pure blue: red ferric oxide hues are visible. the colour of red and yellow stones. But I fear not, for red stones are also found in association with water, such as the remarkable red chalk of the Hunstanton Formation, in north Norfolk. There is more to find out. The colour of brick Bricks are remarkable things. It is astonishing that heat can turn clay, a weak, pliable material formed of sheets of atoms of aluminium, silicon, oxygen and hydrogen (and other elements) between four and ten atoms thick, into a highly resilient material that can support great weight and last thousands of years. But what is a brick? My impression is that much of the knowledge of bricks is based on chemical experiment and centuries of painstaking practical experience and empirical study: a certain clay when fired in a particular way will produce a certain type of brick. But science continues to advance. There have been developments in scientific instrumentation such as X-ray diffraction, and new forms of spectroscopy which allow crystal and molecular structures to be examined and precisely identified. I sense that there is a deeper understanding buried in complicated academic papers that could be shared: what it is about certain clays and firing techniques that produce certain types and colours of brick. At the simplest of levels, brick is an artificial metamorphic rock, where under firing some of
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