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C.Banks

clay plasticity/water content

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I'm working with a clay that has a tendency to flop with little warning. From what little I know this can be a problem with plastic clay bodies holding more water than more forgiving, less plastic bodies.

So with this in mind I'm looking at making adjustments but I'm getting conflicting information on what some accepted water content values are.

Plainsman P700 has a % water of 23.5 - 24.5

Plainsman describes P700 as "our most vitreous cone 10 white body, it is the closest thing we have to a true translucent porcelain body. It is a mix of 50% Grolleg kaolin with feldspar and silica. We also add micro-fine bentonite to improve its plasticity." 

In the Febreuary Ceramics Monthly (thanks Tom) the water contents are significantly higher than the figures from Plainsman.

I'm wondering are clays so different in the Unites States? I'm curious because the clay I'm working with holds aproximately 26% and is mixed up as a high fire, buff, stoneware. It works well for the most part. Fires to an 11 with no issues and kitchen tested absorption showed it was near zero at cone 10. But it doesn't stand up as well as the commercial Plainsman clays and I want to know why.

thoughts much appreciated

 

20 epk

20 OM-4 ball clay

20 silica

20 custer potash feldspar

12 hawthorn Bond Clay

8 pv clay

 

Percentage Analysis by weight

71.13 % SiO2

23.24 % Al2O3

3.06 % K2O

0.73 % Na2O

0.28 % CaO

0.32 % MgO

0.59 % Fe2O3

0.05 % P2O5

0.59 % TiO2

 

Edited by C.Banks

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Looks like plainsman probably uses veegum or macaloid instead of ball clay for plasticity?  I'm guessing that's what they mean by micro-fine bentonite.  That means they can up the feldspar content instead of adding a bunch of ball clay.  That's just my guess.

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C.Banks

in my article: the WOPL (water of plasticity) is given for individual clay varities. WOPL for kaolin is 26 ( grams of water to 100 grams) of clay. Ball clays run 30 to 38.  Bentonites and hectorites run 38-40.  Macaloid is highly refined calcium hectorite. V-Gum T is synthetic, although some suggest it is also a processed from of hectorite. The WOPL for a blended body will be less because you are adding minerals- silica, feldspars.

2 hours ago, C.Banks said:

 

 

20 epk.                  

20 OM-4 ball clay

20 silica

20 custer potash feldspar

12 hawthorn Bond Clay

8 pv clay

 

Percentage Analysis by weight

71.13 % SiO2

23.24 % Al2O3

3.06 % K2O

0.73 % Na2O

0.28 % CaO

0.32 % MgO

0.59 % Fe2O3

0.05 % P2O5

0.59 % TiO2

 

 

Your recipe is basically a 50/50 porcelain blend. 20% kaolin, 20% OM4, and 12% fireclay.  Your SiAl ratio is off: 71% silica is a bit low- should run in the 74-76% range. Alumina in typical porcelain bodies run  17-20% molar. 

Not a fan of EPK in clay bodies: it has a SSA of 28.56- which applied means fine grain. Kaolin holds water on its platelets, ball clay absorbs water  differently. Kaolin being a 1:1 structure means water acts as a lubricant: which is why porcelain is always softer than stoneware. Ball clay is a 2:1 particle, meaning it has an inner core: which absorbs water. Which is why stoneware feels denser than porcelain.

your recipe is a hybrid of sorts: does not really follow porcelain or stoneware standards. A pure porcelain would hold 20% water typically! and stoneware 22-24% typically.  So 26% water content is high and comes from 2 things- 20% OM4 and plastic vitrox. You have a total clay content of 52%:  OM4 accounts for nearly 40% of the total clay content: with an additional 8% PV.  In clay theory: maximum plastic content should not exceed 30%  of total clay content  The OM4 absorbed water when you mixed it, the water from throwing caused it to reach its maximum plastic limit: resulting in fatigue.. Which you called floppy. 

Cut the OM4 to 15%, lose the PV.  Increase EPK proportionately. At cone 10- KnaO should run 4.29% molar. Typical

You should never ask a Nerd why?  You get these protracted answers. Actually I was not finished, but alas.

liam: bentonite is the plasticizer. 

Tom

 

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8 minutes ago, glazenerd said:

C.Banks

in my article: the WOPL (water of plasticity) is given for individual clay varities. WOPL for kaolin is 26 ( grams of water to 100 grams) of clay. Ball clays run 30 to 38.  Bentonites and hectorites run 38-40.  Macaloid is highly refined calcium hectorite. V-Gum T is synthetic, although some suggest it is also a processed from of hectorite. The WOPL for a blended body will be less because you are adding minerals- silica, feldspars.

