Last reviewed 16 Sept 2026 · 10 min read
Brick earth
Composition of good brick earth
| Constituent | Proportion (approx.) | Role |
|---|---|---|
| Alumina (clay) | 20–30% | Gives plasticity for moulding; excess causes shrinkage, warping and cracking on drying and burning |
| Silica | 50–60% | Prevents cracking, shrinkage and warping; gives uniform shape and durability; excess makes bricks brittle |
| Lime | Less than about 5% | In fine powder, reduces shrinkage and helps silica to fuse slightly (binding); excess causes melting and loss of shape; lumps of lime cause splitting |
| Magnesia | Less than about 1% | Gives a yellow tint; reduces shrinkage; excess causes decay |
| Iron oxide | About 5–6% | Gives red colour and helps fusion; improves strength and impermeability |
Harmful ingredients
- Lime lumps — expand on slaking after burning, causing bricks to split (lime bursting).
- Iron pyrites — cause crystallisation, discolouration and splitting.
- Alkalis (salts of sodium, potassium) — cause efflorescence (white patches) and melting.
- Pebbles and gravel — prevent uniform mixing and weaken bricks.
- Organic matter and vegetation — leave pores after burning, making bricks porous.
Manufacture of bricks
1. Preparation of clay
Unsoiling (removal of top soil), digging, cleaning (removing stones, roots), weathering (exposure to atmosphere to soften and mature the clay), blending (mixing ingredients) and tempering (mixing with water and kneading — in a pug mill for large-scale production) to obtain a homogeneous plastic mass.
2. Moulding
| Method | Description |
|---|---|
| Hand moulding — ground moulding | Moulds filled on levelled ground; bricks left on the ground |
| Hand moulding — table moulding | Moulding on a table; bricks carried to drying area |
| Slop moulding | Mould dipped in water before filling — smooth surface |
| Sand moulding | Mould sprinkled with sand — prevents sticking |
| Machine moulding — plastic clay (wire-cut) method | Clay extruded as a column and cut by wires — uniform, many bricks |
| Machine moulding — dry press method | Nearly dry clay pressed in moulds under high pressure — dense, accurate bricks |
A frog — a shallow depression on one face of a hand-moulded brick — provides a key for mortar, reduces weight and may carry the manufacturer's mark. Bricks with frogs are laid frog upward.
3. Drying
Green bricks are dried (naturally in the shade/open air, or artificially in drying chambers) to reduce moisture before burning, preventing cracks and distortion. Natural drying commonly takes one to two weeks depending on weather.
4. Burning
Burning gives strength, hardness and durability through dehydration and partial vitrification of clay; bricks are commonly burnt at around 900–1100 °C.
| Kiln / clamp | Type | Features |
|---|---|---|
| Clamp | Intermittent, temporary | Bricks and fuel stacked in alternate layers and burnt; cheap, but uneven burning, many under- and over-burnt bricks |
| Intermittent kilns (e.g. Scotch kiln) | Loaded, fired, cooled and unloaded batch by batch | Better control than clamps; fuel waste in reheating |
| Bull's trench kiln (BTK) | Continuous, trench in ground (circular or oval); fire moves round the trench | Widely used in India; large output; traditional fixed-chimney kilns cause air pollution |
| Zigzag kiln | Continuous, air flows in a zigzag path through the bricks | Better fuel efficiency and lower emissions — promoted as an improvement over conventional BTKs |
| Hoffman's kiln | Continuous, permanent (circular, with chambers and roof) | Burning even in rain; good-quality bricks; high initial cost |
| Tunnel kiln | Continuous; bricks on cars move through a tunnel with fixed preheating, firing and cooling zones | High quality and output; mechanised factories |
Sizes of bricks
| Brick | Size (mm) |
|---|---|
| Modular (IS) brick | 190 × 90 × 90 (nominal with 10 mm mortar joint: 200 × 100 × 100) |
| Modular brick (thinner variant) | 190 × 90 × 40 |
| Conventional (traditional) brick | About 230 × 110 × 70 (9″ × 4½″ × 3″) |
For modular bricks, about 500 bricks make one cubic metre of masonry (including mortar joints).
Classification of bricks
Traditional (field) classification
| Class | Description | Use |
|---|---|---|
| First-class bricks | Table-moulded, well burnt in kilns, uniform red colour, sharp edges, metallic ringing sound, no scratch with finger nail, low water absorption | Exposed masonry, important structures |
| Second-class bricks | Ground-moulded, kiln burnt, slightly irregular shape or colour, fine cracks | Plastered masonry |
| Third-class bricks | Ground-moulded, burnt in clamps, soft, light-red, dull sound | Temporary structures, sheltered areas |
| Fourth-class (over-burnt) bricks — jhama | Over-burnt, distorted, very hard, dark | Broken as aggregate (brick ballast) for foundations, floors, lime concrete |
| Under-burnt bricks | Soft, yellowish, absorbent | Not used for masonry |
IS classification (IS 1077)
Common burnt clay bricks are classified by average compressive strength into classes (designations in N/mm²) such as 35, 30, 25, 20, 17.5, 15, 12.5, 10, 7.5, 5 and 3.5 — the class designation is the minimum average compressive strength.
Tests on bricks
Laboratory tests (IS 3495)
| Test | Procedure / requirement |
|---|---|
| Compressive strength | Frog filled with mortar, specimens immersed in water, capped and tested; strength = load ÷ bed area |
| Water absorption | Dry bricks immersed in cold water for 24 hours (or boiled for 5 hours); absorption = (wet − dry)/dry × 100; for common bricks it should generally not exceed about 20% by weight for lower classes (lower limits for higher classes) |
| Efflorescence | Bricks placed on end in distilled water in a dish until water is absorbed and evaporated; deposit of white salts rated as nil, slight (up to about 10% of exposed area covered by a thin deposit), moderate (about 10–50%), heavy (more than 50%) and serious (heavy deposit with powdering/flaking) |
| Dimensional tolerance | Twenty bricks placed in contact in a row along length, width and height; total dimensions must be within specified limits |
| Warpage | Measurement of distortion of faces |
Field tests
- Sound test — two bricks struck together should give a clear metallic ringing sound.
- Hardness test — no impression when scratched with a finger nail.
- Soundness/strength test — a good brick should not break when dropped flat on hard ground from a height of about 1 m.
- Shape, size and colour — uniform, with sharp straight edges and uniform colour.
- Structure — a broken brick shows a homogeneous, compact structure without holes or lumps.