Claybody Blues...

[Kevin B.]

So I've been experimenting for the past couple months with blending various local materials into a C6 stoneware clay that I've used successfully for many years in pit and bonfire firings to give it a bit more character for raku firings.  I've settled on two blends for the time being, one lighter and smoother in texture, and the other darker and more gritty.  They both take the thermal shock of firing well and give me the color and character (or taste) that I'm looking for, but they both share one small problem.   I think I've figured out a work-around to fix it but I thought I'd run it by the more knowledgeable folks here to see if there's any variables I'm not taking into consideration, since I am a relative newbie to this clay chemistry stuff.

 

The problem I'm having is small particles (1-2mm) of limestone from the local silts and grits that I use popping out of the clay/glaze surface after firing.  Shouldn't be a huge problem right? Just use a finer mesh sieve to remove larger limestone particles, right?  Well, unfortunately I can't do that because I've (stubbornly) decided the texture of the clay is perfect with the larger particles of grit I use for temper and I don't want to run it through anything finer than a 1/16" screen. (photos below)

 

I have to mention as an aside finding limestone grit in my native iron bearing soils was not a surprise - heck, everyone around here who uses well water knows how hard with iron and carbonates it is.  I just didn't know to what extent it would show up in the clay until I fired it.

 

So, since I decided I can't sieve it through a smaller mesh, I've thought about dissolving the particles with vinegar.  We've always used vinegar to get rid of calcium carbonate build up on our glasses and our house humidifier, so I thought using some apple cider vinegar in lieu of water when I'm sieving the limestone bearing soils before I mix them into the base clay could alleviate the problem.  Plus, if I remember right, I read somewhere that some old time potters mixed in vinegar to help with plasticity.  Unless there's something I'm completely missing this sounds like a win/win right?

 

Please forgive the photo quality (glaring highlights), don't have a lightbox setup yet to photograph my stuff.

 

 

 

 

[glazenerd]

Hi Kevin:

Playing in the dirt again hey? For a frame of reference; the general rule is: 50 mesh and larger is visible to the naked eye. So the smaller white dots are in that area, and you have a few in the 20 mesh range. Judging by the color of the fired piece, your native clay has somewhere in the 5-6% (weight) of iron. So I would have to assume it represents the largest addition in your recipe.

 

I have used vinegar before; not that uncommon to see it added. So you are thinking the vinegar will break down the calcium? It will reduce the PH certainly, but I am doubtful that it will dissolve that large of mesh. Have you dissolved this native clay in water and checked it with a PH meter? If it is calcium, you will get a PH reading somewhere in the 8.3-8.6 range: if other fluxes (sodium and potassium) are present in any measurable amounts: it will run towards 9.0-9.4.

 

The black dots are more than likely ilginite (coal), unless you are adding granular Mno. The only thing that makes me question the reactivity of the limestone is the lack of reaction to it under your glazed piece. I would expect to see some minor bloating issues with that level of contaminants.

 

Nerd

[Marcia Selsor]

Despite the white specs, that is an interesting looking clay body. Nice work.

Marcia

[Joseph Fireborn]

looks amazing. i would be really happy, if you can get rid of the white parts it would be even better. I liked the fired color of the 2nd picture.

[Kevin B.]

Hi Kevin:

Playing in the dirt again hey? For a frame of reference; the general rule is: 50 mesh and larger is visible to the naked eye. So the smaller white dots are in that area, and you have a few in the 20 mesh range. Judging by the color of the fired piece, your native clay has somewhere in the 5-6% (weight) of iron. So I would have to assume it represents the largest addition in your recipe.

 

I have used vinegar before; not that uncommon to see it added. So you are thinking the vinegar will break down the calcium? It will reduce the PH certainly, but I am doubtful that it will dissolve that large of mesh. Have you dissolved this native clay in water and checked it with a PH meter? If it is calcium, you will get a PH reading somewhere in the 8.3-8.6 range: if other fluxes (sodium and potassium) are present in any measurable amounts: it will run towards 9.0-9.4.

