In mineralogical studies of plastic or ‘ball clays’, carbon is typically listed as a ‘minor constituent’. But this undervalues the impact of carbonaceous matter – particularly during ceramics manufacture. In fact, the type and quality of carbonaceous matter can have a profound effect on many of a plastic clay’s properties.

How is carbon found in plastic clays?

When plastic clays are formed, the seams can often be inter-dispersed with organic material; typically derived from local vegetation which was washed from the surrounding area along with the clay minerals during deposition.

Over many millennia the vegetation becomes compacted and develops into distinct lignite seams.

Carbon can be present in two forms; both derived from these lignite seams:

  1. The first is termed ‘lignite’ and is present in lump form, which means the bulk of it can be removed by traditional wet screening, during ceramic body manufacture.
  2. The second is often termed ‘colloidal carbon’ and is made up of resins, tannins, fats, waxes, lignin and humic acids.

The benefits of carbon in ceramics manufacture

The humic acids found in ‘colloidal carbon’ have large cation exchange capacities, which results in a strong buffering action and produces extremely stable casting slips of high fluidity.

Another major advantage of colloidal carbon is its impact on the plastic properties of plastic clay; notably its strength and workability.

In particular, a significant increase in green strength is noticed when clays of similar minerology, but with higher levels of colloidal carbonaceous matter are examined.

Why should carbonaceous matter in plastic clays be controlled?

Though there are notable advantages that carbon can deliver in terms of green strength and the stability of casting slips, uncontrolled levels of carbonaceous matter can seriously affect the firing characteristics of clays:

  1. Black coring occurs when the ceramic body is fired too quickly, or with an insufficient oxygen supply. The result is a black or blue-grey centre to the piece. Where fast firing is used, this phenomenon becomes difficult to avoid.
  2. Pinholing is caused by a relatively large piece of carbonaceous material burning out from the body, leaving a hole at the ceramic surface.

Careful control of process conditions and body formulation can avoid these problems.

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