Wednesday, June 27, 2012

Pretty pictures and testing times










I was tempted to leave this as just a sequence of photos, and probably we should all stop reading at this point, and have another look at the close up of the crystals in the first few photos... (aren't they gorgeous) before running away and having a nice cup of tea, or a beer!


This one has just gone on display today, I love the way the glaze forms such a dark pool around the rim.


I have had a very busy and somewhat frustrating few weeks since I last posted.  Some good progress has been made with crystalline glazed pots and it has been nice having some new ones to look at. In between each crystalline glaze firing I have been firing an earthenware glaze test firing.  I am trying to come up with a lead free clear glaze that fits my clay properly with no crazing, and is clear... not milky.  Rather than take wild guesses (although those can be fun!), I have been working on a fairly systematic approach following the method that digitalfire.com advocate in a good selection of articles on their web site .  This one about developing a majolica glaze was particularly helpful, "The Majolica Earthenware Process".

Making a glaze fit a clay pot without crazing is tricky.  Glaze has little strength when it is in a state of tension, but is strong when it is compressed.  After the pot has been fired, ideally the glaze that covers it needs to be in a state of compression, almost like a jacket that is one size too large for its owner!  To make this really challenging, the clay that the pot is made of expands and contracts as it is fired, and comes out of the kiln rather smaller than what it went in!  The lovely garment that it is clad with has to match this.  This is where something that is called the coefficient of expansion comes in.  It really helps to know what the coefficient of expansion is for the glaze, and for the materials that make up the glaze.  It is a bit like knowing if the jeans that we are wearing for the first time are going to shrink a great deal when they are washed, or not!

The articles on digitalfire.com recommended making a glaze for earthenware clays from glaze frits.  Potters can buy frits as a fine white powder.  Frits are made from materials such as borax, soda, potassium, and calcium, and these are combined with silica, fired in an industrial kiln until they melt together as a glass, and then this molten glass is poured into cold water so that is shatters into pieces, then the pieces are ground to a fine powder.  Some frits are almost a complete glaze by themselves, just add water and maybe some china clay, and you are away, others are designed to be only one part of a glaze.  One good thing about them, is that you can buy high expansion frits, low expansion frits, and frits that are somewhere in between, and it is possible to combine them together.

My first task in making a glaze for my clay, was to sort out the various glaze frits that I have in order of their coefficient of expansion, then select two, a low expansion borax frit (4113), and a slightly higher expansion frit (4124).  From these I made an initial series of 5 glazes that went from low expansion, where 4113 was 80 percent of the glaze, to higher expansion where 4124 was 80 percent of the glaze.  Somewhere along that line up of glazes of different expansion, I hoped to find one that would fit my clay nicely without crazing.  The basic recipe was 80 percent frit, 15 percent china clay, and 5 percent silica.

My first test firing was to the temperature that I would use for a craze free lead bi-silicate based glaze on my clay.  I find somewhere between cone 03 and 02 about right for this, which is approximately 1100 Celsius or 2012 Fahrenheit.  Well, all of the lead free glazes crazed.  They did so "helpfully" as they should, with the highest expansion glazes being badly crazed, and the lowest expansion somewhat better behaved, but it was a bit disconcerting, well, more than "a bit"!  The digitalfire.com site had another step to add to the glaze test before they could be declared "craze free", and that was repeated boiling and iced water immersion tests.  My glazes had failed at room temperature without the further assistance of boiled or iced water!

The other difficulty was that all of the glazes had a somewhat milky appearance to a greater or lesser degree, and the ones that were mostly 4124 would have made a good base for a blue chun glaze.  I do realize that this "bluing" is caused by the high borax content of the frit, but I could see problems looming.

A very pretty blue thanks to borax, but not clear...

After some thought, I repeated the same tests at cone 01 and cone 1 (up to about 1145 C or 2093F) and also expanded the range, adding extra silica to the low expansion glaze to make it even lower.

On the left, added silica prevents crazing, but look at the milky colour!

I also tried several other recipes with different fluxes.  I had read somewhere that alumina can help counteract the "bluing" of borax based glazes, so I made a test of the worst glaze base with extra alumina (and it probably got rid of most of the blue, but created white crystals instead!).  I have a hunch that a small addition of manganese may help clarify glazes, so I did some tests with that too (it didn't!).

