Definition:
The scientific term for the movement of a liquid through a semi-permeable membrane. In the case of an acrylic painting moisture-retaining palette, water seeps from the damp sheet of watercolor paper (the water reservoir) through a sheet of grease-proof paper (the semi-permeable membrane) to the acrylic paint on top of the grease-proof paper. This replaces the water that evaporates from the surface of the acrylic paint, thereby keeping it moist and usable.
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