|Monthly Tech-Tip |
Alternate Names: New Zealand Kaolin, New Zealand PFC
New Zealand China Clays claim that this is the whitest 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. Thus, while often called a kaolin, technically it is a halloysite.
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, 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 common plasticizer addition (e.g. bentonite) to create a plastic porcelain. 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).
These balls are easily broken down by the propeller in a slurry mixer. But they do not break down easily in a dry mixer, even when in a mix with other materials (like silica and feldspar). They just bounce around on a vibrating screen. That means that without some sort of finishing device in the dry material feed stream to break down these lumps before the pugmill, it is difficult to manufacture homogeneous plastic body employing it.
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!
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.
The original bag of this product in 2014.
This cast bowl (just out of the mold and dried) is 130mm in diameter and 85mm deep and yet the walls are only 1mm thick and it only weighs 89 gm! The slip was in the mold for only 1 minute. What slip? A New Zealand Halloysite based cone 6 translucent porcelain. This NZ material is fabulous for casting slips (it needs a little extra plasticizer also to give the body the strength to pull away from the mold surface as it shrinks).
These three materials also fire to a similar color. Grolleg is the most plastic, Dragonite the least.
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.
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!).
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.
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.