Substitute EPK with Georgia kaolin

[Mercedes Montoya]

I have a 5# bucket of Georgia kaolin and no EPK. I was wondering if any one knows if it makes a difference if I use the Georgia kaolin instead of the EPK in a glaze recipe.

[Babs]

Test it.

May not be as white but also Google it. Think their are folk in Georgia who may substitute:-/

[liambesaw]

Short answer: sub away

Long answer: put it into a glaze calculation software and adjust the recipe

[Mark C.]

check this out

http://www.potters.org/subject103612.htm

[neilestrick]

In most glazes you won't notice an difference from using different types of kaolin. I've used 6-Tile, EPK, Georgia, grolleg, and sapphire without any problems.

[Mark C.]

Most of these epks,koalins are very close to the same in a chemical breakdown spec sheet just as Neil says above.

[liambesaw]

Yeah they're almost like different "grades" if anything from what I can tell, some have more "pollutants" like iron, but as long as your glaze isn't super sensitive... I know some of the dichromatic glazes like neodymium oxide based ones require grolleg due to it having no/little iron, but that's the only instance I've seen where a recipe is picky about the kaolin

[Bill Kielb]

I couldn't Agree more in general. Just an add here, when chemically designing glazes we determine the base properties desired, then texture, melt, glaze fit, solubility, etc... and start picking ingredients. For Clay we like to have at least 10% (in General) and use that clay as efficiently as practical.

What I mean to say is if I am designing a glaze that required a high amount of Silica to Alumina I then lean toward getting my ingredients from the clay if practical. I have attached  my spreadsheet so you can see what is chemically in these and understand why one is preferable than another. Of course knowing this chemistry often means one can substitute one for another and compensate as necessary with additions. 

While not the end all approach you will notice each clay can be sorted in any manner so picking most likely candidates is actually easy. Actually the whole process is easy but then must be confirmed by endless testing to insure every component plays well together and as you expect. I have attached the clay portion of the spreadsheet for your information, so if you ever get the chem composition of the GA Clay you can compare. If you look close EPK, Grolleg, OM4 fairly similar to each other but when compared to Red Art, they differ significantly.

I just did a Gerstley to Gillespie theoretical change on several glazes so I included that portion of the worksheet for your perusal. 

I definitely agree with Liambesaw  and favor putting it in  glaze calc software as a start, but there are a bunch of successful testing folks that add a bit, fire see the result then add some more.

In industry we want to apply exactly X number of grams, fully vitrify with the body or mature on an already vitrified body. No surprises, while wasting as little raw materials as practical. Not very exciting or artistic actually.

It is truly part of the fun developing and using glazes!