Why does an efflorescence appear?

In chemistry, efflorescence is defined as the migration of a salt to the surface of a porous material, where it forms a coating. The essential process usually involves the dissolving of an internally held salt in water. The water, with the salt now held in solution, migrates to the surface, then evaporates, leaving a coating of the salt. These efflorescent salt deposits tend to appear at the worst times, usually about a month after the building is constructed, and sometimes as long as a year after completion.

Before bricks and tiles can be fired the majority of water used in its moulding needs to be removed from the clay product. Any excess residual water or moisture left in the article as it enters the kiln to be fired will be removed very quickly leading to major problems such as blow out, warpage or in the extreme – as has been seen on numerous occasions – to explode causing serious damage to the kiln and production.

Generally, refractories are classified as basic, high alumina/silica, fireclay and insulating. There are also classes of “special refractories” which include Silicon Carbide, Silicon Nitride, Graphite, Zircon, Zirconia, fused cast and several others. Most refractory materials are supplied as preformed shapes. However, they also are manufactured in the form of special purpose clays, bonding mortars and monolithic such as castable, plastic refractories, ramming mixes and gunning mixes. In fact, many  refractories materials have been developed specifically to meet the service conditions of a particular process. The characteristic properties of each refractory class are a function of both their raw material base and the methods used to manufacture the refractory products.

Plasticity is the characteristic behaviour of a ceramic material to become permanently deformed after the application of an external force. This property is the most characteristic one in clays, and there are some factors that influence the plasticity and they should be considered in plasticity measurements:

The first process required to begin the production of clay bricks or roof tiles is to make available the raw materials required for the process. The major material used in manufacture are clays and as such brickworks will have been built as close to a clay quarry as possible although some clays are still trucked in to aid in varying colours and technical characteristics.

Breakage and rejection are two of the most common problems in ceramic manufacturing processes and It can come from a wide range of issues. Most  ceramic manufacturers try to reduce  their breakage and rejection as low as possible because small defect in a ceramic product can devalue the  product by more than 50% or even mean a loss for a premium grade producer.

Drying is one of the most critical parts of making quality brick. There are a number of things that can be done to improve drying and reduce drying defects. In order to reduce cracking there needs to be an understanding of the drying process and understanding of the brick body that is being dried. 

All brick and tile manufacturers strive for the elusive 100% yields, but in reality, the norm is anywhere from circa 50% upwards depending upon raw materials, products being produced and processes. These losses have been seen to occur in the extruding/moulding of the clay products, during the drying and firing processes and throughout all handling stages of production.

Various amounts of water are added within brick and tile manufacturing depending upon which process route you choose. A stiff extrusion may have water content as low as 10% while a soft mud factory may be as high as 30%. With hand made and water-struck products, this water addition value may even go higher. However, although these levels of water may be advantageous for manufacturing, they still require to be driven out during the drying process to enable the products to be fired.

Although the EU have downgraded Barium Carbonate from "Poisonous" to "Harmful If Swallowed", it is still treated as a toxic material in various countries across the world. However, its position as the most useful additive for the elimination of salt scumming from clay bricks and tiles is unchanged. For those countries where barium is prohibited, or due to increases in price and reduced availability, manufacturers look for alternatives.