Clay bricks are the backbone of modern construction, providing homes and workplaces for millions. African Pegmatite is a leading supplier of additives and pigments for enhanced brick properties in terms of ease of manufacture, colour and mechanical strength.
Bricks are the ubiquitous and versatile building material that provides the structure to many buildings, walls and decorative installations. As classic as they are common, the standard ‘red’ brick varies in size by country and is used virtually everywhere. Of the many varieties of brick, clay bricks are by far the most common but others such as concrete bricks, mud bricks and others all form part of the building ecosystem for various applications.
The basic concept of mixing, forming, drying and baking (using a kiln) is almost identical across most types of bricks (with the notable exception being chemically set bricks). Typical modern fired clay brick composition is 50 to 60% by weight sand/silica, 20 to 30% alumina (directly from clay) and 2 to 5% lime. The balance is made up with iron oxide and magnesia. Other materials may be added such as refractory materials, glass powders, pigments and dyes for specialist applications, colouring or economic reasons to save cost.
Bricks are strong, good at heat retention and are fire and corrosion resistant. The most common shape is rectangular, however oval, cylindrical and other shapes are possible - particularly when produced using the extrusion method.
Bricks have been around for a large amount of human existence, providing vital shelter. Simple bricks would have been made from mud which was allowed to dry in the sun, before being cured in the presence of a nearby fire. Modern brick manufacture isn’t all that different - especially the outdoor drying step which remains popular in many parts of the world.
The modern brick, in all its forms, is an essential part of the modern, global economy. With advances in manufacturing and progress in the field of additives and better management of starting materials, brick production is set to increase in rate as the global population grows, especially as more economies move towards higher living standards.
The initial step
The first step in brick manufacture is consistent across all brick types, where all the raw materials are crushed and broken in a jaw crusher and a separator. This is done to ensure all minerals are mixed evenly and to achieve a predetermined size distribution. It is at this point that any pigments or additives are added, this is the best time as complete and thorough incorporation of the pigment or additive is ensured. Then, the mixture of ingredients desired for each particular batch is filtered and quality checked before being sent to one of the three brick shaping processes which are extrusion, moulding, or pressing.
Extrusion, out of these three is the most common and is also found to be the most economically scalable process due to the significant degree of mechanisation and automation. Now that the bricks are formed and have gone through any further quality control or surface treatment procedures, they then have to be dried so that they are free from any extra moisture. This is highly important as any excess moisture that might result in cracking during the firing process. Once this is done, they are taken to the ovens ready to be fired, after which they are cooled. Lastly, they are de-hacked, stacked and packaged.
Soft Mud Process
The soft mud process is the oldest and most traditional method of brick manufacture. Firstly, relatively moist clay is taken, which has about 30% water by weight. It is then placed into a mould, either by hand or with a moulding machine and is pressed into the moulds. To keep the clay from sticking to the moulds, the moulds may be dipped in water immediately before being filled. This process produces bricks with a relatively smooth, dense surface and are known as water struck bricks. The same process can be used to create sand-moulded or sand-struck bricks by dusting the wet mould with sand right before forming the brick. These bricks have a textured and matted surface. The major disadvantage to the soft mud process is the greater amount of manual handling compared to extruded brick production.
Dry Press Process
The dry press process is the most commonly used process when making refractory bricks. Unlike the soft mud process, the clay in this has minimal water content, which is only about 10% of the total weight. The clay is placed in the moulds and a hydraulic press is used to give extremely high pressure conditions for the bricks. After they are exposed to high pressure, the bricks are then dried and fired. While they are still moist they can be engraved with functional grooves or different kinds of textures. Because of the time required, this type of production is typically reserved for high-value bricks such as the aforementioned refractory containing ones.
Extruded Bricks
These bricks are known as wire cut bricks and this process is most commonly used today, owing to high production output and scalability. This process is much more modern than the others and requires less human interaction. The clay mixture is fed into a hopper, from where it is pressed through a shaping tool in a single, continuous stream, not dissimilar to sausage manufacturing. A wire then cuts the extruded clay into individual bricks which are then left to dry before being fired as usual. The bricks produced by this method have less moisture content than the other two types.
Classes of Bricks
Bricks are grouped into four classes based on quality. The higher the class, the better the quality, but also the more expensive.
