What kind of Clay to use for large bonsai containers

Hello all,

I'm taking a ceramics class to lean how to make pots for some of my trees. I have been able to make some pots but they all seem to crack during bisque firing (electric). I have been using Laguna clay (Bmix 10 with grog) and a Laguna tile clay (#30 stoneware) without success. I have basicly made rectangle containers (19"w X 13"d X 4"h). Is there some special or secret clay that should be used for larger sized containers? I would love to find a clay that will consistently work for my application.

Thanks in advance for any advise

Randy

Are the cracks along seams or are they across the slabs? If it's the latter, are you sure you are giving them enough time to dry before firing? I'd imagine your slabs are quite thick and if they still have any moisture deep inside when firing, that will want to escape as steam.

Randy, there is a learning curve so don't fall in love with any of the pots before they are finished I don't use Laguna but the Tile clay should be more forgiving than B-mix. At what stage are you joining and are the parts at equal moisture level? What type joints are you making, butt or bevel? Are you reinforcing the seams with small coils? Even when everything goes right with the application of all the techniques there is still the learning of "touch" How to handle the clay that only comes through repetition and reflection. Hang in there and keep going! you'll get there. That first year separates the "men from the boys" Have fun with it.
Wood

Justin: The cracks all seem to appear near a corner but not on the corner and not on a joining seam. I have done 3 pots the same size and all three have cracked in the same manner. I have been using butt joints and making the pots in a wood framework for support while I'm building the pot. I think the pots have been dry before bisque firing as they sat drying for 3 weeks to a month or more. I wasn't sure if special clays were used for larger pots as I have seen some that are quite course with large grog particles but then, not all of them. The Japanese and Chinese pots that I have are rather quite fine particled clay but the method of manufacture is somewhat different (plaster form vs. slab construction. In doing some reading it could be the placement in the kiln, uneven firing or too rapid a rise or fall during heating and cooling during the quartz inversion process in the clay itself. At the moment it's a mystery but I'm going to try different methods (wall thickness) and firing and see how it goes. The problem seems to be more with square or rectangle shapes rather than ovals or rounded surfaces. I guess it's just the learning process that one must survive before it becomes more straight forward. I do however, love being able to make pots for my trees rather than having to hunt for them and then pay an arm and a leg.

thanks for your input.
Randy

Based on my somewhat limited experience, there are many factors at play. I've made most of my bonsai pots with Standard Ceramics #507 Warm Brown. It's got lots of grog and seems pretty tolerant. It also produces some interesting results with salt (and probably wood) firing.

Anyway, dragging out the construction can sometimes be problematic, unless all the clay or slab is stored in a common container so the moisture content is consistent. If I work on a large slab built piece, I usually try to roll out all the slab I will need, interleave it with newspaper and put it in one plastic bag. (I do my stuff at a local community college, so access times are limited.) Another thing is open edges, like rims, tend to dry faster than large expanses like bottoms, especially if the pot is stored bottom down. Once a pot gets dry enough to handle without being too floppy, it's not a bad idea to invert it, resting on the rim. That tends to hold some moisture in the rim and let more moisture escape from the bottom and feet and lower sides. I've not tried it, but I recently heard a suggestion to dampen a single layer of cheese cloth and wrap it over a workpiece as a way of evening out moisture.

The other trick is patience -- dry very slowly. Even in a typical plastic bag, pieces seem to eventually dry, but slow enough to keep shrinkage evened out. If I am still playing around with shaping/carving whatever, I often stick a fairly wet sponge in the bag to keep moisture content high.

After a re-reading -- you mention using a box form. Is your piece inside the box or draped over a box? Even in the transition from clay out of the bag to leather hard there can be a few percent shrinkage. If the clay is over the outside of a form, the form will begin to push against the clay as it dries and shrinks. If it's inside a box, all should be well, as the piece can just pull away from the form as it dries.

The way various structural components intersect can create problematic stress concentrations too. I can't offer too many suggestions there other than careful observation and trial and error.

Very slow ramping up of kiln temperature, and "candling" by holding temperature just below the boiling point for some period of time could also reduce problems when firing.

(Other than that, it's easy! )

Bear with it -- every day is a learning experience!

DaveT

Hello Randy

May I ask how you prepare your clay before working with it?

I personally think that this is one of the main reasons for damage (before and while firing).

Kind regards
Martin

Klaudia & Martin wrote:Hello Randy

May I ask how you prepare your clay before working with it?


Kind regards
Martin

Martin,

I'm not sure what you mean by preparation of the clay? I have not been doing anything in particular with it. I've been reading some other threads and have come to the conclusion that drying is the area that I need to work on, i.e. doing it slowly, very slowly! This is one of those things I guess that just takes time to gather a feel for the material.

Hello Randy

I mean the preparation of the clay before you use it .... the kneading and wedging.

Kind regards
Martin