All Glossary

Flocculation

The flocculation process enables technicians in ceramics to create an engobe or glaze slurry that gels and goes on to the ware in a thick yet even layer that does not drip.

Details

The opposite of deflocculation. Flocculation in a slurry can be a desired or undesired property.

For the latter, a ceramic glaze or clay slurry that would otherwise be thin and runny can be made into a gel by the simple addition of a flocculant. This is typically done to improve 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 on drying 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.

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, thus such glazes are best used soon after they are made or reflormulated such that the needed oxides are supplied by non-soluble materials.

Related Information

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.

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.

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.

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

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.

Which clay contains more soluble salts?

Example of sedimentation test to compare soluble salts water extracts from suspended clay. This simple test also reveals ultimate particle size distribution differences in clays that a sieve analysis cannot do.

Should you throw out the brown water on top of settled glazes?

This is water from the top of a glaze that had been sitting for more than a year. Clearly, the solute contains iron. It is being dissolved out of one or more of the white powders in the glaze recipe (often frits). The iron, at least, is a contaminant. This should be thrown out and replaced with clean water. Why? We do not want anything dissolved in glaze slurries. It either migrates into the body with the water it absorbs during glazing or it migrates to the surface as the water evaporates. Both are bad. How much dissolved material would be lost? It would be measured in tenths or hundreds of a gram. Hypothetically then, if a bucket contains 1000 grams of the material, one ten-thousandth of it would be lost!

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.

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By Tony Hansen