In compositional terms, good brick clay should feature:

• Grain size distribution (sieve line): A high percentage of larger particles requires higher sintering temperatures, resulting in a greater energy requirement and longer firing times.

• Accessory minerals: Quartz, feldspar, and amphibole have an effect on the sintering behavior and might result in undesired colors.

• Organics: Create voids in the finished product. If the organics have a high sulfur content, this might have effects on color, the fumes, and the kiln atmosphere.

• Sulfur: Pyrite and marcasite release SOx on firing, creating large-diameter pores in the brick that might reduce compressive and flexural strength.

• Carbonate minerals: Calcite, dolomite, and other carbonate minerals, such as ankerite and siderite, do have, if finely dispersed, an effect on the release of low — temperature-carbonization gases owing to the formation of channels and funnels but in larger quantities will reduce the compressive and flexural strength of the final product and might lead to chipping on the surface.

• Alkalis: A low alkaline earth content, magnesium and calcium being the most common, is desirable to avoid firing interactions which could promote discolor­ation of the final product.

• Metal oxides and hydroxides: Goethite and hematite, for example, both contain­ing iron oxide (the chief colorant responsible), which ideally should be in the range 5-12% (for good strong color), are the main origin of the red brick color.

• Natural radioactivity of the raw materials (radon).

Adding foreign substances to influence the physical properties of the finished product or the workability is nothing new. It is important to distinguish between “additions” to the clay body (such as saw dust or paper sludge), and “substitutions” (such as sewage sludge, ash, or treated aluminum salt slag) that replace a part of the original clay body. The distinction between “addition” and “substitution” for extraneous materials is not always straightforward or easy: Any substance that is added to the original clay body without substitution of clay modifying its inherent characteristics might be considered an addition. A substitution, on the other hand, may be viewed as any material that for a required volume of brick reduces the quantity of clay needed to achieve that specific volume target. A substitution can also modify the clay body.