First-Class Bricks
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- These bricks are burned very thoroughly and their colours range from dark red, copper, and cherry.
- The exterior should be rectangular and smooth; the edges should be sharp and parallel and the corners should be square with all edges straight
- There should be no visible flaws such as cracks and/or stones
- The texture should be consistent
- If a scratch is made into the brick there should be no impression left
- Lumps or lime should not be present on the fractured exterior of the brick
- When two of these bricks are striked together a ringing or metallic sound should be heard
- Water absorption, when placed in cold water for twenty-four hours, should be 12-15%.
- The crushing strength should also be no less than 10 N mm-2, although this limit will vary according to national standards
The use of these bricks is for exposed face work or pointing in masonry structures, reinforced brickwork and flooring. This type of brick can be used in virtually any scenario.
Second Class Bricks
Broadly speaking, these should have the same requirements as the first class bricks except:
- Small distortions and cracks are permitted
- Water absorption of around 16-20% is tolerated
- Crushing strength should not be be less than 7.0 N mm-2
Second class bricks are recommended for use in hidden masonry works or for exterior non-load bearing walls.
Third Class Bricks
- Are under burnt.
- They’re light-coloured and soft, producing a “dull” sound whenstriking against each other.
- Water absorption is about 25% of dry weight.
Uses: Temporary structures use this class of brick.
Fourth Class Bricks
- Are overburnt, the shapes and sizes are not consistent and are fragile in nature.
Uses for this type of brick include floors and foundations in lime concrete and in road building materials.
Types of Brick Pigment
Primarily for a specific visual appearance reason, brick manufacturers may add a variety of pigments to their clay bricks, but some additional and enhanced properties may be realised through the use of these pigments.
Brown Brick Oxide - Manganese Umber
Manganese umber refers to a broad range of manganese oxides, which are compounds with a broad applicability in pigmentation applications, in addition to having a wide variety of uses well beyond being a source of manganese metal. In addition to the rich brown colourations provided by these pigments - supplied by African Pegmatite - some added benefits such as easier handling and longer brick lifetimes are possible. When used in clay bricks, the depth of colour is viewed as excellent with a superior surface finish. A distinct benefit of the use of manganese umbers is that brick firing temperatures can be elevated - as high as 1,280 °C - at which temperatures other bricks would vitrify and become unstable.
The main uses of manganese dioxide in bricks are as a pigment to change the background colour or used as an additive to manufacture speckled bricks or even both. It's important to note that even though the same basic material is used for both the processes, the determining factor between the two things is the particle size and the amount of umber used. The oxides of manganese occur together and often with iron oxide when mined, these collectively form manganese umber. The umber can be treated by a calcination process after which it is referred to as burnt umber. Burnt umber affords a much darker colouration to the final brick or ceramic tile. Manganese umber can be applied as a superficial coating pre-firing instead of being mixed into the clay. However, when used as in this manner, the water resilience properties normally associated with adding manganese to bricks are less prevalent.
The specification of manganese umber as supplied by African Pegmatite is as follows.
| Appearance | Dark brown powder |
| Manganese:iron ratio | 4.8:1 |
| Manganese content (by weight) | 43 - 47% 42% is available on request |
| Other notable compounds present | SiO2 4 - 6%; CaO 5 - 7% |
| Particle size | 45 μm typical |
| Bulk density (tapped) | 1.53 - 1.55 g cm-3 |
Firing stability for manganese umber is good, and normal brick and ceramic firing temperatures are comfortably tolerated up to 1,280 °C. If firing above 1,150 °C is available, this is preferential, as it means less manganese umber needs to be used to achieve the same colour profile.
Black Brick Oxide – K37
K37 is a ceramic pigment composed mainly of iron oxide, but with a substantial quantity of manganese oxide present. This has the effect of producing a deep black pigment. K37 is long lasting and non-toxic. Added in the clay mixing stage, K37 will rapidly disperse through the mixture and provide a uniformity of pigment and a consistent colouration to the final product. Higher concentrations of K37 will afford black, whilst lower concentrations will produce browns post-firing. Typical additions of K37 are between 6 and 12% by weight for a standard brick manufacture. With the desired colouration level in mind, the amount of pigment should also be calculated with the iron content of the clay in mind. Excess pigment combined with elevated iron levels can prove a hindrance to workability.
