Question to the potters

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Question to the potters

Post  my nellie on Wed Mar 05, 2014 10:24 am

I would like to know what is the difference between clay pots and stoneware pots.
I only know stoneware are fired at higher temperatures and I would like to know some more details, if any...
Do the differences have any impact on pricing?

Thank you in advance.

my nellie
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Re: Question to the potters

Post  prestontolbert on Thu Mar 06, 2014 5:14 am

My Nellie-
    All pottery is made of clay.  It is broken down into many subcategories.  For sake of simplicity I will narrow them down to 3: low fire, midrange, and high fire.  Low fire includes terra cotta, American Raku, majolica, and earthenware. The temperature range is between 1200 and 2100 degrees Fahrenheit. With few exceptions low fire is not frost proof and is not acceptable for bonsai.  Midrange includes most Tokoname pottery and most electric fired pottery.  The temperature range is between about 2180 F and 2250 F.  High fire pottery includes some electric fired and most gas and wood fired pottery. The temp range is between 2300 F and 2450 F.  Sara Rayner, Dale Cochoy, Ron Lang, Robert Wallace and quite a few other western potters fire in this range.    

Generally speaking any clay that is vitrified (glass-like with very low absorption) should be frost proof.  However, pottery fired above 2300 is usually much more durable.  Rodney Clemons told me one time that Sara Rayner pots are as tough as iron.  While this is an exaggeration, it is true that high fired stoneware is generally the toughest pottery.  

All midrange pottery should fall into the category "stoneware".  High fired pottery is either stoneware or porcelain.  True porcelain is fired to 2350 or higher.  Porcelain can be used for bonsai pots, but they are much more fragile.  Porcelain is much more vitreous than stoneware making it frost proof, but more easily shattered.  My wife uses a very vitreous porcelain that is translucent. Smaller bonsai pots can be made of porcelain, but larger bonsai pots are really difficult to make (think $$$) and setting it down on a concrete bench with a large tree in it could spell disaster.
-Preston

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Re: Question to the potters

Post  my nellie on Thu Mar 06, 2014 8:39 am

Dear Preston,
Thank you very much for your time and the detailed answer.

my nellie
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Re: Question to the potters

Post  Khaimraj Seepersad on Tue Mar 18, 2014 9:25 pm

Alexandra,

you can have vitreous pots at cone 010 to 08 [ say 900 to 983 deg.C ]
If you get interested or have a potter friend who is willing to try, see the work of Richard Behrens [ books ]
You can also have translucent, white pots as well, as I noted a few weeks ago on Clayart, at around 1100 deg.c.
So you should be able to make Bonsai pots at much lower temperatures, that handle winter and for much less $$$.

Today, it isn't important to really use stoneware or porcelain clay, just know what temperature you expect durability from and with what use.
The rules changed some 50 or so years ago, but many potters are not trained in Pottery Chemistry and still just buy bags of clay and pre-made glazes, from clay suppliers.
Later.
Khaimraj

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Re: Question to the potters

Post  Marty Weiser on Wed Mar 19, 2014 3:15 am

Pottery becomes vitreous due to the formation of glass in the clay body during firing. The glass melts and bonds the other particles together. In most cases this also reduces the porosity of the pottery to the point where it absorbs very little water and therefore resists cracking as the water freezes (water is one of the few materials that expands as it freezes).

Vitrification can occur at a wide range of temperatures as Khaimraj has stated. There are two reasons that most vitreous pottery is fired at fairly high temperatures (generally cone 5 - 12 where the cone measures time at temperature).
- Tradition. That is what they learned and what is readily available. I have moved from cone 9-10 down to cone 5 by adapting my methods when I get a chance to do some pottery.
- Physical properties. They lower fire vitreous systems tend to be more sensitive to the combination of time and temperature since the fluxes used to create the glass are more aggressive and they must be present in higher quantities. This makes it harder to maintain the shape of the pot, particularly since clays are aluminoslilicates that quite resistant to vitrification at low temperatures, and it is the clay that allows you to form a nice shape.

I envision that frost resistant bonsai pots will be made from lower fire clay systems in the future as we develop our techniques and become more familiar with them.

Marty
BS Ceramic Engr.
MS & PhD Materials Science - focus on sintering (what happens when you don't get vitrification)

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Re: Question to the potters

Post  prestontolbert on Tue Mar 25, 2014 3:27 am

Khaimraj-
There are potters that have made translucent "porcelain" at much lower temps than 1100c I have seen it fired as low as 950c. My point wasn't vitreosity, it was durability. Vitreous lowfire and midrange is generally not as durable as highfire stoneware. Marty is exactly right that the difficulty in vitreous low fire clays is in precise firing. I have used a cone 04 vitreous black clay that is underfired at 1910 F and slumps at 1940 F. My cone 10 clay has a much wider firing range. It is mature at cone 10, and I have yet to overfire it, even at cone 13.

