Merry Christmas!!

To all who celebrate it, I wish you a happy and peaceful Christmas and New Year. I hope all the artists out there have an enjoyable, creative and successful 2012!

Turkey, Christmas pudding, mince pies, chocolates, wine, beer....sigh, the diet will have to wait again until next year. Shame..

Global warming? Really??

Somehow, the thought of plunging my hands into buckets of water and throwing clay doesn't seem very appealing today..

Looks like we might be in for the same kind of winter we had last year and the year before.. absolutely freezing!

John Leach, Potter.

I've just been watching a very pleasant interview with John Leach, the Potter. For anyone who would like an introduction to the world of handmade ceramics, it offers an interesting overview of life in a traditional pottery workshop. John Leach has a fair amount of experience, having been a potter for over fifty years!

More interesting glaze results!

I fired the electric kiln to cone 9 this week and some exciting pots and test pieces have emerged! This large tea bowl was decorated mainly with a red clay and iron oxide slip..the thinner lines were made using the black slip (which contains red clay, manganese dioxide, cobalt oxide, red iron oxide and Masons black stain).

The glaze is a traditional shino comprising mostly of nepheline syenite and it has bubbled vigorously on the outside. What's really interesting is that the same glaze hasn't produced this volcanic effect on the inside, or over the bare clay.

Large tea bowl, fired to cone 9

Large tea bowl, fired to cone 9


The bubbling seems to be caused by a combination of slightly thicker glaze application and higher temperature, but I suspect that the main reason is decomposition of the red iron oxide which releases oxygen and this has to try and escape from the glaze. Red iron oxide Fe2O3 lets go of an oxygen atom at around 2250 F (1232 centigrade) and becomes black iron oxide (FeO). Where the glaze is thicker, and there is a heavier application of slip, I am guessing that the oxygen can't escape the glaze layer so easily and produces bubbles and craters.

In the case of this tea bowl, the bubbles have produced a smooth, sugar-fosted kind of texture, which is very pleasantly tactile, so on the whole, I'm pretty happy with it.

Here are two lidded jars, decorated in a similar way:

Lidded jar, approx. 6 inches tall

Lidded jar, approx. 5 inches tall

I also brushed on more of the orange matt, calcium glaze onto a previously fired vase and it came out anything but matt! It was fired in the saggar under quite heavy reduction and the titanium dioxide has created some wonderful glaze effects:

Carved "waterfall" vase, approx 5 inches tall

Carved "waterfall" vase, approx. 5 inches tall

The only problem I'm experiencing with this glaze is some bubbling and pinholing. Really not sure why this is happening on some pieces and not on others..perhaps the bisque surface wasn't completely free of dust and dirt etc, as some of the ware has been sitting around for a while.

The test piece below was coated with a stoneware matt transparent glaze, and the decoration brushed on top with a mix of 50/50 red iron oxide and yellow iron oxide. It's quite easy to paint onto this unfired glaze, but there's always a danger of disturbing or smudging the surface..I much prefer working directly onto bisqued clay. The glaze is very unobtrusive and just enhances the natural colour of the clay slightly. I rather like the combination of muted colours here.

Red & yellow iron oxide over matt transparent glaze

The next trial pieces are interesting where I tried out a Japanese recipe for nami jiro glaze. This is a very simple recipe consisting of 50 ball clay, 50 wood ash and 5 to 20% kaolin (according to Lee Love, the proportion of kaolin is varied depending on where the piece is to be placed in the kiln. Thank you, Lee, for sharing this information!). I used washed oak ash and 5% kaolin over a clay body which is 50% original raku and 50% Earthstone Original stoneware. Where raw ash is used, I'm thinking that such a high dosage must make the glaze slop pretty caustic, so care should be taken in handling.

Black slip & red/yellow iron oxide under nami jiro

The trial on the left shows nami jiro over black slip, with the slip and glaze applied to bone-dry clay. This glaze has produced a pleasant oatmealy colour on the clay body, with a very matt finish. On the black slip itself it's created a slight sheen. Used on its own, I would prefer the glaze to be a little glossier, which could no doubt be achieved by adding a bit more potash feldspar to the mix. But combined with a glossy oribe glaze in contrasting areas of a pot, it could work really well. The brushwork has stayed clear and dense, which is what I was hoping to achieve.

The piece on the right is the same glaze but the brushwork was done with 50/50 red iron oxide and yellow iron oxide. It's clear that the yellow iron oxide is a weaker pigment (as it contains some clay) and the decoration has faded somewhat, especially at the top where the tile was double-dipped.

