SECTION 2
BRICK MANUFACTURE

2.1 General
2.2 Manufacturers
2.3 Sourcing Clays and Shales
2.4 Sales and Marketing
2.5 Brick Making
2.6 Brick Firing
2.7 Brick-making Variables
2.8 Colour Range
2.9 Size Tolerance
2.10 Investment
2.11 Manufacturers and Responsibility
2.12 Recommendations for Architects

2.1 General

Brick manufacture in New South Wales indicates the following trends:

• a decline in the number of brickmakers since 1945;
• the amalgamation of several smaller brickmakers into larger companies;
• a gradual increase in production capacity to almost 1 billion per annum;
• extrusion (stiff-plastic) remains the dominant brick production method;
• the long-term trend away from dry-pressed bricks has been arrested;
• the wet-plastic moulded production method has increased fourfold in recent times;
• the continuing mechanisation of brick-making and delivery processes;
• the continuing high proportion of brick sales for housing hence that sector's influence on production.

2.2 Manufacturers

2.3 Sourcing Clays and Shales

In most cases, clays and shales are extracted from one large pit. However, brick manufacturers may draw from a number of individual pits to both maintain production quantities (when one colour in Pit A is exhausted, the same or similar colour will be taken from Pit B), and to cater for a variety of brick colours from seams of different colours in different locations. It is much more likely that a variety of clay colour seams will appear in any one pit. The Sydney Basin, which serves the major three manufacturers, has good continuity in most colours in the cream to brown range. The significant exceptions are white/creamy white material, which is in reliable deposits only from Bowral, Bathurst and Albury pits, and deep red which is obtained in southern New South Wales.

As clay seams are exhausted, manufacturers will, in some cases, withdraw a brick colour from the market. This often causes resentment and has influenced brickmakers to blend clays to guarantee colour uniformity over time, usually at the expense of a slightly wider colour range.

2.4 Sales and Marketing

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.

2.5 Brick Making

There are currently three methods of brick making employed in New South Wales.

Semi-dry pressed process

After grinding and mixing the clay to a relatively dry content (7-10% water), the clay mix is fed into steel moulds or die boxes and compressed. Dry-press brick machines operate at various levels of automation. Manpower levels are much higher than extruded or wet-plastic processes and can be of the order of 5,000 bricks per man-day compared to 20,000 for the wet-plastic process. The high shale content (70%) of dry-pressed bricks results in the familiar gritty texture of New South Wales bricks. In comparison, Victorian stiff-plastic bricks are produced with a higher percentage of clay, with a resulting higher degree of surface crazing. The higher cost of dry-pressed bricks (typically 40% more than extruded face bricks), reflects the higher labour content of the manufacturing process. Both larger and smaller brick manufacturers have, in some instances, improved dry pressed productivity. These manufacturers (Boral, Bowral, Willis), have invested in integrating dry pressing machinery with drying and kiln plant.

Extruded (stiff-plastic) process

Wet-plastic process

Clay of higher plasticity and moisture content than in the stiff plastic process is extruded, cut into clots and thrown into steel moulds. The throwing process creates creases in the brick face and frogs in the flat bed faces, in a manner similar to the old technique of sandstock brick production. The two types of wet plastic bricks are sand-struck and water-struck. Sand-strucks are formed in moulds coated with a thin layer of sand preventing clay adhesion to the mould, whereas water-strucks are formed in moulds dipped in water to prevent clay adhesion.

Re-pressing

This is a two-stage process in which a solid clay column is extruded producing cut bricks that are shorter, narrower and higher. They are pressed in a mould to the normal dimensions. A frog is also formed.

2.6 Brick Firing

The various kiln types most commonly used in New South Wales are:

Patent kiln

A circular beehive-shaped kiln in which coal fuel is fed through openings at the top of the kiln. By rotating the heat source around the kiln, brick stacks are variously fired and cooled sufficiently to remove and replace with unfired bricks.

