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hershey8

What To Do With Old, Dry, Moldy Clay?

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I came across two bags of prehistoric clay in my studio (ok...basement) a few days ago. They were over in the corner, right under Jimmy Hoffa. Like Jimmy, the clay was cold and dry, and very stiff. I was able to cut it with a wire, barely, and the cross section revealed what appeared to be a 2-inch thick band of  yellowish and blackish moldy looking clay. Yuck! I poked holes in the clay and fill them with water and let it set for several days. Tonight I tried to cut and slam the clay. Every time I cut the clay I would observe smears of black stuff that almost looked like charcoal, When I dug into where the smear ended, there would be a solid deposit of black stuff, kinda hard but I  could rub it between my finger and thumb and break it up. The clay body is called "Spotted Owl", not because it's on the endangered species list, but presumably because of its spots.  I'm thinking that exposure to this  black mold ,may be endangering ME. Has anyone come across this level of mold in clay? I don't think I can wedge out the lumps of mold, and I think that is strange. Perhaps I'll just roll it into tiny balls and throw petri dishes. I didn't sign up for this when I became a novice wannabe potter. This is just sooooo unfair.  ja

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Some people are allergic to mold and should not work with clay at all as most clay likely has some mold in it. But it actually helps the aging process and improves the workability of the clay. The more mold, the better aged the clay. Wet a towel, wrap the clay in it and put it back in the bag for a few days to even out the moisture. Wedge and enjoy. It may be the best clay you'll work with so far. 

 

Ruth 

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I had no idea you could add peroxide or bleach to clay; that may be the answer. Some mold can "gettcha" whether you are allergic or not. So the idea of killing it is attractive to me. Still, I'm not sure that the solid chunks will wedge out, but it might be interesting to leave them in and see what happens. Thanks. ja

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Ruth, I've heard this, but I've never had it confirmed as fact.  Is it because there's less organic material in the clay because it has decomposed?

I'm not sure which part you're asking about, but here goes. The aging process is complex. Most commercial bodies are made from air floated clay of fairly uniform mesh size. This clay has nearly been bisque fired. Most clay companies do not blunge the air floated clay to throughly wet the materials. It takes time for bacteria and mold to break this clay back down into various sizes of particles and for all the particles to be throughly wetted, both important factors in a strong clay. Over time the flocculation, plasticity and general workability and responsiveness of the clay increases. If you've ever get the pleasure of working with a clay that has been aged at least a year, either a commercial body or preferably with a clay that has been naturally mined and minimally processed by blunging and filter pressing, you will quickly realize the benefits of well aged clay. It practically throws itself. 

 

Ruth

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A better way of rehydrating your clay is to keep it in its plastic bag and add 1 to 1 1/2 cups water (maybe a capful of bleach in the water in this case 50/50 water and bleach in this case), and reseal the bag with a twist tie. Then put the bag of clay in a 5 gallon bucket and fill the bucket with water until it just reaches the top of the clay block. The water pressure from the bucket will force the water/bleach into the dry clay and rehydrate it. Let it sit in the bucket for a couple of days, or until it is soft and pliable. By not poking holes and filling with water, you eliminate the potential for air bubbles when you wedge the clay.

I've found mold on some clay just out of the box; it burns out during bisque. I've also had mold grow from recycled clay . . . always figured it was dead cells from my hands, etc. that were decomposing.

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Really old clay from a friend who'd mined and processed it herself, mold  black and green, once wedged was beautiful to work with, smooth and plastic.

Thought I'd read somewhere rwhere one generation puts down clay for the next. I'll think of where I read this sometime when it is totally irrelevant to the discussion.

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When I lived in Spain in a pottery village, the clay was brought in by mule cart..three types dug locally. The professional clay maker who worked at several potteries, combined the clays, slaked and agitated the tank ( 8' x8' x5' deep)after a few days the runnier clay was released into a more shallow tank, parts of that were put onto a brick patio to thicken, cut into block and put into a mixer where a little salt was added. They said this was for the water jugs called "Botijos" and helped with the taste of the water. There were 20 potteries at the time. The clay makers worked at 2-3 potteries and also fired the kilns -2-3 story wood kins that fired for 5 days.

I have found that clay that is slaked then aged to a workable thickness, is more plastic than clay dry mixed and run through a pugger. When I hand mixed clay and dried it in flower pots to a workable consistency, it was a very plastic clay.I mixed clay daily, first thing in the morning. I did this in the basement where I was a caretaker. I had access to hundreds of terra cotta flower pots from the old estate green house.