Lastly, I re-tested the Leach 4321 tin glaze, but added more tin oxide, which takes the tin oxide content from 2% to around 3.5 to 4%. The test tile has come out quite nicely, but I was pretty amazed to see that the glaze was glossy and completely clear! It's really gobbled up the red, black and yellow iron oxide decoration (on the other side to the one shown), but the black slip has stayed fairly well defined..I think because only the 15% black stain has survived. So I could probably get the same effect by just adding Mason's black stain to a kaolin/ball clay slip.

Black slip under Leach 4321 + 3.5% tin oxide

Why has all the tin oxide disappeared completely? I'm not sure, but I came across something online today which says that the Leach 4321 glaze is a cone 8 recipe. So maybe taking it to cone 9 is over firing and makes the tin oxide act more as a flux instead of an opacifier. Food for thought...

NB. All the test pieces and the large tea bowl were fired in oxidation.

Never give up..the best is yet to come!

Some experiments with black underglaze

Most of the glazes I applied for the last firing were successful, but I was a little disappointed with the black slip I used for the underglaze decoration (brushed onto bisqued clay). I tried out several different clear glazes, glossy and matt, and most of them "ate" the black slip to some extent. I was hoping that by adding 15% Mason's black stain to the slip recipe I was already using, I would obtain a denser, more solid, line. The colour certainly turned out much blacker, but on some pieces the decoration was still too faded under the glaze. Of course, it's possible that the slip was slightly too diluted..

One surprise was that the Leach 4321 glaze with 2% tin oxide came out almost clear..just slightly milky in the thicker areas, but the underglaze painting has remained reasonably well defined on this bottle-vase:

Bottle-vase. Approx 6 inches tall. Cone 9 oxidation.

I read yesterday that on traditional oribe ware, Japanese potters paint brown iron oxide (a kind of burnt umber? We can't get brown iron oxide here) on top of a very thin transparent glaze, and that this glaze is made by adding a little wood ash glaze to feldspar. Unfortunately, I don't yet have a recipe, but perhaps painting onto the glaze helps prevent it fusing and being absorbed. But I find it hard to believe that all oribe work is done this way, because I've watched Goro Suzuki painting directly onto raw clay..also, many of the pictures I've seen on the web seem to show the iron decoration under patches of thicker, cloudy glaze. If anyone can enlighten me as to the exact process, please drop me a line! Painting directly onto unfired glaze would also be problematic if the glaze surface is loose and powdery (as it is with Hamada clear glaze), which may explain why they use a form of feldspar to help harden the surface. Having said that, I imagine that most oribe potters will be raw glazing (applying glaze to leatherhard work), which could make a huge difference to the qualities of the glaze surface.

Most clear glaze recipes I come across, contain some form of calcium oxide, which in large quantities can have a bleaching effect on iron oxide. So, I'm guessing that oribe potters use a glaze which avoids using whiting (calcium carbonate), for example.

The bottle vase shown below was glazed with a low-clay shino, and the decoration has stayed very clear and solid under the glaze, with no fuzzy edges. I was really pleased with the finish achieved here and will be using this combination again.

Bottle-vase, approx 3 inches tall. Cone 9 oxidation.

Bottle-vase, approx 3 inches tall. Cone 9 oxidation

Another approach might be to spray wood ash onto the bisqued clay, before decorating with pigments..in a gas kiln, it gives an attractive flashing effect on the clay, but I don't know if it would work in an electric or whether it would harm the elements (in the same way that soda ash does).

The next step is to try applying the clear glaze a little thinner and see how different iron pigments behave on top. (Just thinking out loud..any craters and pinholes in the glaze surface are also not going to help with fine brushwork..). I'll mix up a few combinations of yellow, red and black iron oxides and also combine them with a clay slip to see how it affects brushability etc.

And tomorrow, I really must tidy up the studio a little..experimenting with new glazes makes plenty of mess!

Never give up...the best is yet to come!

Frustratingly frothy glazes!

The last few days have been fun and games, trying to make larger batches of the Hamada and Leach clear glazes, the main problem being that both glazes contain little or no clay. The Hamada glaze has no clay at all and consists mainly of feldspar, which means it settles out incredibly quickly..it needs a helping of bentonite and epsom salts (many thanks to John Britt for his helpful advice on that!) to stop it deflocculating into an immovable lump at the bottom of the bucket. The Leach glaze contains only 10% china clay..it's basically the Hamada glaze with the addition of a little clay.

