Saturday, 28 September 2013

Post 123 Making bonsai pots

This picture documents a milestone in number of changes in the way I've been working. What you see here are five bisqued pots about to be glazed.

The three at the front are compound ovals or bowed wall rectangular pots. I've pretty much abandoned making straight walled rectangular pots. They really are a great challenge to make and keep the sides flat and straight. If you don't get it quite right then they very clearly are not right. There's something to do with the unresolved drying stresses at the corners which are fine at bisque firing but then come into play at maturity, causing the right angle corners to come in a little dishing the walls inwards. This can be countered at the right time in the drying cycle by opening out the right angles and then bringing them back again, but it's all a bit subjective, and hard to get just right.

This new shape which can be replicated easily for any size is far more forgiving and the rounded corners distribute the drying stresses much better. Not to mention also that it is a good looking pot shape that will accommodate a wide variety of tree species and styles. I've made a few like this before but it's going to become the new normal from now on.

You will see also that the smaller pot of the front three is a different colour. That's the last pot I've made in RGH, and I hope it is the last I do too. The other four pots are made from Clayworks YG Yellow Stoneware. It's yellow when fresh and a more terra cotta colour at bisque and apparently sandstone colour at maturity. It is a 45# clay and so just a little grittier than RHG but good to work with. Shrinkage has been 5 % wet to dry/bisque. These are the first pots in this clay that I've taken to this stage, and three of them are commission pots so they need to work out.

And the other milestone is the work table and slab roller. As you see this is my universal work space with the slab roller bench at the end. I've made all these pots with slabs rolled out with the roller and it has been a treat to use. Making a wider slab produces a much better result with fewer joins, more easily. Joins are a problem not so much because they fail, which they don't, but because sometimes you just can't entirely incorporate them so they can't be seen in the finished pot.

So it's a matter of new clay, new technique and new shape all about to be tested in a 5 pot glaze firing. Not quite the right experimental process but ................... watch this space!

Sunday, 22 September 2013

Post 122 2013 QBS Exhibition

It was the 2013 Queensland Bonsai Exhibition at the Botanical Gardens Auditorium this weekend.
There were three trees I really liked, two chinese elms and an old Queensland small leaf fig.
The elms are a testament to what can be achieved in 20 years. The fig somewhat older has an amazing layering of ageing and patina on the trunk and branches, The style of the fig is typical of how they grow, much more broom like.

The spot lighting on this elm was great, creating a very complementary shadow on the wall behind.

While I was there I went for a walk in the gardens and reaquainten myself with the succulent area. They have a nice assortment of plants and trees from Madagasgar and thes are two I have admired for many years. It's been a while since I've seen them and they certainly have put on some trunk mass to show great taper. They are not exactly bonsai material with very heavy branchlets and as I recall large leaves when in season but they offer a very interesting structure at at this time of year.

Monday, 16 September 2013

Post 121 Pot drying

Drying the base of pots and at the same time keeping them flat and in the right position can be quite a challenge if you are after a great result.

It has long been my practice when I make the pot feet to make up small pieces of clay of the same thickness to use as supports for the floor of the pot while drying. I use a small single piece of newspaper to separate the pot clay from the support clay so they don't stick together. I then take the same supports through bisque firing and then again final firing.
There is no movement in the clay during bisque firing becaues that's really just elevated drying, so I don't put them under the pot, but definitiely do so for the final glaze firing as the clay is likely to sag if not supported during vitrification.

So that's what I've been doing for a while. I have also dried the pots standing on their feet rather than their rims. It's much easier to support the floor that way and if you stand the pot on its rim you then have to accommodate the shrinkage in the wall height in a similar shrinkage in the support. Trying to do all that in clay is a big job. So I dry them right side up and the shrinkage of the supports matches the feet shrinkage and so there is no relative floor deformation in drying. I should say also that there is no point trying to dry a pot sitting on its rim as the final fired rim stability will be influenced more by how the clay is manipulated when wet and firm than when drying.

That's all good but then as in all things perhaps there is something lost and in this case it is that the drying speed of the floor is now less than the walls. Air comes into contact with both sides of the wall whereas it only touches the inside of the floor. Furthermore the inside of the pot is enclosed compared to the outer wall surface which sees some air exchange with the outside world, even with everything covered in plastic. This is compounded by the supports because there is now more clay mass to dry and all that moisture can only be removed by migrating to the inside of the pot. Not surprising that the walls are dry before the floor. This is gong to set up stresses that may return to crack the pot at the wrong time. It would be really good to eliminate this drying stress and I've been turning my mind to it for some time.

An option I considered and rejected was to place the wet pot on a piece of plaster wall board. That stuff is certainly very effective in pulling water out of clay, but how much is enough and how do you regulate the speed so the floor doesn't dry first. You could just end up with the opposite problem.
Then I started to think about how I could dry the pots on a rigid mesh support that was ventilated from underneath. That way, while covered in a sheet of plastic the moisture content of the air under the pot would equilibrate with the air on the outside of the pot and reduce the drying differential between floor and walls. Rack my brain as I did I couldn't come up with a cheap and easy way to do this.

