|Monthly Tech-Tip |
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.
Key phrases linking here: agglomeration, agglomerates - Learn more
In powders and slurries, particles can aggregate, sticking together with varying degrees of tenacity. The mechanisms of the bonding can be physical or chemical. Measures must be taken to avoid the problem or deal with it.
The fine mineral, oxide and clay particles used in ceramics often agglomerate during storage. Mixing equipment must be able to break these down, if not then sieving may be the only way to remove them. Pigment agglomerates will produce specking that can render ware unsaleable. Agglomerates in bodies, especially porcelains, will reduce fired strength, impact physical properties that are dependent on the particles being dispersed and mar the homogeneous fired surface.
Agglomerates in slurries (casting bodies, engobes and glazes) almost always ruin the fired surface appearance. In slurries, the agglomerates are often the fine particled clays, their presence means the clay particles are not fully performing their suspending, hardening or gelling function.
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.
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).
Both the fritware body and glaze contain significant percentages of New Zealand kaolin (NZK). White agglomerates of it have ruined both. The body was slurried by propeller mixing (at the highest speed) and dewatering on plaster. The glaze was slurried and propeller-mixed in a similar manner. But in both cases, the action of our lab mixer, a very capable device, was not enough to break up the kaolin agglomerates! This relates to the stickiness and particle dynamics of NZK. The glaze is the easiest to fix: Sieving at 100 mesh. But the body is just about impossible to sieve because it contains significant VeeGum which gels the slurry. But since I make smaller quantities of both of these, as a potter, blender mixing is much easier, it totally smashes them. However caution is required, the slurry needs a high enough specific gravity that it circulates freely in the blender jar. But a low enough one to enable the maximum RPM of the blender.
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! 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.
This is a cone 04 glaze on a terra cotta body. Two 300-gram test batches were made. Both have 5% tin oxide added. The one on the left was high-speed propeller-mixed for 10 seconds on a closed container. That was not enough, small agglomerates appear as white specks floating in the glass. The one on the right was mixed for 60 seconds. Now the tin particles, which are incredibly small, have been dispersed and can do their job of opacifying the glaze. Notice that 5% is not quite enough, more is needed.
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.
This brushing engobe is thick and gooey (because it contains CMC gum), so it is practically impossible to sieve. While our propeller mixer was not able to break up the tiny agglomerates of New Zealand kaolin 30 seconds of bender mixing made 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. It does not require Veegum (like glazes) because it already contains plenty of clay. The CMC gum greatly improves the brushing properties so additions of stain make it useful for colored underglazes also. For best performance we apply them to leather hard ware.
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.
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.
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.
Same recipe, same clay, same firing schedule. What went wrong?
Many ceramic products, especially tile, are formed by pressing high-moisture or binder-containing dust or pelletized dust into steel molds at high pressures.
Particle Agglomeration on Wikipedia
|By Tony Hansen|
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