 

Marcia

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Norm, I have seen a documentary, that focused on a Japanese "Pottery Village". The type, where each person had a specific job.

I remember the small woman, who was responsible for wedging. Her forearms were the size of the average person's calves.

 

Anyway, the wedged clay came from a large pile, where the processed clay had be thrown to age. The pile was not covered or anything. It sat in the corner of the room, on the floor, completely uncovered.

 

I have zero experience with that of course. Maybe once I teach my daughter to throw, I can get a pottery mill goiing....

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Ruth  -  I've never had the experience of clay becoming more plastic over time, but I have routinely seen the pugs become so hard as to be unusable for throwing.

 

There's clear differences in particle orientation when comparing the outside of a lug of clay vs the interior, which is a well-documented result of the blunging process,and this can change slowly over time, especially in the outer edges of each clay pug.

 

I'd be very curious what John Baymore has to say about this topic.  For some reason my experience doesn't confirm any of your comments about aging clay.  Is there a specific type of clay you use where you've experienced this?

 

We keep our Laguna Clay in the 25 pound bags inside the 50 pound cardboard boxes they come in.  These in turn are stored in an airtight locked metal dumpster with a thick plywood lid, located under a large tree which keeps the clay container in the shade year round.  By weighing the pugs of clay after 12 to 18 months of storage, they weigh the same so have not lost any moisture.  But most harden over time.

 

When the clay is newly delivered it's very plastic.  As our clay ages it becomes progressively harder as deflocculating salts leech out of many of the fluxes, like nepheline syenite, feldpspars and man-made frits over time.  The same things happens to many glazes, leading them to hardpan.  This is a well-known process. This deflocculation occurs more quickly with Cone 6 clay than Cone 10 because Cone 6 clay contains more fluxes.

 

People at our studio who like throwing on the potters wheels with newly delivered clay, but they claim to have problems with most clays which are a year or more old. They say wedging in some Calcium Chloride restored some of the lost plasticity, but they have thrown out several older pugs of Cone 6 clay because it's not worth the trouble.  I've also never seen mold develop in any Laguna clay, even after 18 months of storage.  For there to be mold, the clay would have to contain a lot of organics.

 

Paper clay is of course a completely different deal, in my experience.  When first delivered paper clay is a bit mushy, almost buttery like a porcelain.  After being stored for six months the paper clay is more plastic and less mushy, but it begins to show mold by 12 months depending on our weather.  I'm not sure if a bactericide is added the Max's Paper Clay.

 

The Chinese hoping to hide their secret of porcelain, which was Kaolin mined from the mountains,  told European merchants many fanciful stories that their porcelain clays were mixed with eggshells and other materials organic material and buried for decades. Grown children would supposedly dig up the porcelain clay their parents had buried in their youth leaving it to mature into porcelain clay they used to create porcelain ware.  Of course this story was complete nonsense, as Europeans later discovered when they began mining their own local kaolins.  But this misdirection maintained the Chinese monopoly on porcelain for quite some time.  And a surprising number of potters still believe these stories today.

 

Porcelain clay bodies are often made of kaolin of uniform particle size, but most other clay bodies are deliberately made of mixed particle sizes to give the clay better body characteristics.

 

I'm not sure which part you're asking about, but here goes. The aging process is complex. Most commercial bodies are made from air floated clay of fairly uniform mesh size. This clay has nearly been bisque fired. Most clay companies do not blunge the air floated clay to thoroughly wet the materials. It takes time for bacteria and mold to break this clay back down into various sizes of particles and for all the particles to be thoroughly wetted, both important factors in a strong clay. Over time the flocculation, plasticity and general workability and responsiveness of the clay increases. If you've ever get the pleasure of working with a clay that has been aged at least a year, either a commercial body or preferably with a clay that has been naturally mined and minimally processed by blunging and filter pressing, you will quickly realize the benefits of well aged clay. It practically throws itself. 

 

Ruth

 

 

I have no idea how Laguna processes their clay, apart from blunging it, but I can't confirm your experience.