Downdraft kiln

A downdraft kiln consists of a rectangular space with a barrel-arch roof and a slotted or perforated floor which leads to flues below. Green bricks (40-100,000 at a time) are stacked in the kiln, fires are lit in fireboxes along the sides, and the hot gases pass up to the curved roof, down through the bricks and thence to the chimney stack. Wood is still used in some country areas, but most are coal, gas or oil-fired. When the desired temperature has been reached the fires are allowed to die, the kiln cools and the fired bricks are replaced by another batch of green bricks.

Hoffman kiln

Tunnel kiln

Austral Bricks 158 m tunnel kiln at Horsely Park, NSW
Photo: Austral Bricks

2.7 Brick-making Variables

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:

Colour

Brick colour is determined by the mineralogy of its clay, kiln temperature and atmosphere, and, to a small degree, the kiln fuel.

Texture

Grind size

Re-pressing

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.

Size tolerance

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 A.S. 1225-1984 dimensional tolerance category ST3.

± 60mm in a length of 20 bricks
± 40mm in a width of 20 bricks
± 40mm in a height of 20 bricks.

Manufacturers advise that they have no difficulty in complying with this tolerance category (the higher tolerance ST2 is rarely requested).

Shape tolerance

Impurities

Manufacturers must comply with the levels of efflorescence specified in A.S. 1226.6. This ensures that efflorescence is reasonably infrequent.

Hardness

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.

Oxidation

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

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

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.

Oxides

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.

Weather

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.

2.8 Colour range

The difficult colours to achieve in monochromatic bricks are:

Cream

Light cream material is relatively scarce. Although brickmakers offer a range of creams, few have the warm yellow hues associated with bricks from earlier periods. Creams now available tend to be either heavier in colour (tan), "dirtier", or are very bleached (parchment and putty colours).

Red

White

Blue blacks

2.9 Size Tolerance

Although there have been previous times in the history of Australian brickmaking when considerable variation in brick size occurred, there remains very real frustration among architects and contractors about current size variation, given the current sophisticated mechanised production techniques.

Typical dimensions of dry-pressed face brickwork in some older Sydney buildings are:

Although length variation is smaller and perpends are tighter than today's brickwork, it should not be concluded that there was less size variability in the past, as there was more non-face work to absorb the out-of-size bricks.

It is not unusual to find modern extruded bricks from a single order varying up to 6 mm in length, and 8 mm in some cases. This is technically permissable under A.S. 1225, dimensional category ST3 (which deals with 20 bricks on aggregate, rather than individual brick comparisons). Category ST2 is more readily achievable with recent plant improvements.

Out-of-plane tolerance for modern extruded bricks can be considerable. Warping, hogging ("bananas") and lipping is fortunately more frequent in cheap bricks which manufacturers denote as unsuitable for face work. 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.

2.10 Investment

Polychromatic brickwork creates a relatively small annual demand for (usually) monochromatic bricks, and is thus not likely to influence the investment decisions of brick manufacturers.

Needless to say, post World War II brick plant investment has followed the major economic cycles, with bursts of new investment in the early 1960s, late 1970s and late 1980s. Brickmaking plant (especially pressing and extruding equipment) is considered to be a 15-year investment. Recent years have seen:

• Dry-pressed equipment purchased from brickworks that have closed down (e.g. Statebricks and Goodlett & Smith). Plant from 1960s and 1970s is being purchased by the specialised dry press producers. Dry press production receives new investment more in the setting, drying and kiln firing areas than in brick pressing.
• Extruded brick plant equipment is being progressively upgraded.
• Wet-plastic production equipment has been introduced to create new markets primarily within the residential industry.
• In a few cases, new repressing equipment for the 1990s has been purchased.
• More attention being given to devise plant that can achieve long term colour consistency.
• The phasing in of fully automated and computer-controlled tunnel kilns and brick handling equipment.

