Cracking in drying

[vgarth]

Does anyone want to offer advice? The clay is Feeneys red raku, rolled out with a rolling pin, cut to a template, joined with slip, left standing on newspaper on a board, with some small blocks of same clay between feet and under middle to support it, in a slightly open plastic bag in my teacher's workshop in country Victoria, to dry slowly. Cool damp weather. Came back to this.

P1010111.JPG

I know Pat Kennedy would say bang the clay out a bit, then slice it, don't roll it, so I'll try that next.

The suiban done by the same method, dried just fine, then cracked in the bisque, (still at my teacher's).

P1010116.JPG

This is a Feeneys white earthenware, and had an oval flat sheet of same clay under it, inside the feet, same thickness, to support it, but that oval seems to have warped in the firing,crackig the pot.

I only used earthenware cos that came with the class. I intended to learn the methods, then apply to stoneware at home. But although I will use Feeneys BRT for many pots, I want a porcelain for a suiban, and that won't have the grog of BRT.

Most of my wheel thrown pots don't crack, most of my (bigger) handbuilt do. I can make mini and shohin pots that don't crack, but I'm trying to move uo a bit. Any advice would be received with thanks - oh, and if I make something without feet it's usually a success, but bonsai need feet. How is it done
Val Garth

[pjkatich]

How big are these pot Val?

From personal experience, the bigger they are the harder they fall.

Once you get over approximately 36 cm in length, the dynamics of the drying process start to take their toll. Especially with slab built pieces and the standard, store bought clay bodies don't perform very well.

I have had similar problems in my quest to produce larger pots.

A few things to consider:

Make sure that the moisture content of all your pieces are the same before you join them.
Try an avoid picking up your work unless it is properly supported.
This is very important, you can never dry a pot to slowly. Uneven drying is your biggest enemy on larger pots.
For larger pots, you will need to amend (add grog) or change your clay body.

I hope this helps.

Cheers,
Paul

[vgarth]

Thanks Paul, I'll add those points to the things I'm doing.

[vgarth]

Sorry, you asked how big - the bonsai pot was about 30 centimetres, or a bit more (can't measure now, it's back in the clay bucket). The suiban just over 60. (As big as I could fit in teacher's kiln!)

[Justin Hervey]

Hi Val
I would drop someone like Peter Krebs a question or two, a very experienced artist and is very helpful:
http://www.peter-krebs.de/

[vgarth]

Thanks Justin,
Val Garth

[pjkatich]

Sorry, you asked how big - the bonsai pot was about 30 centimetres, or a bit more (can't measure now, it's back in the clay bucket). The suiban just over 60. (As big as I could fit in teacher's kiln!)

G'day Val,

Based on the size of the suiban, I would recommend using a clay body that has more grog.

I have talked to several bonsai potters here in the US about these types of failures. They all have told be the same thing, use a clay body with more grog. All of these potters have their own proprietary clay bodies that they make up themselves or have made for them. Depending on the size of the individual piece, you should be using a clay body with up to 20% grog.

If you don't have the facilities to make your own clay bodies try using a sculptural clay body or even a raku clay body. They are a bit hard on the hands and not very easy to throw but they do perform well on the bigger pots.

I have started amending the store bought clay bodies with additional grog and have had better luck with the bigger pots.

What type of kiln and what cone do you fire?

Cheers,
Paul

[vgarth]

P1010112.JPG

Another view of one of the pots shown above

Thanks again Paul,
It's not thrown pots I have cracking trouble with, so using grog is not a problem & I will. I can throw fairly large pots, and they're pretty successful, though adding feet later often means they then warp in firing. So that's another problem. But thrown pots are pretty much round. One can alter them but not to look like rectangular etc pots.

That bonsai pot shown above is a raku clay, but doesn't feel groggy to me.

I can buy some grogged clays, and will change to a more heavily grogged clay and try that. I wonder if I could use what's called a paperclay for a suiban? I can buy a white porcelain paperclay here, and mabe that would have the same effect as grog. I think its purpose is structural strength for handbuilding.

Problem is, that's for a stoneware firing. In the class I'm making these in, teacher is using a fairly large electric kiln and only firing to earthenware temps, as far as I know. For students, anyway. I could discuss possibility of a stoneware firing with her.

Personally, at home, I have a small, very old electric kiln. I do stoneware or earthenware in it. I have a pyrometer, and haven't used cones. So if I do stoneware, I fire to 1280, & wait there half an hour more, then turn it off. It takes about 18 hours to get to 1280! (I'm sure it used to only take about 13.) Its capacity is only about a cubic foot, (a little less,) and that has to include shelves, props & the pyrometre probe. But even with nothing but a floor shelf in, I can't fit in these sorts of pots. They are just too big for the space between the walls.

