This information was found at https://gharpedia.com/good-clay-bricks/
“Clay bricks are the oldest and most popular building materials because they are cheap, durable and easy to work with. Clay bricks are used for exterior walls, partitions, piers, footings and other load bearing structures.”
Brick Earth:
Brick earth should be free from stones, kankars, organic matter, saltpetre and harmful chemicals.
Size:
The bricks should have a uniform size, plain, rectangular surfaces with parallel sides and sharp straight edges. Good bricks should not exceed 3 mm tolerances in length and 1.5 mm tolerances in width and height.
Standard size of brick is 190 x 90 x 90 mm
Shape:
Good bricks should be uniform in shape.
Colour of Brick:
Good bricks should be well burnt and should have uniform copper colour. Over burnt and under burnt bricks losses uniformity of colour on its surface and its strength.
Soundness:
Well burnt brick should give a metallic sound when struck with a hammer or another brick.
Hardness:
The brick should be so hard that when scratched by a finger nail no impression is made.
Strength:
The compressive strength of brick should be minimum 3.5 N/mm2 as per IS code.
Texture and Compactness:
The surface of bricks should not be too smooth to cause slipping of mortar. The brick should have precompacted and uniform texture. A fractured surface should not show cracks, holes, grits or lumps of lime.
Water Absorption:
Water absorption of a good brick should not exceed 20 percent of its dry weight when kept immersed in water for 24 hours.
Frog in Brick:
Bricks should have a proper frog so that the mortar can be properly filled in. The size of frog should be 100 mm in length, 40 mm in width and 10 mm depth. “