Wednesday, February 4, 2015

Kitchen and Co-op Windows, Plum Jam, Boiling Glaze, and Peter's Clear Desperation Glaze for Cone 2 - 3!


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!


I did not manage to photograph the pots before bringing them into Dunedin, so I took a few photos as I set up the display.

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. 
This glaze is watery clear with no milkiness evident. It is craze free thus far over my earthenware, and looks nice over slip and raw clay. All the main oxides look good over it. I need to now test it on small pots, and to do boil/freeze testing on the glaze to see if it will craze when stressed.

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
Whiting 5
China clay 6
Quartz 23"


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
 Kaolin                    6.00
 Silica                   23.00
 Gerstley Borate          39.00
                         100.00

 CaO       0.62*
 MgO       0.11*
 K2O       0.01*
 Na2O      0.26*
 P2O5      0.00*
 TiO2      0.00
 Al2O3     0.27
 B2O3      0.51
 SiO2      2.81
 Fe2O3     0.00

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
Silica  21
Soda Feldspar  17.5
China Clay  14
Whiting  3
Dolomite  2

Thermal Expansion 6.91

 CaO       0.62*
 MgO       0.10*
 K2O       0.00*
 Na2O      0.27*
 P2O5      0.00*
 TiO2      0.00
 Al2O3     0.34
 B2O3      0.49
 SiO2      3.16
 Fe2O3     0.00 

You might like to test the glaze and let me know if it works for you!

12 comments:

Arkansas Patti said...

I love your sentence "Windows give a little glimpse of the world, a world that is held in place, and kept in order by a frame." Brilliant and perfect. I will never look at a window the same.
Your window display is just beautiful. I hope you did wonderfully--you should.

Melissa Rohrer said...

I wish there were someone to tell me exactly how to solve the problem when something goes wrong! You know so much more about glaze formulation than I do.

I love the photo of the pitchers with the berries on the kitchen table. Just where they belong!

Anonymous said...

Very beautiful pitchers in the window photo. First time at this site, but I will be back for sure. Wonderful spirit in the pottery...

gz said...

The first thing I thought when I saw those glaze pictures was....Calcium Borate.......

best of luck with that.

Wish we had a potters' co-op....the shop window looks good and those goblets are lovely

Armelle Léon said...

Bonjour Peter,
Love the photo of your garden with the plums fruits, it smells good the summer. It's nice to have the window for your pots at the Co-op, I am so sorry for the bubbles in the glaze, always frustrating !!!
I am quite away from my blog, here is winter and I had got an ear infection since 3 weeks and I am so weak :-( .So I lost some of your posts as well.

Wish you a beautiful summer and good sales in the Co-op

Peter said...

Hi Patti,
Thank you for the "Windows give a little glimpse..." appreciation! The sentence was one of those brief flashes of inspiration that seem to occur between 4 and 5 in the morning... before the rest of the brain kicks in and replaces the poetic with "must do...."!

Hi Melissa,
Good to hear from you. I know what you mean.... It can be so frustrating trying to solve things on your own! Potting in relative isolation is difficult, but it does mean that you have to find solutions, and the hard won victories tend to stay with you!

There are times when it would be great to be able to climb in the kiln and see what is going on when the glaze is near top temperature... I am sure that many of the mysteries of glazing would be solved that way... !

The good thing is that blogging makes for less isolated potting!

Hi Anonymous,
Thank you for your comment, welcome to the blog and for your encouragement. I am glad you enjoyed your visit to the site.

Hello Gwynneth,
It sounds like you too may have suffered from an encounter with low fired glazes at some time! "...calcium borate...."!

Often galleries seem reluctant to take pottery, I guess they prefer to cover their walls with paintings that yield a higher value per square inch ....), so a Potter's Co-op is certainly a useful outlet!

Bonjour Armelle,
Sorry to read that you have not been well for 3 weeks. I hope that the ear infection goes away and you are healthy and strong again soon. I am glad that the photo of the garden and the plums reminds you of summer. Kind thoughts from us all, P, L & NS

Linda Starr said...

beautiful photo in front of the windows, nice you have wild plums, when I lowered my bisque temperature just a bit I got much better results with glazes not crazing.

Peter said...

Hi Linda,
Good to hear from you. Ahh, the joys of summer and fresh fruit from the trees! We certainly need to make the most of it here in our rather short growing season, the last few nights have been decidedly chilly with the feeling that frost is not far away!

Interesting about your better glaze results after lowering the bisque temperature. I can't quite understand why that would work, but it is wonderful that is does!! There are so many mysteries with all of this! I had been thinking of trying the opposite regime, of firing the bisque to the maturity of the clay, then formulating the glaze for a lower temperature... much the way that industrial pottery is done..., but then I would almost certainly have to spray my glazes, and I don't really want to do that! So it is nice to hear that you have had success going the other way with temperatures!

mikeintonbridge said...

I have a little bowl by John Solly, which more than likely has that glaze on it - or the honey variation of it. I heard John talk at a Kent Potters master day years ago. On leaving college he developed about three glazes and half a dozen slips, and then made a whole career using them and was still finding new ways. John used supplies from Potclays and latterly The Clay Cellar. From an old Potclays catalogue I have, their calcium borate frit is listed as: 1 CaO, 0.097 Al2O3, 0.609 SiO2, 1.5 B2O3.

Peter said...

Hello Mike,
Thank you so much for the helpful information regarding John Solly and the frit that he most likely used. How delightful that you have a little bowl by John Solly that may have the clear glaze.

It is nice to have this information about John Solly, it makes the glaze recipe even more precious to have a little glimpse of the person that made it.

I am looking forward to running the glaze recipe through Insight Glaze software with the calcium borate frit that you give the formula for,and seeing how near, or far, my "Clear Desperation Glaze" is to the original!

Best Wishes, P

Michèle Hastings said...

Your summer fruits and sunshiny window are making me envious! We are having an arctic blast here in southern USA.
You copper red goblet is a stunner. The glaze reminds me of many of the pots that Jeff either photographed in S. Korea, or brought home with him.

Peter said...

Hi Michèle,
I had wondered how those of you in the US were faring with the weather. I don't often listen to the news these days, but I had picked up from somewhere that you were experimenting with a new ice age in your part of the world! I do hope you can curl up beside a warm kiln and things improve outside soon!
Kind Thoughts from us all, P

PS.. the "copper red" is a chrome/tin that I work with at cone 10 in oxidation... I started with a June Perry Cone 6 recipe, and then took it off on a journey of exploration to Cone 10. The glaze is really different now from the Cone 6 one, but it was useful to start with one that worked and figure something out from there. It is a very useful addition to my range of cone 10 oxidation glazes, and can look just like a reduction copper glaze.