Helmer Kaolin

Mineralogically, Helmer kaolin contains kaolinte and halloysite clay minerals (and small amounts of others of course). Its halloysite content is one of the factors that makes it useful for certain special purpose non-ceramic applications. Plainsman does not currently monitor the mineralogy (e.g. halloysite content) of this material.

Helmer Kaolin is prized by potters for its flashing properties during firing (flashing refers to brown and red surface coloration on unglazed areas that develops during wood firing). The chemical or physical phenomena responsible for this behavior has not been clearly explained and there is much disagreement in currect speculation.

Helmer kaolin was used for many years in the refractories industry. It was calcined to produce low-quartz thermal-expansion-stable aggregate used to make kiln furniture.

The physical properties of Helmer kaolin are similar to typical commercially available medium plasticity kaolins. In raw form it is quite white and has a plasticity lower than a typical pottery clay.

Like any kaolin, it has a high water of plasticity (26-28%) and LOI and it has an ultimate particle size that is significantly larger than ball clays and bentonites. Additions of Helmer kaolin to bodies made from other Plainsman native clays helps diversify the ultimate particle size distribution to produce whiter bodies with better drying properties, greater fired strength, and higher maturing temperatures.

Helmer kaolin has a fired shrinkage that is higher than typical kaolins, even if fired to temperatures far below its maturing range. This is because of the shape of the halloysite mineral particles it contains and the way these collapse and pack during firing. Thus there is some need for caution as to how much is used in a body. However if other materials in the recipe have ranges of particle sizes that complement it fired shrinkage will be minimized.