Post 23 Clay Memory

I've looked at lots of pictures of pots and at my pots and and other potter's pots and everywhere I see the evidence of that intractable phenemon of clay memory. It behaves just like any other plastic material - to get it to where you want it you have to take it past an elastic limit so it comes back part of the way to where you want it. Just like bending a piece of wire which is why using wire to shape bonsai branches can be problematic too. You have to bend the wire, and the branch, past where you want it to finish up, putting the branch at risk in the bending.

In shaping a piece of clay into the form you want, depending on how wet (elastic) it is at the time, every change in position will leave residual stresses in the material which will only be relieved if and when the material is soft enough to move again. The time for that is up near the clay's final curing temperature and that's the time when it might move, to yield to the stresses and set up in a position you may not have planned.

The coaster below is a great example. After working it was dead flat, also at bone dry and dead flat after bisque only to move in the final glaze firing. It was fired sitting flat on a shelf so the corner actually lifted during firing.

As you can see the corners from left to top are ok but that right hand edge has lifted. It has to have happened as I lifted it off the table from working it, put a small bend in it and then pushed it flat again as I set it down to dry, but in doing so locking in a memory about being just that little bit higher than the other corners. Arguably I should have taken it beyond flat and then back to flat before setting it to dry.

This stuff is much more metallic in its behavour than you would imagine and perhaps thats not a bad mindset to adopt. How to apply that learning to potmaking. Well there's something there about drying and positional changes as the clay dries. Dry it slowly so there are no moisture differential stresses locked in and any shape adjustments need to be made early and to go beyond and come back; perhaps. I've got a couple of pots drying right now and have them well wrapped. The first of these was made over two weeks ago and I'm going to give it more time yet. The experiments go on!

Post 22 New mould part 2

After a day or so the plaster has a bit more strength and it is ready to flip over. Here it is again with the dam removed, still a little rough around the edges but then what the outside looks like is not so important.

Once inverted it is simply a matter of removing the armature which is easy with the thin layer of clay between it and the mould.

With the armature removed you can see the moulded clay which then just peels away from the paster. With a smaller pot its probably just as easy to shape a full block of clay as use an armature but at this size (430mm length) this is a more comfortable way to go.

The plaster is still very wet and quite soft. Best now to leave it entirely alone for a few weeks to really dry out. It can't be used until then anyway. Once it's dry it is more easily touched up with the odd scrape or sanding. I'm really looking forward to turning a pot out of this mould but will just have to occupy myself with other challenges for now.

Post 21 New Oval Pot Mould

I promised a few shots of the development of my next mould that I have been wanting to make.

The model for this pot is the one in the following shot.

It is a great ficus and the pot is beautifully suited to the trunk mass and inertia of the tree. I'd like to lay claim to it but unfortumately it's certainly not one I've got in my backyard. The pot has great proportions and visual appeal with just enough relief decoration to not distract from the tree.

So the place to start working this one up is to make what I would call an armiture, in sculptural terms. I first cut out the upper and lower oval shapes from a thin sheet of fibre board. These are then mounted either side of a shaped framework representing the body of the pot. I wrapped the framework with masking tape to provide a surface to clad with clay to shape the mould profile. This doesn't need to be too sophisticated - just robust enough to support the clay during shaping.

This is what the sandwich armiture then looks like. As you see it starts to look like the pot upside down.

 Then the armiture is packed out with clay - in this case just a good fine white terracotta clay. Any fine clay will do the job.

 Then a custom scraper, crafted to replicate the side profile of the final pot, is used around the outside surface to remove excess clay and cut down to the right size with a nice smooth surface. This shot shows the start of that process.

This shows what it looks like when the profiling cut is finished and ready to cast.

And if you invert a picture you begin to see what the body of the final pot will look like.

This is now ready to cast so it's time to get the dam in place. Obviously it is very important to make sure the dam is leak proof. There's nothing worse than having plaster squirting out onto the ground and trying to reseal it from the outside. Trust me I speak from experience. Any clay you use as as sealant will be exposed to the moisture in the plaster and water as the plaster dewaters if it does, so care is needed to make sure there is enough in place.

And here it is with the plaster poured into place, sealed nicely this time with no leaks.
Now sit back and reflect on a good job while it firms up.

The dam came come off pretty much after and hour or two but it otherwise it needs to sit for a couple of days undisturbed to firm up even more before removing the armiture.

 

As is usually the case the devil is in the detail and a few carpentry skills and equipment don't go astray as well.

Post 20 Cone 6 Glazes

As I continue to put myself through glaze school it feels more like I'm taking one step forward and ending up two back. Without doubt with each new small understanding you also see just how much further away is the destination. Fortunately in this day and age we can all buy vast amounts of knowledge and experience of anything.
If cone 6 glazes are going to be a point of focus then it was time to invest in Hasselberth and Roy's book. Trial and error just wasn't going to cut it.