The tests at higher temperatures did fit a little better, with less crazing, and the lowest expansion ones were craze free, even after boil and freeze testing, but the problem of glaze clarity remains.  As I write this (all huddled up with coat and woolly hat on as sleet splatters the windows) I am firing more tests to cone 3 (I should find that around 1165 C or so, 2120F).  The good thing about this higher temperature is that some other fluxes start to become useful, and I am trying a series of glazes that use far less borax.

I currently do not own glaze software (although I may yet purchase some from digitalfire.com), but I did make use of http://www.glazesimulator.com/  and found this a helpful place to do a lot of co-efficiency of expansion calculations and came up with a series of low expansion glazes that should be clear.... in theory!
To check how clever such software could be at predicting how a glaze would perform I did feed in some glazes recipes that I know to work well... and got mixed results.  Just like the weather forecast, it is useful to have an idea about what might happen, but there are really too many variables for a completely accurate prediction.  In the end you have to weigh materials, mix them with water, sieve them repeatedly, apply them to a pot and fire it!

The clear lead free glaze is important to me at the moment, as I have a commission that requires it.  As a comparison I did put a lead bi-silicate based glaze through the same testing, and it has performed beautifully... , crystal clear, no crazing, stands up to boiling and freezing repeatedly.... darn it!

Lead bi-silicate fits like a charm and is crystal clear just like the lead crystal wine glasses that you happily drink out of!

The tests have been useful in terms of understanding the clay that I use and its firing temperature, although they have swallowed up an enormous amount of time, and most of the results have been dispiriting.

Sorry that this post has been tedious to read, but it does make a useful record of events for me anyway!
Stay warm... I'm not!  P

12 comments:

Judy Shreve said...

First - those crystals are amazing! And it is fun to see the photo of you resting after all those tedious glaze tests.
One of the reasons I quit working in clay was my inability to get a clear that worked on my clay body. I became increasingly frustrated with throwing most of my work out due to the milky-crazing clears!
I'm hoping you discover the solution! :-)

cookingwithgas said...

WE are going to be very warm indeed.
It is going near 100 for a few days.
can I send you some.
Lovely pictures, really lovely.

smartcat said...

Beautiful crystals. There are deep worlds there.

Happy to see you managed a little rest after all that work.

Welcome to the Wonderful World of Low Fire! Finding clears that don't craze and/or cloud is an ongoing problem. Are you familiar with Ron Philbrick's work? (www-dot-ronphilbrickpottery-dot-com) He has a clear which looks great.

Do you bisque and glaze at the same temp? I find that if I bisque at my glaze temp or even a little higher I don't have to deal with shrinkage problems. Is there a reason for firing at 03-02? You might try lowering your temp to 04-03.

It's still pretty early here...if I think of anything else I'll let you know. Don't lose hope...when you get there you will be delighted!

Angie said...

A closeup of the beautiful crystals and then enlarged,would make an amazing wall poster.
Love that last glaze ...and the fact it withstands high and low temperatures is brilliant ...but I take it that is not the one you wanted to do this.
You looked so sweet lying there with your feline companion also asleep. xx

Peter said...

Hi Judy,
I do remember your courageous battle with glaze testing and your work with slips and surfaces. It is frustrating that there doesn't appear to be an easier way for clear glazes at this temperature... well, I suppose there is probably a 2 pot polyurethane gloop that could be sprayed over without the need to fire at all...just joking! Another interesting area is to do what the Iranian potters did years ago, and make a fritted body that fitted their alkaline glazes better than local clay would have.

Hi Meredith,
100 degrees! Goodness, always sounds even hotter to me because I have to translate from C to F, but it still is amazingly warm. We've only just got our power on after icy cold gales brought down power lines, so coffee this morning was over the pot belly stove in my studio.

