Improvements to the Brickmaking Process

The quality of brick is important for a successful brickmaking program. Different aspects of the brickmaking process can be improved easily in order to increase the quality of brick. Listed below are some defects and their causes.

Defects in Size

Defects of Shape

Defects in Raw Material

Defects of Appearance

Defects After Manufacturing


Defects in Size

Defect Reason

Oversize in all 3 dimensions
The brick may be underfired. The presence of too much sand will decrease the amount of drying. The brick will be lighter than normal, and there will be a dull "ring" when knocked with another brick instead of a metallic sound.

Oversize in width and length
The brick may have been squashed while still wet. This may have happened when the brick was set down on the ground or drying rack after being molded or if a slop molded brick was pressed to flatten out a distortion.

Oversize in thickness
This is a common fault with wirecut bricks. In the process of wirecutting, the block of clay is forced through a row of wires. The force on the wire may cause movement that changes the dimension of the opening which the clay passes through.

Undersize in all dimensions
The brick may have been made with an undersized mod, but another cause could be a problem of too high of a clay content in the batch, or using too much water during the mixing stage. The undersizing should be noticeable at the end of the drying stage. Another cause could be overfiring, common to bricks around the tunnels in a field kiln. The bricks can be separated as a separate grade.


Defects of Shape

Defect Reason

Slumping
The clay mix was too wet and the brick slumped under its own weight or it was set down too hard between the molding and drying processes.

Rounded corners
When molding, the worker may not have pressed enough clay into the mold, or it may have broken between drying in firing. The brick is brittle at this point.

Raised corner
The brick may have stuck to the mold when being removed from the mold.

Lip on bed face
The excess clay may not have been removed with the bow cutter and a small "flashing" is left around borders the top edge.

Flashing on top face
On a frogged brick, where the brickmaker's name in impressed upon one surface, the mold is made so air will escape around a fixed bottom, the side used for impression. If the air gap is large enough, clay will get into the gap and cause flashing.

Distorted or contaminated under-surface
The drying surface may be uneven or dirty. This happens more often with slop molding, because the wetter mixture can pick up more particles.

Stacking marks
If bricks are moved too soon to drying stacks from individual drying, they may distort. Also, fingerprints or other marks may get on the surface.

Banana shapes
The brick may become distorted if two sides dry at such different rates that it curves. If the bricks are not turned over during the initial drying stage before the top gets hard, this defect is more likely to occur.

Multiple distortions
Some bricks may inevitably become distorted in multiple ways. This may have happened in the forming stage, and the brick should have been rejected before being fired. Bricks may also distort if overburnt at the base of the clamp.


Defects in Raw Material

Defect Reason

Underfiring
This is one of the most common reasons for a brick to fail from low compressive strength, and can be identified when the brick is oversized. The clay will be a lighter color, and when knocked together, the bricks will not have the expected metallic sound, but rather a dull clunk.

Straight cracks at right angles from one of the long surfaces
These cracks form if the drying process is too fast, and the brick may break even if fired correctly.

Multiple surface cracks in random directions
Differential drying can cause shrinkage, and lumps of drier material will shrink differently than the rest of the brick. One correction can be to make sure mixing is sufficient before molding. Pebbles in the clay mix will also cause the same defect.

Large crack with bulge in surface
This "bloating" can occur if a brick is heated too quickly and the surface of the brick vitrifies before the chemical reactions inside of the brick. These bricks should still be strong enough, but since misshaped, may be used where not seen, such as in footings.

Extrusion laminations
The defect occurs in solid wirecut bricks where the turning effect of the equipment pushes the clay out of the machine: the center of the brick is turning faster than the outside clay. These defects are weak spots because they can be harmed by frost damage or spalling. It can be fixed by inserting auger dies into the machine to restrict the turning movement.

Lamination crack
This defect may occur in the sand-molded process if a piece of clay covered with sand is mixed into another piece. The film of sand may separate the two pieces of clay if stresses occur during drying or firing. This may also happen with oil molding, and proper training of workers should prevent this defect.


Defects of Appearance

Defect Reason

Spalling
If residual water is in the brick after drying, pieces of quickly heated bricks may chip off or explode because the water will turn to steam inside the brick. This process of water heating and exploding inside the brick is called spalling.

Dog-eared corners
This happens most commonly in extruded bricks when the brick leaves the machine. A solution to this is better lubrication of the column or to make sure the material is finely mixed.

Ragged arrises
If the long face of the brick from a wirecut is deformed, the wires may not be tightened properly.

Drag marks
Along the length of the wirecut bricks or on a sand-molded or slop-molded brick, defects can appear from residue on the wire, bow cutter or smoothing stick. This can happen because of trigs or leaves in the clay getting stuck on the smoothing and cutting implements. The tools can be checked to prevent this defect.

Barmarks
Barmarks are the result of the firing process where kiln gases do not reach the entire surface of a brick. If the desired color appears on the exposed surface, the bricks should be arranged so that the entire side of the brick is exposed to the gases. In the same way, the entire side of the brick should be covered if the desired color is the area not exposed to the gases.


Defects After Manufacturing

Defect Reason
Efflorescence Efflorescence is what appears as a crystalline deposit on the surface of a brick, caused by soluble salts in the water used for the brickmaking process. The problem is usually temporary and will disappear in time. In places where the land is waterlogged, salts will tend to rise through the soil. This defect can be avoided by not using the top layer of topsoil and clay in extraction.
Lime-blowing Lime-blowing is caused by limestone lumps in clay that re-hydrate and expand after firing. To identify the affected bricks, they can be dunked in water, and fractures will be caused with white powdery lumps exposed. To avoid lime-blowing, clay mixtures should not have particles larger than 2mm incorporated. Another technique is to dead-burn the bricks, which is to heat the kiln to higher temperatures. The problem with this is that fuel is dramatically increased and the high temperatures are not even possible in many clamps.


All figures on this page are from Brickmaking in Developing Countries by J.P.M Parry.


Background Conditions Process Improvements Efficiency More Resources