Back to the thinking.
Lucky I've really only got two important things to think about at the moment - growing bonsai and making pots; what a dilemma! You see an engineer can only cope with so much.
So more thinking and enlightenment observation and perhaps just a little chemical engineering was called for.

You see if I could get some air under the pot it would wick the moisture to the outside. Did I say wick? Is there something else that would serve the same purpose, that would be easier than a piece of rigid mesh and static air? I have known for a long time that newspaper very effectively pulls moisture out of clay and not only that but continues to do so quite a distance from the point of contact - it is a great wick! The answer was right there, I'd just been using the newspaper for something else.

So what I've been trying lately is to set the freshly made pots on 6 pieces of newspaper, on top of a plastic sheet, then cover everything in a plastic bag. The newspaper wicks the moisture from the floor to outside the wall space where it will saturate the air in that space and thus slow the drying of the walls, or at least bring the wall and floor drying closer together. I have 6 pots drying this way right now and so far it seems to be very effective, as well as reducing the overall drying time. With the wick pulling the moisture from the floor you don't have to let poor old slow and challenged equilibrium do the same thing over a longer time. Why 6 pieces. Well no I haven't done the mass transfer calcs but it feels about right. Let's hope our bridges aren't designed the same way.

During drying the air under the bag becomes saturated and no more drying will take place until that air is changed out. Every day I do just that by reversing the bags, inside to outside and the inside surface is always wet, showing the air is saturated, doing it's job. It's important that the newspaper wick is also entirely under the cover bag for obvious reasons.

I don't know why I'm telling you all this, giving away my secrets, but it's a great tip and looks like a new SOP for me. It's probably something that most experienced potters discovered long ago and I'm just catching up. Let's see how they go in firing.
You'll note the colour of the pot in the picture too. Yes a new clay, a yellow staneware that fires a sandstone colour. I've switched after recent problems the reformulated red stoneware.

Friday, 13 September 2013

Post 120 Celtis update

Back in late July I collected a number of Celtis yamadori and showed these two in Post 112.
With our very early spring and rapidly increasing temperatures they have shot and really got underway. We are now getting temperatures in the high 20s C and overnight only down to 15 or 16. But the air is sooo dry that its hard keeping water up to things.

Anyway with the new shoots advancing it was time just to set their direction and slightly reduce the number of them. At any particular node there have been up to 6 shoots and never less than 3.
On this first one I had left two vertical branches which was looking a bit like a rude gesture, so one of them had to go. There was also a short one in the 'front' which needed to go too. I'm still thinking that the remaining 'trunk' will be shortened at some point but for now, and without many lower shoots it is time to let it grow and develop some roots. The next thing to do will be remove the branch remains in the front and rear and get those scars under recovery with good vertical sap flow before doing any more further up. That may take this growing season.

It has some great movement in the trunk and I have been inspired by pictures of Taiwanese Celtis bonsai, styled in a much more 'tropical' than 'traditional' style. So I have something to aim for and a development path in mind; the two important things to settle on when you start a new project.

The second one follows. It resulted in a just above ground level chop and a couple of years of free growth. It was just on a footpath area so I'm guessing it was the local council that did the chop, thanks very much. Fortunately I got there before they came back and did it again.

Like the first one it has freely shot. This preparatory styling was also just about thinning the number of shoots at each node and fanning them outwards. Before starting they were all vertical and heading for the sky. I'm looking forward to seeing how this one goes this summer. My intention now is to let it run for the summer, subject to just how much weight they put on and then before the next season cut each back to a few nodes, to get movement and branching, etc etc. I may even reduce the number of trunks - it is a little crowded there on the left.

The poly boxes are great for this job, thanks to our local fruit and veg man. They are the standard vegetable boxes that come with a close fitting lid. I just glued the lid on and then cut the box in half to make two from one. Cut some drainage holes and they're ready.

Tuesday, 3 September 2013

Post 119 New pots

A few of the latest pots from the kiln.

The first one is a deepish oval for a big old swamp cyprus;  Pot 85  (390 x 280 x 85) glazed in a 'parchment' glaze with a hint of green tones.

This next one, Pot 80 is a round pot of 285 diameter x 75high. It is highly textured. The last one I made like this was an oval pot and it was pretty popular. This on is a little darker but has all the same 'lava' like flow marks on the surface.

At the other end of the scale is this long low oval, Pot 81 (580 x 400 x 44). Another long pot that just fitted in the kiln diagonally. The glaze is slightly more matte than usual and a good brown brown, vs red brown or grey brown colour. The pot has a fair bit of movement in it which will make a good contribution to the final composition.

And then at the other end of the scale is this commissioned shohin, Pot S30. It is a compound oval shape at 185 x 130 x 48, glazed in a nice blue. A nice pot for an Aussie native pine.