|Monthly Tech-Tip |
Alternate Names: New Zealand Kaolin, New Zealand PFC
Thought to be the whitest globally available clay in the world. The white primary clay deposits mined at Matauri Bay are derived from the alteration of acid volcanic rocks. The aluminosilicate feldspar minerals in the parent rhyolite have been broken down to their constituents by low-temperature hydrothermal alteration and have then reconstituted as halloysite. It has a tubular crystal structure, which is markedly different from the booklet or platelet crystal structure of kaolinite.
The company has developed a unique beneficiation process (which includes filter pressing and thus the designation PFC or Premium Filter Cake) to ensure a high degree of purity. They claim 0.1% on a 240 mesh screen. This material is exported to many parts of the world.
We have found that porcelains made using this fire a little whiter and are more translucent than with Grolleg kaolin. However, that comes at a cost. As a pure material, New Zealand Halloysite (NZK) is less plastic than pure Grolleg (being barely workable enough to wedge and roll into a slab, for example). Notwithstanding this, NZK produces a body that is noticeably stickier. Grolleg requires less plasticizer addition (e.g. bentonite) to augment plasticity. However, NZK responds especially well to VeeGum (or similar highly refined smectites and hectorites) to create highly plastic, yet very white-burning porcelains. But NZK will produce a significantly less vitreous body because it comes closer to the theoretical A12O2:2SiO2 chemistry (Grolleg contains significant KNaO). That means, for example, that a cone 10 porcelain having near 50% kaolin will need 5-10% more feldspar, that will need to come at the expense of the kaolin. That will, in turn, necessitate an increase in white plasticizer (thereby increasing body cost).
This kaolin is also ideal in glaze slurries, it suspends and dry-hardens them well and the low iron and TiO2 content mean that clear glazes will be more transparent, often significantly so. Its stickiness means more issues with agglomerates so it is important to intensely propeller mix or sieve glazes to break these up. Grolleg kaolin has a similar performance so it can be substituted into most recipes.
The original bag of this product in 2014.
These three materials also fire to a similar color. Grolleg is the most plastic, Dragonite the least.
The whitest test bar here is a New-Zealand-kaolin-based cone 6 porcelain (NZK). NZK has low plasticity, so this body employs VeeGum to improve it. Immediately to the left of it are three North American-koalin-based bodies using standard bentonites. The bar to its right is a Grolleg-based body that uses a standard bentonite rather than a white burning one. All are plastic.
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.
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.
This is Polar Ice casting, a New Zealand Halloysite based cone 6 translucent porcelain. The body, as is, would never have enough plastic strength to pull itself from this mold without some tearing, but the addition of Veegum gives it that strength. This dried cast bowl measures 130mm in diameter and 85mm deep, it only weighs 89 gm! The slip was in the mold for only 1 minute before pour-out. Of course, there is a price to pay for adding Veegum to a slip: Increased casting time and more difficult deflocculation. Regular bentonite can be used in most bodies, but for super-whites like this, Veegum (or equivalent) is the choice. Testing is needed to determine what percentage gives the needed strength yet does not increase the casting time too much. This approach can be employed with any slip that is not releasing (assuming of course that you are using quality molds having good absorbency). This is also a testament slip casting process.
A transparent glazed. It is a made from Plainsman Polar Ice in 2014 (a New Zealand kaolin based porcelain) and fired to cone 6 with G2926B clear glaze. 5% Mason 6306 teal blue stain was added to the clay, then this was wedged only a few times. The piece was thrown, then trimmed on the outside at the leather hard stage and sanded on the inside when dry.
The recipe: 50% New Zealand kaolin, 21% G200 Feldspar, 25% silica and 3% VeeGum (for cone 10R). These are the cleanest materials available. Yet it contains 0.15% iron (mainly from the 0.25% in the New Zealand kaolin, the VeeGum chemistry is not known, I am assuming it contributes zero iron). A 50 lb a box of pugged would contain about 18,000 grams of dry clay (assuming 20% water). 0.15% of 18,000 is the 27 grams of iron you see here! This mug is a typical Grolleg-based porcelain using a standard raw bentonite. A box of it contains four times as much iron. Enough to fill that cup half full!