Norm, let me preface this by stating that I am not a scientist and my evaluation of clay is empirical and what works for me. The detailed science of clay can make my head hurt. But I think I have absorbed a bit information. Every clay I've worked with benefits from aging and I've been working with clay for, umm, a number of years. As recently as last week, I threw some new and aged clay of the same body. The aged clay had hardened, same as your experience. I softened it by wrapping the clay in a wet towel and putting it back in the bag for a day, but stiff clay can also be "awakened" by slamming on a concrete floor a few times. I can only make a subjective statement, but the aged clay was more responsive and stronger, centering more quickly and I was able to throw a taller thinner piece with less effort. The clays I use do not contain Neph Sye or frits. It's possible that our different experiences arise from the fact that my clay has not deflocculated and yours has. If calcium chloride helps your clay, I would say that you are correct that it has become deflocculated. But flocculation/deflocculation is only one property of clay workability.I can't make any other statement about your experience with Laguna's clays as I've never used them. 

 

Some of my information comes from clay suppliers, some from conversations with other potters whose studios I have visited.  I make it a practice to learn how a particular company process the clay; I've been able to visit a couple. Most US clays are made up from a recipe; the ingredients have been air floated.  Burgess Pigment's website states that "The crude kaolin is ground,air classified to acceptable size distributions, and dried. An air floated clay is generally poor in color, higher in residue, and abrasive, as compared to more refined grades." Air floating creates a very consistent product, which is obviously very important in an industrial setting.  A conversation with Ian Currie added the information that this air is heated almost to bisque temperatures. 

 

While living in Europe, I used naturally occurring German and French clays (not formulated clay bodies), processed by blunging and filter press. The workability was impressive and my friend there called it "God's own clay". I visited the Solargil plant in France and saw the mounds of dug clay drying in the sun before being blunged and filter pressed. Their parking lot is surrounded by huge chunks of clay as a decorative element.... something only a potter would love! The La Borne wood firing potters use a clay so high in organics that it is black. Very messy, and wonderful to use.  StarWORKS clay in Star, NC digs and processes a NC clay in this way. I know at least one Seagrove potter who won't use anything else. The Star White with grog is my current clay.  I'll be going there next week and will take some pictures of the process to share. 

 

The selection of a clay is extremely important. I've worked with a lot of different commercial bodies and a number of mined bodies, both new and aged. I've not found much difference among all these with slab work. The difference really shows on the wheel. 

 

Clayart has had a number of discussions about the nature of plasticity and organics. Following are a couple of quotes:

 

From Neon-Cat, June 17, 2009

 

Microbes, fungi, and their derivatives are quite useful clay body components

or additives and they enhance plasticity while helping to hold the clay body

together. Slime, organic matter, and synthetic plasticizers ease friction to

help clay particles, flocs, and other clay body components slide past one

another or, when needed, they can be great wetting agents, binders,

adhesives, fillers, pH stabilizers, etc. while helping the clay body hold

and retain moisture. 

 

From Vince Pitelka, Oct. 19, 2009

 

The finer the particles, the more water layers needed to

mobilize the mass and reach a plastic state - the so-called "water of

plasticity." Of course that's also the reason that clays featuring very fine

particles shrink so much in drying - there is more water to evaporate.

That's why we normally do not make a claybody with too much ball clay or

bentonite in it - the drying shrinkage would be excessive and would result

in serious cracking.

 

Plasticity results from the nature of the clay particle - it's flat shape,

ultra-fine size (one cubic inch of clay contains at least five trillion

particles), and affinity for water, and of course involves a balance between

the water layers providing lubrication, and the contact points creating

friction that provides working structure.  Too much water, and there is too

little friction and poor working structure.  Too little water, and there is

too much friction and not enough lubrication to achieve plasticity.

 

This relates directly to the recent discussion of "green packing," which I

have always referred to as the "distribution of particle sizes."  If you

modify a claybody to increase the distribution of clay particle sizes, you

1) maintain the water layers that create plasticity, 2) reduce the overall

water content because there will be less large water-filled voids that do

not contribute to plasticity, 3) increase the number of contact points

between particles and thus increase working structure, 4) decrease firing

shrinkage by eliminating the large voids, 5) increase dry and bisque

strength because of the increased number of contact points between

particles.  It's a win-win situation.

 

Hope all this helps!

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Guest JBaymore

On caveat before I comment based on Norm's question...... don't beleive everything you read in books, hear in seminars or workshops, or read on the internet. Research information from diverse sources, and look at the qualifications/ experience of the individial making statements in the particular field of discussion*, before coming to conclusions.  Question everything.

 

Clay is known to improve in plasticity with aging.  Two main factors are involved.... wetting of the plastics particles in the body, and the gradual alttering of the chemistry of the water by bacterial growth and by-products.  But of course if the clay is allowed to slowly dry out, like with the modern habit of slightly water permeable plastic bags with poor seals, that affects this potential process. 

