These are partially full urns (1% - 99% full) made by Woodcutting while having a strong woodcutting urn (r) in your inventory. You can only have one of these at a time, as a new urn won't start to fill while you have another partially filled urn of the same type in your inventory or bank, or when you have 10 full urns of that type.
This is collecting woodchips when I chop wood at or below Woodcutting level 75.
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