 

The black dots are more than likely ilginite (coal), unless you are adding granular Mno. The only thing that makes me question the reactivity of the limestone is the lack of reaction to it under your glazed piece. I would expect to see some minor bloating issues with that level of contaminants.

 

Nerd

 

The iron comes directly from the decomposed granite subsoil I added into the clay body - so you're right on the mark with that guess.  From what I know about our local granites, they're predominantly very iron rich K-feldspars locked up in a matrix with lots of mica, quartz, and varying amounts of other trace minerals - you'll also find occasional pockets of pyrite and garnet crystals in them.  Actually the subsoil itself looks exactly like the body in pic 1, which is great because I wanted a warmer, redder body to put iron dependent glazes on.  Otherwise, the base clay just turns out light grey to yellowish/pinkish buff under various levels of oxidation - really unattractive in my opinion which is why I wanted to alter the claybody in the first place.

 

Are we talking bloating during firing or post firing moisture absorption?  I know the larger of the particles have swollen enough a couple weeks after to the point where they've popped off small pieces of body/glaze sitting on the counter (pics 2+3).  I should have specified before that the limestone in question is at the very center of each pic, the majority of the remaining white specs are actually silica in the form of decomposed quartzite granules. 

 

If you meant during firing, the reason why you're not seeing a reaction to the glaze is because there is no glaze in the traditional sense.  I was experimenting with sodium bicarbonate vapor firing in a saggar, and that thick green in pic 2 is where some of the molten sodium splashed out of the crucible I overfilled and onto the bottom of the piece - was a fortuitous accident, which I am now replicating and trying to perfect into a process.  So it's not glaze like one would normally think, but rather the reaction of molten flux acting on the clay components and melting a sort of glaze right into the pot face.  I can tell you that there's no lignite in my soils, so I'm going with naturally occurring manganese from the parent rock - it would explain why a "glaze" on an iron rich clay would go yellowish-green and not orange or red at 1000-1100C (manganese naturally forms green crystals by itself, right?)

 

I'll have to try the pH tests for the clay, like I said all this chem is new to me - I know I cant use my own tap water though, our water softening system uses a potassium based salt.  Could trace amounts of potassium also cause the glass to go green?  I just realized I've been using tap water to mix up my slip dish and to process all my additives.  Man, now I've got a looong list of variables to backtrack

[glazenerd]

Hmm... that confirms my hunch about the white specks.

 

 

Issue 1 ---"in a matrix with lots of mica,"   2.  a potassium based salt.

I had a hunch the white spots were either soapstone or microline, not limestone.  Calcium in concentrated amounts will bloat the glaze, but not rupture. Mica typically runs 13% potassium (weight): so I will go with microline for the moment. In a cone 6 firing, that large of mesh size will be very problematic because it would require an extended hold to off gas. So the green would be from potassium, but more so by sodium: most likely both. Potassium is an off gassing hound in a cone 6 firing; the reason it tops my list.

 

Easy enough to confirm: make a simple form and glaze it with a simple (regular) cone 6 glaze. Fire it either to 2190F with a 20 minute hold, or to 2230F with a five minute hold. If this is some kind of microline (which is my hunch), then you should end up with blisters and pinholes. Might seem like a waste, but it will confirm what you are dealing with. if it is in fact some tectosilicate, dealing with it will get easier.  Just run the test with a regular load of cone 6 in the future.

 

Otherwise, the base clay just turns out light grey ---- which would be higher magnesium levels.

o yellowish/pinkish buff under various levels of oxidation -- which would be low iron with higher titanium levels.

Nerd

 

I use bottled water to rinse and calibrate my PH meter. I keep it simple.

[Kevin B.]

Hmm... that confirms my hunch about the white specks.

