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Struggles Developing Clay Body From Local Clay

clay body formulation local clay clay/glaze interaction

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#1 Rockhopper

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Posted 05 October 2013 - 06:44 PM

Apologies for the lengthy post, but i'm hoping providing lots of detail will increase my chance that someone will here will have the missing piece to my puzzle...

I have been working off & on for nearly a year, trying to come up with a workable ^6 body from locally dug clay of uncertain composition.  This clay comes from the ground gray, and fires to beige/buff color at ^6.

Tested first at low (^05-^06) fire.  After second firing w/ low-fire glaze, test-tiles were like shortbread cookies.  Very dry & easily broken, and the glaze looked like all of the water had baked out of it, leaving behind a layer of dry ingredients - much like you would see if you put a thin layer of slip on a block of wood and let it dry.

Test fire at ^6 was a better - but still very porous (20+% absorption).  Glaze soaked into clay like a sponge - came out looking like it had been painted with water-colors.

Early tests had lots of lime-pop, so watered raw (dug) clay to thin slip consistency and poured through 80 mesh screen after reading the lime particles smaller than 70 mesh are usually not a problem.  Apparently this is true, as I've had no more lime-pops.

Have now tried over 3-dozen different variations, most of them with 50% - 75% 'raw' clay, and the balance made up of different combinations of OM4, EPK, Flint, Potash Spar, and Goldart.

For each test, I make a small batch of clay, roll it into a slab, and cut into 2cm x 10cm strips, which I drape over a plastic tube to form an arch.  I bisque fire both strips, then glaze one and fire the other unglazed - at ^6.  Several have looked promising at the bisque stage, and actually came through the second firing in good shape unglazed -with around 4% absorbancy...  But I'm having lots of confusing results with the glazed pieces.

I've tested all of the mixes with the same two glazes - Randy's Red, and a Tan - dipping each end of the test strip into one glaze, so there's a small overlap at the middle.  Both glazes are mixed by the potter that owns the studio.  Her RR recipe is the same as found on digital-fire, minus the bentonite. (Unfortunately, she's never worked with anything other than commercial clays, and knows very little about clay-body formulation.)

Here's where the frustration begins: Several pieces have come out of the glaze test looking great on the tan end - but with the RR end ranging from dull to lava-like (very porous).  With several tests, the RR end has deformed severely, while the tan end held it's original shape. 

 

The photo below is a test with 75% 'raw' clay and 25% EPK.  Both pieces were the same shape & size after bisque firing, and were fired together on the same shelf at ^6.  The unglazed piece retained its shape, but the glazed one sagged heavily on the RR end, while holding shape on the tan end.  (The unglazed piece in the pic is wet, as I started soaking for absorption test before taking the pic, so color looks a little darker than it really is.)
 

I know different glazes react/interact differently with different clay bodies - but have not found any info that suggests why there would be such a drastic difference between glazed and unglazed pieces fired together.

 

Attached File  Img_1476b.jpg   149.92KB   4 downloads  **Edit:  Got my pic's mixed up.  The sample in this pic is 65% local, 25% EPK, 10% OM4.



#2 Rockhopper

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Posted 05 October 2013 - 06:54 PM

PS - On the chance it will help lead someone to an answer for me, here are the glaze recipes used on my test pieces.

 

Tan
Nepheline Syenite    36%
Dolomite        6%
Gerstley's Borate    13%
Flint (silica)        18%
Talc            13%
EPK             4%
Tin Oxide         6%
Iron Oxide         2%
Manganese Dioxide     1%


Randy's Red
Soda Feldspar        17%
Gerstley's Borate    28%
Flint (silica)        26%
Talc            12%
EPK             4%
Red Iron Oxide        15%



#3 Biglou13

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Posted 05 October 2013 - 08:31 PM

i suppose seeing most succesful clay formula would help

local clay is the unknown

also seeing results of other clays

what formula had best absorbtion

what is absorbtion of above pictured clay, how strong is it

what happened to local clay only

did you try firing local at lowfire temps

 

looking at picture above something in the glaze is "melting" structure. lots of what ifs here.

it also seems like your testing too many variables at one time.   body  and body and glaze.

