|Monthly Tech-Tip |
Alternate Names: Grolleg China Clay
A blended English china clay, combining moderate plasticity, low titania content and relatively high flux content, low shrinkage and blue-white fired color. The low TiO2 content is a key factor for the usefulness of Grolleg. It is excellent for making translucent throwing or casting porcelains. The pottery community uses many Grolleg based porcelains.
Its chemistry is different from a typical North American kaolin, it contains some fluxing oxides (e.g. it has almost 2% K2O). Thus porcelain bodies employing it require quite less feldspar to vitrify (5-10% at cone 10, for example).
For use in plastic porcelains there is dilemma that body formulators face: This material has a lower plasticity than American materials like EPK and much lower than #6 Tile and Sapphire kaolins. Thus porcelain bodies employing it require additions of a plasticizer like bentonite (up to 5% to get high plasticity). Since raw bentonites having the necessary plasticity are also exceptionally high in iron, people must balance the advantages of using a white burning and more costly material like Grolleg against the lower plasticity that makes the addition of dirty plasticizers necessary. Theoretically making translucent bodies employing 5% bentonite having 5% iron content should only increase iron in the body by 0.25%. This does not sound like much, but it is. Bodies having 0.1% iron burn much whiter than those having 0.2%. Thus either lower plasticity should be tolerated or white plasticizers (like VeeGum or Bentone) should be used.
Glazes: While the chemistry difference between this and more typical kaolins is certainly worth noting for bodies, it is still likely close enough to the theoretical 1:2 alumina:silica to be substituted in recipes that call for kaolin or china clay. Grolleg may also be preferred because the titania in other kaolins is detrimental to the development of a color (titanium can turn celedons to a greenish tint). Transparent glazes employing Grolleg as-the-kaolin used over Grolleg-based porcelains can produce very white and even blue-white color in reduction firing. Another factor making this material an excellent glaze suspender is its inherent stickiness, which is exhibited to a degree much in excess of its actual plasticity. This property also makes for great hardening and durability of the dried glaze layer.
As far as commonly available materials in North America, Grolleg competes against New Zealand Halloysite where the whitest possible result is desired.
Modulus of Rupture, Kgf/cm2
At 80% Relative Humidity: 10.0
Dried at 110C: 25.7
Casting Rate @ 63% solids (mm2/min): 0.8
% P84 Deflocculant Required for 5 Poise Slip: 0.65
1180C 1280C 1410C
Brightness: 86 86 75
25 Porcelain refers to a recipe of about 25% each of ball clay, kaolin, feldspar and silica. In this case the 25-porcelain employs Tile #6 kaolin and a Kentucky ball clay. Both of them contain raw bentonite to augment the plasticity and both have about 50% clay in the recipe. The Grolleg body requires more bentonite to achieve the same plasticity. In spite of the fact that the raw bentonite has a high iron content and it darkens the color, the Grolleg porcelain is still much whiter firing.
Yes. The two specimens are both the same Grolleg-based porcelains. Both of them are glazed with the same glaze: 1947U transparent. But the glaze on the left is using EP Kaolin and the one on the right Grolleg kaolin. The Grolleg glaze is dramatically better, the color has a bluish cast that is more attractive. The Grolleg does not suspend the slurry as well, however it responds well to gelling (using vinegar, for example) more than compensating to create an easy-to-use suspension.
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.
Perhaps you are shocked that a material this dark and dirty (the bars are fired from cone 1 to 7 oxidation, bottom to top), would be used in porcelains. Why? Bentonites are very difficult to process. This is just raw bentonite (HPM-20), dry ground to -325 mesh (to guarantee no fired specks). That grinding does not reduce the soluble salts (that melt by cone 4) or the iron (which accounts for the dark-burning color). These undesirable properties must be tolerated (as whiteness loss) to get the plasticity supercharge 3-5% of this can impart. Why not use super-white bentonites or smectites instead? They can cost ten or even twenty times as much!
