Reading is located in an area that has no indigenous building stone. The local geology comprises non-marine sands and mottled clays from the Palaeogene Period around 65 million years ago when the land around Reading was a river delta with marshy mud flats of clay, sand and silt. These clays are specifically known to geologists as the Reading Formation. The Reading Formation lies over an older bedrock of chalk. Both the chalk and the Reading Formation clays are poorly consolidated materials that in their raw state are not generally suitable for use as building stones.
What is clay? Clay is a sedimentary rock made of tiny particles which come from the weathering of other rocks and minerals which may be of differing geological ages and composition. It is a kind of mineral stew.
Clay is flexible and malleable when it is wet but it becomes hard on drying or firing. Other properties of clay include shrinkage under firing, colour after firing, fineness of grain, cohesion, and capacity of the surface to take decoration. The fact that wet clay is easily moulded into a form that is retained when dried enables clay to be widely used in the making of bricks, tiles and terracotta. The different clays extracted from a number of areas across the country often dictate the colour and properties of the finished brick, providing a clear indication of the clay’s origin.
The presence of the Reading Formation clays enabled local clay bricks to be made on a small scale from the medieval period but from the 1850s clay brick production in Reading expanded greatly in both quantity and range of colours. There were brickworks at Katesgrove, Coley, Caversham, Calcot and Tilehurst using the local Reading Formation clays from the adjoining pits. Terracotta was also produced at these brickworks from the clay. The Town Hall and Queen Victoria Street are good examples of buildings from this time using the local clays. However what is really special about Reading is sheer number of small terraced houses which were constructed with brickwork of red, cream and light grey in different patterns all from the local clays.
Where did the colours come from? The colour of the resultant brick was dependent on the minerals or impurities in the clay, or the qualities of the clay. Differing processes during the firing of the clay also produced differing colours of bricks. The red colour associated with Reading bricks comes from iron oxide in the clay. Clays with a higher lime content produced white, cream or yellow bricks. Grey, particularly when on the surface of the brick only, is produced as a result of firing.
Clay is liable to weathering. There are three types of weathering, physical, biological and chemical. Physical weathering can be caused by the effects of rain and changes in temperature such as freezing and thawing. Biological weathering can be caused by the growth of plant roots. Chemical weathering can be caused by acid rain or pollution.
Bricks manufactured from clay weather by spalling, cracking, and dusting. When rainwater penetrates the bricks and the water subsequently freezes and expands it causes the surface to disintegrate. It is not necessarily the coldest or wettest winters that lead to this type of failure, but rather recurring freezing and thawing cycles in saturated brickwork. The rate of damage may vary according to the particular clay used in the manufacturing process and soft red bricks appear more prone to the effects of weathering and are less resistant to frost. Failure of bricks is often associated with particular locations on a structure such as at ground level due to the upward movement of ground water or under sills particularly where the sill does not extend past the face of the bricks below. Brickwork is unlikely to become saturated where projecting features shed run-off water clear of the walling below.
The start coordinates for this earthcache are located one of the entrances to Reading Town Hall. The outer walls of the main Town Hall and clock tower were built in 1875 using red and grey bricks and terracotta ornamentation manufactured at brickworks in Tilehurst using the local Reading Formation clays.
In contrast, the Concert Hall, Museum and Library in Blagrave Street which were added in 1882 were made from harder, darker bricks produced from clays which originated in the Etruria Formation of the Upper Coal Measures in the Staffordshire area in the Carboniferous age around 359 – 299 million years ago. These clays resulted from the weathered material from the coal measures and produce much darker blue grey bricks. The particular mineralogy of this clay and the low levels of impurities make it suitable for the manufacture of high strength bricks with low water absorption.
TO LOG THIS EARTHCACHE, please email me the GC number and the cache title together with your answers to the questions below. Please do not put the answers in your log.
1. What is the name given by geologists to the type of clay used to make the bricks in this area and when was it formed?
2. At the start coordinates look at the light grey bricks. Another colour can be seen just below the surface on some of these bricks. What is that colour and where does it come from?
3. Now walk to the left of the entrance until you reach the spot where the façade changes from light grey bricks to much darker grey bricks. Compare the texture of the light and dark grey bricks. What are the two main differences and why do you think the textures are different?
4. Turning back again, between the darker grey façade and the entrance at the start coordinates there are two pairs of windows set behind some low railings (the lower railings). Some of the brickwork here shows signs of weathering. It seems to be concentrated in a specific location. Where is this brickwork located and why do you think it has weathered more quickly here than elsewhere on the building?
I hope you enjoy my earthcache and good luck with the answers.