Another problem with these glazes is that after mixing with water, they froth up tremendously, so when you dip a pot in it, the glaze surface is left covered in bubbles and craters, large and small. I waited a full 24 hours for the bubbles in the Hamada glaze to subside, but it was just as bad the next day. I was thinking that it might be the rough surface of the plastic container which was causing the frothing, so i transferred it to a glass bowl and this seemed to calm things down a bit. The strange thing is that by day three, both glazes had settled down and didn't bubble up nearly as much when stirred..why this improvement happens I have no idea, but the lesson is to try to mix up glazes several days before I need to use them.

And the final issue I faced was heavy drips and runs forming on the rims of pots after dipping them. With a clear glaze this can cause problems because thicker areas may fire white or milky rather than clear. After glazing a couple of pots, I suddenly remembered that I'd bisqued them at a slightly lower temperature than the previous batch (1020 rather than 1050 degrees C) and it's surprising how much difference that makes to the speed of water absorption. Effectively, the glaze was now too thick for this lower temperature bisque ware, so the drips were drying before they had chance to run off the edges.  It's very easy to forget to take into account one of the factors which affect how the glaze behaves..not very exciting, but I'll list them here mainly for my own benefit, so I don't forget next time:

1. The thickness/density of the glaze slop. For years I tried to judge this by eye but it's much less risky to measure it with a hydrometer (or weigh the glaze exactly, relative to an equal volume of water).

2. The temperature of the bisque firing.

3. The thickness of the pot walls. The heavier the pot, the more glaze it will suck up. Glazing the inside of a thin pot will then affect the amount of glaze the outside surface can absorb. One may need to wait a day between glazing the inside and outside of a very thin-walled vessel.

4. The amount of time the pot is dipped for. I usually dip my pots for between 3 and 8 seconds, depending on what thickness I'm aiming for.

5. If gums like CMC have been added to aid suspension or glaze hardness, they may also affect speed of glaze drying and how much glaze is absorbed by the clay.

Anyway, by early evening today, I'd managed to glaze a number of pieces reasonably well..and the proof of the pudding will be in the firing tomorrow!

Never give up, the best is yet to come! 

This week's glaze results

This week's firing produced mixed results. Several pieces were fired in the saggar but only one (a small cup I had glazed for the previous firing but couldn't fit in the saggar) came out with the finish I was looking for:


Cup with orange matt glaze, fired to cone 8

 The other pieces I prepared for this firing came out way too dark, basically because the glaze was applied too thinly:

I have a feeling I may have added a little extra water to the glaze slop, thinking it would evaporate before I next used it, since the bucket lid isn't airtight. I'd also mixed in some CMC  to help with glaze suspension ( carboxymethylcellulose is a kind of organic glue ) and this appears to have rotted as it now stinks dreadfully, like no glaze I've ever experienced! Smells just like a stagnant pond! I've since learned that a side effect of CMC is that glaze dries alot slower, which I assume means that more glaze pours off the pot after you dip it. Normally, the first layer of a glaze will dry almost instantly on the bisqued surface. I think in future I will stick to bentonite for glaze suspension, not least because it doesn't go off.

Here are some of the test pieces I put in as well:

Test pieces, fired to cone 8 in oxidation

Whenever I take trial pieces out of the kiln, they usually raise another half dozen questions I would like to have answered. I'm trying to achieve a dark (preferably black) slip or engobe decoration under transparent and semi-transparent glazes at high stoneware temperatures (cones 8 to 10). The Hamada clear glaze on the right is interesting in as far as the black lines have stayed very black (which is odd, since it's absorbed the iron oxide from the background slip, which was originally orange). The other two tests were glazed with a mixture of Hamada and Leach clear glazes with a little tin oxide added. This glaze formulation was done by rule of thumb, not very scientific, but I was more interested in the effect on the slips under the tin glaze. For the middle one, I used an iron-rich clay body (a 50/50 raku and terracotta mix) and on this piece the decoration has come out somewhat darker than on the test on the left (a lighter clay body). The tin oxide in the glaze also gives the black slip a bluish tinge, with the result looking a bit like cobalt decoration on 18th Century delftware.

The lidded pot below was simply coated with two layers of black slip-glaze and it came out perfect, but another small vase blistered in several places. Based on previous firings, I had hoped that this black slip-glaze was very reliable, but it appears that more than one coat and there is a chance of peeling. It might be worth making it with a little flux added to see if it makes any difference to adhesion.

Lidded vessel approx. 3 inches tall. Fired in oxidation.

Never give up, the best is yet to come!