One of the advantages to using K37 over synthetic stone is that K37-pigmented ceramics will not fade over time. Instead of post-firing treatments such as glazing and painting, the complete and uniform inclusion of K37 provides a much more natural looking pigmentation, whilst being much more stable over a long timeframe. A similar visual effect and resilience to basalt can be achieved with K37 pigmentation of clay bricks, albeit with lower cost. Mechanical and strength properties of pigmented bricks using K37 is analogous to untreated bricks. Because of the complete incorporation of the pigment through the clay, the persistence of colour through the finished brick and/or tile is notable and much more effective than surface painted or glazed bricks, while also having the advantage of not being able to be chipped off or damaged on the surface, and K37 pigmented bricks are highly tolerant of adverse weather. Long term piment stability contributes to long brick and thus building lifetimes, to the benefit of both residents and contractors., in addition to an almost zero maintenance requirement.
K37 is best suited for use in clay bricks at the standard firing temperature of 1,130 °C, including and especially for red clays. K37B is a developed variant of the K37 pigment that is optimal for use at lower firing temperatures around 1,080 °C - particularly useful for regions with less stable or poorer quality clays.
K37 ceramic pigment is supplied as standard as a finely milled black powder, with greater than 98% of the material being ground to below 45 um (325 mesh). The specification of K37 as supplied by African Pegmatite is as follows.
| Appearance | Dark brown powder |
| Manganese:iron ratio | 2.9:1 |
| Manganese content (by weight) | 13.95 - 14.7% |
| Other notable compounds present | SiO2 2 - 3% |
| Particle size | 45 μm ± 3 - 4% |
| Bulk density (dry) | 0.34 g cm-3 |
| Moisture content | 2.5% maximum |
Grey Brick Oxide - Chrome Flour
Naturally white firing clay or ivory clay is used with chrome flour to form different shades of grey colour in your ceramic bricks.Often concrete or cement is used to achieve these colours however concrete is very limited in the shades of grey used and may not offer any thermal or water tolerance property enhancement. Iron chromite is used as a grey brick oxide and can create a vast range of grey colours as well as the benefits of having a fired ceramic rather than a concrete or cement brick. This is a very effective grey brick pigment which has the added benefit of being at least semi-refractory in nature owing to the presence of the chromite; a refractory material in its own right. Because of the refractoriness afforded by the chromite, chrome flour is also used in the production of chrome magnesite refractory bricks, which are used in the construction of certain furnaces and kilns.
Due to its highly corrosion and chemical resistant nature, chrome flour is used to produce magnesia chrome refractories that are especially well suited to non-ferrous metals like copper, lead, and zinc furnace linings. These bricks are suitable for the following:
- Open hearth furnaces
- Electric arc furnaces
- Metallurgy furnaces
- Cement rotary kilns
- Glass kilns
- High temperature industrial furnaces
Chromite-containing bricks are rarely used as primary building bricks for buildings or walls, owing to the refractory content making them more expensive, even if they are harder wearing and longer lasting. Owing to density of the chromite, bricks made using this material tend to be heavier than their standard clay counterparts.
The specification of chrome sand for brick applications as supplied by African Pegmatite is as follows.
| Appearance | Brown powder |
| Cr2O3 content (by weight) | 43.5%, typically 44% |
| Other notable compounds present | SiO2 3 - 5%; FeO 25 - 26% |
| Moisture content | 0.2% maximum |
| Particle size | 45 μm ± 3 - 5% typical |
| Bulk density (dry) | 4.46 g cm-3 |
Summary
- Clay bricks are vital in modern society, providing the means to build homes for millions
- The process of manufacture is well established, but several additives to the traditional clay may be used to afford enhanced properties
- Manganese umber, K37 and chrome flour are finely milled products that can be added to brick manufacture to afford brown, black and grey coloured bricks, some of which possess enhanced thermal stability and mechanical strength properties
African Pegmatite is a leading supplier of pigments and additives for the production of the highest quality clay bricks for many applications. Providing a wealth of experience and unparalleled service, African Pegmatite is the ideal partner for the building and construction sectors.
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