 

ugh

:)

please excuse the reading comprehension failure

and thanks for the reply

 

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Clay holds water. Silica and feldspar, not so much. The less clay you have in it, the less water it needs. A porcelain body might on be 50% clay, so it needs a lot less water than a stoneware body. When I was manager of the clay production line at A.R.T, I'd adjust the moisture level of our porcelain by 1 gallon of water at a time to an 1850 pound batch.

As for how much water a body should have in it, I don't think there's a number that can be assigned. Some people like softer clay, some people like stiffer clay. Some companies mix the same body two ways- one softer for schools where kids will be using it, one stiffer for potters.

Your clay body is almost an equal parts body, which should work really, really well. We used a lot of that in grad school, and it's a great all around throwing body. There's no need for the PV Clay in it though. Just go 20% Hawthorn and you'll probably see a big difference.

I'm a huge fan of kaolin in clay bodies. It contributes to particle size distribution, and reduces the overly-plastic and sticky feel of ball clay. I like Tom's recommendation of cutting the ball clay to 15% and increasing the EPK.

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I removed the pv clay, reduced the ball clay and adjusted the kaolin.

33 epk

15 OM-4 Ball Clay

20 silica

20 custer potash feldspar

12 hawthorn Bond Clay 

 

Percentage Analysis by weight

68.68 % SiO2

26.24 % Al2O3

2.66 % K2O

0.70 % Na2O

0.26 % CaO

0.21 % MgO

0.59 % Fe2O3

0.08 % P2O5

0.58 % TiO2

 

I also adjusted according to Neils suggestions.

25 epk

15 OM-4 Ball Clay

20 silica

20 custer potash feldspar

20 hawthorn Bond Clay

 

68.63 % SiO2

25.98 % Al2O3

2.72 % K2O

0.71 % Na2O

0.26 % CaO

0.26 % MgO

0.67 % Fe2O3

0.07 % P2O5

0.72 % TiO2

 

I can see some trouble and how I might proceed but a previous version turned out short so I'm reluctant to change too much for fear of losing what I have.

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C & C is one of the original versions that never got included because of the ready supply of om4.

I remember thinking it looked good and was disappointed I couldn't find easy access. I'm going to look a farther afield.

Thanks for the suggestion!

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@C.Banks, I can mail you a sample of C&C if you want to try it out with a test batch of clay. Don't know where in BC you get your supplies from but Greenbarn ordered it in for me. (PM me your address and how much you need and I'll drop some in the mail)

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30 minutes ago, oldlady said:

thank you neil, i always use C&C for its whiteness.  can you tell me what makes it "better" in this case or am i getting too far from the topic?

C&C is a blend of Champion and Challenger ball clays. It is more consistent, and just has a better feel than OM-4.

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55 minutes ago, Min said:

@C.Banks, I can mail you a sample of C&C if you want to try it out with a test batch of clay. Don't know where in BC you get your supplies from but Greenbarn ordered it in for me. (PM me your address and how much you need and I'll drop some in the mail)

That's a generous offer.

I'm thinking of trying the A2 Ball Clay and Plainsman Fireclay from Plainsman. I know the nearest supplier uses the Greenbarn catalogue so I'm hoping they can include a few odd pounds in addition to some C and C.

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1 hour ago, neilestrick said:

C&C is a blend of Champion and Challenger ball clays. It is more consistent, and just has a better feel than OM-4.

I will miss the iron content.

Originally all I wanted was a off-white/buff stoneware. After a few versions in different atmospheres all I want now is an off-white/buff stoneware that throws well and flashes in wood/soda atmospheres.

or maybe just two clays

:)

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How are you mixing your clay?  Something that has not been mention is that commercial clays are typically pugged in a de-airing pugmill. This alone makes the plastic clay much more dense than if it was mixed to a plastic consistency from dry materials (Soldner mixer). Mixing clay in a slurry and drying out will produced a more thoroughly wetted and denser clay body  but it is much more work. If you are mixing in a Soldner style mixer there will be a lot of tiny air pockets in your clay. I have not read this anywhere but I have noticed it from my own experience...maybe others can confirm or deny. Either way, a vacuum de-airing pugmill will produce the densest and most homogenous clay body.  

