Saturday to Wednesday, a bit busy.... but here are more glaze testers!

Saturday was glaze test day.  In the manner of great and famous chefs, I was stirring, sieving, adding pinches of this and that, and hoping that the "flavour" would come out just right!  The tests were partly for the tile commission that I am working on, and also some others that I am trying to develop.

 This is a glaze for something sculptural.  Magnesium carbonate shrinks madly as it is being fired.  If you make a simple glaze with equal parts of Magnesium carbonate and Nepheline Syenite and put it over another glaze, you can get effects like the ones above.  I fired all the tests on this post to Orton Cone 9, or 1255 Centigrade (2291 Fahrenheit).

Sunday was packing and firing the glaze tests in the electric kiln, doing a little work in the shed, and being around in case someone visited the gallery.

The glaze that I tested on the cup in this photo makes this lovely optical blue when put over a dark iron rich glaze.

Monday was power tools, sawdust, and the construction of another work bench for the shed.

I like to test glazes on "real" pots or mugs as soon as I can.  Flat surfaces are OK at first, but you soon need something with some verticals to see how much a glaze will move. This pot shows me a dark and light version of the same glaze, and also reveals the behaviour of the glaze when it overlaps the glaze that I have used inside the pot.

Tuesday I unpacked the test tiles first thing, and got the kiln ready for a bisque firing.  The phone rang just as I was about to load the kiln, and someone from of the local primary schools asked very nicely if I could fire some little clay pigs for them that the pupils had made.

The timing was good as I had lots of room in the kiln, so I got them to deliver the pigs right away.

I had to do a careful operation on each pig, as they had been made hollow, but no holes had been put in them to allow for steam and air to get out as they were being fired.  Without the operation, there would have been a dozen or so loud explosions in the kiln, and the pigs would have become little porcine hand grenades!  As a precaution I also put the pigs into a *sagger that I had made some time before, and put a lid loosely on it, so that small pig fragments would not fly around the inside of the kiln if one did blow up.  I also put a kiln shelf between them and the tiles that I was bisque firing!

This is one of the glazes that I am testing for the tile commission.  I needed to see how it would work over incised detail, and I have been fine tuning its maturing temperature and trying to make it a little more lively and interesting.  The mug, with its strong throwing lines, showed me how the glaze would pool and thin when moving over slight obstructions, and the detail in the "medallion" shows well through the thick glaze.


Whilst the kiln was starting to heat up, I photographed the test tiles.

I then travelled through to Dunedin on the local bus to join Laura looking after the Stuart Street Potter's Co-op for the rest of the day.  Once home we went out to a talk that was given by a local artist, Juliet Novena Sorrel, about sketch books. Juliet was charming and had a delightful attitude towards making art. The whole process sounded natural and fun.  Juliet illustrated her talk with photos of sketchbooks that are part of the Hocken Library collection in Dunedin, but also brought examples of her own sketchbooks, and very generously  handed them around for us to look at.  In spite of Laura and I both feeling "all in" after rather a long day, we both enjoyed the evening very much.

I tested this glaze over incised decoration.  I filled the lines of the tile on the left with a wash of manganese, and wiped the rest of the tile clean before dipping the tile in glaze.  The tile on the right has nothing in the lines, and they are hard to see.

The kiln took all day to gently steam out its small farm and tiles, so I fired the kiln through the night, and had to get up to check on progress a few times.  Around 5am, a very strong wind got up, and I had to keep reasonably alert, because we sometimes have power cuts when there are gales.  Just a power outage for a second will cause my kiln to switch off, and it doesn't come on again by itself.

This is an iron red glaze.  The wiggly line along the top was made by mixing some dolomite with water and applying it with a watercolour brush to the freshly glazed tile.  The dolomite adds extra flux to the glaze, and really livens things up.  The lower wiggle was a brush stroke of the same glaze that I used on the tile on the right in the previous photograph.

Today intended to throw some mugs and bowls, clean kiln shelves, and get some other things done, but I ended up mostly sorting out raw materials for glazing.  I had a storage problem that had got completely out of control, and realized that my little work space was no fun any more!  Glaze materials are best kept in plastic lidded containers (Plastic! .... A potter recommending it ...., what ever next!).  Trouble was that I have been so busy over weeks/months that materials have just tended to stay in their original packaging, and my storage shelves have been a nasty unhealthy jumble of dusty plastic bags and odd glass jars.  Not good to let it get like that, but... there we are, the truth is out!!  It all took several hours to tame, groom and organise, but it is much better now.  Tomorrow I won't know the place when I unpack the kiln and start to glaze tiles and pots!

This glaze is the same on the inside and on the outside of the cup.... but, what a difference!  I had a simple liner glaze on the inside of the cup, and glazed over it. The combination of the two glaze bases really brings out the blue colouration from the cobalt that is in the glaze.

Well, that's me done!  I'll just add some photos and post this.

This very quiet glaze is a "rutile blue" glaze...  Well, it probably would be blue if fired in a reduction atmosphere in a fuel fired kiln.  In the electric kiln, it fires an interesting off white with hints of blue.

We are all thinking of the people in Christchurch who have been affected by more earthquakes over the last few days.  This latest shake up will mean that many will probably never be able to return to their homes, but will have to relocate.   Laura did feel Monday's 6.3 (that was centred near Christchurch) and came outside a little wide eyed and worried, carrying a cat under one arm, but I did not feel the quake at all as I was in my little shed using a power tool..   Probably the shed is much safer than our old brick building anyway... maybe we will move in their permanently!

*Sagger (also saggar): a pot that looks a bit like a cake tin.  Pots that may need to be protected whilst they are being fired can be put in a sagger.  This was often the case when kilns were fired with coal and glazes could be sensitive to the sulphurous smoke or direct contact with the fire.

Added on 16 June for those who might be interested in a glaze recipe.
The Recipe for the Dark Blue glaze that is on the mug and the medallion tester.
The recipe started life as one that I found in "The World of Japanese Ceramics" by Herbert H. Sanders. (A wonderful book that was first published in 1967, and I enthusiastically recommend it if you can find a copy). 
Common Limestone Glaze, Cone 9 - 10
Eureka Spar  49.38
Whiting  14.67
Kaolin  12.09
Flint  23.60
Magnesite  0.26
 
I have limited ability to pick and choose what feldspar I can get here so I use any Potash Feldspar in place of the Eureka Spar, and also use Magnesium Carbonate in place of the Magnesite.  I find it slightly underfired at Orton Cone 9, and have fired it much higher than cone 10 in my wood fired kiln.  The book is an old one, and the cones may be Seger, rather than Orton. 

In the blue example that I have photographed I have added 5 percent Fritt 4108 (similar to 3134) to lower the maturing temperature, and have 2 percent cobalt carbonate and 3 percent rutile. 

The small amount of magnesium in the glaze will act as a flux at cone 9, but it also helps give the glaze a pleasing wax-like feel.

In the Comments section of this post "gz" mentions "Leach 1234".  For those of you who are not potting, that probably sounds like code.  The recipe is as follows:
Potash Feldspar  40
Silica  30
Whiting  20
Kaolin  10

This makes a very useful base recipe for stoneware temperatures, and is slightly milky to clear at cone 9, depending on thickness.