The ancient Egyptians also used sun dried mud bricks as building materials, evidence of which can still be seen today at ruins such as Harappa Buhen and Mohenjo-daro. Paintings on the tomb walls of Thebes portray slaves mixing, tempering and carrying clay for the sun dried bricks. These bricks also consisted of a 4:2:1 ratio which enabled them to be laid more easily.
The Romans however distinguished the bricks which had been dried by the sun and air and those bricks which were burnt in a kiln. Preferring to make their bricks in the spring, the Romans held on to their bricks for 2 years before they were used or sold. They only used clay which was whitish or red for their bricks.
Using mobile kilns, the Romans were successful in introducing kiln fired bricks to the whole of the Roman Empire. The bricks were then stamped with the mark of the legion who supervised the brick production. These bricks differed from other ancient bricks in size and shape. Roman bricks were more commonly round, square, oblong, triangular or rectangular. The kiln fired bricks were generally 1 Roman foot by 1 Roman foot, but with some larger bricks at up to 3 Roman feet. The Romans preferred this type of brick making during the first century of their civilisation and used the bricks for public and private buildings all over the empire.
The Greeks also considered perpendicular brick walls more durable than stone walls and used them for public edifices. They also realised how the modern brick was less susceptible to erosion than the old marble walls.
During the 12th century bricks were reintroduced to northern Germany from northern Italy. This created the brick gothic period which was a reduced style of Gothic architecture previously very common in northern Europe. The buildings around this time were mainly built from fired red clay bricks.
Brick Gothic style buildings can be found in the Baltic countries Sweden, Denmark, Poland, Germany, Finland, Lithuania, Latvia, Estonia, Belarus and Russia. The brick gothic period can be categorized by the lack of figural architectural sculptures which had previously been carved in stone. The Gothic figures were impossible to create out of bulky bricks at that time, but could be identified by the use of split courses of bricks in varying colours, red bricks, glazed bricks and white lime plaster. Eventually special shaped bricks were introduced which would imitate the architectural sculptures.
During the renaissance and Baroque periods, exposed brick walls became unpopular and brickwork was generally covered by plaster. Only during the mid 18th century did visible brick walls again regain some popularity.
Modern Brick Making
Bricks are more commonly used in the construction of buildings than any other material except wood. Brick and terra cotta architecture is dominant within its field and a great industry has developed and invested in the manufacture of many different types of bricks of all shapes and colours. With modern machinery, earth moving equipment, powerful electric motors and modern tunnel kilns, making bricks has become much more productive and efficient. Bricks can be made from variety of materials the most common being clay but also calcium silicate and concrete. With clay bricks being the more popular, they are now manufactured using three processes soft mud, dry press and extruded. Also during 2007 the new ‘fly ash’ brick was created using the by-products from coal power plants.
Good quality bricks have a major advantage over stone as they are reliable, weather resistant and can tolerate acids, pollution and fire. Bricks can be made to any specification in colour, size and shape which makes bricks easier to build with than stone. Brickwork is also much cheaper than cut stone work. However there are some bricks which are more porous and therefore more susceptible to dampness when exposed to water.
For best results in any construction work, the correct brick must be chosen in accordance with the job specifications.
Composition of Bricks
Building bricks are mixture of clay and sand which is mixed with water to create the correct consistency. Sometimes the bricks also have added lime, ash or organic matter which speeds up the burning of the brick.
The clay mixture is then formed in moulds to the desired specification ready to be dried then burnt in the kiln.
Clay: The properties and quality of bricks depend on the type of clay used. The most common form of clay used for everyday bricks, is that with a sandy consistency, silicate or alumina, which usually contains small quantities of lime or iron oxide. Silica, when added to pure clay in the form of sand, prevents cracking, shrinking and warping. If there is a large proportion of sand used in the mixture the brick will be more textured and shapely. An excess of sand, however, renders the bricks too brittle and destroys cohesion. 25% of silica is said to be advantageous.
Iron oxide in the clay enables the silica and alumina to fuse and adds considerably to the hardness and strength of the bricks. The iron content of the brick is evident in the colour of the brick and can be used to add the colour red into the bricks. However a clay which burns to a red colour will provide a stronger brick than clay which burns to a white or yellow brick.
