All Glossary

Agglomeration

The fine mineral, oxide and clay particles used in ceramics often aglommerate during storage or even in the latter stages of production. These must be broken down later.

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Details

Agglomeration is the clustering of fine particles into larger groups. In ceramic powders and slurries, these clusters can contain one material or several different materials. Their bonds range from weak attractions to strong mineral bridges. Unwanted agglomerates can interfere with mixing, application, color development and fired quality.

Fine powders can agglomerate during manufacture, transport and storage. Moisture, pressure and drying can strengthen the clusters. When powders are mixed with water, inadequate wetting and insufficient mixing energy can leave agglomerates intact. A slurry can look smooth while still containing clusters large enough to cause fired specks, bumps or uneven color. When clay remains in lumps, it cannot contribute its full suspending and dry-binding properties.

Effective mixing must disperse the particles, not merely circulate the slurry. A propeller mixer can break down many agglomerates, but results depend on multiple factors. Blender mixing can be particularly effective for small batches. If agglomerates are resistant to even that, a recipe change may be more practical than milling (it changes particle size and doesn't prevent reagglomeration). Sieving is useful, but passing a screen does not prove complete dispersion.

Re-agglomeration during storage

Particles that have been dispersed can cluster again. Breaking up existing lumps does not necessarily remove the conditions that caused them. Changes in dissolved salts, pH and particle surface chemistry can change the stability of a suspension. Settling can also bring particles into close contact. For example, a potter reported repeatedly screening a wollastonite-containing glaze through 80 mesh, only to find new lumps after several weeks. Wollastonite can slowly release calcium into water and affect its pH. These changes could encourage aggregation involving wollastonite, clay and other ingredients.

Of course, not every lump appearing during storage is a re-formed agglomerate; it is assumed you can tell the difference between thousands of tiny agglomerates and precipitated crystals or lumps of dried glaze from the lid or bucket walls. Smear a lump between a spatula and a smooth surface. A soft cluster that readily disperses suggests weak aggregation. A gritty grain that remains intact suggests a strongly bonded cluster. Try high-shear mixing (e.g. blender mixing). Lumps that disappear are likely dispersible clusters. Persistent hard grains warrant closer examination. White lumps suggest a pure material like wollastonite.

For recurring problems, it may be worth trying to identify the cause. Do this by preparing comparison jars using the suspect material and water alone, and the complete glaze with the usual water and distilled water. Record mixing, lump formation and, if possible, pH and conductivity over time. Compare different material lots when available. A change in pH or conductivity indicates changing solution chemistry, but does not by itself identify the cause. Or, just try a batch of glaze with distilled water and a dispersant.

If the problem recurs, consider a recipe change (e.g. sourcing CaO from calcium carbonate instead of wollastonite). Rescreening may be a practical remedy, but consider blending mixing also. A quartz-size mixer can quickly process glaze if done in 30-second batches.

Related Information

Wollastonite containing glazes should be sieved

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Screen a glaze to break down the wollastonite agglomerates (which often form in storage). This is an 80 mesh plastic sieve (the actual screen is a metal insert inside), I am using a spatula to encourage it to pass through the screen. If you do not do this, the small lumps you see on the freshly glazed piece will fire to surface bumps and ruin the glaze.

Agglomeration of New Zealand kaolin

In both a fritware body and the glaze

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White agglomerates of New Zealand kaolin (NZK) have ruined both glaze and body (Zero4 fritware). Both were slurried up by propeller mixing (the latter dewatered on a plaster table). But in both cases, the action of our lab mixer, a very capable device, was not enough to break up the NZK agglomerates! The glaze appears to be the easiest to fix: Sieve it at 100 mesh. But does that really work? No. The particles are 10-20 times smaller than the openings so countless agglomerates could easily remain intact. The body is another matter. It is just about impossible to sieve because it contains significant VeeGum that gels the slurry. However, I am a potter and don't need to make thousands of gallons. Blender mixing is the answer, on high speed, it smashes the agglomerates in seconds. Even for many gallons it is easy to process the slurry in batches using the 2 litre jar of my mixer.

A wollastonite glaze slurry has agglomerates:

Is sieving the solution? Not necessarily.

