Flocculation in Art Glazes

One of the more frustrating features of using art glazes is the fact that what you see is not what you get. Additionally, the act of glazing can be a very intricate operation which requires the potter to follow each step absolutely correctly and in the correct order to obtain a perfect glaze effect. It is necessary to have the glazes well-prepared in order to prevent common mistakes such as having glaze settle out, or having glaze curtain and drip down the sides of the pottery. In order to adjust glazes it is necessary to understand certain concepts about glaze in itself.

The first of these is that not all of the particles in it are the same size. Although they may all feel fine to the touch, nonetheless the size range is enormous. If the smallest particles are taken to be pea-sized, then the largest particles by comparison are as enormous as large buildings; with everything in between represented in the mix as well. The smaller particles tend to attract one another to form flocs which slow down the settling of the larger particles. Glazes which are kept in suspension by flocs are termed flocculated glazes, and they tend to settle slowly into an open, soft layer. Deflocculated glazes settle into thin, rock-hard layers. Therefore, in applying glaze by pouring or dipping to a soft bisque (that is, a bisque ceramic which was fired to earthenware temperatures), it is necessary that the glaze be flocculated. On the other hand, in applying glaze to vitreous bisque which has undergone high temperature firing, the glaze should be deflocculated (commercial china is manufactured this way).

Another thing to take into account in understanding how wet glaze acts is that tiny clay particles in suspension behave like magnets, either attracting or repelling each other. When they repel, the smaller particles do not floc together to hold the larger particles up, which causes the larger particles to settle faster. When a pot is dipped into this deflocculated glaze the particles align in parallel layers which seals off the pores in the bisque. In the ideal case the glaze should appear to dry almost immediately upon withdrawing it from the glaze bucket.

Deflocculated glazes, on the other hand, remain wet on the surface, which makes the glaze drip and takes a long time to dry. Extremely deflocculated glazes only form thin layers and when they dry they have a drippy, crystalline appearance. Sometimes entire sections of the glaze slide downward to make a curtained effect. Deflocculation can be counteracted by adding a flocculant to the dry glazes – magnesium sulfate (epsom salts) and calcium chloride are the commonest – which brings the glaze back into its flocculated state. Flocculation is not needed if the glaze has a high clay content, doesn’t settle out, or tends to gel. But glazes which settle out quickly usually require the addition of a flocculant. You can add the dry flocculant to the glaze bucket and mix it well; but a better technique is to make a concentrated solution of magnesium sulfate or calcium chloride in hot water in a blender adding more and more water until no more dissolves. Add this solution a little bit at a time as you stir the glaze. The glaze will thicken as it flocculates. It can take two or three tablespoons of the solution to flocculate a 5 gallon glaze bucket.

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