Claybody for large scale ceramics

[Maulik Oza]

hi everyone.. just joined in. By profession i am an architect from India and very much interested in ceramics. so started my own workshop where we work in ceramics and handmade papers.

recently started with large sculpture but in firing it got cracked. can anyone help me with the stoneware claybody for large scale ceramics and pots.

waiting for the reply..

NOTE: Replies are posted from oldest to newest, so the most recent posts will be at the bottom of the last page.

[Norm Stuart]

I'd like to hear about other methods and recipes from other artists here.

I've found the key to working large is using a low-expansion "clay body", similar to "flameware" recipes, decorated with matching low-expansion glazes. Much larger ceramic sculptures are often a collection of smaller pieces attached to a metal framework.

This sculpture material requires the use of materials like Pyrophyllite and Spodumene combined with a Smectite like Macaloid to give the material plasticity.

http://www.studiopotter.org/articles/?art=art0017

http://digitalfire.com/4sight/material/pyrotrol_2270.html

http://digitalfire.com/4sight/material/spodumene_1287.html

http://digitalfire.com/4sight/material/macaloid_3052.html

Here's a typical low-expansion "clay body" for use with a metal armature, using a lot of sand or grog:

17.0%  Wollastonite
35.3%  Fire Clay
17.6%  Fine Grog
17.6%  Medium Grog
12.5%  Coarse Grog

Another method is to use a low-expansion refractory cement which provide high levels of greenware strength.

Refractory cement "clay" body

50%  Lumnite Alumina Cement
50%  Clay
http://www.hca.com/index.php?id=15

I've experimented with Calcium Phosphate cement as a sculpting medium, although it's much softer and very quick setting. You mix 1.51 parts of Wollastonite by weight with 1 part of Phosphoric Acid 80% concentration. You can add water to make the mix more plastic, but this reduces the final strength of the ware. It's better to form this as is, and smooth out the surface with water. Unkneaded the reaction of the Wollastonite with the acid produces a low-density material like firebrick, which I primarily use to repair or sculpt replacement firebricks for our electric kiln.

Of course, the other way to work large is to use slip-casting so the final piece is quite thin. Jeff Koons had an Italian ceramics factory manufacture his larger than life "Michael Jackson and Bubbles" ceramic sculpture with slip-cast porcelain. I can only imagine how many of these pieces had to be made in order to create the two final pieces and a third "artist's proof" without cracks or other flaws. One was sold at auction for $5.6 million.

[Norm Stuart]

Wollastonite is a naturally fibrous material, but adding other fibers can also be helpful in providing dry strength to large pieces.

I use 1/2 inch long Nylon Fibers (Dupont 105) from Bailey Ceramics.

http://www.baileypottery.com/clay/clays-chemicals.htm

New Mexico Clay sells a prepared clay with these nylon fibers included.

http://www.nmclay.com/Amazing/items.asp?Cc=Clay-FiberClay&iTpSt...

[Norm Stuart]

Barnaby Barford, a London artist, makes porcelain assemblages on metal armatures.

His current series is a group of seven mirrors and frames called "The Seven Deadly Sins".

His earlier work were assemblages of ceramic pieces cut from commercial figurines.

http://www.videonewsindex.com/play/porcelain-artist-barnaby-barford...

This is a close-up of a section of his mirror titled "Pride".

Gluttony

[Maulik Oza]

hi norm, thank you for your reply.

The problem is i want to throw big pots on wheel as well. so too much of grog in clay body might create a problem in throwing. and other materials which you suggested like fibers and all they are difficult to get in my locality. i am right now trying to use my regular stoneware body with 10 percent addition of fine grog. lets see how it works. i will fire it and get back to you. the whole discussion i have started because i made one piece in my studio and it got cracked in firing. i am attaching some photos of it just have a look. and  if you can tell me that what could be the reason. it will be a good help.

