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?

Two samples, the same engobe runs but not the other

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.

Would it be possible to glaze a stainless steel spoon?

This is a stainless steel spoon that has been dipped into a ceramic engobe that has been flocculated using powdered epsom salts. Without the salts the slip completely runs off leaving only a film. But with the right amount 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 fired to cone 03 with no cracks. If this were fired high enough it would transform to a glaze. But it would craze. Special low expansion frits are available to make enamels for metals.

How to stop an slip from running

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

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

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.

Links

Glossary	Deflocculation Deflocculation is the magic behind the ceramic casting process, it enables slurries having impossibly low water contents and ware having amazingly low drying shrinkage
Glossary	Thixotropy Thixotropy is a property of ceramic slurries of high water content. Thixotropic suspensions flow when moving but gel after sitting (for a few moments more depending on application). This phenomenon is helpful in getting even, drip-free glaze coverage.
Glossary	Rheology In ceramics, this term refers to the flow and gel properties of a glaze or body suspension (made from water and mineral powders, with possible additives, deflocculants, modifiers).
Glossary	Glaze Gelling 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.
Glossary	Mocha glazes 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.
Recipes	G1916Q - Low Fire Highly-Expansion-Adjustable Transparent An expansion-adjustable cone 04 transparent glaze made using three common Ferro frits (low and high expansion), it produces an easy-to-use slurry.
Recipes	L3685U - Cone 03 White Engobe Recipe A white burning body with enough added frit to produce a cone 03 stoneware or white slip for use on the matching red Zero3 stoneware.

By Tony Hansen
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