The NZK body, Polar Ice, is on the left. The Grolleg one, L3778D, is on the right. They are not the same recipe, the feldspar content in the L3778D has been adjusted to match the degree of vitrification (Grolleg contains some feldspar). Clearly, the NZK has better translucency. And it fires whiter.
The very whitest porcelains are made from New Zealand kaolin. However, while Grolleg kaolin does not fire quite as white, it requires up to 10% less feldspar to produce a vitreous porcelain (it contains natural feldspar). That 10% less spar can be made up in kaolin, imparting better workability and dry strength to the body (and Grolleg is known for its dry strength). Assuming that 25% silica is needed for glaze fit, one only needs to discover what blend of feldspar and kaolin in the remaining 75% achieves the desired degree of vitrification (e.g. we like zero porosity just-reached at cone 6). We found 25% nepheline was too vitreous (pieces warped) and at 20% porosity was not yet zero. While the Grolleg version fires a little darker, the better workability imparted by the extra kaolin makes up for that. The plasticity needed for good throwing requires the addition of bentonite (4% for NZK and 3% for Grolleg). Both of these can be made into casting bodies by reducing the amount of bentonite (~ 1% for NZK, 0.5% for Grolleg). Do your testing to discover the % of bentonite needed for the leather hard to pull away from a mold without cracking but not take too long to cast.
Left: Cone 10R (reduction) Plainsman P700 porcelain (made using Grolleg and G200 Feldspar). Right: Plainsman Cone 6 Plainsman Polar Ice porcelain (made using New Zealand kaolin and Nepheline Syenite). Both are zero porosity. The Polar Ice is very translucent, the P700 much less. The blue coloration of the P700 is mostly a product of the suspended micro-bubbles in the feldspar clear glaze (G1947U). The cone 6 glaze is fritted and much more transparent, but it could be stained to match the blue. These are high quality combinations of glaze and body.
Electron micrograph showing Dragonite Halloysite needle structure. For use in making porcelains, Halloysite has physical properties similar to a kaolin. However it tends to be less plastic, so bodies employing it need more bentonite or other plasticizer added. Compared to a typical kaolin it also has a higher fired shrinkage due to the nature of the way its particles densify during firing. However, Dragonite and New Zealand Halloysites have proven to be the whitest firing materials available, they make excellent porcelains.
Both of these Zero4 fritware porcelain mugs were bisque fired at 1450F and glaze fired at cone 04. The Grolleg version of the porcelain is burning a much pinker color (both the bare body and under our G1916Q3 glaze). In typical feldspar porcelains the color difference between these two kaolins is not nearly so much but here the extra glass development is likely amplifying the presence of even just a little more iron oxide.
These are two cone 6 transparent-glazed porcelain mugs. On the left is the porcelainous Plainsman M370 (Laguna B-Mix 6 would have similar opacity), it is semi-vitreous and has no translucency. Right is a highly vitreous, New Zealand kaolin based porcelain, Polar Ice. The secret to making this porcelain super-white is the NZ kaolin. The secret of its impossibly-high plasticity is the very expensive plasticizer, VeeGum T. What about the translucency? Nepheline syenite is used as the feldspar, but it alone cannot deliver this kind of translucency at cone 6. Amazingly the 4% Veegum acts as a translucency catalyst, it is the secret. Commercial manufacturers could never use a sticky and difficult-to-dry porcelain like this, but a potter can do incredible things with it (e.g. throw thinner, lighter, bigger than any other clay he/she has ever used!).
From a purely physical properties point-of-view, halloysite is a clay mineral similar to kaolin in f
Imerys Halloysite Information Page
A clay of incredibly small particle size. It has the highest plasticity of any known clay and acts as a suspending and gelling in slurries.
A white burning kaolin from the UK, commonly used in porcelain bodies and as a glaze suspender. Sticky when wet, low plasticity.
|By Tony Hansen|
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