2.11 Manufacturers and Responsibility

Colour continuity

It is possible that some brick colour ranges may become unavailable because of the exhaustion of a particular clay deposit. Manufacturers will advise of impending run-down of stocks if a large order is received, but will rarely make such a run-down common knowledge. This represents a difficulty for jobs constructed in stages over several years. Although the problem has always existed (and notably for monochrome brick types), it is becoming increasingly common in the rarer clay colours.

Impurities

Some manufacturers of cream bricks will warn architects of the presence of vanadium salts, however, sales information does not usually contain information on the presence of vanadium or other salts.

Size and shape tolerance

All brickmakers will usually warn architects during the design selection of bricks for polychromatic work that brick sizes will vary from colour to colour. They are less vocal about the actual size tolerance within single brick types and over which they exercise control. Out-of-shape tolerance problems are raised where low-price bricks are requested for high quality facework, but again, discrepancies in higher cost face bricks (such as arris and corner breakages) are matters of caveat emptor.

Culling and classifying

The classifying and culling of sub-standard or inappropriate bricks in typical dry-pressed operation is often thorough, and is as much a product of the heavy labour content present in the operation as it is a gesture of goodwill. There is an understandable inertia on the part of large volume brick manufacturers to classify and cull bricks with the same vigour in the fast-moving world of tunnel kilns and conveyor belts. Reliance on machine-based quality control to produce a defect-free brick is preferred.

2.12 Recommendations for Architects

The above summary of current brickmaking shows trends which affect architects' attitudes, brick selection habits, and the feedback from architects to brick manufacturers.

Dry-pressed brick availability

The decline in the number of manufacturers offering dry-pressed bricks, the reduction in colours available, and the difficulty in filling large orders unless early pre-purchase is possible, are problems that are basically irreversible for architects who prefer the dry-pressed product.

Maintaining the present range of dry-pressed bricks will depend on continuing demand. Increased demand may result in the purchase of better presses and setting equipment, and unit costs will only decrease by the introduction of more automated presses and/or the firing in tunnel kilns. Architects have some influence on demand, but no control over investment decisions.

Monochromatic extruded bricks

These bricks tend to be seen as too spartan for domestic work, but are in increasing demand for architect-designed non-residential work, thus adding to architects' lobbying potential. An example of this is pressure to cease the practice of edge setting extruded bricks with marble chips on the surface to ensure the burning out of hearts. As this can result in blemished brick faces, architects' market pressure could persuade brickmakers to face-set, eliminating the need for marble chips.

Re-pressing

A similar argument can be made for re-pressing, that is, the pressing or smoothing by roller after extrusion. A manufacturer who is selling mostly monochromatic bricks to an architect-led market, may well decide to re-press a brick judged unsaleable without it, for example, due to too much particle drag on the surface, or warping.

Fine grinding

In a bid to improve the "pasty" drawn texture of wire-cut bricks, the alternative to re-pressing is to reduce the grinding size. Manufacturers who may lose a brick through brittleness or excessive crazing if the grind is too fine will be cautious of this move. For a large project, however, a prototype may be feasible.

Cream and red deposits

Manufacturers are very aware of the relative scarcity of good deep red and light cream burning clay deposits, but demand alone is having some impact on the search for new pits. In one case, cream clay has been burnt to a light cream brick instead of the more common tendency of adding manganese dioxide to produce a grey brick.

Quality assurance

A new Australian Standard is currently being written to supersede A.S.. 1255-1984 Clay Building Bricks. This is badly needed, as several key brick quality variables are absent from the current standard. In the immediate future, architects should insist that trade literature include the following basic data:

dimensional tolerances and histogram showing their distribution

• durability class
• transverse strength
• compressive strength
• characteristic expansion
• efflorescence
• lime pitting
• water absorption properties
• initial rate of absorption.

Other information would include:

• likely continuity of colour supply from clay source
• vanadium presence
• batch colour range (actual, as opposed to ideal)
• description of culling standards for arrises and corners
• date of removal from kiln.