How to lower a cone 10 clay body to cone 7?

[jrgpots]

I have been thinking of adjusting Aardvark Coleman white cone 10 to cone 7.  I'm having problems finding good reference material about quantities and types  of fluxes.

 

Jed

[Kelly in AK]

Is this premixed, or do you have a recipe you’re looking to adjust?

I’m assuming this is a premixed clay body and also assuming (from the name and what I know of Tom Coleman) you want it to look very white when it’s fired. Adding a flux to lower the maturation temperature is not simple. It’ll be either a frit or nepheline syenite, and adding enough will lower the percentage of plastic material in the body. You can’t very well take out the feldspar or silica already in there. If you add clay to make up for plasticity you raise the maturation temperature again. Bentonite could help, but that can push it off white. Veegum is cleaner firing and super plastic but requires even more special handling than bentonite to get it into an already mixed clay. No matter what it’s going to be lots of testing, not just of absorption, but of forming and firing characteristics.

Another thought would be to combine a lower firing white clay body with the Coleman White. Again, lots of testing. This one seems like an easier approach. Finding the right body and the right ratio could take some time. Most premixed clays are proprietary recipes, so it’ll be guess and check (systematically, of course). Somewhere on Digitalfire Tony Hansen has written about combining two existing clay bodies to make a very nice stoneware, I just can’t recall the details. That could be a good resource. 

If you have the recipe and are mixing from scratch, it’s a whole different story. Looking at recipes for mid-range clay bodies could give you an idea of which frits/fluxes people use. Matt Kelleher has also written stuff about tuning clay bodies with frits to adjust their temperature range. 

Without a doubt someone has done something similar, and with any luck they’ll post their experiences here.

Hey, you’re the guy who built the sweet new kiln! Hope that’s going well. 

[PeterH]

2 hours ago, Kelly in AK said:

Somewhere on Digitalfire Tony Hansen has written about combining two existing clay bodies to make a very nice stoneware, I just can’t recall the details. That could be a good resource. 

Can anybody can provide a reference for this, or more details to aid my searches?.

 I did stumble across https://digitalfire.com/picture/1068

Also a tantalizing comment in https://digitalfire.com/glossary/stoneware

PS I also found this of interest https://digitalfire.com/picture/c6Ho2e5Fsn

[jrgpots]

Thanks.  I now have starting point

[PeterH]

Again, only the mildest of hints from  Digitalfire.

Formulating a body using clays native to your area
https://digitalfire.com/article/formulating+a+body+using+clays+native+to+your+area
-What might appear to be a worst-case scenario is that you find a fireclay. What good would that be for pottery? Fireclays are often just the crude form of ball clays. Slurry and screen and add about 30% feldspar and you have a cone 6 stoneware. A super plastic one.

[Min]

Given that it is probably overfluxed at cone 10 (references below) I would try a couple things. First I would fire some as is in the hottest part of your kiln and measure the porosity, it could just be low enough for your purposes as is. Will likely loose translucency but perhaps that isn't an issue.

If that is a no go then I would try adding 5% nepheline syenite to it. Coleman is so plastic I doubt this will make a significant difference to the working properties of it. 

From this link: 

From this link: 

"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."

edit: I'll go through my bags of old clay and see if I kept any leftovers of Coleman, if I have some I'll  fire it to cone 6 electric and measure the porosity just for curiosities sake.

 

Edited November 10, 2022 by Min
added a thought

[neilestrick]

If you're adding to moist clay, you're going to have the mix the heck out of it in order to get good dispersal of the neph sye, especially since you're adding such a small percentage. I would definitely mix the neph sye with water and hit it with a blender so there are no chunks before adding it to the body.