Post 11 Glaze layering

Of all the glazes I trialled recently there were only a very few that were compelling and I mean irresistibly compelling. That’s got to be the criteria, not just ok. I have to admit though, the longer I look at some of them the better they look. Its a bit like looking at a paint swatch and wondering what it would look like in the wall. I did get a couple of clear glazes that worked out very well and if ever I need one, I now have a good choice.

Layering of glazes gives interesting results and adds an additional spontaneous flavour to the outcome. It also offers an alternative to a homogeneous surface finish and colour from a single glaze finish. Testing a number of combinations will definitely be a priority for the next tests. Amaco offer a great range of commercial glazes and have a full pictorial offering to demonstrate the results that can be achieved with glaze layering. http://www.amaco.com/amaco-school/layering-amaco-glazes/  The results have as much if not more to do with chemistry as colour.

And so it can only be back to the drawing board. I’ve looked at the recipes and where they were too glossy, or too transparent or lacked colour, made adjustments for another trial set. All it takes is a few hours and kilos of clay to produce another few dozen test tiles. That’s the easy part, before the weighing out of the multiple ingredients for each test glaze.

While getting ready for this I’ve produced another couple of pots too and so now have the kiln set for a bisque firing with three pots and test tiles to go. 

Here is a picture of a visitor to my garden this morning.

Post 10 Glaze trial results

This trial firing very much confirmed my expectations that in testing glaze formulae it is much more about the local source of the ingredients and the firing of the kiln itself, than the recipe, which deternmins the outcome. Many many of the tests produced results that were quite unexpected. Surface finish of glazes that were suggested to be satin or matte ended up high gloss and in very few cases were the colours as expected. Certainly where ingredients have different chemical compositions the colourants will behave differently.
I do now have a few formulations however that I will use on the next set of pots. The following picture show a set of the more prospective;

One of the goals I had in going into this set of trials was to find glazes that simulated antique bronze or cast/wrought iron colouration and surface finish. I now have a few recipes that fit the bill. In preparing for the firing I made up a few small figures from medieval or ancient history to give the glaze some context.

This is the first in the shape of a lion. Nice glaze with metalic look and feel and a certain suggestion of tarnish.

And the second is a copy of a Roman sheep figure in a slightly glossier glaze although the test tile itself is a little more satin.

 

A copy of an Egyptian sphinx with a greenish tinted glaze.

 

And finally another Egyptian figure - of a sheep.

There's no doubt that even in application the glazes look different for the test tiles.

The following are a number of the prospects I'll now be working on for final pot glazing.

 This rather exhausting testing does raise almost more questions than it answers. With the formulation variability of raw ingredients the focus has to be on the target surface finish as a first priority and then the colourants second. All that means is that this testing process is just beginning. This has just been stage one of an unknown number in the series!

Post 9 The big oval pot

In Post 7 I covered the making of a new mould - which is quite a deep oval. It took a couple of weeks for the mould to finally dry and now I've pulled the first stoneware pot. The clay has a little iron in it so at this stage has a pink tone but will finally fire to a buff brown. You can see the colour in the pots of Post 5.
In this picture it is very close to bone dry and has already shrunk about 4%.  For those into the detail, at this point it is about 455 x 340 x 108. I think it will finish up after the final firing at about 420x320x100. 

In the conditions that prevail here I like to have a little more than the minimum potting medium for my trees to help them retain some moisture through the summer days, so am quite happy to have the deeper pot. Using the same mould I'll also be able to turn out more shallow pots as well as be able to apply surface profiling after they are pulled.

This is pot number 14. With an oval pot the drying stresses are better spread giving better stability or at least less noticeable distortion. Now I need to get on and make another 3 or 4 to fill the kiln for a bisque fire.

With this post the history has been covered and the blog is up to date. Tomorrow I'm going to have a glaze run to fire those test tiles from Post 8.

Post 8 - Glaze trials

I’d been collecting recipes over the last year and put together a ‘shortlist’ of about 90 coloured and clear glaze recipes. The time has come to ‘just do it’. I want to put together a group of glazes that I can work with and which give me the tonal and textural qualities I’m looking for.

The first choice to be made aloong with clay selection is glaze firing temperature – in ceramic terms the cone number. Now there is a fascinating science in itself. I’ve decided on cone 6 glaze firing at about 1200 to 1240C. This is a good compromise of clay characteristics, kiln element life and glaze formulation.

On the table in the picture below are the first ~50 all weighed out and ready to water mix and then apply to small test tiles. With a precision scale you can mix small quantities easily for trial purposes and not feel like you are wasting too much material. It’s an essential step to build that glaze portfolio. A glaze needs to be tested using the ingredients you can get and fired in the kiln you are going to use. 

 

One more batch after this set and then I'll be ready for a kiln run. I’ll post some pictures of the test when complete.

Here are some pictures of some of the glazes I'm trialling:

  Pete's matte

 Shino

 Tomato red

 Bright sky blue 

Post 7 - Making a new oval mould

For some time I'd been thinking about developing a mould for an oval pot. In January this year I had some time on my hands at last and decided that it was time.

The first step is to make the equivalent of a casting pattern – a dummy of the final pot to be produced. This starts with a kind of armature over which a veneer of clay is positioned and then shaped to create the outer surface. Once this is done it is then a simple matter of building a dam around the pattern and covering it with plaster. Getting a good seal all ‘round is important.

 

Here it is just after the plaster is poured.

 

And then with the dam removed and the mould inverted into its normal orientation.

The oval shape you can see is the base of the armature which served as the template for the oval shape of the top rim of the pot.