 

ive been studying clay bodies,  and making  clay... but local unkown is a wild card.... (could be blind leading blind)

 

i have a feeling the 75% local 25 epk may be off,  the epk will need some flux to "glass" or vitrify, not to mention some silica and filler which  is prolly in local.  @cone 6  nepsy may be better choice,  or at least partial part of spar %.

50 -75% may be too bold a percentage of local.   the majority of primitive clays im aware of mature at low temperatures,   assuming this is the same with your local, im guessing you need to beef it up to take cone 6.

 

another idea is to mix local only  to a body texture

then wedge into a cone 10 clay body in different percentages

here you know you have a stable base

most commercial cone 10 clays are "relatively" stable at cone 6

while the assumption may sound like local will lower vitrification may not be the case

 

you could always do same with cone 6 commercial body.

 

get a rock solid body before glaze testing.

 

here is my "wild guess"

 

35   local

25   nepsy

15   epk

15  om4

10   silica

 

 

here is a known (ish) cone 9 ish porcelain

35   kaolin

10   ball clay

30   nephsy

20   silica

 

replace with  local   by subtracting ball clay and silica kaolin


Caution big brother is watching.
The beige is blinding!!!!!!
The middle of the road is boring

The true sign of intelligence is not knowledge but imagination.
-Albert Einstein

#4 Biglou13

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Posted 06 October 2013 - 09:19 AM

Yeah more people making clay.
Caution big brother is watching.
The beige is blinding!!!!!!
The middle of the road is boring

The true sign of intelligence is not knowledge but imagination.
-Albert Einstein

#5 JBaymore

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Posted 06 October 2013 - 09:48 AM

A good reference for basic clay geology and basic testing procedures is Michael Cardew's "Pioneer Pottery" (out of print).  Not the book "Pioneer Potter" which is ABOUT Michael Cardew.

 

best,

 

....................john


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Professor of Ceramics; New Hampshire Insitute of Art

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#6 Biglou13

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Posted 06 October 2013 - 11:18 AM

I got the same directions,regarding sticky.

If your worried about kiln make "trays" and or saggars. to protect.

Another is to understand what geologically your looking at.
Caution big brother is watching.
The beige is blinding!!!!!!
The middle of the road is boring

The true sign of intelligence is not knowledge but imagination.
-Albert Einstein

#7 Rockhopper

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Posted 06 October 2013 - 03:21 PM

A good reference for basic clay geology and basic testing procedures is Michael Cardew's "Pioneer Pottery" (out of print).  Not the book "Pioneer Potter" which is ABOUT Michael Cardew.

 

best,

 

....................john

 

Thanks John.  Looks like my local library has a copy.



#8 Rockhopper

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Posted 06 October 2013 - 04:15 PM

As noted in edit to original post, I got my pic's mixed up - the pic I posted was actually 65% local, 25% EPK, 10% OM4.  Another sample fired at same time with 55% local, 25% EPK, 10% OM4, 10% silica came out similarly deformed, but more pitted.

Local only test strips have held shape at ^6, both glazed and unglazed - but were very porous (22% absorption) and glaze seemed to soak into clay, rather than forming glass on surface.

Tried mixing 3:1, 2:1, and 1:1 with Standard's #112 (^4-^6) body, and got worse results from all 3 than most of my other tests.

I've had several tests that seemed to do well un-glazed - but then melted when fired with glaze. So, with each batch of tests, I bisque two samples of each formulation, then fire at ^6 one with glaze and one without .  To minimize variables, I use the same 2 glazes on each.

Another sample in this latest round was actually a little closer to your suggestion, at 50% local, 25% EPK, 10% OM4, 10% Flint/silica, 5% potash spar. It melted worse than the one in the pic.

Absorption test on the sample pictured is <3% after 24hr soak, and no measureable change with additional 2hr boil. (My scale only weighs to nearest gram, so accuracy is limited. The unglazed sample weighed 33g both before and after soaking, so I know it absorbed <1g.  1g/33g = 3%.)

Since I'm getting fairly low absorption, and partial melting, it would seem that there's plenty of flux (maybe too much).  Would I add more silica (or other filler) - or increase % of EPK and/or OM4 ?


