EPK vs #6 Tile

[Jeff Longtin]

The recent post makes me wonder if anyone can explain the differences between 6 and EPK?

At the pottery shop we found that it was best to let a glaze mixed with EPK to sit overnight to let the mix thoroughly wet. Otherwise, upon mixing, the EPK glaze tended to gel up. (Get thick, in other words.) Whereas glazes mixed with #6 tended to mix up beautifully from the get go.

The method for measuring consistency was to fill a 500 ml beaker with glaze and weigh it. The 6 glazes weighed fine but the EPK glazes weighed off the charts until they sat a day.

I read that EPK is actually "washed" before it is processed whereas 6 is just dug out of the ground and bagged. (essentially) Does anyone know if this is why the usage difference?

 

[Callie Beller Diesel]

Most glazes I’ve used benefit from slaking overnight before sieving. Given Tony says that Tile 6 “is relatively impermeable to water compared to other kaolins, it is thus the last choice for casting bodies,” I’m curious what the comparison is between the glazes a week later.  

Side by side, there seems to be a few more impurities in EPK than Tile 6. While the flux comparisons have some variation that might need watching in a glaze formula, the silica and alumina balances are close enough that part might not matter. 

Tile 6  is known for its plasticity in clay bodies, which I can confirm from personal use. The plasticity may have an effect on some high clay glazes where shrinkage could affect application, but that’s theory on my part, and if someone knows better, speak up. 

As to why a material might be washed and another isn’t: we have to remember that potters are a secondary market for almost all of the materials we use. EPK I think gets used to make glossy magazine paper. If tile 6 is mined for something else, the washing may not be necessary. 

Edited to add:

Links to Digitalfire materials pages for comparison. 

Tile 6 writeup
EPK writeup

[Min]

My hunch would be the difference in cation exchange rate. EPK is 4.5, I don't have one for 6Tile though, do you have that info Jeff?

[Jeff Longtin]

Here's the curious thing learned at the pottery shop: Mason Color (the folks behind Mason Stains) strongly suggested EPK to solve a "glazes too thin" issue. The gelling was seen as a benefit as it allowed the glaze to be applied thinly and yet get good coverage. (For the most part.)

This change was motivated by an unusual experience with #6: a few months after covid lockdown we got a batch of #6 (maybe about 300lbs.) and it was terrible. It was almost yellow in color and it gelled up incredibly. And worse, it continued to get thicker, as time went on. Unfortunately, the young glaze crew, just added water to thin the thick glaze. This led to glaze cracking before the firing. It was really a mess for a while.

EPK somewhat solved the problem but another batch of #6, a few months later, was much better and we returned to using it as the kaolin component.

[neilestrick]

Could the difference be in solubles, since one is washed and the other isn't? Just throwing out ideas...

[Min]

On 8/17/2023 at 4:00 PM, Jeff Longtin said:

Mason Color (the folks behind Mason Stains) strongly suggested EPK to solve a "glazes too thin" issue. The gelling was seen as a benefit as it allowed the glaze to be applied thinly and yet get good coverage.

I wonder why they didn't suggest a floc? 

edit: I found the cation exchange rate for 6Tile, it's 9.8 so in theory it will not deflocculate nearly as easily as EPK.

 

[glazenerd]

Kaolin is a 1:1 particle, which means it only holds water on its surface. Ball clay is a 2:1 particle, and pending the CEC (cation exchange) will absorb water into its inner platelets. Naturally occurring calcium plays a large role in CEC: EPK has 0.18%, and #6 Tile has 0.30%. May seem like nothing, but its enough to create better suspension. Particle size also plays a role in suspension.

Tom