Substitutions

[BlackDogPottery]

Hello again. In my small pottery studio I am often looking for substitutions and learning what different materials do. But do to financial limitations I cant often do special test, so sorry for the flux of questions. I was reading up on ball clay, EPK, and Kaolin. Theoretically, what is the difference if you were to substitute these in a glaze recipe with clay? As in the clay your pot is made of.

I've heard changing feldspars in a glaze recipe are somewhat the same but can change the tint of the glaze. Anything like this?

[glazenerd]

EPK is a kaolin. Pottery uses two varieties of clay: kaolin and ball, although a third group ( chlorite) does exist. There primary distinctions between the two are:

1. Kaolin has a 1:1 particle structure that will only hold water on the face of its platelets. This translates to; it will not add any appreciable viscosity to the glaze. However, most all kaolin has 37% alumina, and under 50% silica. The expansion co-efficient of kaolin is also lower than ball clay. So kaolin will boost alumina levels quickly, and lower over-all COE of the glaze slightly more than ball clay. The other benefit is that most all kaolin has very low metallic oxide content: so it will not change the color hue intended. EPK is the usual kaolin of choice due to it's price point and smaller particle size. However, NZ kaolin is much cleaner and produces a brighter/ clearer sheen than EPK. In general, most kaolin run around 13% LOI.

2. Ball clay has a 2:1 particle structure, that will hold water on it's platelets. This can be beneficial to some degree when blending with glazes that contain high % of magnesium or frit. If the CEC value of a given ball clay is high enough: it can add minor degrees of viscosity. A CEC value of 11 or higher, means it is then classified as a "swelling" clay. (Bentonite has a CEC of over 100.)  most all ball clays have between 25-29% alumina (some exceptions) and over 50% silica (generally). Their COE typically run higher than kaolin, that can play some minor differences when trying to lower the COE of a glaze.

The big problem with ball clay is metallic oxide levels. There can be appreciable amounts of iron, magnesium, and titanium: which can slightly alter color. If firing in reduction, this problem can be amplified. The darker a ball clay becomes, the higher the oxide levels become. Ball clay also has higher carbon and sulfur content, whereas kaolin has none. While carbons typically burn off, vaporizing sulfur produces a sulfide: ( bonds molecularly with other oxides) which can produce black specks in the clay if the sulfur content is high enough. Even white ball clays that most assume as "clean", are not. The acidity of the surrounding soil and water table bleaches the color over time.

Nerd

[neilestrick]

Most recipes call for kaolin rather than ball clay, because kaolin is whiter. Most people in the USA use EPK, as it is cheap and easy to get- most suppliers carry it, however you can generally use any kaolin without having to alter the recipe.

The clay your pots are made of is not actually a single clay. It is what we call a 'clay body'. It is a blend of different clays and other materials. It may contain kaolin, ball clay, fireclay, stoneware, or red clay. In the case of mid-range and high fire clays, it also contains feldspar, which melts and fuses the body tight in order to reach vitrification. It may also contain silica, to help with glaze fit. It may also contain silica sand or grog, to give the body texture/grit. Each company that mixes clay bodies has their own proprietary formulas that they've developed.

[oldlady]

as a total chemistry failure, i have always used EPK  (edgar plastic kaolin) whenever a recipe calls for kaolin or china clay.  china clay seems to be in recipes from england .  

ball clay is a totally different ingredient. 

there is a website that defines the  names you might find in recipes.  now that i have started this, i cannot search for it among my bookmarks, but i will and edit this.

 

darn!  it is gone.  the site had glaze recipes and definitions of everything i ever wanted to look up.  do not know where it went.

[Tyler Miller]

I feel the need to point out that kaolin and ball clays are not different at the molecular/atomic level but rather ball clay is ghe product of unique geological forces acting on a kaolinitic clay deposit.  It is a sedmentary mixture of kaolinite, quartz, and phyllosilicates like mica and the weathering processes ghat produce this are what cause the plasticity.  This is why very pure kaolinite found in its place of formation is extremely non-plastic.  It is not like the differences between kaolinite (which ball clay contains), illite, and montmorillonite.

Anyway, kaolins are pretty easily interchangeable among one another in glazes.   There is more silica and less alumina  in OM-4, than EPK and 0.5% more iron oxide (among other tiny differences).   But I'm not going to lie, in a pinch I've used OM4 instead of EPK.  I've also tested shinos with both, with not huge differences --(ball clay might better for carbon trapping, kaolins for "toast effects")

[oldlady]

sorry, tyler, of course you are right.  my comment was only to note the  differences in the two types of clay.  my assumption, and you know how that goes, is that if the recipe wanted me to use two different kinds of clay, kaolin and ball clay there must be a reason i should do so,  just as if there were two different kinds of flour in a bread recipe.   

 i do have a problem with substitutions of a recipe ingredient for something else that might be "just as good".    since so much of what we do depends on factors beyond our control, i feel that the ingredient list is sacred.  it worked for whoever developed it.  changing an ingredient  is defeating the whole idea of having a recipe.

 

hmmmm.........  maybe that is why i don't like jazz.....................