gumming

Gumming is the removal of clay and other adhesive fine material that clings to freshly exposed coal surfaces after cutting. When a coal seam is broken open by pick, drill, or saw, the clay bands and shale interbeds that run through the coal stick to the coal's face and edges. A miner with a shovel, scraper, or pick physically removes this "gum", the tacky clay matrix, before loading the coal into tubs or onto a belt. Without gumming, the coal carries excess weight of useless material to the surface, raising haulage costs and reducing saleable yield.

The gum itself is typically soft, plastic clay, often kaolinite or illite, that has absorbed groundwater and becomes adhesive. It differs from the coal itself in color (lighter, greyer) and consistency (stickier when wet). In thick seams with clay partings, gumming is routine work; in thin or clean seams, it may be unnecessary. The thickness of gum adhering to the coal face can range from a thin film to several centimeters, depending on the geology and how recently the coal was broken.

Gumming in practice and cost

Gumming is labor-intensive and adds time to the cutting cycle. It must be done at the face before or immediately after loading, or the clay sets hard as it dries. In deep mines with high moisture, the clay remains plastic for longer; in dry conditions or after exposure to air, it hardens and becomes harder to remove, sometimes requiring rewetting. The work is physically demanding and repetitive, causing strain injury in miners who perform it continuously.

The ash content of coal is a direct economic penalty, power stations, coke plants, and other end-users dock price for ash that exceeds contract limits, typically around 10-15% for raw coal. Every kilogram of gum left on the coal reduces the price per tonne. Conversely, aggressive gumming wastes labor and time without adding value once the coal is acceptably clean. Modern longwall and room-and-pillar operations in Europe and North America now rely partly on preparation plant screening to remove fine material downstream, reducing reliance on manual gumming, but the term persists in older workings and in regions where surface processing is less developed.