Any ideas why the glaze containing soda spar would make the clay melt and get full of bubbles - but the one with neph-sye seems to be OK ?   I've pretty much concluded that there's something in the RR glaze reacting with something in the clay. At this point it kind-of looks like the glaze 'reactant' might be the soda-spar.  When I compare the two glaze recipes, the biggest differences are soda spar vs neph sye, and different amounts of gerstley's borate.

Haven't tried adding neph-sye to the clay.  Maybe I'll pick some up and try a few test mixes. Unfortunately, I don't have a test kiln, and am doing my test firings at a friend's studio - so I have to wait 'til she has a full load to get fired.



#9 TJR

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Posted 06 October 2013 - 05:46 PM

Rockhopper;

Sorry, I didn't read through your entire post

You mention soda psar and neph sy. Hate to break it to you but Neph Sy is a soda spar. So you are comparing soda spar to soda spar.

 

Soda spars are very strong fluxes, used in glaze calc to lower the temperature of a cone 10 glaze to cone 6.

I would leave them out.

Another possibility is that you may have soluable salts in your clay. they may be causing it to flux

Keep trying.

TJR.



#10 Rockhopper

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Posted 06 October 2013 - 06:02 PM

Rockhopper;

Sorry, I didn't read through your entire post

You mention soda psar and neph sy. Hate to break it to you but Neph Sy is a soda spar. So you are comparing soda spar to soda spar.

 

Soda spars are very strong fluxes, used in glaze calc to lower the temperature of a cone 10 glaze to cone 6.

I would leave them out.

Another possibility is that you may have soluable salts in your clay. they may be causing it to flux

Keep trying.

TJR.

 

Hmm... if NephSy is a soda spar - then there must be something else different between the two glazes that causes them to react differently with the clay...  Maybe the different amounts of Gerstley's Borate ?   (Not trying to troubleshoot the glaze - it works fine with the commercial clay bodies they use at the studio - just trying to figure out why these two glazes react so differently with my clay and, hopefully, in the process figure out what to add to my clay to make it work.)

 

Any suggestions how to determine presence of soluble salts (and/or how to neutralize or counter-act them) ?



#11 JBaymore

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Posted 06 October 2013 - 06:31 PM

Wish I could handle this fired stuff.  The pictures don't show enough of the character of the pieces.  You need to share REAL macro close ups of a number of surface locations, glazed and unglazed.

 

Apparent porosity (absorbtion) of a clay body will decrease steadily as the clay heads toward the vitification point.  Then there will be a SMALL period of increasing firing heat work whre thers is almost no change in poriosity.  But then as the heat work is ioncreased, the porosity increases.... as the clay becomes overfired and it starts to evolve gasses withiong the structrue... forming voids that then can be "filled" with water in a porosity test.  A graph of this looks a bit like a "U" with spread out sides at the top.

 

I am wondering if the body is already well overfired at the cone you are firing to.  Add in the interaction of the high boron glaze with the clay/glaze interface layer... and the body is REALLY getting overfired.

 

best,

 

..................john


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Professor of Ceramics; New Hampshire Insitute of Art

http://www.JohnBaymore.com

#12 JBaymore

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Posted 06 October 2013 - 06:34 PM

Technically Nepheline Syenite is not a feldspar.  It is a syenite.  Separate beast.  The ratios of the oxides it supplies are different than those of a true feldspar.  But is is a significant source of sodium oxide in glazes and clay bodies.  One mol of neph sy provides more mols of sodium oxide in the melt than one mol of a feldspar will.  The silica for a spar is at about 5 ... while the silica for syenite is 6.

 

Spar ...........1 flux 1 alumina 5 silica

 

Syenite ........ 1 flux 1 alumina 6 silica

 

best,

 

....................john


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Professor of Ceramics; New Hampshire Insitute of Art

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#13 Rockhopper

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Posted 06 October 2013 - 08:03 PM

. . .  I am wondering if the body is already well overfired at the cone you are firing to.  Add in the interaction of the high boron glaze with the clay/glaze interface layer... and the body is REALLY getting overfired.

 

best,

 

..................john

 

Boron - from Gerstley's Borate, I assume.  That would make a lot of sense, since the RR has more than 2x as much as the tan that seems to be working OK.



#14 Rockhopper

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Posted 06 October 2013 - 08:26 PM

Here are some close-up pics.  Not quite pro-quality macro, but as good as it gets with my Canon powershot..

