A technique used by ceramic artists to decorate pottery. It happens when bleeding occurs at the edges of a thin colored acidic mixture painted over a still-wet slip.
Mocha diffusion is a technique of layering slips onto ware so that the edges of the upper one bleed outward into the lower. An alkali/acid mechanism is employed. The lower layer is a typical water based slip (usually white or cream) that is gelled enough to stay stable on the ware and wet long enough to apply the upper layer. Plastic slips (those high in bentonite or ball clay) work the best. The upper layer is acid-based and stained and very runny, people use a wide range acidic solutions (like tobacco or lemon juice, vinegar). When the stained acid is painted over the wet base its edges bleed outward in accordance with the fluidities and degree of difference in pH. The effect survives firing well.
M370 mug using M370 clear glaze by Victor Duffhues.
Creating a Non-Glaze Ceramic Slip or Engobe
It can be difficult to find an engobe that is drying and firing compatible with your body. It is better to understand, formulate and tune your own slip to your own body, glaze and process.
The flocculation process enables technicians in ceramics to create an engobe or glaze slurry that gels and goes on to the ware in a thick yet even layer that does not drip.
Glaze slurries can gel if they contain soluble materials that flocculate the suspension. Gelling is a real problem since it requires water additions that increase shrinkage.