All Glossary

Flocculation

The flocculation process enables technicians in ceramics to create an engobe or glaze slurry that gels and goes on to ware in a thick yet even, non-dripping layer.

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Details

Technically, flocculation is particle aggregation in a suspension, but in ceramic circles the term generally refers to the gelling that visibly occurs in a ceramic glaze or clay slurry, that would otherwise be thin and runny, by the simple addition of a flocculant (the opposite of deflocculation). It is regarded as a process rather than a phenomena. Flocculation can make a slurry thixotropic, improving its suspension properties or enable application of engobes and slips (and sometimes glazes) in a thicker layer that does not run or drip. To achieve the gel the flocculation process normally requires a slurry of higher water content. It will thus have a higher shrinkage and likely take longer to dry completely. But technicians learn how to balance these issues to make the process successful. Common flocculants include calcium chloride, vinegar and epsom salts.

A slurry can be gelled by the addition of a high-surface-area clay (e.g. Veegum, Bentone), this is not technically the same as flocculating it. 3% Veegum, for example, can double the amount of water needed and still leave the slurry in a gelled condition. Suppliers often employ this phenomenon, in consort with a CMC gum addition, to make glazes more paintable and lay down in a thinner layer. Glazes gelled in this manner are potentially more rheology-stable. Slurries gelled in this manner do not settle whereas flocculated slurries can settle.

Flocculation in a slurry can be a desired or undesired property. Glazes can change their viscosity with storage, when they thicken they are said to 'flocculate'. In these cases slightly soluble materials in the mix (e.g. nepheline syenite, gerstley borate, boron frits, clays containing sulfates) can act to change the viscosity of the slurry. It can be difficult to deflocculate these slurries and make them usable again, these are best used soon after they are made or reflormulated such that the needed oxides are supplied by non-soluble materials.

Related Information

The same engobe. Same water content. What is the difference?

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The engobe on the left, even though it has a fairly low water content, is running off the leather hard clay, dripping and drying slowly. The one on the right has been flocculated with epsom salts (powdered), giving it thixotropy (ability to gel when not in motion but flow when in motion). Now there are no drips, there are no thin or thick sections. It gels after a few seconds and can be uprighted and set on the shelf for drying.

The engobe on this stainless steel spoon has not cracked in drying or firing. How?

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This spoon was dipped into a ceramic dipping engobe, L3954B. It contains no CMC gum, it was only flocculated using powdered Epsom salts. Without the Epsom salts, the engobe runs off, leaving only a film. But, when turned into a thixotropic slurry, it stays on the spoon in an even layer (as a gel), then hardens as it dewaters (left) and finally dries completely (right). With no cracks! It also fires to cone 03 with no cracks. Of course, if this were fired high enough, it would begin to shrink, crack, crawl, melt and then craze, ceasing to be an engobe. Of course, special low-expansion frits and additives and mixing, preparation and application techniques make enamels, which do melt, possible for metals.

How to stop an slip from running

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The flocculated slip (left) hangs on, stays even and does not run. The normal slip (right) is thin and running on verticals and thinning at the rim.

Fundamentals of Fluid Mechanics - book

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Many aspects of ceramic production relate to the control of fluids (mostly suspensions). This is also true of material production. If you want to solve problems and optimize your process this is invaluable knowledge. This book is available at amazon.com.

When to use vinegar and when to use epsom salts to flocculate a slurry

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Slurries with more clay (like engobes, slips) generally respond better to epsom salts. However the extra clay also makes them more likely to go moldy, so you may need to add a few drops of Dettol to kill the bacteria (if they are stored for any length of time). Vinegar works better for glaze surries, but only if they have sufficient specific gravity. Many people like to make an epsom salts solution and add that, but if you have a good mixer you may find it more intuitive to add the crystals (which you should crush to a powder) and wait 30 seconds for the viscosity to respond.

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