 

The first three are cross-section of broken test-strips.  1) Un-glazed;  2) RR end of glazed piece; 3) Tan end of glazed piece.  (2 & 3 are opposite ends of same piece)

 

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#15 Rockhopper

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Posted 06 October 2013 - 08:29 PM

These two show the wide surface, middle portion of the two pieces.  4) Shows tan and RR side-by-side and overlapping in center; 5) un-glazed strip

 

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#16 Rockhopper

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Posted 06 October 2013 - 08:33 PM

And last, but not least, close-up of the glazed surfaces.  6) Randy's Red;  7) Tan  (again, these are opposite ends of the same piece.)

 

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#17 JBaymore

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Posted 06 October 2013 - 10:16 PM

From the first three close ups I'm guessing that what you have is a LOW firing natural clay.....an earthenware.   It is looking overfired on all the XC images.  The part with the RR glaze is looking SERIOUSLY fluxed and seriously outgassing (hence the holes). 

 

You likely either need to shift to a lower cone for the firings, or decrease the percentage of the local clay in the body forumlation.

 

And a reminder....... this is sort of like doing surgery by telling someone how to hold the scalpel over the phone and were to cut.  NOT the most accurate way to diagnose this.

 

best,

 

........................john


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Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art

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#18 JBaymore

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Posted 06 October 2013 - 10:27 PM

Randy's Red is not a great glaze recipe.  The boron is about double the normal cone 6 limit, the alumina is WAY low, the silica is a bit low from limits, and it is way over limits for CaO.  Clear that this will tend to attack the clay body trying to get the silica and alumina from that source. 

 

That tendency to eat into the body makes the glaze "reactive" for visual effects ....... but as a functional glaze...... bad news.

 

  Soda Feldspar................    17.00 
  Gerstly.....................    28.00 
  Silica......................    26.00 
  Talc........................    12.00 
  EP Kaolin...................     4.00 
  Iron Oxide Red..............    15.00 
                              =========
                                 102.00

          CaO       0.69*   
          MgO       0.24*    
          K2O       0.04*    
          Na2O      0.03*    
          P2O5      0.00*   
          TiO2      0.00     
          Al2O3     0.13     
          B2O3      0.64    
          SiO2      2.26    
          Fe2O3     0.27   
 


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Professor of Ceramics; New Hampshire Insitute of Art

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#19 Rockhopper

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Posted 07 October 2013 - 06:27 PM

From the first three close ups I'm guessing that what you have is a LOW firing natural clay.....an earthenware.   It is looking overfired on all the XC images.  The part with the RR glaze is looking SERIOUSLY fluxed and seriously outgassing (hence the holes). 

 

You likely either need to shift to a lower cone for the firings, or decrease the percentage of the local clay in the body forumlation.

 

And a reminder....... this is sort of like doing surgery by telling someone how to hold the scalpel over the phone and were to cut.  NOT the most accurate way to diagnose this.

 

best,

 

........................john

John -

 

I really appreciate your taking time to share your insights.  I knew there had to be something in the RR glaze reacting with my clay - but didn't know enough of the chemistry to figure out what.  About half of the glazes they use at this studio share the same base as the RR, with different oxides for color, so that lets me know I can probably expect similar results from those.

 

I'll have to talk to my friends about possibiity of firing lower. Nearly everything they fire is ^05-^06 bisque and ^6 glazed. I'm going to get a copy of Cardew's book from the library - but in the mean-time: When decreasing % of local, would you recommend keeping the EPK & OM4 at the same proportions relative to each other, and just reduce the local  - or is there something else you would suggest adding to help raise the maturing temp ?



#20 Rockhopper

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Posted 15 March 2014 - 02:05 PM

Update:   after much trial & error - 60+ recipe variations, 100+ test tiles, and failed pots ranging from crumbling to melted - I have a useable clay body.  Still need to tighten it up a bit before I can use it for vases & such (paper towel placed under bowl filled with water, gets damp pretty quickly) - but at-least my pots aren't coming out of the kiln looking like lava :)

 

Current mix;  Local Clay 41.5%, EPK 35%, Silica 15%, Custer Spar 8.5%

 

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