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This glaze has just been applied to a bisque tile. It contains wollastonite, a curious mineral because it has a shelf life (it can agglomerate in storage as a powder). Surprisingly, it can also form them in slurries. Other materials can also agglomerate; these include barium carbonate, iron oxide, cobalt oxide, clays, tin oxide, zircon and titanium dioxide.

This glaze was propeller-mixed, but that was not enough to break down the white lumps. Is sieving the answer? Maybe not. While screening can reduce the size of the lumps, it does not prevent their regrowth in the slurry (because smaller agglomerates that survive sieving can become starting points for larger clusters). A better option is blender-mixing. When that is done and re-agglomeration occurs, then the problem is more complicated.

The difference propeller-mixing makes in a titanium glaze

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The glaze has 5% added titanium dioxide. These were fired at cone 6. The titanium in the one on the left remained agglomerated, it did not disperse in the slurry during hand mixing (the agglomerates can be seen as white particles floating in the glass). On high-speed propller-mixing the effect on the right was produced! Blender mixing would be even better. This incredible difference occurs because the mixer is able to break up the titanium agglomerates, dispersing and wetting all the surfaces of the incredibly tiny particles. In this state they do their magic during the firing, opacifying and variegating the otherwise transparent base matte glaze. Would sieving be as good? Only partially, the particles are 200+ times smaller than the openings in an 80-mesh sieve, high energy input is needed to separate them.

Don't waste expensive tin oxide, propeller mix glazes well

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This is a cone 04 glaze on a terra cotta body. Two 300-gram test batches were made. Both have 5% tin oxide added to a base transparent, G1916Q. The one on the left was high-speed propeller-mixed for 10 seconds. That was not enough, small agglomerates appear as tiny white specks floating in the glass. The one on the right was blender mixed for 60 seconds. Now the incredibly small tin particles have been dispersed and can do their job of opacifying the glaze. Would sieving the slurry have worked? No, the particle agglomerates can be far smaller than the openings in an 80-mesh sieve (individual particles could be 1000 times smaller!). The thorough dispersion of the expensive tin oxide particles has real benefit: It enables making an accurate assessment of whether the 5% addition is enough. In this case it isn't, more is needed.

This is how New Zealand kaolin powder agglomerates

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These lumps do not break down easily in a dry mixer, even when with other materials (like silica and feldspar). And they just bounce around on a vibrating screen. That means that without some sort of finishing device in the dry material feed stream is needed to break down these lumps before the pugmill.

Here’s how I make a base brushing engobe

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This brushing engobe has already passed my EBCT test for use on the intended body. It is thick and gooey (because it contains CMC gum), so it is practically impossible to sieve. My regular propeller mixer is not able to break up the tiny agglomerates of New Zealand kaolin. But 30 seconds of blender mixing makes it as smooth as silk. To make this liter of brushing engobe, I use 800g of powder and 10g of CMC gum in 800g of water. Because of the very high clay content, this does not require Veegum to gel it. The CMC gum greatly improves the brushing properties. It also thins the slurry enough to enable its lower-than-normal water content, making it more suitable for painting onto leather-hard ware, often in as little as one coat.

Comparing the fired glaze specks from different iron oxide brands

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Five different brand names of iron oxide at 4% in G1214W cone 5 transparent glaze. The glazes have been sieved to 100 mesh but remaining specks are still due to agglomeration of particles, not particle size differences.

4% iron oxide in a clear glaze. Unscreened. The result: Fired specks.

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Iron oxide is a very fine powder. Unfortunately it can agglomerate badly and no amount of wet mixing seems to break down the lumps. However putting the glaze through a screen, in this case, 80 mesh, does reduce them in size. Ball milling would remove them completely. Other oxide colorants have this same issue (e.g. cobalt oxide). Stains disperse much better in slurries.

Blue specks in a pugged porcelain. Be careful when adding stain.

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This is the cut-line on a wet, plastic slug of porcelain. These specks are agglomerates of a blue stain and existed even though the porcelain was dispersed under a powerful slurry mixer for ten minutes. Pure cobalt, if used to stain a porcelain, is known to do this. So stain is often used as an alternative. Some stains disperse much better than others (and do not agglomerate like this). The lesson is to test the colors of the stain available to you to make sure and use one that does disperse well.

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