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!

Eric Knoche's wood-fired ceramics

Yesterday I became an admirer of Eric Knoche's functional and sculptural ceramics. His work is partly influenced by the Japanese ceramic tradition, and he was fortunate enough to spend several months working as an apprentice to Japanese potter and Living National Treasure, Isezaki Jun. A video was produced, giving an overview of his time in Japan, and providing a fascinating insight into the daily working practises of a Bizen pottery workshop.  It demonstrates the arduous processes involved in collecting and preparing the clay, and the quiet, steady discipline required to produce work of the highest standard.

 http://vimeo.com/2929505   (the password required to access the video is "woodfire")

A gallery of Eric Knoche's work may be viewed here: http://www.ericknoche.com/

Jim Malone, potter

I thoroughly enjoyed this wonderful film about Jim Malone, the potter, made by the Goldmark Gallery, Uppingham. Jim's philosophy is that "the best pots are born and not made".


 

Youtubelink: http://youtu.be/3Er5PUXpo5c

Goldmark Gallery: http://www.goldmarkart.com/exhibitions.php

Slideshow of 2011 exhibition: http://www.youtube.com/watch?v=Qr_P7ECKFCg&feature=related

Nice weather for the time of year!

Autumn is upon us, but the weather this has been rather nice this week. That is to say, we've had some clear skies and it's not freezing cold, and there's alot to be said for that when the only heating you have in the studio is a small electric fan heater! All the bright sunshine we've been having seems to have stimulated the right side of my brain, and I'm having a creative spurt just now. I'm busy developing some new ideas for vessels in various shapes and sizes, mostly with lids. I've not made many pots with lids before, and I know that the problems of firing increase exponentially due to problem of glazes sticking and lids warping and so on. Partly for this reason, I'm going to start by low firing most of the lidded vessels I'm making at the moment, and use slips and terra sigillata to decorate them. I think a low-fired finish will suit many of the forms I'm throwing at the moment, so I'm happy to go down that road.

This carved pot on the other hand, will probably need to be fired to stoneware:

The vessel was thrown as a closed form, then carved once it became leatherhard (which takes over a week this time of year as closed forms take longer to dry out of course). I didn't really consider the pot's function as I was making it..but perhaps it would make a nice tea caddy.

Then there is the bottle sculpture.. :

This is one of those pieces that comes about by complete accident. I'd thrown a bottle on the wheel and didn't much like the form or the heavy weight of it, so decided I'd cut it in half to check the wall thickness and then chuck it in the recycle bucket. Once I'd chopped it in two, I noticed how much the halves resembled two figures, sitting opposite each other, especially when I squeezed the middle and the necks kind of leaned back a little. Anyway, I joined them back together with slip, and this was the result..it's giving me some ideas for other similar pieces I could make, so I thought I'd make a note of it here.

The ceramic work of Jenny Mendes

Jenny Mendes is a hugely talented ceramic artist and an expert in the use of terra sigillata. Her work is made from terracotta clay and she applies an enormous variety of coloured slips to create fantastical and intriguing designs. I can highly recommend a wander through her Etsy online store, where she is displaying over 250 of her imaginative pieces.