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2 hours ago, tinbucket said:

How are you mixing your clay?  Something that has not been mention is that commercial clays are typically pugged in a de-airing pugmill. This alone makes the plastic clay much more dense than if it was mixed to a plastic consistency from dry materials (Soldner mixer). Mixing clay in a slurry and drying out will produced a more thoroughly wetted and denser clay body  but it is much more work. If you are mixing in a Soldner style mixer there will be a lot of tiny air pockets in your clay. I have not read this anywhere but I have noticed it from my own experience...maybe others can confirm or deny. Either way, a vacuum de-airing pugmill will produce the densest and most homogenous clay body.  

This is a good point.

I'd love to own a vacuum pug but for now my clay gets mixed in a Soldner styler mixer.

I've noticed the little air bubbles - they sound like tiny bubble wrap. I always figured the clay was just young and probably not wedged well enough.

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Clay that hasn't been de-aired will be 'short'. All those teeny tiny little air bubble prevent the clay platelets from sticking together as well as they should. This is probably a bigger problem than the clay formula. You either need to age your clay longer, like months or more, or mix it by making a slurry and letting it dry out.

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CB:

you have a Soldner, so we have to work this around for a Soldner. 

The biggest mistake potters make is blending clay bodies so they are highly plastic after mixing. Plasticity is subject to a rate law: simply applied means the plasticity level will increase at a set rate over a period of time. Every potter loves clay that has aged for a few months: that is because it has reached its maximum exchange rate. The chemistry that creates plasticity actually occurs in the water film; the clay just supplies the negative charges needed. As @neilestrick pointed out: air bubbles delay that process; so you need to work around that

After you mix in your Soldner: set your slab roller for 1/2" and pass it through a few times to get rid of some of the air: then bag it and let it  sit for 7-10 days.

plasticity is measured in plastic limits (pl). Let's say a recipe is short at 20% water content and fat at 24% water content. This means when you throw: at 28% water content clay fatigue will begin. Clay bodies have a narrow window of plasticity limits before they begin to fold.  The problem is; potters judge plasticity in terms of water content which is not accurate. Plasticity is developed from a negative ionic charge in the water film: which is supplied by the CEC of any given ball clay. There only has to be enough water to carry the ionic charge uniformly: adding  more water only makes it softer: not more plastic.

tom

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1 hour ago, neilestrick said:

Clay that hasn't been de-aired will be 'short'. All those teeny tiny little air bubble prevent the clay platelets from sticking together as well as they should. This is probably a bigger problem than the clay formula. You either need to age your clay longer, like months or more, or mix it by making a slurry and letting it dry out.

I usually try to let the clay sit for at least 3 weeks. Sometimes I get impatient for fired results. I use apple cider vinegar so that helps a bit but I will try to heed this sound advice.

 

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1 hour ago, glazenerd said:

After you mix in your Soldner: set your slab roller for 1/2" and pass it through a few times to get rid of some of the air: then bag it and let it  sit for 7-10 days.

I like this idea. I've been known to use a slab roller as a impromptu pugmill before.

Lots more to think on here Tom thank you.

I'm looking at increasing the silica in a few tests but I need to learn better about flux/si/al ratios - all the while trying to preserve/understand better the mechanisms of plasticity.

Commercial clay is easier in just about every way I can think of but this is worlds more 'fun'.

 

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2 hours ago, C.Banks said:

 

I'm looking at increasing the silica in a few tests but I need to learn better about flux/si/al ratios - all the while trying to preserve/understand better the mechanisms of plasticity.

 

Silica is a good idea. I worked your recipe, but did not post it. You strike me as someone who wants to learn on their own terms. Keep following Ceramics Monthly; there is an upcoming article that explores SiAl ratios, flux molarity: teaser :)  I will give you a plasticity test later.

tom

 

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I like the idea of these substitutions. I had to change software because my copy of hyperglaze is buggy and difficult to input custom material.

Looks like I may need to find some flux and there may be issues with drying but I like using more local material.

A2 Ball Clay................    20.00  
  Plainsman Fire Clay.........    20.00  
  EP Kaolin...................    20.00  
  Custer Feldspar.............    20.00  
  Silica......................    20.00  
                              =========
                                 100.00

  Oxide   Formula  Analysis    Molar%
  CaO       0.00     0.20%w    0.24%m
  MgO       0.00     0.16%w    0.26%m
  K2O       0.03     2.82%w    2.01%m
  Na2O      0.01     0.88%w    0.96%m
  P2O5      0.00     0.05%w    0.02%m
  TiO2      0.00     0.39%w    0.32%m
  Al2O3     0.20    21.63%w   14.26%m
  SiO2      1.13    72.84%w   81.49%m
  Fe2O3     0.01     1.03%w    0.43%m