 

Vince as quoted there has concepts of body development principles pretty well nailed (as one would expect).

 

There are few suppliers in the US that mix using blunging.... most mix to wet plastic clay.... a poor (but cheaper) way of preparing bodies.  Tuckers  and Dave's apparently are the only two that use blunging and filter pressing... and that approach produces a FAR superior product. 

 

I've seen mold grow in the manner you describe in the original posting.  As Norm mentions, pugging is well known to NOT produce an even blend....... there are strata in the extruded pugs.  Some of the "blend" of clays in the orginal batch might have had more organics for mold to feed off of than the other materials... and the mold started growing in the nutrient rich strata. 

 

The blaskish molds are typically anerobic........ not getting much oxygen in there.  These are the same kinds that form in the bottoms of the wash-off buckets in studios that stink all to heck when they are disturbed (rotten swamp smell).

 

Without seeing the actual stuff....... there might be other factors.  But it is probably mold.

 

The "Spotted Owl" name makes me wonder if there is added manganese dioxide in the body...... a common "speckling" agent....... that MIGHT by VERY poorly mixed in the body... and is concentrated in seams.  This is very unlikely.....it would be a BAD manufacturering boo-boo for that to be THAT far screwed up.  But such stuiff does happen..

 

And yes, for some folks mold can be a real issue.  And some types of mold can affect everyone (not common in clay). 

 

best,

 

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

 

* - by this comment I mean.... if you are speaking about aesthetic issues, someone with strong background in that area likely has good comments, if you are speaking about technical issues, someone with a strong technical background is likely providing useful comments.  Don't assume the artist is a technician, and don't assume the technician is an artist.  Occasionaly, you get perople who are strong at both (like Vince). 

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On caveat before I comment based on Norm's question...... don't beleive everything you read in books, hear in seminars or workshops, or read on the internet. Research information from diverse sources, and look at the qualifications/ experience of the individial making statements in the particular field of discussion*, before coming to conclusions.  Question everything.

 

Clay is know to improve in plasticity with aging.  Two main factors are involved.... wetting of the plastics particles in the body, and the gradual alttering of the chemistry of the water by bacterial growth and by-products.  But of course if the clay is allowed to slowly dry out, like with the modern habit of slightly water permeable plastic bags with poor seals, that affects this potential process. 

 

Vince as quoted there has concepts of body development principles pretty well nailed (as one would expect).

 

There are few suppliers in the US that mix using blunging.... most mix to wet plastic clay.... a poor (but cheaper) way of preparing bodies.  Tuckers  and Dave's apparently are the only two that use blunging and filter pressing... and that approach produces a FAR superior product. 

 

I've seen mold grow in the manner you describe in the original posting.  As Norm mentions, pugging is well known to NOT produce an even blend....... there are strata in the extruded pugs.  Some of the "blend" of clays in the orginal batch might have had more organics for mold to feed off of than the other materials... and the mold started growing in the nutrient rich strata. 

 

The blaskish molds are typically anerobic........ not getting much oxygen in there.  These are the same kinds that form in the bottoms of the wash-off buckets in studios that stink all to heck when they are disturbed (rotten swamp smell).

 

Without seeing the actual stuff....... there might be other factors.  But it is probably mold.

 

The "Spotted Owl" name makes me wonder if there is added manganese dioxide in the body...... a common "speckling" agent....... that MIGHT by VERY poorly mixed in the body... and is concentrated in seams.  This is very unlikely.....it would be a BAD manufacturering boo-boo for that to be THAT far screwed up.  But such stuiff does happen..

 

And yes, for some folks mold can be a real issue.  And some types of mold can affect everyone (not common in clay). 

 

best,

 

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

 

* - by this comment I mean.... if you are speaking about aesthetic issues, someone with strong background in that area likely has good comments, if you are speaking about technical issues, someone with a strong technical background is likely providing useful comments.  Don't assume the artist is a technician, and don't assume the technician is an artist.  Occasionaly, you get perople who are strong at both (like Vince). 

 

 

Thank you, John, for rephrasing and confirming my response to Norm that the plasticity of clay improves with aging and that blunging and filter pressing produce a superior product.  

I also agree that researching diverse sources on any subject is a good idea.  As Ben Franklin said, "Believe none of what you hear, and only half of what you see."I see that both you and I reference

Vince Pitelka as a knowledgeable resource. I've been following Clayart since the beginning and have saved posts from Vince such as the one I quoted and those of a few others, to my own database for future reference.