Hi Smartcat,
Yes Ron does great work, I should get in touch with him and see if he will share his clear glaze recipe, it would be interesting. The reason I am firing so high is related to the earthenware clay that we have here, it really is more refractory than what seems to be the normal range for earthenware in the States and in the UK. I have a feeling that Australia may also have rather high fired earthenware clay there too??? This has been an issue for me as I assumed that a temperature of around 03 would have been just right, but I suspect that our clay doesn't develop enough of the glassy gloop in it at that temperature to assist it with a good squeeze that should come at around 225 C as the pot is cooling. This means that the glaze is difficult to put under compression. I had been wondering about changing to a high fired bisque to what ever temperature the clay really needs for maturity and a lower temperature glaze firing, so that I can use the more common 04-03 glazes, but I wondered if I would have less success making a good interface between glaze and clay that way? The other thing I have started to play with is a soak as I go through the last couple of cones to peak temperature, I believe this could help a bit. Anyway, any advice you have and ideas are most helpful and appreciated and could well trigger a useful solution to this.

Hi Angie,
Good to hear from you, and thanks for your encouragement. "Sweet", I like that, it is not everyone that glimpses that side of me through my rugged, flinty, careworn exterior! Old Ginger loves to sleep like that.
Mmmm, that last glaze works really well, but it does contain some lead, although it does so in a fairly safe form. Sadly I have to find an alternative to this one for use on domestic ware. I read somewhere that the actual lead release from a glaze like that is about one sixth the amount of lead that people would normally consume through eating fruit and vegetables!!! Quite a thought!

Armelle said...

Salut Peter,
Your round pots are very nice, I love these shapes, the one on photo n°4, is really nice with beautiful crystals.
I can see how lead free clear glaze is difficult to find !!! I have got a recipe from Daniel Rhodes, it's for 1165° celsius and had some colemanite, maybe you did test it ? I didn't, I have no colemanite, and I prefer to fire stoneware clay, probably because it's easier to make glazes.
Best wishes to you

Julia said...

The results of your testing (as painful and tiring as it sounds) - are breathtaking!

Amy said...

wow- fascinating! your posts are often so rich in being filled with information- I learn so much! thank you! I have saved this blog post to read closer to learn about glazing. I have much to learn, especially about how to measure and if needed, convert the numbers for the dry ingredients. If you know of any webpages to recommend to learn more, please let me know where to find them. Thanks a bunch! Stay warm- as I read on another blog that it is cold there now.

Peter said...

Salut Armelle,
Thank you for the Daniel Rhodes recipe. In the firing that I was doing when writing this post, I included a test of a glaze that had colemanite, but not identical to the Rhodes one, it produced an interesting and rather lovely glaze, but it looked like mother of pearl with lustrous rainbow colours!

Hi Julia,
Thank you for your encouragement, it is much appreciated!

Hi Amy,
Thank you for letting me know that you are finding the information on my blog useful, that is really good to know.

I hope to do a follow up post quite soon with an update about the glazes that I was firing when I wrote this post. I had some good results from the firing, and some glazes that I may be able to use.

I will give some thought to adding some links to useful web pages too when I do the next post. From time to time I have found real gems of help on fellow blogger's sites, and I make use of the search function that is built into many blogs to dig up helpful posts. Ron Philbeck has put a Technical tab on his site (which is a great idea) and has organised glaze information there (I must do this myself!!!).
http://www.ronphilbeckpottery.com/technical/

The digitalfire.com site that I mentioned in the post has a lot on it that is very helpful, you'll find an index of articles they have written at
http://digitalfire.com/4sight/education/index.html

Amy said...

Peter, Thanks a bunch for that digital fire link. Will check it out! This in itself is encouraging... And please send some of that cold weather to North Carolina, U.S. :) All the best-

gz said...

glazes are fascinating and frustrating.
That last pot and the pooling in the neck is lovely...you never know, I might be able to see that in person...we have plans, if we can get the ££££...and friends in Morrinsville, Waikato......
meanwhile, back to the research!!

Peter said...

Hi Amy,
We're at 2.8 degrees Centigrade as I write(about 37 F), it is actually warmer inside our refrigerator! but... that is "warm" compared with a few days back at -6 C at this hour of the morning! So I hope that makes you feel cool in NC!

Hello Gwynneth,
I do hope your plans blossom and flourish, it would be so nice to see you if you do managed to visit this part of the world. Very welcome to Email me if you want further information about our (South Island) part of NZ.