                        Cost:    0.08
              Calculated LOI:    6.88
                 Imposed LOI:        
                       Si:Al:    5.72
                      SiB:Al:    5.72
           Thermal Expansion:  232.64
              Formula Weight:   93.12

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CB

there are 2 stoneware "classic" recipes:

80/10/10. Which is a total of 80% of various clays, 10% silica, and 10% spar.

the second: 5/20. 5 ingredients of 20% each. Which you are following above.

for grins: try 15% silica and 25% EPK.  Two fundamental principles applies to stoneware, but both relate to the same issue: cristabolite.

the conversion being: metakaolin converts to spinel, spinel converts to mullite. Mullite is the dense alumina silicate that gives clay it's strength. To achieve that, there has to be X amount of silica to combine with X amount of alumina to produce mullite. Any excess silica is ejected from the spinel formation as crystallyttes. This is where spars come in: the molar levels of spar must be high enough to incorporate the ejected crystallyttes into the glassy matrix. If not, then cristabolite forms from free silica. 

At cone 10 -2.89 to 3.19% molar KnaO.   Si/Al should not exceed 5:0 , the lower this number is: the stronger the body  if you are wood or salt firing where thermal resistance/ shock is needed: keeping SiAL closer to 4.0 helps.

spinel to mullite begins at 2050F, which is why I note that in my posted firing schedules.

Tom

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Code Number: new Plainsman clay                          Code Number: original
=========================================     =========================================
                                              
  EP Kaolin...................    25.00         EP Kaolin...................    20.00
  A2 Ball Clay................    20.00         Custer Feldspar.............    20.00
  Plainsman Fire Clay.........    20.00         Silica......................    20.00
  Custer Feldspar.............    20.00         OM #4 Ball Clay.............    20.00
  Silica......................    15.00         Hawthorne Bond..............    12.00
                              =========         PV Clay.....................     8.00
                                 100.00                                     =========
                                                                               100.00
  Oxide   Formula  Analysis    Molar%         
  CaO       0.00     0.21%w    0.26%m           Oxide   Formula  Analysis    Molar%
  MgO       0.00     0.16%w    0.28%m           CaO       0.00     0.27%w    0.32%m
  K2O       0.03     2.86%w    2.07%m           MgO       0.01     0.26%w    0.43%m
  Na2O      0.01     0.89%w    0.98%m           K2O       0.03     3.03%w    2.17%m
  P2O5      0.00     0.06%w    0.03%m           Na2O      0.01     0.78%w    0.85%m
  TiO2      0.00     0.41%w    0.35%m           P2O5      0.00     0.05%w    0.02%m
  Al2O3     0.22    23.82%w   15.87%m           TiO2      0.01     0.75%w    0.64%m
  SiO2      1.08    70.49%w   79.71%m           Al2O3     0.21    22.73%w   15.04%m
  Fe2O3     0.01     1.08%w    0.46%m           SiO2      1.12    71.42%w   80.22%m
                                                Fe2O3     0.00     0.70%w    0.29%m
                        Cost:    0.08         
              Calculated LOI:    7.62                                 Cost:    0.08
                 Imposed LOI:                               Calculated LOI:    6.07
                       Si:Al:    5.02                          Imposed LOI:
                      SiB:Al:    5.02                                Si:Al:    5.33
           Thermal Expansion:  237.63                               SiB:Al:    5.33
              Formula Weight:   92.38                    Thermal Expansion:  239.49
                                                            Formula Weight:   93.93
                
                                              

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I think I have an idea what you are suggesting with regards to si:al ratios and KNaO.

I posted these because I wanted to confirm the original fired well in reduction, soda and wood kilns. It did well up to cone 11 and for the most part performed ok on the wheel.

These adjustments are hopefully changing the particle size distribution enough to reduce the water of plasticity or provide more 'tooth' in order for this otherwise functional clay to 'stand up' a bit better.

I know there is lots I don't understand but there's a bunch I do even though I may not be able to represent as well as some.

I also know very well the back and forth that happens when we start trying to make small adjustments in glazes and now clay bodies.

Information such as:

5 hours ago, glazenerd said:

At cone 10 -2.89 to 3.19% molar KnaO.   Si/Al should not exceed 5:0 , the lower this number is: the stronger the body  if you are wood or salt firing where thermal resistance/ shock is needed: keeping SiAL closer to 4.0 helps.

along with explanations how Si:Al ratios work with fluxes and their relation to free silica is important and I very much appreciate the effort.

cheers

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