Clay molecules bind with alum and sink as they become heavier than water. Stirring up the bottom could cause them to suspend again temporarily, but you aren't breaking the bond between the alum and clay molecules so they should sink again. I suspect your initial treatment might have been too light, but I'm not an expert by any means. Rex should be able to direct you.

Many men go fishing all of their lives without knowing that it is not fish they are after. ~ Henry David Thoreau

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