Thursday, 27 December 2012

Post 64 Some pots from my collection

Another Christmas has come and gone and this will be the first for many years that I didn't score a new bonsai pot for a present. For a long time that has been my standard request for and ABC event ( aniversary, birthday, Christmas). At least I can't be accused of being boring anymore- well at least not for that reason.

The motivation for getting into making my own pots was about getting bigger and better for less. I didn't know a lot about pots before making them myself and I think many enthusiasts would be the same.

Most people at some point in their lives will have bought ceramic tiles, for a reno job, a new house or simply repairs. If you reflect on that purchase you might recall the information available about the different types and classes of tiles for different applications and what they may be exposed to.
 There is a clear distinction between non-vitreous, semi vitreous and vitreous tiles. This classifcation has much to do with the type of clay and the firing cycle it may have been matured to. You would chose between them depending on whether they were to be located inside or out, on the floor or the wall etc. There is a clear quality, service capability and price relationship at work with ceramic tiles and you get what you pay for.

Such a pity then that the same level of information is generally not available when purchasing bonsai pots, beacuse the same considerations should apply; what sort of clay was a pot made of, how was it fired and how will it perform in service.

This pot for example is a nice shaped little unglazed oval pot about 370mm long. Before taking the picture I gave it a vigorous wipe down with a damp cloth and then let it dry, no brushing. The pot has been out in the weather for years. It has provided good service but before even thinking about using it for a displayed tree some serious work would be needed. Actually now I wouldn't even think about it. It's just a grow pot and I'm just keeping it long enough until I have one of my own to take its place..

As you see there is a whitish deposit on the surface of the pot. This might be lime scale but I have others of a different origin which do not suffer from that so I am more inclined  to think it is efflorescence, the migration of slightly soluble salts from within the ceramic material to the surface; the sort of thing often seen on new brickwork. This is a reflection on the quality of the original clay body and what may have been added to it for various reasons. I suspect this is a low fired earthenware ceramic material which in tile terms is non or semi vitrified, which gives it a level of porosity that allows water to move through the pot wall and carry salts to the surface. That porosity also allows and supports unattractive biological patina to grow on the pot, futher diffusing the appearance and colour. Unfortunately in the absence of information about where it came from, how it was made and its qualities you don't know if the pots you buy will look like this one in a few years time.

I now know you can tell a low fired pot from a high fired pot by the bell test. Support the pot on a finger through a drainage hole and wrap the pot with a knuckle - to get anywhere from a thunk, to a ping to a resonant bell.  If you don't want a pot that looks like this one then if they don't ring like a bell don't buy them. A drop of moisture on the pot wall will also help you understand its porosity, a key influence on future appearance.
Knowing what I know now and having made and used pots in both earthenware and stoneware, I would never ever buy an earthenware pot again. If it wasn't stoneware I wouldn't touch it no matter what, I'd rather use a plastic pot.

Glazed pots are a different challenge to assess.

 This pot is a shohin sized glazed oval. It also is a few years old and has spent all that time out in the weather. When I bought it the glaze was a most attractive high gloss and you can see where it has run in the firing to give a gradation in colour under the rim. The picture is a truthful illustration of its current state. The glaze now looks like it's been sandblasted to remove the gloss leaving a very dull surface appearance. Once I just thought that a glazed ceramic surface was glasslike and forever. Now I understand the chemistry and that is only so under certain circumstances.

Most folks will at some time in their lives have been in a limestone cave - stalactites and stalagmites etc. They take a long time to get like that and have come about through dissolved limestone redepositing from drops of calcuim carbonite rich water evaporating and leaving the crystaline material behind. That's why the mites and tites sparkle. But of course how did the limestone dissolve in the first place. Well easy really; as rain falls carbon dioxide in the atmosphere dissolves in the rain, turing it into dilute soda water, otherwise known as carbonic acid. Dilute acid will also attack the surface of a glaze, if it is not correctly balanced and stable. Glazes which are used on ceramics for food service by professional suppliers will be formulated to be resistant to attack by dilute acidic food materials; it is possible and simple enough.

