TM05 Carbon and Sulphur Analysis

If clays contain a significant amount of carbon, an exothermic reaction takes place, which may help to fire the clay products. This may be very useful using old firing technique such as the Hoffman chamber kiln or clamp kilns. High carbon contents (above 3%) make modern controlled tunnel kiln firing very difficult to control. 

Much lower levers of carbon in excess of 1% tend to lead to other production problems such as bloating and hearting. Some manufacturers produce aesthetically pleasing hearted bricks intentionally and require a high carbon content to achieve this effect.

Where glazes are applied and fired using the once fired method, carbon content within the clay body above 0.1% tend to give rise to glaze faults.

Between these values it may be useful to assess when the carbon is burnt out during firing. Differential thermal analysis techniques and fast firing billet techniques have been useful. A common misconception is that the carbon is all burnt out around 900^oC. The carbon burn out range varies with the raw material, usually starting at about 400^oC and in some materials depending upon the firing profile still contain carbon when fired up to 1100^oC.

Carbon core analysis is useful in association with DTA for enhancing the firing profile to achieve either a more consistent product or increase the firing efficiency.