That's also why Hamer and Hamer's long standing reputation makes The Potter's Dictionary of Materials and Techniques  one of my go to resources on technical questions. 

The sections on Preparation of Clay and Workability are too long to reproduce here, but would be useful reading for any one wanting a better understanding of the properties of a good clay body.

And I've been fortunate to visit primary sources of information, Solargil and StarWORKS, for a better understanding of clay preparation. 

 

Ruth Ballou

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Ruth  -  I've never had the experience of clay becoming more plastic over time, but I have routinely seen the pugs become so hard as to be unusable for throwing.

 

There's clear differences in particle orientation when comparing the outside of a lug of clay vs the interior, which is a well-documented result of the blunging process,and this can change slowly over time, especially in the outer edges of each clay pug.

 

I'd be very curious what John Baymore has to say about this topic.  For some reason my experience doesn't confirm any of your comments about aging clay.  Is there a specific type of clay you use where you've experienced this?

 

We keep our Laguna Clay in the 25 pound bags inside the 50 pound cardboard boxes they come in.  These in turn are stored in an airtight locked metal dumpster with a thick plywood lid, located under a large tree which keeps the clay container in the shade year round.  By weighing the pugs of clay after 12 to 18 months of storage, they weigh the same so have not lost any moisture.  But most harden over time.

 

When the clay is newly delivered it's very plastic.  As our clay ages it becomes progressively harder as deflocculating salts leech out of many of the fluxes, like nepheline syenite, feldpspars and man-made frits over time.  The same things happens to many glazes, leading them to hardpan.  This is a well-known process. This deflocculation occurs more quickly with Cone 6 clay than Cone 10 because Cone 6 clay contains more fluxes.

 

People at our studio who like throwing on the potters wheels with newly delivered clay, but they claim to have problems with most clays which are a year or more old. They say wedging in some Calcium Chloride restored some of the lost plasticity, but they have thrown out several older pugs of Cone 6 clay because it's not worth the trouble.  I've also never seen mold develop in any Laguna clay, even after 18 months of storage.  For there to be mold, the clay would have to contain a lot of organics.

 

Paper clay is of course a completely different deal, in my experience.  When first delivered paper clay is a bit mushy, almost buttery like a porcelain.  After being stored for six months the paper clay is more plastic and less mushy, but it begins to show mold by 12 months depending on our weather.  I'm not sure if a bactericide is added the Max's Paper Clay.

 

The Chinese hoping to hide their secret of porcelain, which was Kaolin mined from the mountains,  told European merchants many fanciful stories that their porcelain clays were mixed with eggshells and other materials organic material and buried for decades. Grown children would supposedly dig up the porcelain clay their parents had buried in their youth leaving it to mature into porcelain clay they used to create porcelain ware.  Of course this story was complete nonsense, as Europeans later discovered when they began mining their own local kaolins.  But this misdirection maintained the Chinese monopoly on porcelain for quite some time.  And a surprising number of potters still believe these stories today.

 

Porcelain clay bodies are often made of kaolin of uniform particle size, but most other clay bodies are deliberately made of mixed particle sizes to give the clay better body characteristics.

 

I'm not sure which part you're asking about, but here goes. The aging process is complex. Most commercial bodies are made from air floated clay of fairly uniform mesh size. This clay has nearly been bisque fired. Most clay companies do not blunge the air floated clay to thoroughly wet the materials. It takes time for bacteria and mold to break this clay back down into various sizes of particles and for all the particles to be thoroughly wetted, both important factors in a strong clay. Over time the flocculation, plasticity and general workability and responsiveness of the clay increases. If you've ever get the pleasure of working with a clay that has been aged at least a year, either a commercial body or preferably with a clay that has been naturally mined and minimally processed by blunging and filter pressing, you will quickly realize the benefits of well aged clay. It practically throws itself. 

 

Ruth

 

I have no idea how Laguna processes their clay, apart from blunging it, but I can't confirm your experience.

 

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The Chinese hoping to hide their secret of porcelain, which was Kaolin mined from the mountains,  told European merchants many fanciful stories that their porcelain clays were mixed with eggshells and other materials organic material and buried for decades. Grown children would supposedly dig up the porcelain clay their parents had buried in their youth leaving it to mature into porcelain clay they used to create porcelain ware.  Of course this story was complete nonsense, as Europeans later discovered when they began mining their own local kaolins.  But this misdirection maintained the Chinese monopoly on porcelain for quite some time.  And a surprising number of potters still believe these stories today.