So back to the pot. The little blue / green pot glaze is what I call a showroom glaze; looks great in the showroom, but never ever take it out in the weather or it will wash off! Yes it really does start to wash off over time and it's the surface, the reason you bought the pot,  that goes first. I've got a few pots like this, how about you? This is a really difficult issues to deal with at purchase time. The only way to beat it is to know where your pot comes from and how it has been made.

If you are interested in how your unglazed and glazed pots will look after a few years out in the weather and your retail pot supplier offers no information about their pots other than price, then pretend you are buying ceramic tiles and ask more questions. This is why I now make my own but there was a time when I decided to go right up market; next time I'll show you my two Tokonome pots.

Thursday, 20 December 2012

Post 63 Ceramic dovetail

We all know about timber dovetails, those traditional little reverse wedge shapes used to connect pieces of timber in a really strong joint. Dovetails in drawers continue to be the hallmark of a well built and valueable piece of furniture.

But what about stone or ceramic. I got thinking about this recently when a pot came out of a bisque firing with a crack in the side wall. This is only the second time this has happened and it is a bit annoying. After having invested the time and energy to get it here it would be disappointing to throw it away, but to continue to go to a glaze firing would amost certainly result in the crack opening up and rendering the pot almost un-usable. I've looked for but haven't found an approach on the net about how to repair it for a reasonable glaze firing result. If anyone has an answer I'd love to hear it.

I think I know how the crack happened and how to avoid it in future; a question of differential drying stresses.

 In this photo the crack is at about 7 oclock, just left of centre on the rim. Here is a closeup. It would be a nice pot and I was making it for a japanese box forest.

So how to fix it. There are well established practicesboth new and old for repairing broken ceramics, but none of those methods will survive the changes and temperature of firing to maturity. What ever you use has to contract 6 or 7% with the body of the pot, retain the pressure on the crack faces and not burn up at 1230C.

The ancients used metal  'clamps' and dowels to join masonry in the days before reliable cement / mortar technology. The Greeks used pieces of iron set in holes in the masonry around which molten lead was poured to secure the joint, as above.

This one is a double dovetail or bowtie from Egypt at the ancient temple of Dendera

This picture is of the Temple of Apollo at Delphi in Greece and on the foundation blocks you can see the interconnecting iron clamps - installed like staples to keep the blocks together. This was an early technology to earthquake proof a building.

 And for the students of history can anyone tell me what this picture is all about. Well in the chaos of the post Roman world when iron production went into disarray you had to get it wherever you could. So the ancient world's ruins became both stone quarry and iron mine, where it was sometimes easier (cheaper?)  to chisel through the stones in a wall and extract the iron clamp than source freshly produced material.

I do remember too, though could not find a picture, of the stonework on Fort Denison in Sydney Harbour. As I recall stone bowtie joiners were used there to tie together the monumental stonework on the wall of the big gun emplacements. Perhaps this was commonly used in 18th and early 19th C defensive military stonework. If anyone has got a picture of this please send it to me and I'll post it.

 Well all very interesting indeed and mostly well before the time of bonsai so not really relevant. I agree of course.  
But with all that perhaps there is some precedent for me to fix my pot with a bowtie double dovetail clamp made of the same material as the pot.

So on the inside of the pot I've placed a bowtie that is about 2/3s of the wall depth and also drilled a small hole at the limit of the crack to stop propagation. Bisqued clay is nicely workable. Before firing I'll sprinkle a little galze in the base of the bowtie recess and around the join to secure it in position, then glaze right over the top both inside and outside. The crack may still be apparent, but after reaching maturity it will be totally stable and with luck the wall will now not gap open. Time will tell.

That's as far as I've got on this one and will now have to wait for the next firing opportunity. Speaking of production disarray I'm doing a few little renovations in my workshop at the moment and both pot making and firing is in total disarray!

This will likely be my last post for 2012. Season's greetings to one and all.

