A glance at any brick manufacturer's sales literature will indicate that promotional material is chiefly for the housing market, because 80% of brick sales are for housing.
Not all of the 20% non-housing market share is controlled by architects. The fickleness of brick demand in the commercial, retail, civic, and special purpose building sector because of economic cycles, changing architectural preferences, and the relative cost of brickwork compared with alternative cladding assemblies, make it not surprising that marketing information has not been specialised and targeted at architects as much as they might like.
This trend is being changed, particularly by the larger suppliers, who are starting to present brochures and sample displays specifically for architects. Interestingly, the literature and sample displays often show polychromatic brick work, perhaps reflecting greater public acceptance.
The range of brick colours and textures available in Australia now is probably greater than ever before giving great potential for rich and diverse designing in brick. The illustrations sample this diversity, including smooth, matt and rough surfaces and show the effect of using different coloured mortar.
Special shapes are available both as "standard" and "specials", but can be also be made to order for a project.
Clay bricks exhibit variations of colour, texture, size, density, hardness, expansion, absorption and purity determined by the nature of the clay deposit and moulding and firing methods. The following key variables should be noted:
Brick colour is determined by the mineralogy of its clay, kiln temperature and atmosphere, and, to a small degree, the kiln fuel.
Clay consists of hydrated silicates and aluminates with varying amounts of impurities such as iron oxide which affect brick colour. For example, purer clay burns to form white bricks because of the highly refractory nature of the material, but red-burning clays contain more iron oxide.
Most New South Wales kilns are gas fired. Where sawdust (North Coast and Launceston) is used, the resultant brick colours are redder. The use of oil (Wagga) assists in producing a black and blue-black finish. Oil can also produce a glaze-like finish. Firing with excessive fuel (reducing kiln conditions) is a common technique for the manufacture of blue-black bricks and to simulate "old world" type bricks.
Inherent in the semi-dry pressed brickmaking process is the granular clay/shale texture ranging from a fine, smooth finish through to a speckled pebbly conglomerate with no real sense of a binding matrix. In contrast, the extruded brick will invariably "read" as a mixture of particle and binder. The complaint often made about extruded bricks when viewed closely is the "pasty" and processed feel of the brick exacerbated by some textures when contrasted with the more stone-like character of the semi dry-pressed brick.
Closely allied to texture is the size to which clay and shale particles are ground. Grinding down to 1.5 to 2.0 mm maximum particle size is common. When drawn across the brick face during the wire cutting of the clay column the larger particles score the surface and help open it up and produces the mexicut finish. This shortens the green brick drying period (due to increased surface area) and minimises hairline cracks and crazing. Other particle sizes, such as coarse particles (+3mm) can produce various face finishes and textures.
Some manufacturers will re-press extruded bricks after wirecutting. This can achieve a smoother texture (with little or no scoring by larger particles), as well as reducing brick out-of-plane deformities prior to firing. For manufacturers, re-pressing has the disadvantage of adding another step to the brickmaking sequence.
Bricks undergo shrinkage as the initial drying process drives off free water from the green brick, and during kiln firing. Manufacturers generally test fire each new clay deposit coming into production, to ensure that the end product complies with the dimensional tolerance of +/- 60 mm in 20 bricks for category DW2 of AS/NZS 4455-1997, "Masonry Units and segmental pavers." This category applies to most face and common bricks with sharp arrises, with DWO applying to bricks with overall irregular or rough surfaces (eg. "rumbled" bricks) and DW1 for bricks with irregular faces (eg. textured or rock-faced). The higher tolerances of categories DW3 and DW4 can be sought, but are unlikely to be needed in most applications.
Manufacturers advise that they have no difficulty in complying with tolerance category DW2.
Out-of-plane variations for modern extruded bricks can be considerable. In the authors' experience, boom times and high brick demand and sales tend to create an atmosphere in which brickmakers are reluctant to notify architects of out-of-plane defects.
Out-of-plane tolerances for either header and stretcher faces or for the bed faces of bricks are rarely regarded by manufacturers as a matter for concern, as warping, hogging ("bananas") and lipping is fortunately more frequent in cheap bricks which they denote as unsuitable for face work. This is because most modern plants can achieve an acceptable degree of shape tolerance. When problems of warp occur, brick manufacturers can adjust the moisture content of the green brick or the firing regime. The problems that architects encounter in this area require greater attention.
The most difficult impurity to eradicate from clay bricks is vanadium. Yellow, brown, green, blue or pink vanadium stains can occur on light coloured bricks. Inert vanadium is trapped in the shale particles and is very difficult to detect pre-production. One solution is to fine-grind all shales to assist vanadium detection, but too fine a grind results in an excessively brittle brick. Often glucose or barium carbonate is added to inhibit premature migration of salts to the surface during drying. Salts leading to efflorescence are mostly sulphates of calcium, magnesium, aluminium, sodium and potassium, with carbonates occurring to a lesser extent. Brickmakers are fairly successful in choosing material free of salts.
Vanadium salts are typically present in light coloured bricks. Inert vanadium is trapped in the shale particles and is very difficult to detect pre-production. On occasion this may lead to yellow, brown, green, or black vanadium stains on light coloured bricks in situ. This staining is a natural by product of clay products and is not harmful. The stain will gradually weather away, the brick supplier can provide guidance on methods for quicker removal.
Hardness is imparted by vitrification. Plastic clays, usually red-burning, vitrify at lower temperatures than the more refractory clays commonly used to produce lighter-coloured bricks.
The proportion of air (oxygen) to fuel affects colour and tone. Oxidation brings out colours, such as reds developed from iron oxide, and promotes colour uniformity.
Reduction occurs when the fuel is starved of oxygen, resulting in a darkening of the brick colour. If the bricks are edge set, the darker colour is concentrated around the brick edge. Blue-black bricks are produced by heavy reduction and high temperatures.
Flashes, or hearts, result from incomplete burning off of the carbon on the centre of the brick face. This can be deliberately achieved by edge-setting bricks causing the hot kiln air to move unevenly across the brick faces. To avoid flashing, bricks are flat-set.
Pigments can be added to the clay mix. For example, the addition of manganese dioxide to a cream base creates a grey brick, and when added to a red clay base creates a brown brick.
As modern brickmaking plants bulk store clay material under cover, the brickmaking process is relatively immune to the vagaries of weather. Smaller manufacturers, and in particular intermittent kilns in country areas, suffer serious delays from rain. Extra pre-drying of clay following prolonged rain can result in marked batch darkening after firing.
Frit and Surface Additives
Manufacturers sometimes add a glass based material known as frit to create a spattered surface effect. Salt, clay particles, carbon, or even small pebbles can be added to the surface. Other surface additives such as slurries can produce various colours and styles.