 

 

Issue 1 ---"in a matrix with lots of mica,"   2.  a potassium based salt.

I had a hunch the white spots were either soapstone or microline, not limestone.  Calcium in concentrated amounts will bloat the glaze, but not rupture. Mica typically runs 13% potassium (weight): so I will go with microline for the moment. In a cone 6 firing, that large of mesh size will be very problematic because it would require an extended hold to off gas. So the green would be from potassium, but more so by sodium: most likely both. Potassium is an off gassing hound in a cone 6 firing; the reason it tops my list.

 

Easy enough to confirm: make a simple form and glaze it with a simple (regular) cone 6 glaze. Fire it either to 2190F with a 20 minute hold, or to 2230F with a five minute hold. If this is some kind of microline (which is my hunch), then you should end up with blisters and pinholes. Might seem like a waste, but it will confirm what you are dealing with. if it is in fact some tectosilicate, dealing with it will get easier.  Just run the test with a regular load of cone 6 in the future.

 

Otherwise, the base clay just turns out light grey ---- which would be higher magnesium levels.

o yellowish/pinkish buff under various levels of oxidation -- which would be low iron with higher titanium levels.

Nerd

 

I use bottled water to rinse and calibrate my PH meter. I keep it simple.

 

 

So microline expands from atmo. moisture weeks after firing like limestone, interesting, I didn't know...  Interesting though, the rocks my materials are coming from are in the middle of a territory bordered by soapstone deposits to the north and limestone beds to the south, so there is some possible geological overlap.  The reason I went to limestone as a first thought was because I literally read about this similar phenomena two days ago in Leach's book and he talked about limestone grains the size of small birdseed being big enough to pop pots and glaze weeks after firing.  Hmmm, this might be tricky to test since my kiln is a small wood/charcoal fired raku-type muffle which only really gets to cone 1 or 2 max.  And it means I'd also have to learn some "real" glaze chemistry - was trying to do the vapor firing to negate that - oh well... lot to think about.  Would all your tests still work at my low fire temps, or do they need to be up near ^6 for the chemistry to work correctly?

[glazenerd]

Cone 1-2 will work just fine; plenty enough heat to get potassium/sodium to start burping. Use a premix cone 6, just add some NEP SY to it to help the melt along at lower firing temp. (10%).  Do a soak for20 minutes or so at top temp to off gas the glaze; the large particles in the clay would require more than that to off gas at that temp.

 

http://community.ceramicartsdaily.org/topic/15155-stoneware-limit-study/   lengthy discussion about stoneware bodies.

Nerd

[alabama]

I'm thinking vinegar dissolves lime...if so your problem is solved.

If not, you'll have to find another source of natural clay.

 

Pour a bottle of vinegar in a bucket of clay and see if it fizzes...if it does, then it's working... The way you duplicate "hole tempered" pottery is to make and fire shell tempered pots, then soak them in vinegar and the shell dissolves. Early 20th century amateur archaeologists found pottery that the ground had leached out the shell, and it was classified as hole tempered.

 

Good luck...

Alabama

[Magnolia Mud Research]

vinegar will dissolve lime (CaO).  However, it is not likely to dissolve limestone (CaCO3), especially if there is any magnesium carbonate in the limestone.  It will take a stronger acid than vinegar to decompose the limestone. 

 

If large 'bits' of limestone are not reacted to form either a glassy phase, or a crystalline mineral during the firing, you will end up with pure lime where the 'bit' was.  The lime reacts with moisture from the air, and this may take some time if the fired clay has a very low water absorption value, to form hydrated-lime.  Hydrated-lime takes up more room than lime, and this creates expansion stresses, and often results in cracking and/or spalling -- some potters call it 'lime pops'. 

 

My question what is the firing temperature and what is the water absorption value for your fired clay?  I have fired a red high iron and sandy clay from my pond to cone 10, and while hard, it is still very porous - almost like bisque from 'store bought' cone 10 stoneware.