This is not a light weight glossy read. It's a pretty technical manual. I've talked about the issue of formulating glazes from ingredients which may have a different chemical basis in different locations. A recipe isn't repeatable unless the molecular composition is repeated. Now John Hesselberth is a retired Chemical Engineer so I'd say he know's what he's talking about. It is very clear after reading the book that blind recipe tryouts have a very low prospect of success and the only way to get close is to understand the composition of the ingredients available to you and then to work out an answer using the Seger glaze analysis conventions. I had a feeling when I set off on this path of formulating my own glazes that it was going to get harder before it got easier. For anyone setting off on the same path, aiming to get a particular result, I'd say you really need to get your head around Seger and the basic chemistry.

Post 19 Latest pot - number 17

This is my latest pot having tweaked a few design elements to get closer to the picture I want to achieve. This one has a deeper foot with lower recess connecting the feet, a deeper flange and greater flange radius. All these little details add to the visual appeal without necessarily being separately apparent. Isn't that always the way.

At the moment it is just leather hard and in need of a little tidying up around the edges. I'll leave those until it is bone dry when the surface is less sensitive to accidental touches. It looks really good at this stage and from experience I think I've got drying sorted to maintain stability; if only they would tightly hold shape through the firing cycles.  If there is any experience to share on that subject I'd love to hear from you.

I'm also starting to work on the bits and pieces for a new oval design and mould; watch this space. This time I'll post a few more pictures of the process as it unfolds.

Post 18 And more new pots

Ok so the last 3 for a while - at least until I make some more. And as I forecast last time, a few more surprises.

The first one is with a glaze I call 'Sea Mist'. As you see a good satin/matte surface and a good foggy blue/green.

  This is a digitalfire.com glaze number G2571A. It is formulated as a cone 10 glaze and so has lots of Custer Feldspar. I've changed it a bit by adding a little extra Silica to get it back into the satin range.

 

Here's a picture of the trial test tiles with three different oxides. Sea Mist is in the middle, no 59. The other two have nice earthy tones, on the left using RIO and Nickel on the right. Sea Mist has 2.5% copper and a little cobalt.
Nice one I'll use it again and reformulate with other colourants.

Next up is derived from Val's Satin Black base and in the test produced a nice mottled green tint. In the pot firing it has come out quite homogeneous with more grey tones. Not unattractive but a surprise. Good surface finish.

And finally for now is one in the test that was quite a mottled rusty red but in the pot firing produced much more of the lighter tones to produce a colouration not unlike a satin terracotta finish. Again not unattractive but another surprise.

All three pots are also around the same size of 380 x 275 x  92, stoneware clay fired to cone 6.
It's coming into winter here in Australia at the moment so my trees need to sit quietly for a couple of months before I can press these pots into service.

These firings have given me quite a lot of information, what it means exactly might take me a little time to analyse! An early observation from a comparison between these pots and the ones I produced last year is that the moisture level of the clay when pressing the pots might be far more influential on stability in glaze firing than you might think. That's something else that I need try to measure and make notes of at the time, in future.

First prioirity is workshop shelves, then back to pressing out some pots. Somewhere in there is making another oval mold too. So much to do and so little time.

Post 17 New bonsai pots

The first 4 have gone through firing and again there are mixed results.
Starting with the two oval pots.

The first one was glazed with a good mid range satin blue.

I'm very happy with the colour and surface. Final dimensions were 425 x 315 x 100 and it fired with minimal deformation, looks good.

The other oval was fired with an overlay glaze, a reasonably transparent dark blue/green over a nutmeg shino. Hard to believe looking at the individual test tiles and the combined result but the overlay on the pot fired very close to the test tile. Final size 415 x 320 x 100

                        

  

 Very nice satin surface and a good mottled colour. This was the first oval out of the mold and it didn't dry as well so a little sag in the pot. All part of the individuality of a hand made pot!

The next one cracked in the bisque firing but I took it to glaze for the test of its survival. Well the crack opened up a little but the pot is still usable, but not for display. This one also developed quite a bit of movement in the firing which I do not understand and would be happy for any advice about that. None the less the glaze turned out really well and I will use it again. Final size 380 x 282 x 91.

 

 The glaze was heavy on the spodumene and both Red Iron Oxide and Yellow Ochre as colourants with some addet Tin Oxide, great mottled colour.

The last one did not fire faithful to the colour test at all. It should have been a cream breaking brown mottle but fired to a clear brown with feint blue speckle, and quite glossy.Final size 373 x 270 x 94.

 Why indeed. It was on the top shelf and may have got a little hotter or the glaze may have been applied a little heavier than the test tile. You can see why all the advice points to finding a glaze that works and sticking to it. Not there yet.

 The final picture just shows the foot detail on the rectangular pots, from underneath.

There are three more in the kiln today for their final firing so more surprises coming up!