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

In powders and slurries, particles can aggregate, sticking together with varying degrees of tenacity. Agglomerates can be thousands or even millions of particles. The mechanisms of the bonding can be physical, electrolytic or chemical. If measures are not taken to recognize and avoid the problem process and ware quality are detrimentally affected.

Fine powders used in ceramics often agglomerate during storage. Mixing equipment capable of injecting plenty of energy (e.g. a propeller mixer) can break these down (if used appropriately). Sieving is also helpful, the finer the mesh size the better the breakdown (however openings in even the finest sieves can still permit agglomerates of thousands or particles).

Pigment agglomerates can produce specking that can render ware unsaleable. Agglomerates in bodies, especially porcelains, will reduce fired strength and affect fired surface appearance and character. Agglomerates in slurries (casting bodies, engobes and glazes) will also are fired surface appearance. In slurries, the agglomerates are often the fine-particled clays - their presence implies the clay particles are not fully performing their suspending, hardening or gelling function.

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 fritware body and 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 agglomerates of hundreds 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. Even if I need to do multiple gallons it is easy to process the slurry in batches in the 2 litre jar of my mixer.

A wollastonite containing glaze slurry was not sieved before use

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This glaze has just been applied to a bisqued tile. It contains wollastonite, which can agglomerate in storage. It was propeller-mixed at high speed, but that was not enough to break down the white lumps (agglomerates). But they can be broken down by sieving the slurry through 80 mesh or finer. Many other materials behave in a similar manner (e.g. barium carbonate, iron oxide, cobalt oxide, clays, tin oxide, zircon, titanium dioxide).

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 we make a base brushing engobe

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This brushing engobe is thick and gooey (because it contains CMC gum), so it is practically impossible to sieve. Our 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 we 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, minimizing the number of coats needed.

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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