 

This is the armature coming out to reveal the clay veneer which formed the wall shape.

 

Here the clay is being pulled easily away from the wall of the mould.

 Leaving a nice clean smooth surface for future pot moulding.

The mould inner dimensions are about 470 x 350 x 115 – remember it will come down by 15% to the finished fired and glazed pot.

As in slip-casting the plaster works by pulling moisture from the clay. It accelerates the initial drying and initiates the first of the body shrinkage. This releases the new pot from the mould. For this to work the mould has to be thoroughly dry before it can be used for the first time and that will take a couple of weeks.

Next on the agenda is the rather daunting task of glaze testing – about 100 different recipes coming up.

Post 6 - Japanese Box & new stoneware pots

Here’s the second glazed pot from Post 5 put into service with a Japanese Box.

The size of the Australian ceramics market clearly keeps a lid on the number and range of readymade commercial glaze offerings, particularly for higher fired clays. The stoneware choice for pots is pretty non-negotiable so I need to do more about making my own glazes. Watch this space.

I never really liked the mould pattern I used for my first mould – the one with the sharp corners. So last year I took to it and filled in those corners and reprofiled so both the corners and rim flange were nicely curved. That was harder than it sounds. When you pour fresh plaster on old the old sucks the moisture out of the plaster very quickly and then the new plaster sets rock hard – very hard to shape in a tight position.

 

This is a completely bone dry example of a pot from the reprofiled mould - much prettier. As you see also I’m climbing the learning curve and getting better dimensional stability. I usually give them a little light sanding before the bisque firing to remove little imperfections and soften sharp edges. It’s sometimes easier to do it this way than when the clay is moist. After bisque firing there is another chance with the sand paper, when it works a bit like hardwood.

These pots will finish off at about 370x280x85.

 

 

This is another of the same vintage from mould 2. Again this is starting to look like a marketable product, much more flat, straight and square. Similar size as the one above.

These pots have now been bisque fired and will now wait unitl I test some home mixed glazes and make a selection for use on the pots.

Post 5 - Glazed stoneware pots

Somewhere along the way the dreaded work got in the way and here we are by now in November 2011.

Making pots is still the focus and for that first glaze firing I applied some of those previously mentioned commercial glazes to a couple of my stoneware pots. These are not small pots and so dipping is out of the question – the volume of glaze needed would be huge. I masked the areas I wanted to remain unglazed (regular masking tape), like the feet and then applied the glaze with a soft brush; three coats.

 

 

  

This one was with a Northcote glaze – SG277 Mottled Blue Grey. As you see more gloss than not. Note also the crack in the pot on the far side. This happened in the bisque firing and I tried to use the glaze as a filler and bonding agent, which worked reasonably well. Next time I’d try to pressure inject the glaze into the crack. Clay is interesting stuff – it has a strong positional memory. Unless it is ‘well trained’ when still mobile it can and will struggle against the position you put it in to go back to where it was, right through to final firing. So the learning here is train it well and then don’t leave it too late in the air drying process to make a correction to the shape or those stresses you impose late will find a release in firing.

 

 This one was with a Northcote glaze – SG277 Mottled Blue Grey two coats with a top coat of SG624 Jade. An interesting mix of colours and closer to a satin finish.

 

 And this one is with three coats of the Northcote SG624 Jade. It was fired to 1240C and came out quite matte.

The three pots demonstrate the challenge of maintaining dimensional stability from shaping through drying and then firing. Through all that time the pot is shrinking – first through the loss of moisture and then through vitrification – by about 15%. So don’t be surprised when you look at hand made pots which have funny bends and deformations. And the bigger the pot the greater the challenge. Different manufacturing processes for factory made pots are designed for greater scale, faster production and greater stability. The craftsmanship in a handmade pot is in its individuality of form and finish, it is in the bumps an bends and happenstance. I can provide that individuality right now; the straight, square, aligned pot may take a little practice, but it is the goal. I talk to my kids about being really good at doing anything as long as you put in your 10,000 hrs of practice. I might be really good at it by the time I don’t need to make any more!

Post 4 - Glaze testing

Conventional advice is to begin glazing with commercial mixes and that is the approach I took. By this time I’d also pretty much committed to one type of stoneware clay as the best for the next stage of exploration.

Now an electrical kiln operates in an atmosphere where the oxygen in the heated air is at normal levels (oxidising) and so various components of a glaze when in molten form tend to be relatively more stable than the alternative. In a fired kiln (timber or gas) the oxygen is consumed and the atmosphere in the kiln is a reducing one – which tends to extract the oxygen out of many glaze components which are basically metallic oxides. This can cause interesting colour responses. With modern analytical tools and a thorough understanding of the interaction between the key components it is possible to achieve just as interesting results in an oxidising atmosphere.

Glazes are amazing mixtures of clay, minerals, silica and metallic oxides nearly all of which have individual melting temperatures well beyond the maturity or vitrification temperature of clay. The really interesting thing is that when you mix these powders together, they then melt at a temperature below the vitrification temperature. These eutectic compositions have been the subject of plenty of scientific investigation and are well understood. So the ancients did pretty well to get started on that path in the 8^th or 9^th century BC.

That makes me a late starter and here is my first  effort at testing variously sourced glaze recipes. At this point I’m principally chasing glaze formulations which have a matte or preferably satin finish. I’m not keen on the shine of high gloss and the satin surface is just as easy to keep clean. That said there are some pots which look quite good in gloss. It’s an interesting call to make.

Just for fun I thought I’d post my winged griffin. Modelled on an old European stove tile - clear Italian Green gloss over terracotta.