The lime content in a brick has two different effects. It stops the raw brick from shrinking and drying out, and it also acts as a flux during burning which causes the silica to melt and creates the bond which binds all the components of the brick together. However, too much lime can cause the brick to melt and loose shape. Any amount of quicklime within a brick is detrimental to its quality and can cause the brick to split into pieces.
For the best qualities of pressed brick the clay is carefully selected both colour and composition. Clay from different sources is also often mixed together to create the desired mixture.
Handmade Bricks
Handmade bricks used to be very commonly used throughout the UK. The process involves putting the clay, water and additives into a large pit where it is all mixed together by a tempering wheel generally still powered by horse power. Once the mixture is of the correct consistency, the clay is removed and pressed into moulds by hand.
To prevent the brick from sticking to the mould, the brick is coated in either sand or water. Named ‘slop moulding’ when dipped in water and ‘sand struck’ when coated in sand. Coating the brick with sand however gives an overall better finish to the brick.
Once shaped, the bricks are laid outside to dry by air and sun where they will be drying for three to four days. After this process the bricks are then transferred to the kiln for burning. If green bricks are left outside for the drying process and are left out during a shower; the water leaves an indentation of the brick is considered very undesirable. However this does not affect the strength properties of the bricks.
Machine made Bricks
Bricks are now more generally made by large scale manufacturing processes using machinery. This is a large scale effort and produces bricks which have been burned in patent kilns.
There are three different types of manufacturing process for brick making; the soft mud process, the stiff mud process and the dry clay process for which machines are specifically designed.
Soft Mud Process
The soft mud process is similar to that of handmade bricks. In the Soft Mud process that clay contains too much water to be extruded as the clay is left to soak in water for 24 hours. For this process three pits are usually in operation at any one time to keep the production flowing. Occasionally the clay is worked in a pug mill before being thrown into the machine. Due to the 20% water content of the clay, wooden moulds are generally used and are lined with either oil or sand to stop the clay sticking. After being drawn from the machine the filled moulds are emptied by hand and the bricks taken to the dry shed. So the soft mud bricks can be dried properly, both handmade soft mud bricks and machine made (more mass produced) bricks will both be placed in a large dryer which is separate from the extrusion dryer.
Stiff Mud Process
This process differs because only enough water to create plasticity is added to the clay, approximately 12% water. Clay is then extruded through a ‘die’ to produce a long stream of pressed clay which is then cut to size by the machine. The die sizes and cutter wire are calculated to compensate for the shrinkage of the brick during drying and firing. Attachments can also be added to the die which gives the brick its texture from brush, roll, and scratch to roughen. Green bricks are then dried out carefully to ensure a consistent colour and strength.
Differences between Soft Mud and Stiff Mud bricks
Because Soft Mud bricks have been created under little or no pressure, their density is not as great as that of Stiff Mud bricks. It has been argued that when Soft Mud bricks have been made and burned properly they are possibly the most durable brick. However Stiff mud bricks can have defects or planes of separation which can affect the bricks durability. However as Stiff Mud bricks are becoming increasingly cheaper to produce these are becoming the more popular.
Repressing and Pressed bricks
The repressing of a brick is to re-shape the brick or round of any corners dependent on specification. Both types of soft mud and stiff mud bricks can be repressed when they are only partially dried. This is done by placing the bricks in metal moulds and putting them under great pressure before burning. Pressed bricks however are machine moulded bricks where the clay being used is already nearly dry. This process can make a significant difference on the appearance of the bricks. Bricks made using this process generally are more difficult to compress. Dry pressed bricks however are now commonly used for face bricks. Pressed bricks generally mean dry pressed bricks, but many face bricks are made by repressing soft mud bricks.
Cement bricks
Made from Portland cement, these bricks are machine moulded into size and shape to match the size of clay bricks. These are extensively used in some regions.
Hollow, Terra-cotta or Tile
This type of product can made into practically and size or shape for any kind of use. Blocks made of terra-cotta are light and durable. For use in partitions the terra-cotta is mixed with sawdust which burns off in the kiln, but creates a more porous brick. Terra-cotta can be glazed or unglazed.
