|Monthly Tech-Tip |
Mined and processed in the Tintic Mining district in Central Utah. The company claims it is the highest quality in the world (99% Halloysite) and explains on its website: "Halloysite is an aluminosilicate clay exhibiting a rare, naturally occurring hollow tubular structure. Halloysite tubes have a length in the range of 0.5 - 3.0 microns, an exterior diameter in the range of 50 - 70 nanometers and an internal diameter (lumen) in the range of 15 - 30 nanometers."
Surface area: 65-70 m2/g
Particle size: 91% < 10 microns
80% < 5 microns
70% < 2 microns
65% < 1 micron
63% < 0.5 microns
57% < 0.25 microns
54% < 0.2 microns
They are claiming a higher dry strength than Imerys Super Standard and Diamond Star, and a dramatically lower iron content (0.15 vs 0.45). The company has very detailed data and information sheets.
Our tests demonstrated the following (when comparing a highly vitreous cone 7 porcelain made using this material vs. an equivalent porcelain made using New Zealand Halloysite):
-In the dry powder and pugged form, Dragonite produces the whitest body we have ever seen.
-Plasticity is similar, both bodies needed 4% Veegum to produce a plasticity that makes it possible to throw very thin and delicate shapes.
-The Dragonite material exhibits higher drying shrinkage (8% compared to 7% for NZK body).
-Dry hardness is as good or better than NZK and it dries faster.
-It takes less water to produce a slurry using Dragonite.
-The fired colours are comparable and both produce high translucence when the body is fully vitrified.
-For the high-feldspar formulation we used both bodies had blistering when over-fired to cone 7.
-Dragonite requires a little more feldspar (3-4% in our tests) to produce a body of the same degree of vitrification (likely due to the lower K2O content).
-When the body has the same degree of vitrification, fired shrinkage is similar.
-Fired color of the vitreous porcelains are very similar.
-In a casting slip recipe Dragonite behaves similarly and can be cast very thin. Like the NZ material, a VeeGum addition of 1% was needed to produce a slip that had the strength to pull itself from the mold for very thin casts.
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.
A Dragonite Halloysite porcelain mug (center), New Zealand Halloysite (right) and a typical one made using North American kaolin (left). All have the G2926B glaze and fired at cone 6 to zero porosity. Although not clear on this photo, the two on the right are much whiter. The Dragonite has about the same whiteness as the NZK based body.
Halloysite forms over long periods as kaolin sheets roll into tubes.
These three materials also fire to a similar color. Grolleg is the most plastic, Dragonite the least.
Few raw materials are as clean as this before processing.
New Zealand Halloysite
The whitest burning kaolin we have ever seen. It is very sticky when wet, suspends glazes well & makes super white porcelain (with help from a white bentonite).
From a purely physical properties point-of-view, halloysite is a clay mineral similar to kaolin in f
Pure clay mineral, there are many brand names of varying purity and iron content.
|By Tony Hansen|
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