The cone 6 porcelain on the left uses Grolleg kaolin, the right uses Tile #6 kaolin. The Grolleg body needs 5-10% less feldspar to vitrify it to zero porosity. It thus contains more kaolin, yet it fires significantly whiter. Theoretically this seems simple. Tile #6 contains alot more iron than Grolleg. Wrong! According to the data sheets, Grolleg has the more iron of the two. Why does it always fire whiter? I actually do not know. But the point is, do not rely totally on numbers on data sheets, do the testing yourself.
These two cone 10 porcelains have the same recipe. 50% Grolleg Kaolin and 25% each of silica and feldspar. But the one on the left is plasticized using 3.5% VeeGum T and the one on the right uses 5% regular raw bentonite. The VeeGum is obviously doing more than making it more workable, it is fluxing the body to make it much more translucent. Although not clear from this picture, the entire mug on the left is covered with blisters, it has over vitrified (while the one on the right is stable).
Left: A porcelain that is plasticized using only ball clays (Spinx Gleason and Old Hickory #5). Right: Only kaolin (in this case Grolleg). Kaolins are much less plastic so bentonite (e.g. 2-5%) is typically needed to get good plasticity. The color can be alot whiter using a clean kaolin, but there are down sides. Kaolins have double the LOI of ball clays, so there are more gasses that potentially need to bubble up through the glaze (ball clay porcelains can produce brilliantly glassy and clean results in transparent glazes even at fast fire, while pure kaolins can produce tiny dimples in the glaze surface if firings are not soaked long enough). Kaolins plasticized by bentonite often do not dry as well as ball clays even though the drying shrinkage is usually less. Strangely, even though ball clays are so much harder and stronger in the dry state, a porcelain made using only ball clays often still needs some bentonite. If you do not need the very whitest result, it seems that a hibrid using both is still the best general purpose, low cost answer.
From these (SHAB test bars) EP kaolin appears to have a much higher fired shrinkage. But half of that happened during drying. Still, EPK shrinks 4% more during firing. Yet Grolleg produces more vitrified porcelains. The EPK bar also appears be whiter. Yet in a porcelain body Grolleg fires much whiter. That higher drying shrinkage proves that EPK is much more plastic, right? Not really. Throwing porcelains containing either require plasticity augmentation using similar percentages of bentonite. What do we learn? To compare materials like this we need to see them "playing on the team", in a recipe working with other materials. Don't rely on material data sheets, do the testing.
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!
These are cone 10R porcelains made using Grolleg kaolin, feldspar and silica. Plainsman P700 (left) has 20% G200 Feldspar. Coleman porcelain (popular among potters in the US) has 30% of the same feldspar. The Coleman porcelain is certainly more vitreous. In fact, it is too vitreous. It is much more likely to warp during firing. And, it is much more problematic with regard to plucking (lower left), foot rings glue to even kiln-washed shelves. But, notice that the Coleman porcelain is not any more translucent than P700 (it appears less so because I could not throw it as thin). So why is 30% feldspar even used? I have a theory: American kaolins do require 30% feldspar to vitrify at cone 10, but not Grolleg. it contains significant natural feldspar, so much so that far less feldspar is needed in the recipe. I think that, in the past, Grolleg was simply substituted for an American kaolin a standard recipe. P700 benefits in a special way by recognizing this different in Grolleg: It dedicates the 10% gain to kaolin, increasing body plasticity. On top of that it adds bentonite, producing a porcelain whose high plasticity would come as a shock to people used to the Coleman body.
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.
|Materials||New Zealand Halloysite|
|Materials||Standard Porcelain Kaolin|
|Materials||Super Standard Porcelain|
Pure clay mineral, there are many brand names of varying purity and iron content.
|Firing Shrinkage||1180C 0% 1280C 14% 1410C 18%|
|Frit Softening Point||3000F|
|Ultimate Particle Size Distribution||>53 microns .05 max >10 microns 10 <2 microns 57|
|Water absorption||1180C 14% 1280C 6% 1410C 0.5%|
|Body Maturity||The use of kaolins containing significant alkalies is an excellent way to achieve greater body maturity coupled with white color. Lower feldspar enables replacing dirty ball clays with kaolin.|