Sunday, 16 December 2012

Post 62 A branch hidden may be a branch wasted

I have mentioned previously the value I get out of having one of my trees in the house on rotation. Each one gets a turn for a week and I get to look at it every time I go backwards and forwards. It is always hard to find the flaws in your own trees, but then quite surprising some times what you can see when there is only one to look at.

This willowleaf ficus (Nerifolia) has been styled with a well defined branch structure which is a little different from the umbrella canopy style you often see it in. The second branch connects to the trunk on the back of the trunk. Up until now it has been oriented to be pretty much in a sight line directly in line with the trunk, away from the viewer. The foliage pad can be seen and is arranged to provide the background depth to the tree. The prominent space between the left hand branches is too prominent. So why not fill the space and at the same time get some value out of that second rearward facing branch.

The next picture shows the temporary restraint in place to set the branch in a new position. This fills the space a little, provides some visibility of the branch and retains the background foliage pad for depth.

Courtesy of a little photoshopping I can take out the restrainer to more clearly see the final result. It will be a matter of viewing height but a possible problem this creates is the close alignment of the rear branch with the first right-hand branch. This can be resloved at a later stage with a little 'S' bend in the rear branch to lower the visible part by about 10mm to get a result.

I like the way taking a picture of a tree  and thus compressing it into two dimensions sometimes makes it easier to take a uncompromising clinical view about styling You get to see it in a different way. A photo shop like tool is also a good way to move branches around and experiment before you take irreversable steps.

Thursday, 13 December 2012

Post 61 More clay formulation tests

This is going to be a little technical for most folks but for the ceramics affectionados it might carry some interest.

Some time ago I ran a series of tests adding various materials to the clay body I have been using to evaluate the impact on deformation, shrinkage and porosity. Nothing stood out as a winner which was good enough to go to the trouble of adding and combining a dry material to the clay.
The best result came from the addition of Kaolin, but it wasn't good enough to take up the option.

A little further research suggested two further alternative prospects. One is like a calcined version of Kaolin called Molochite and the other is a calcined Alumina. Calcination drives out all forms of water both free and molecular. The composition of Molochite is principally silicon and aluminium oxides and the Alumina clearly just aluminium oxide.

So I designed as set of test samples using these materials at concentrations up to 20% using two Clayworks stoneware bodies - RGH and CTS.

 The photograph above shows the stack of 12 deformation test samples, stacked in order of performance. The RGH samples - darker colour - performed less favourably to the CTS.
CTS also showed less shrinkage which was consistent with the deformation characteristic.
Unfortunately this superior performance was achieved it seems, by the action of the additives in elevating the vitrification temperature of the clay body, demonstrated by relatively poor levels of permeability at increasing levels of the additives. The CTS also had an unsatisfactory permeability when fired to the same schedule as the RGH.

Taking the clay to vitrification temperature is all about achieving impermeability and so to include a refractory additive which might improve the physical dimensional characteristics (stability) at the cost of vitrification and permeadility would be going in the wrong direction. That's likely to be what the Chinese mass bonsai pot market producers seem to be doing. This might be ok if you are making a sculptural object where the dimensional and shrinkage properties have a higher priority but for a bonsai container exposed to the elements it's the wrong end of the compromise stick.

Interesting as the idea of an additive was it's looking like a dead end. Besides incorporating a dry powder into a clay body in sufficient quantity, up to 20%, would be a huge pain in the asp so I'm pleased not to have found a compelling reason to do it. The answer might well be the simple matter of testing just how much heat work is needed to achieve vitrification; where is the tipping point, the elbow in the curve. I'll just have to fire up the test kiln and run some different firing schedules to learn a little more.

Monday, 10 December 2012

Post 60 Moonrock

In the last glaze firing I also fired my latest root over rock, rock; the moonrock.

The colour of the clay itself when fired is quite a good stony buff colour and although once fired the stoneware is as good as impervioue to moisture, I still wanted to put a layer of glaze on it.

The glaze I used is one formulated with yellow ochre  as the colourant which for some reason offers a few more of the brighter red tones than the Red Iron Oxide which at the same level goes more brown.