I have bought an old gas kiln, more than twice the size of my electric kiln, but having problems getting gas to it. Seems the laws have changed, or something, (since the person I bought it from got it connected) and gasfitters refuse to come. I need a certificate from a special type of gasfitter so the company will deliver gas bottles, but that type of gasfitter is rare, and the two I've been able to contact won't do the job. One is a company that does jobs in the hundres of thousands of dollars bracket. The other seemed to think it was illegal. Again, this is a separate problem, and I'll find a solution somehow, I hope. Legally. I intend to use cones when I start firing it. Then I should be able to fire these bigger pots myself, up to stoneware temps.

I'd never be able to see cones in the little electric kiln. I don't have a stand for it. it's a top opener, and on the shed floor, and the peep hole is lower than eye-level even if I'm down on all fours.

So, I hope that's covered what you asked. I will definitely find groggier clay, and yes, I could knead in more grog, though it's a bit tedious. I have, just a few times, thrown with a highly grogged clay. It did feel rough, but my hands felt lovely later! And those pots didn't have the warping problems in firing. Didn't sag between the feet. So I'll ask my current teacher if there's any chance of my having pots fired to stoneware, and if she can/will, I'll try that clay for handbuilding these bigger pots.

It's great to discuss these things with someone who knows about bonsai and pottery. The pottery teachers I've had access to don't know about bonsai, or bonsai pots. They only know about pots with proper flat bases that sit flat on the shelf. Other friends do sympathise, but don't care about the details. Even much-loved hubby mutters "it's just a hobby" so I can't really talk it over with him. Now I've got some ideas to try.

Many thanks again,
Val Garth

[mudlarkpottery]

Hi Val.
Feeney's Red Raku has plenty of grog for what you want. Your problem looks like uneven drying where the outside edges have dried faster than the centre causing them to want to pull up and puts stress on the middle. The pressure you put on the clay when making the holes (it doesn't need much) wouldn't have helped either. Try to dry your pots upside down and covered in plastic. if the edges are uneven, use something soft to sit them on so the weight is distributed evenly. I use cheap plastic painters' drop sheets from Bunnings and cut them into manageable sizes.
Pat is right about bashing the slab out. You can do that to get the rough height. This compresses the clay and aligns the particles much as you would when throwing a platter. I'm sure you've experienced S-cracks. You then use a roller (I use a large rolling pin), piece of PVC pipe, to get an even flat surface. Make sure you flip the slab each time you roll it. And, as Paul says, make sure the pieces all have the same moisture content before you join them. I use Feeney's RR often - it's easier on the hands than BRT. Mostly, I use Keane's clays. They have a bigger range of suitable clay bodies. Don't use straight earthenware - it's too fine and limp for slab work. You can add sand to what you have for some tooth. That will help - just wedge it in. The extra silica won't affect the clay at low temps.
Penny.

[mudlarkpottery]

I forgot to mention that the crack in the bisqued pot looks a different type. It looks as if one or both sides heated up faster than the middle and expanded again causing stress resulting in fracture. You can tell the difference as this crack is wider at the edge than the drying crack which is wider in the centre. To stop the uneven heating during firing, you need to keep the pot away from whatever source of heating you are using. Put it in the middle of the kiln and place other pots around it to protect it from direct heat. I have props made from sliced bricks which I put alongside my pots as a barrier from the direct flame when I have to place them at the edges of the shelves.
Penny.

[vgarth]

Hi Penny, thanks so much. Really useful advice, like the other I have received. I'll put it all together and post results soon. I'm feeling a lot more hopeful. Cheers,
Val Garth

[pjkatich]

The pottery teachers I've had access to don't know about bonsai, or bonsai pots. They only know about pots with proper flat bases that sit flat on the shelf.

I have experienced the same situation Val.

As bonsai potters, the forms we make are some of the hardest to execute successfully. The low, wide, flat shapes of our pots are very unforgiving and there are many opportunities for failure during the process.

Penny has offered some great advise.

The only other thing I might suggest is during the bisque firing, spread some grog or sand on the shelf. This helps minimize stress cracks as the pot expands and contracts during the firing process. Another thing I do is I will run an 8 hour preheat cycle. This will help drive off any residual free water before it can turn to steam.

Good luck,

Cheers,
Paul

[vgarth]

And thanks again Paul. I'm very touched by the kindness of the people responding to my request. Thanks everyone. Also, I've finally got a Class B gasfitter coming to look at my gas kiln, so perhaps in the foreseeable future, I'll be struggling with firing them myself. I'm now going to look for a way to print this all out, or I'll sit down and hand write out each bit of info, and as I work I'll refer to it and make sure I get it right!
A grateful Val Garth

[MattA]

Hey Val,
Here is a word doc with all the info you need from the thread. I have had a few pots crack while drying now I know a bit more about how why & how to maybe prevent it. Look forward to seeing some more of your work, that first pot would have been a beauty.
Matt

[vgarth]

Beautiful, Matt, thanks very much. And thanks for your opinion of the raku pot. I wil get back to that template when a sinpler rectangle has some success
Val Garth