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