A Question of Kaolins from Mary Jane

[George Lewter]

At 12:40pm on December 23, 2010, Mary Jane said…

Hi George - Another question.  The recipes I got at the Steven Hill workshop just say kaolin.  Lakeside Pottery website specifies tile #6.  I am buying EPK and hope to not have to buy tile #6 as well.  Do you mix these with tile #6 or with EPK, and have you noticed any difference?  BTW - do you fire using the Steven Hill firing schedule provided in the old cone 8 workshops, or have you fired these to cone 6?  Thx again.  MJ

George Lewter replied - The two kaolins EPK and #6 Tile are similar but not identical.  The handout I got at Steven's workshop in Sept 09 calls for EPK in Strontium Crystal Magic and #6 Tile in Pier Black. Those are the only two recipies I got that called for either.  Take a look at the comparison in the glazemaster page below, and see if you can tell us reasons to use one or the other in a glaze.  You can get some sense from the weight percentages, but the molecular percentages allow you to reach better glaze composition conclusions.

[Mary Jane]

Hi George - I have a feeling that some of the properties of T6 and EPK may have changed over the years.  Someone directed me to Imery's website (http://www.matweb.com/search/datasheet.aspx?matguid=b49b49a8f9f5466...) and the properties for both EPK and T6 look a bit different from what was in the Glazemaster software.  I am new to the software so I may be reading it incorrectly.  To be honest, I am just starting to try to look at these formulas - so I have LOTS to learn about predicting results based on the information Glazemaster produces.  I would guess that EPK and T6 might melt differently, but it is really just a guess at this point.  Looking forward to working up an understanding of this over the next few months (years?).  Cheers, MJ

[George Lewter]

OK, lets look at the new data for #6 tile

And also for EPK Kaolin

By weight #6 Tile clay is 38.1% Al2O3, 45.5% SiO2, and 13.5% volitiles that are driven off in firing and don't become part of the glaze.  So these three components make up 97.1% of this material.  In my older composition on glazemaster we had 38.1% Al2O3, 45.5% SiO2 and 13.8% volatiles the three added up to 97.4% of the material. 

The remaining 2.9 or 2.6% of the #6 Tile are fluxes where we see what appear to be large variations, but you need to remember that altogether the fluxes are only a very small percentage of the material's composition.

EPK Kaolin's major components are 37.4% Al2O3, 45.7% SiO2, and 13.9% volitiles for a total of 97% (very close to the numbers for #6 Tile).  The fluxes are different, but again they are only 3% of the total. 

We don't use kaolins for their fluxes.  They melt above 2,700 degrees F.  One reason to consider the fluxes in kaolins might be to totally eliminate or minimize a flux that would adversely affect a glaze quality or color we are trying to achieve. We add kaolins to glazes to cheaply source alumina and silica in a form that will give us good suspension qualities in the bucket and to help the glaze coating contract with the clay body in drying and on the heating side of the firing cycle when raw glazing and single firing.

To help understand how fluxes in kaolins are very minor components, let's look at couple feldspathic materials and how they compare to the kaolins.

   In the two materials above you see that fluxes have become much more prominent in the composition of these materials.  You can also easily discern that the mix of fluxes can be much different and that they are present in large enough quantities that they might have different secondary effects beyond just fluxing power.  These are materials you want to be very careful with in formulating your glazes.   

[Mary Jane]

Wow George - thanks so much for taking the time to do that.  It is really helpful.  I am very excited about learning more about glazes other than just learning about the final application.  Thanks again.  Cheers, MJ