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This defect in pugged clay is often termed "hard chunks". Two are visible in this picture, dried powdery chunks have been exposed while working the surface. There is always some gap between the pugmill blades and the sidewalls - as the pugmil runs a hard crust develops against the barrel in this space. Harder pieces can then break off at the upper edge on the side of blade rise (as the shaft rotates). The harder pieces are thus composed of the same clay. Still, if meticulous attention can be paid to prevent this, it should happen rarely. Several factors play to make the problem worse or more likely.
-A pre-mixer is not being used. When pugmills see double duty as the mixer and the pugger dry clay is fed into the head of the barrel and mixed with water by the blades. This problem is exacerbated by several factors. When the powder entering a mixer/pugger has zero water content or has a recipe having a high percentage of bentonite (it resists being wetted). When the pugmill blades are angled too much they can push the clay down the barrel before it has adequate chance to wet.
-When the pugmill is used for extended periods without cleanout.
-When the pugmill is not fitted with a bar to hold the dry layer in place.
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