 

I am interested in the results of you baking soda experiments.  Continue to post your results, including what 'don't work'.

 

LT

[Kevin B.]

vinegar will dissolve lime (CaO).  However, it is not likely to dissolve limestone (CaCO3), especially if there is any magnesium carbonate in the limestone.  It will take a stronger acid than vinegar to decompose the limestone. 

 

If large 'bits' of limestone are not reacted to form either a glassy phase, or a crystalline mineral during the firing, you will end up with pure lime where the 'bit' was.  The lime reacts with moisture from the air, and this may take some time if the fired clay has a very low water absorption value, to form hydrated-lime.  Hydrated-lime takes up more room than lime, and this creates expansion stresses, and often results in cracking and/or spalling -- some potters call it 'lime pops'. 

 

My question what is the firing temperature and what is the water absorption value for your fired clay?  I have fired a red high iron and sandy clay from my pond to cone 10, and while hard, it is still very porous - almost like bisque from 'store bought' cone 10 stoneware.

 

I am interested in the results of you baking soda experiments.  Continue to post your results, including what 'don't work'.

 

LT

 

 

Thanks for the input, here's the specs. for the base clay before I add my grit/sand and other native clays/soils to it - I haven't tested absorption on the final recipes yet.  I gloss fire it around cone 1 in a neutral to reducing atmosphere (depends on amount of NaHCO3 in the particular "glaze" recipe).  When I bisque fire in oxidation it turns both claybody recipes various shades of buff to light salmon pinks.  With the additions I use in the claybodies, I know I'm opening up the body and making it more porous, but even with the chawan I've made, they stop weeping after only 10-12 uses, so there's actually not a lot of space in the earthenware even after I open it up.  And that's on pieces that aren't completely glazed either as a result of brushing on the bicarb.  Once I figure out a the correct consistency and concentration for dipping my bicarbonate solution so I get an even coat, I'm guessing the porosity of the inside surfaces will decrease even more

 

I'm planning on posting all the results of my experiments once I have them - I'm realistically only in stage 2/5 for what I need to figure this process out.  I'm basically going off of an old snippet of an article I found from a NCECA conference summary from back in the 70's.  And I can tell you that the little glazing "accident" from my second pic above is already sending me in a different direction than what they were trying to attempt in the article.  Trust me though, the process I'm developing is a little unorthodox for the potters who like nicely formulated clays with predictable glaze results in a completely controlled kiln, but very cool at the same time.

 

I'm a primitive technologist being a little creative, not a potter.  With the exception of the clay, the rest of my materials come from a grocery store, hardware store, or the woods behind my house - no fancy heavy duty fire brick or potentially dangerous glaze chemicals in sight.  Just good ol' down to earth simple materials.  My goal is to ultimately come up with a process any hobby potter could do on a budget for attractive and functional low fired & glazed earthenware - especially if they're into tea ceremony and want to make their own bowls, like I do.  Necessity is the mother of invention especially with these experiments, because as much as I love wood fired ^10 stoneware with natural ash glazes from protracted firings, I currently don't have the budget or the space or time to put an Anagama in the back yard.  (maybe one day, but not now)  However, if I can completely justify a small wood fired muffle kiln that takes only a couple armfulls of kindling or a few bags of charcoal to fire, then I'm going to try and take full advantage of what is possible with it.

[glazenerd]

 

https://en.wikipedia.org/wiki/Hygroscopy               see also CME   coefficient of moisture expansion

Calcium, sodium and potassium will draw atmospheric moisture.Have to look at this slightly different because you are dealing with them in their after firing state of oxidation: as LT pointed out in his post. CaCO3 goes in, and CaO comes out--same for sodium and potassium. It is extremely rare for CME to apply to ceramics: but you are the exception. Being a primitive guy, you would know what salt does to meat-dries it out (draws moisture). simplistic, but you get the idea.