 

Norm,

There are two takes on this myth. One is that the name "porcelain" came for por cien anos "for 100 years"

the other is that the name came from the Portuguese word for a certain shell fish resembling porcelain products.

 

Marcia

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I cannot speak towards the science behind it, nor historically, but I like moldy clay. I have acid added to it to speed up the aging process.  Its going to be a good day throwing when I open the bag and see  lots of spots of black, green, white, and pink!  when I cut it beautiful streaks of black mold.

 

My newer student prefer the mold free, but gradually change over once they have some basic throwing skills.

 

Hershey8, reclaim it you may find you love it!!

 

Chad

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Over the years, I have had to do a lot of research on diverse areas of interest. If there is one thing you find out there it is to really check your sources. When the only source was print, and those things made from print ie scanned print materials, you had to check sources, and weed out questionable information. With the advent of the internet this became so much more important, since it seems it is a place of opinion made to look like fact. Authors are often anonymous and sometimes their agendas are quite hidden. Someone said to me the other day, "well in wikipedia it says . . . . . I asked them who writes wiki, and who oversees it. Just because its there, it is not necessarily true.

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Guest JBaymore

Someone said to me the other day, "well in wikipedia it says . . . . . I asked them who writes wiki, and who oversees it. Just because its there, it is not necessarily true.

 

In a lot of the collegiate academic world, Wikipedia is not a source that students can use for their writings.  It is fine for casual stuff, I guess.  But because of the "crowdsourceing" idea behind it... if you happen to catch a listing before the crowds have had a chance to settle down the facts from the fiction posted there........ you get bad information.

 

best,

 

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

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Someone said to me the other day, "well in wikipedia it says . . . . . I asked them who writes wiki, and who oversees it. Just because its there, it is not necessarily true.

 

 

In a lot of the collegiate academic world, Wikipedia is not a source that students can use for their writings.  It is fine for casual stuff, I guess.  But because of the "crowdsourceing" idea behind it... if you happen to catch a listing before the crowds have had a chance to settle down the facts from the fiction posted there........ you get bad information.

 

best,

 

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

B,b,but it was the first result that came up when I Googled it!!! Obviously it's the best source, and by best I mean easiest to find.

 

And the Wiki information is spot on.....if it pertains to a fictional character, item, world from a book, movie, videogame etc.

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Marcia,

There may be a grain of truth in the suggestion that the Chinese had a special kaolin,

thanks to that wonder ingredient of many old technologies - stale urine.

 

For those who have access to the scientific journals, its well worth reading:

Weiss, "A Secret of Chinese Porcelain Manufacture"
http://onlinelibrary.wiley.com/doi/10.1002/anie.196306971/abstract

 

To make up from the brevity of the abstract, I quote from the 2nd page of:

Rytwo. "Clay Minerals as an Ancient Nanotechnology"

http://www.ehu.es/sem/macla_pdf/macla9/macla9_15.pdf

 

Chinese Porcelain:. Porcelain has been made in China probably since
the 6th or 7th century A. D. Quartz, feldspar, and kaolin were then,
as now, the raw materials employed. The name “kaolinâ€, for china
clay derived from old deposits on the mountain Kao- ling in China.
Observations had been made since the 9th century about the fact that
“chinese have particularly fine clay which they use to make drinking
vessels with the delicacy of glass; although they are made of clay.â€
Such items had great strength, were shaped perfectly by hand, and have
wall thickness of less than 0.4mm! During Mongol domination (1280-1368)
the knowledge was lost.  Ceramics from the following Ming dinasty was
not as thin as before, even they tried to use the most plastic kaolin
deposits they found. Only till the end of the Ming dinasty (1644) thin
ceramics appear again, but in this case those were made from illite,
which has a-priori a considerably higher plasticity than kaolin,
making it easier to handle. The secret of ancient Chinese kaolin-based
porcelain must therefore lie in some technique enabling delicate items
to be formed from kaolin of poor plasticity, which would normally be
expected to require clays of extremely good plasticity. Weiss (1963)
showed that by mixing kaolinite with urea and aging it “The kaolinite
crystals did not dissolve, but the urea-based chemicals penetrated into
the crystal lattice and increased the distance between the kaolinite
layers from 7.2 to 10.7 A.  Rheological behavior of kaolin does indeed
increase strongly with this pretreatment. A kaolin of low thixotropy
subjected to this treatment yields a material surpassing the best ceramic
kaolins and approximating sodium bentonites.

 

Regards, Peter

 

 

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