As the name suggests a Black Core is a circular or semi-circular shaped black or gray spots formed around the core or middle portion in fire clay refractory bricks (aluminosilicate refractory bricks mostly containing good percentage of raw clay).
These are not desirable since presence of Black Core spots reduces the normal life of a refractory brick and sometimes, also cause bloating in the furnace.

How to stop formation of Black Cores (Black Reducing Spots)  
Due the fineness of this clay, gas diffusion during firing may be inhibited. The colour change from black core to red rim in the brick appears to arise from the oxidation state of iron i.e. from Fe3O4 magnetite core to the Fe2O3 hematite outer region. If the firing atmosphere lacks oxygen, no red colour develops and brick material remains in a dark grey colour both inside and outside of the fired refractory brick piece.

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The following factors determine the extent of black core in refractory bricks:

  • Firing time - a longer firing time can eliminate the black reduction core.
  • The oxygen atmosphere during firing. Lack of oxygen promotes the formation of black reduction cores.
  • Iron oxide content in the raw clay. 
  • Carbon content and burnout or oxidation of carbon during firing of the raw clay. 
  • Fineness of clay and degree of compaction which influences gas permeability: If gases do not have sufficient time to diffuse into or out of the interior of the brick during firing, the brick bloats. ​
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Considering the above mentioned factors, the three steps that should be taken in order to avoid the formation of Black Core in fireclay refractory bricks especially, are:-

  1. The kiln atmosphere must be kept oxidizing during firing of bricks. 
  2. Soaking time should be prolonged.
  3. If it is not eliminated even after taking the above two steps then the percentage of the raw clay used in the brick composition need to be reduced or may be replaced.      

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Black Core defects in Fire Clay Refractory Bricks - Causes and Remedies

24-Jan-2019.

Chemically, fire clay contain mainly silicon and aluminium - or since these elements usually occur as oxides, as silica and alumina - with smaller amounts of iron, calcium, sodium and other elements. In aluminosilicate or fireclay refractory bricks, raw clay is added as a sintering aid or even as a main constituent depending upon its application, while firing temperature is ideally tailored as per the raw materials used.
The two main reasons attributed to the formation of Black Core in fireclay refractory bricks are:

  1. Presence of Carbonaceous matters in plastic / non-plastic fire clays which have been used as raw material in manufacturing those refractory bricks. Sometimes, excess carbonaceous matter and sulphur have been found to occur in the fire clays available particularly nearby a coal-belt. ​
  2. Because of too much reducing atmosphere in the kiln during firing of these refractory bricks.
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Reasons behind formation of Black Core spots in Refractory Bricks

What is ‘Black Core’ in Refractory Bricks?