Face Bricks
Face bricks are uniform in colour and shape and can now be made to any almost any specification, texture, colour and size.
Wire Cut Bricks; for this type of brick the clay is extruded and cut by wore into individual bricks. This is a very cost effective way of producing bricks and is done by an automated production process. These bricks are readily available in a variety of styles and colours.
Stock bricks; are usually slightly more expensive than wire cut. These are a soft mud brick which are sometimes irregular in shape.
Handmade bricks; as previously discussed above, handmade bricks are very desirable and individual in shape and colour. This brick is one of the most expensive sorts of brick.
Fletton or London Brick; is a brick made from clay extracted from the south east of England which contains traces of coal which is burnt off during the kiln process. Reclaimed London bricks are highly sought after.
Arch and Clinker bricks
This term is used for bricks which are burned immediately. They are over burnt and sometimes distorted in shape.
Body, Cherry and/or hard bricks
These bricks are of a higher quality and are generally the bricks that were in the centre of the pile of bricks which have been burned. These bricks are top bricks as they have a higher overall quality and finish. Cherry is used as a term when the clay which has been used burns red.
Salmon, Pale or Soft bricks.
These are the bricks which were nearer to the outside of the kiln during burning which means they are slightly under burnt. These bricks are generally softer than the bricks taken from the centre of the kiln are therefore are of a lesser quality, although this does not affect the overall shape of the brick. These bricks are generally used for the interior of walls.
Waterstruck Brick
This type of brick is a soft mud moulded brick. It uses alluvial clay which deposited at the end of the last ice age. The clay is pressed into mould lined with silicate. When the bricks are removed from their mould, they are left with a textured effect which can only be achieved using this method. This type of brick looks old and handmade even when new.
Engineering Bricks
Engineering bricks are called so due to their overall strength and water absorption. The Class A brick has strength of 125N/mm² and water absorption of less than 4.5%. Class B engineering bricks have a strength greater than 75N/mm² and water absorption of less than 7%. Traditionally used in civil engineering, these bricks are also useful for damp courses and structural design.
Bullnose Bricks
These special bricks are used when round edges are needed, for gate recesses, quadrants or arches.
Off Shades
Offshaded bricks are batches of bricks which are generally consistant in colour but do not match the product which is marketed. These are cheap bricks but are good quality which are not misshaped they are just purely off colour.
Trial Bricks
Trial bricks are also cheap bricks due to when the manufacturer is trying out new product ranges they have to manufacture normally quite a large batch and if they decide not to run the product as a standard line the bricks become discount bricks and are sold cheap.
Different uses for bricks
Dependant on their final use, the bricks are named accordingly.
Radial Bricks either have one edge shorter than the other or vary in thickness. This type of brick is used for walls with curved edges.
Arch bricks are used for arches as they have one end thicker than the other.
Ordinary bricks or face bricks and have regular shape and colour used for the outside of building etc.
Fire bricks are generally yellow in colour and used in places where they would be subject to high temperatures.
Paving bricks are of uniform size and colour and have been made by burning hard clay or shale. Good brick to be used where toughness and water tightness is essential.
Aesthetic appearance
Bricks can be made to virtually any specification, examples of appearances are;
Heavy Rustic, Light Rustic, Drag faced, Stock, Rolled, Smooth, Rumbled,
Overall strength and water absorption of clay bricks
Compressive strengths vary from 5 N/mm2 to 125N/mm2 (125 being very structural bricks from this factory were used in the empire state building Hansons Accrington Works)
Water absorption varies from 6-26% dependant on brick type
Brick Dimensions Metric and imperial
215 × 102.5 × 50 Metric
215 × 102.5 × 65 Metric
225 × 107.5 × 67 Imperial
230 × 110 × 70 Imperial
230 × 110 × 73 Imperial
230 × 110 × 76 Imperial
230 × 110 × 80 Imperial
Brick Sizes
Metric bricks are a little smaller than the old imperial one. New bricks can be bonded into old brickwork by slightly increasing the mortar bed joint. Brick sizes have remained fairly constant over the years although, in the UK, the depth used to be less (about 2 ins/51mm) whereas modern bricks are about 2.5 ins/64mm.
Brick Cutting
Thermal Efficiency and Compliance with Building Regulations insert link
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