Wednesday, June 15, 2011

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. 


Angie said...

Love the glaze on/in the small cup ...and the textured brown/cream tile. What a lot of work.
I am sure some of the pigs were elephants lol

gz said...

A nice dark blue-what cone do you fire to?
I'm firing cone 9 down, I have a nice blue similar to that- Leach's 1234, plus 2pc cobalt carb and 1pc manganese dioxide (gives more depth to the colour) It is good in oxidisation, but better reduced.

Peter said...

Hi Angie,

Loved the "elephants"!! Just as long as they didn't suffer a fatal build up of gas whilst firing, they will be fine!

Judy Shreve said...

What a great lot of tests -- love the manganese cat! And that cobalt blue is so rich! I like it over the iron glaze too.

Loved the pigs -- and I agree with Angie - some did look like elephants -lol

I need to sort through my chemical closet - not a chore I am looking forward to. In fact I bought some plastic containers months ago . . .

Peter said...

Hi Gwynneth,
I'm firing to cone 9 with tip touching, but not absolutely flattened. No soak at top temperature with this one. I think this glaze base has a little more character than the 1234 in oxidation, and a nice "feel" as there is a little magnesium in it. I'll post the base recipe tomorrow on this as a comment. Just got to get to bed now. Kind thoughts, P.

Peter said...

Hi Judy,
I'm excited about the manganese cat, as I tried a new technique (for me) for doing the incised decoration. I used a little electric drill thing, a bit like a dentist, and buzzed out the decoration with it on a bisqued tile. Great fun too!
Now I really must go to bed as is 24 past 11!

cookingwithgas said...

I admire your stick to it and get it done testing! This is really where I fall short-very short.
I will do anything not to test and mix glaze.
But i feel it coming.
Really- maybe today- then there are those ----- to make and --- well maybe tomorrow!
Yep! tomorrow....
You have some real winners there!

Linda Starr said...

Love the crawly glaze and the dolomite over the iron and the second mug, I might just have to try a couple of these, thanks for sharing.

Armelle said...

Hello Peter,
Nice tests, I love the first cup, is it your chün ? Lovely pigs and what a delicate opération !!!
The crawl tests are nice too.
Best wishes to all

Arkansas Patti said...

So glad you didn't have any effects from the earthquake. Those poor people in Christchurch must be worn down with all the damage.
I adore that blue mug but have to say I was totally inchanted with the pigs. How nice of you to do that for them.

Peter said...

Hi Meredith,
Good to hear from you. Yes... I am a reluctant glaze tester too, believe it or not!!! It just seems to take forever and can be so frustrating when the kiln is unloaded and you realise that the one glaze test that you thought of doing before the firing, but was simply too exhausted and frazzled to do, was definitely the one that would have unlocked the puzzle that you were trying to solve!! Saturday afternoons are a good time to test glazes here as our local radio station plays old rock and roll hits, and I like to sing along in 4 part harmony!

Hi Linda,
The crawly glaze is great fun, and it is something that I have intended to test for about the last two years!!! It is definitely a sculptural type glaze as the texture would be the sort of thing that would be useful for sanding floors! The thickness of application makes a difference to how big and chunky the texture is, and you can vary all that lots more by putting it over another glaze.

Hello Armelle,
The first cup is the chun. It is interesting to see how the blue colour will develop where the glaze is thick enough, even over white clay. Ah, the pigs, I can see a future career move for me as a veterinary surgeon! I love the work that young children make out of clay... so much life and fun!

Hi Patti,
The earthquakes have been going on for such a long time now, with the first in September of last year. The poor people in the Christchurch area just cannot plan their futures, and many have houses and places of work that are slowly being shaken to bits by the aftershocks. The pigs..., firing them is a pleasure really, and came at a good time when I had a lot of space in the kiln.

Peter said...

Oh, I was going to add that I have put a footnote to the post with extra information about the dark blue glaze and its recipe. P.

Pat - Arkansas said...

My goodness! I see I have missed out on a lot of your doings while my daughter was here. That's what I get for not making time to read my favorite blogs. Your glazes are beautiful; I'd be hard pressed to select a favorite, but the dark blue is wonderful.

Sorry to learn of Ginger's problem but glad to read that things are better.

Love, love, love the leaf impressions and the photos of the process.

It's hard to comprehend that you're already wearing winter woolies when this morning at 7 a.m. it was already over 80F in my back yard, with very high humidity. Arrgh!

Oh, another thing: it's been so long since I've visited that you've changed the format of your blog. Very nice.

"See" you soon.

Peter said...

Hello Pat,
Great to catch up with you again, and I'm glad that you had such a good time with family visiting. Today was lovely and sunny here, and we actually felt a little warm whilst going for a little walk up the street at lunchtime..., it was then about 44 degrees Fahrenheit! I do think that your 80 degrees is a bit excessive! Glad you like the new blog format, I think that it is easier to read (which suits my ageing eyesight!), and I hope others find it easy too.
Best Wishes, P xx

gz said...

Thanks for sharing the dark blue glaze recipe

Peter said...

A pleasure, its always nice to share these things, and fun to see someone else use them and adapt them.. It's like having a big workshop with lots of potters in it then! P :)