Windows give a little glimpse of the world, a world that is held in place, and kept in order by a frame. I love summer for being warm enough to make it possible to have the kitchen window open for much of the day, and to let the air and sound and scent of the garden enter and invade the space within!
We picked plums from the trees that tumble together at one end of our garden. These are wild trees and wild plums, small and sweet. We made jars of jam and also put some stewed plum in the freezer for later in the year. I hope we remember it, and this summer fruit doesn't become part of an archaeological dig in future years when we investigate mysterious containers who's labels have become unreadable at the bottom of the freezer!
Laura and I were in Dunedin on Monday, where I did my day a the Stuart Street Potter's Co-op that is situated quite close to the railway station. It was my turn in the window, so I put together a selection of earthenware pitchers, bowls, and some porcelain goblets. I had hoped to have made a display purely from the pitchers that I had made recently, but.... all was not plain sailing with them after all, and I only had half the number that I had intended, however...., my window plan B made for more variety!
It is quite fun being in the window putting a display together. Shop windows are a strange place, a sort of "no man's land" between shop and the street that functions like a stage at a theatre. Unlike theatre, the performance is rarely a "live" one, and it can provide quite a surprise to passers by when something in the window moves!
One year I put my potter's wheel in the window and actually made a few pots in the tiny improvised studio. A young child watched me intently as I worked and asked me to make the smallest pot that I could do! Which was quite a change from adults to whom size is everything!!
I was pleased by the porcelain goblets that I made recently. These feel good and solid in the hand, without being excessively heavy, and are taller than what they seem in the photos... I am guessing about 7 inches (I should have measured them, but there we go!!).
The display in the window of the Co-op will be there for the next two weeks, so... if you are passing you might like to have a look!
Boiling, Bubbling, Toiling.....
I shared a glaze recipe on the post I did prior to this one, and all was looking well with it at that point in time... I have had some difficulties since then!
I had great trouble with 5 or 6 of the pitchers, all have the alkaline glaze, or similar.
|Poor old Pitcher... still bubbled after 3 glaze firings....|
|A particularly nasty example of the problem.|
The problem is extremely frustrating, but quite interesting, in that the glaze has worked OK over white earthenware, and acceptably well over the red earthenware in tests, and on some actual real live pots...., but this time around I ran into severe bubbling of the glaze that left a surface like craters of the moon!
I can see evidence of gas coming from the clay body, there is a pinhole in the centre of some of the craters that goes right through the slip.
The problem is also worse in most cases when over exposed raw clay, rather than over the areas protected by the slip. It is interesting and probably significant that the slip does make a difference with this.
What I have Done Thus Far....
I have tried re firing the affected pots two more times (so they have had one bisque firing and a total of 3 glaze firings thus far).
The first extra firing was with a really long soak at top temperature and a peak temperature slightly below that of the first firing. I also did a slow cool where I fired down, all this was to allow gas to bubble out and the glaze to heal and settle down... it didn't!
The last firing I took to a temperature 1 cone above the original glaze firing temperature to see if extra glaze movement might help in the healing process.... it didn't!
It may all be my fault!
If the problem is due to gas coming out of the body of the pot, then I have fired the bisque firing too fast, and possibly the glaze firing as well!
I am fairly careful with my bisque firings, and try not to do them too fast, but I will take even more time with them in future particularly from red heat to the peak of the firing, just in case this clay has a lot of stuff in it that needs to burn out prior to glaze firing. The confined space and lack of air movement in an electric kiln also makes it harder to get rid of impurities in clay through bisque firing, so more time may well be needed.
There does seem to be a problem with this particular glaze/body combination though. Other glazes do seem to be able to work on this clay. One exotic theory that I have is that the high sodium and lithium content of the glaze may give it enough spare flux to nibble away at the clay that it is sitting on and cause a chemical reaction of some kind. Maybe the iron oxide in this red clay is being persuaded to let go its oxygen content in a more enthusiastic way than usual at this temperature?
|This glaze works OK. Same clay and slip with commercial clear glaze over it, with 1 percent manganese dioxide and 1 percent red iron oxide added.|
I had a good look at Tony Hanson's great Digitalfire glaze site, and there were many helpful ideas about the causes of craters, pinholes, and other glaze defects for me to think about.
Anyway, like a pack of cigarettes, the alkaline glaze comes with the Surgeon General's warning.... Test first on something that doesn't matter!
Peter's Clear Desperation Glaze!
Whilst going through the misery (it was!) of trying to sort out the boiling pitchers with a Co-op window deadline looming, I did formulate a new clear glaze for Cone 2 or 3. I have to report that the first test looks wonderful... I know that such things may later prove disappointing, but....
|Peter's Clear Desperation Glaze, fired at cone 2, With stripes of copper, chromium, cobalt, manganese and iron painted over.|
How The Glaze Began...
The idea for the glaze came from reading a comment on Marshall Colman's blog
where a reader, Linda Bloomfield, suggested a glaze recipe,
"Have you tried the following alkaline recipe 1060C (1940 F ... Cone 04) by John Solly:
Calcium borate frit 39
Soda feldspar 27
China clay 6
The glaze recipe looked like the sort of thing I have been trying to do, however the recipe was for a much lower temperature, and mentioned calcium borate frit, without specifying the frit number.
Using Insight Software, I had a look at the recipe to better understand the chemistry of the glaze see how near "sensible" limits such things as sodium and calcium were, and I realised that I had to make assumptions straight away about the glaze, as I did not know what calcium borate frit John Solly had used.
Still crunching numbers on the computer, I tried several frits with the recipe, and they made considerable differences to important things like, the ratio of Alumina to Silica which has a great bearing on the maturing temperature of a glaze, and also to the Thermal Expansion of the glaze, that determines how well a glaze will fit the clay that it is going to adorn.
Having realised that even a simple looking recipe like this was full of mystery, I decided to run with the assumption that the calcium borate frit was Gerstley Borate... My logic there is that calcium borate frits were made in the first place to replace such things as Gertley Borate, so ... I thought it good to wind the clock backwards! I liked the look of the theoretical glaze with Gerstley Borate. important things like Thermal Expansion came down, and other numbers looked promising too.
c04 John Solly Alkaline Clear (with Gerstley Borate)
Soda Feldspar 27.00
Calcium Carbonate 5.00
Gerstley Borate 39.00
Thermal Expansion 7.10
Inspired, I made some assumptions as to the amount of Silica and Alumina a glaze like this would need to mature at cone 2 - 3. I then replaced as much of the Gerstley Borate as I could with Frit 4108 (a calcium borate frit) whilst adjusting everything else in the glaze to keep the fluxes similar to how they were in my Gertley Borate version of John Solly's cone 04 glaze.
And this became....
Peter's Clear Desperation Glaze Cone 2 - 3
Frit 4108 21.5
Gerstley Borate 21
Soda Feldspar 17.5
China Clay 14
Thermal Expansion 6.91
You might like to test the glaze and let me know if it works for you!