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Re: Question to the potters

Post  Khaimraj Seepersad on Tue Mar 25, 2014 12:22 pm

Preston,

three situations, yes, translucence can be had at low temperatures, but if you had asked for a recipe, I would have had to say, sorry, personal research and recipes, so I left a direction to a public recipe.

Secondly, with no winter, the home recipes I use give chemical durability, as I have pots in use for over twenty years and no failures.

Thirdly, the Japanese and Chinese originally used a high fired porous earthenware, as guidelines for use in books, started with soak the pot before using for planting. For us, this is excellent advice as the J.B.pine in our climate prefers a porous pot.
The practice used to be to put your trees away for winter, with the older books showing how they did it.
Today, I guess laziness has set in?????????????
Thanks for responding,
Apologies for the deception.
Khaimraj


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Re: Question to the potters

Post  prestontolbert on Tue Mar 25, 2014 1:28 pm

Khaimraj-
Where is the deception? I'm most definitely not looking for any recipes.

I think we have a breakdown in terminology. "high fired porous earthenware" "High fired" generally means it was fired above cone 8, cone 6 at the lowest. The highest I have seen any "earthenware" fired was cone 6, and it was most definitely not porous.

An excellent example of durability occurred at a conference last week. I had a cup in my backpack, and while looking for something, dropped it from about four feet to the concrete. Though it did crack from rim to foot, when I tried to finish it off and break it the rest of the way, I wasn't able to. I passed it around a group to try, and they were all amazed that it was still very solid and amazingly water tight. Earthenware, midrange, or porcelain would have simply shattered.

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Re: Question to the potters

Post  Khaimraj Seepersad on Tue Mar 25, 2014 1:39 pm

Preston,

earthenware, can often go to 1170 or 1180 deg.C and it begins to deform. If you fire to say 1130 or 1140 deg.c it will be high fired.

Sub-stoneware is how our clays are listed, they will fire well, at just below 1170 deg.c and after that will deform.

I can also drop my 980 deg.C Egyptian paste fritware as a mug from 3 feet and it will not break. It is not porous and also works outdoors as a bonsai pot. Normal body thickness as a mug, and my floors are terrazzo.

Just in case you mis-understood, I was the one doing the deception. Apologies again.
Later.
Khaimraj

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Re: Question to the potters

Post  prestontolbert on Tue Mar 25, 2014 2:30 pm

I understand that you think you are doing the deception, I just don't see where you are trying to deceive me. My point about terminology is that when you fire a clay to the point of slumping, it isn't porous (unless you consider bloating porous). I'm not try to get into a spitting contest about "from how high can you drop your mug"  Laughing 
Industrial ceramic insulators need to be very durable, impervious to the elements, and inexpensive. The engineers want to keep costs down, and firing costs are higher than material costs. The low range that insulators are fired to is 1200c. Most are fired between 1300c and 1400c.
-Cheers, Preston

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Re: Question to the potters

Post  Khaimraj Seepersad on Tue Mar 25, 2014 4:31 pm

Preston,

just to clarify - earthenware clay is "EARTHENWARE CLAY: Natural low-fire secondary clay - fluxed with iron, fires porous. Often called "common" clay, found almost everywhere, matures below 2000 degrees F. " [ 1093.33 deg.C]

source -http://iweb.tntech.edu/wpitelka/syllabi-handouts/handouts/glossary_of_ceramic_terminology.htm

______________________________________

Not talking about firing a clay to below it's natural vitrification point, so it remains over 5 to 8 % porous as the body
goes.

When I left the information in my first post, it was to give Alexandra, a choice. Greece has many potters, and she lives somewhere between zone 9 to 11. Very mild winters.

So to save money, she could find a potter willing to fire a body designed to fire at a lower temperature and be vitreous, hopefully durable to frost. Marty has agreed with the idea.

She could also ask someone to do Attic ware, for a more local feel.

Then she would probably just have to worry about when not to water, and have her soil possibly expand.
Hope this clarifies.
Khaimraj

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Re: Question to the potters

Post  little wing on Wed Apr 09, 2014 10:02 am

Say i have a beautiful pot and want to use for a little tree some day but the inside is not poris. Can i sand it or something to make it suitable for a tree ? Bet someone is laughing at me now but Im really asking.

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Re: Question to the potters

Post  JimLewis on Wed Apr 09, 2014 12:26 pm

If it has a decent sized drainage hole, it won't matter whether the inside is porous (note spelling!) or not. Porcelain pots are as good as any.

_________________
Jim Lewis - lewisjk@windstream.net - Western NC - People, when Columbus discovered this country, it was plumb full of nuts and berries. And I'm right here to tell you the berries are just about all gone. Uncle Dave Macon, old-time country musician

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Re: Question to the potters

Post  little wing on Wed Apr 09, 2014 1:55 pm

Thankyou for your correction and patience with your answer. My first bonsi club meet next week. Lucky me to have people on this fourm to answer my odd questions. Inspiring.

little wing
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