Slips are great for adding highlights and decorative details to pottery, and potters often want to apply them with a brush. Trouble is, brushing can get messy because slips naturally flocculate, making them flow off the brush in clumps rather than in a nice smooth fluid. Adding water doesn’t improve the flow, it just dilutes the slip, thinning out the coverage, and causing the brush lines to bleed.
These issues can be corrected by adding a trace amount of a deflocculant to the slip. Deflocculants neutralize the charges on the platelets, whether they’ve occurred naturally or through the action of flocculants, so that instead of attracting one another, the platelets actually repulse.
This feature reduces the clumpiness of the slip because the platelets all separate. Still more, deflocculants disrupt the structure of the water films surrounding the platelets, thus allowing the slip to flow dramatically easier.
To understand the action of the water films in a slip, we need to delve into a text by Lawrence and West: Ceramic Science for the Potter.
….. Stay with me !
First, water is a polar molecule (see Figure 2) meaning it has a positive and negative side. The polarity causes it to stick to other water molecules at an angle, building up into hexagonal rings, as shown on the right side of the figure.
The rings are just the right size to bind to the edges of clay platelets and hold the clay and water film tightly together, thus promoting plasticity.
Two platelets bound by tight hexagonal water films are illustrated here to the right. But….in a slip, plasticity is not desirable. What we really want is for the platelets to be loosly held, to actually flow apart rather than hold together.
Defloculants make that happen. They release large ions into the slip that force their way into the “donut hole” in the water ring and break it apart, as illustrated in Figure 4. This weakens the grip of the water film and dramatically reduces the slip’s viscosity – meaning it pours and flows a lot easier, even though it’s still just as thick as it was.
Deflocculants, like Darvan 7 – a Sodium Polymethacrylate, thus improve a slip’s brushability, allowing it to flow much more evenly and readily while still retaining thickness (ie, clay-to-water ratio). Even though the slip seems thinner, it contains the same amount of water and dry material as a thick, gloppy slip.