[Norm Stuart]

From the photos your walls are thicker than what I hand-build large pieces with. At 550C a tremendous amount of chemically attached water is released from the clay as steam. This PDF file calls this "Metakaolin Formation". Even if you raise the temperature around 550C very slowly, if the clay is too thick this steam will create cracks or even explode the clay. There is also quartz conversion from α to β-quartz, but this is a different type of cracking problem.

http://claystore.alfred.edu/rawmats/presentations/Raw%20Mats/Class-...

http:// claystore.alfred.edu/rawmats/presentations/Raw%20Mats/Class-8%20%28firing%29.pdf

If you add grog (previously fired clay ground-up), sand and paper pulp to the clay, these will help make the clay more porous allowing the water released at 550C to exit with less damage. Grog and paper pulp will also provide more structural strength while you are building the piece.

Increasing non-clay material like grog, sand, wollastonite ( Ca Si O₃) or paper also solves another problem clay develops. Look at this electron micrograph of kaolin clay and you can see it is made up of plates which tend to line up in a stack against each other when pressure is applied by rolling, throwing, or all by itself during the drying process. As the water is removed between the plates, the height of the stack of plates get much shorter but the clay plates never get less wide. This means when you roll-out a slab of clay, the slab will get thinner as it dries but tends not to shrink much in length or width. Using your arrangement of internal ribs, clay shrinkage could create a cracking problem.

Grog, sand or wollastonite interrupt these plate alignments giving the clay less directional shrinkage. Rolling the clay out in different directions and flipping it over after each roll also helps prevent this structure from forming. But this clay structure can also be helpful if you use it to your advantage.

individual plates of clay stack as they dry

Look at these three examples of prepared clays.

1.) "Danish White" is terrible clay for hand building. It has no grog, sand or paper. http://www.lagunaclay.com/clays/western/wc841.php

2.) "Big White" clay is excellent for hand-building large forms. It has 60 mesh sand and 30 to 60 mesh grog. Once dry, the clay is very porous allowing the water produced at 550C and easy exit. I can even build with clay slabs 20 mm thick without cracking in the kiln. http://www.lagunaclay.com/clays/western/wc381.php

3.) "Max's Paper Clay" is also excellent for hand-building. It has a significant amount of fine and medium grog, and also has about 10% paper pulp. After the paper burns-out, paper-clay will be much less dense. While this allows easy exit for water at 550C, the clay has less strength with the paper missing, which can lead to structural cracking problems. http://www.lagunaclay.com/clays/western/wc953.php

I would ask Jeff Poulter about throwing large pots http://cone6pots.ning.com/profile/JeffPoulter

But clay for throwing large pots has many of the same requirements.

Maulik Oza said:

hi norm, thank you for your reply.

The problem is i want to throw big pots on wheel as well. so too much of grog in clay body might create a problem in throwing. and other materials which you suggested like fibers and all they are difficult to get in my locality. i am right now trying to use my regular stoneware body with 10 percent addition of fine grog. lets see how it works. i will fire it and get back to you. the whole discussion i have started because i made one piece in my studio and it got cracked in firing. i am attaching some photos of it just have a look. and  if you can tell me that what could be the reason. it will be a good help.

[Norm Stuart]

Wollastonite is mined locally in India and is sold by many local suppliers.

http://www.indianyellowpages.com/mineral-metals/minerals/wollastoni...

http://www.garg-mineralandchemicals.com/wollastonite.htm

http://digitalfire.com/4sight/material/fibrous_wollastonite_313.html

[Maulik Oza]

hey norm,

thanks a ton.. for sharing all this information with me. Specially the firing PDF. till now i was doing firing and knew little bit about quartz inversion and all but not in this detail. its definitely gonna make difference in my firing results. and thanks for sharing supplier links for wollastonite. i have asked them before but they don't supply in small quantities but now i think i will ask some ceramic factory guys to give me in small quantity.i am very happy that you are taking out some time to reply my quarries. thank you.