Not wanting to mix up a large quantity of glaze to dip it, I painted the glaze on, which wasn't easy with all that surface texture. That said I could have used a little more glaze. Even with glazes which are formulated to produce a matte surface, where it is applied thinly it will often darken in tone towards the browns and develop a more shiny surface. So there is a little more shine here than I wanted but all those nooks and crannies will soon catch a little patina. I've got it out in the weather now, just waiting for my tube planted ficus to develop a little more this season.

Tuesday, 4 December 2012

Post 59 Another 4 new pots

Another glaze firing done and some happy results. The kiln gods have smiled again for now. That's a bit like that old one about the harder you work the luckier you get, but I might have said that before!

This first one is Pot 34 a short Taraibune oval at 425 x 310 x 78. The glaze is a light blue tone applied to simulate a little patina. A nice size and very useful pot.

This is my original Wasen design again, Pot 36 with a finished size of 407 x 300 x 84. You might recall some time ago - Post 46 in mid October - I tried something new in a glaze firing and as a direct result caused a Wasen oval pot with the same glaze as this one, to be badly deformed. If I'd really thought about it, the result was predictable, but not at the time. Bob offered to take it off my hands for the feight cost but I relented and decided to keep it where I could see it just to be reminded, about fallibility and the danger of hubris. Speaking of which I put a tree in that pot and unless you really look for it you would hardly notice the flaw. Imperfect perfection - the power of wabi sabi. The antidote might be wearing off.
So here is Pot 36 in the same glaze - light green tones breaking beige and in great shape.

To show the flexibilty of mould made pots here is a Wasen with a squared rim flange; Pot 37 at 412 x 300 x 84. Just a small change but a different look. The picture doesn't do the glaze justice and sometimes with digital photography true colour rendering is elusive. Suffice to say it is formulated as a cream beige glaze but the different depths of application break the surface and colour. There are a number of stoney earthen colours here that make for a great composition. It looks like it's been carved from a piece of stone and buffed. I don't think I can part with this one, at least not before trying to make another one like it.

And finally for this firing we have Pot 39 which is a Tenmasen rectangular pot at 407 x 300 x 84, in a glaze used before too. But it makes such an interesting surface that it's hard to resist using it, making a very handsome pot.

So all in all a good result; all nice additions to a collection.

Sunday, 2 December 2012

Post 58 Bekabune

The Bekabune is another traditional inshore Japanee fishing vessel. This picture from Douglas Brooks' excellent website shows one he made as an apprentice in Japan. The craftsmanship is exceptional. This museum quality build was done entirely with hand tools only - now that's dedication.

The edge joined planks and flush sides of the craft offer enough similarity (never mind the pointy end) to attach the name to my latest pot model.

I made the mould a few weeks ago and this is now the first one produced, pictured full frontal and three quarters, wet clay yet to dry out.

Final fired dimensions for the pot will be about  340 x 260 x 55, so it's a useful intermediate size a little smaller than my other ovals, which are both a little over 400mm long.

In my first design for the pot I toyed with the idea of making it without the rim flange to give a very clean line. But I think the composition works, the proportions are sound and having incorporated the rim flange in the mould gives me the flexibility of making a pot without it. It would be more challenging to do it the other way around.
There are as usual with mould made pots, opportunities for post production alterteration to break the pattern. Things like setting the feet back slightly to break the line, changing the rim flange profile, outer or inner rim flange, wall planking or other textural detail and even pot height; one mould many pots.
It will be good to get a few of these down the line. I have a number of my trees in imported pots of unknown origin which are about this size and they are looking very ordinary. So it will be good to change them out and there will be a bunch of secondhanders on the sale table at the next exhibition.

I have a friend who refers to my kiln as 'the blast furnace' and suggests that every time I fire it his lights dim a little. Ha Ha. Well his lights should be doing just that today with 4 more pots in for glaze firing as well as my moonstone rock which I have glazed with what I'm hoping will be a super matte iron reddish brown glaze.