 

Nerd

[Magnolia Mud Research]

Nerd, et.al.

 

Sodium and potassium oxides at temperatures ~800 C are corrosive enough to stoneware and porcelain bodies to react with the clay body to form a glassy phase.  There is a very low probability that after firing that any of the sodium or potassium oxide remains as such in the fired clay body.  Calcium oxide reacts much much slower at ~800 C than does the alkalis (Na&K) and CaO becomes an 'active ingredient' at much higher temperatures (my memory says ~ cone 4 and above).   
   
LT

[glazenerd]

LT: not in disagreement per se except to make a point. The U of I study traced potassium and sodium up to 2190F, then it was no longer visible on x-ray diffraction. Secondly, drawing from my own experiences:

 

That 20 + plus mesh size piece of soapstone remained after firing. We have been trained and taught in 200-325 mesh.

 

Nerd

[tinbucket]

Kevin B you ought to try some of Sheffield's 42 stoneware if you haven't. It's their high iron stoneware, cone 6-10 (slightly undercooked at cone 6). The color in oxidation is similar to the first picture you posted and it has plenty of tooth. Even if it doesn't replace your current body it's still very nice to work with

[Kevin B.]

Kevin B you ought to try some of Sheffield's 42 stoneware if you haven't. It's their high iron stoneware, cone 6-10 (slightly undercooked at cone 6). The color in oxidation is similar to the first picture you posted and it has plenty of tooth. Even if it doesn't replace your current body it's still very nice to work with

 

It's funny you should mention that clay, I almost bought some last time I was up there to play around with.  Maybe when I get my wheel finished that could become my standard claybody for throwing.

[Magnolia Mud Research]

Nerd:
 
Loss of signal by X-Ray Diffraction (XRD) means that "the specie" is no longer in a crystal. XRD depends on the atoms being arranged in a particular pattern over a long range (relative to the size of an atom) and is not a reliable analytical tool for elements that are in an amorphous (aka glassy or liquid) phase. In a silicate melt the atoms do have a 'short range' structure, but not a 'long range' structure.  XRD is often used by geologists studying melts to prove that the material they are testing does NOT have crystals in the sample. 
 
 
Think about sodium and potassium in melts from the view point of bottle and window glass.  The three major elements in these glasses are oxygen, silicon, and sodium. The major raw materials are, cullet, soda ash, potash or feldspar, clay, and silica sand being dumped into a furnace tank that runs somewhere around 1500 C.  In good glass none of these elements show up in an XRD.
 
Is the "I of U study" available, I would very much like to study it.
 
I am not sure what your message is regarding the soapstone in your clay body test.  To me, soapstone means a magnesium silicate rich rock that withstands high temperature well, in other words, somewhat refractory.  It would react via sintering or by being partially dissolved by a surrounding low temperature melt, but a large particle would only be partially consumed if used in a clay body.  I would also expect the same response for a large particle of silica.
 
LT

[glazenerd]

LT:

 

Was thinking microline and typed soapstone.. old age I guess. My point being: would enough time/heat work take place in a typical c6 firing to completely melt/off-gas large particles?.... Soon as i find which thread I put the U of I study in.. will PM it to you.

 

Nerd

[Kevin B.]

Don't know why I didn't think of this first... duh... So I went and tested all my added materials separately with a vinegar soak to see if I could find out where the carbonates were coming from, and I found the culprit... turns out the river sand I use as temper must have some limestone in the parent rock upstream.  Hard to take a clear pic since the silt/clay in the sand left the vinegar a little cloudy, but if you take a look below, all the little dark grey spots in the liquid above the sand on the bottom are bubbles.  I'd say this shows that I will be able to dissolve out the offending calcium carbonate so pops in the claybody from large granules can be avoided.  Also shows I had quite a bit more carbonate than I suspected, amazed I didn't get more pops on my pots.  Only thing left to do is mix some treated sand into my next batch of clay and see if the pops are eliminated.