Glazes are mixtures of finely ground minerals (ie: rocks), clays, and water. The clays and minerals unavoidably settle to the bottom of the bucket; the heaviest falling first, followed by the lighter ones in succession. As potters, we know we have to completely stir up a glaze, “homogenizing” all the ingredients, in order to get the best results.
Most of our glazes settle either pretty slowly, or are used often enough that the settling stays minor, and the minerals can be mixed back into suspension without too much trouble. But periodically, there’s a glaze that’s been left too long, and the ingredients have settled out into a hard lump that just can’t be mixed back up.
We can treat glazes that settle out hard with an additive called a Flocculant. Flocculants encourage a soft and slower settling of the minerals. Our preferred flocculant is Epson Salts (magnesium sulfate), which dissolves in the glaze, releasing ions that cause the clay to have oppositely charged surfaces and edges (Illustrated in Figure 1a). Having opposite charges, the platelets attract one another’s edges to one another’s surfaces, forming flocs, the simplest of which is illustrated in Figure 1b.
As flocking continues, a structure known as a House of Cards forms little compartments throughout the bucket. The compartments encapsulate glaze droplets, preventing them from settling, or softening the collection that does form at the bottom of the bucket. See illustration.
Flocculating a glaze needs to be done sparingly since it also causes the mix to act thicker than it should, even though the specific gravity is still the same. A glaze with too much flocculant can even appear to be jelled upon first opening the bucket – although it starts to thin when stirred. If this occurs, a studio potter would add a small amount of deflocculant until the fluidity is back to normal.
Extremes of both flocculation and deflocculation have to be avoided. They lead to the jelled effect on the one hand; and the hard lump at the bottom of the bucket on the other. Opposite to the flocculated House of Cards effect, platelets fall parallel to one another in a deflocculated mixture, compressing platelets beneath them into a so-called Deck of Cards pile, illustrated schematically below. As the weight of the pile increases, water is squeezed out of the lower layers, leaving a thick hard dry lump of minerals.