I've had a few conversions recently about glaze durability. Here is my article that I published in the 2016 NCECA journal on that subject.
I looked into the the question of why glazes are durable or not and found that we are able to correlate the overall durability of a glaze to its underlying chemistry.
We are also able to put to rest the "Lemon Test" which we found was a test of little value.
We've been working on a long term research project on Oil Spots, with our research assistant TC @coneinfinity . We came up with this Orange Oil Spot glaze and we thought it was too much fun, not to post.
Shout out to some of our class members who have also helped in the discovery of this glaze. Jeannine Vrins @jeanninevrinsceramics and Jake Corby @jakecorboy
We continue the conversation about Cone 6, from our upcoming, online workshop. "The Middle Glazes: The Story of Cone 6 Glazes"
This time we are looking at how temperature effects a glaze. Above you will see four glazes, but they are all the same glaze. This is a Cone 10, Chrome-Tin Red Glaze that we developed. Its a slight tweak on CMW Gilbert's Red, which you can find here https://glazy.org/recipes/26657 . This glaze is designed to be a Cone 10 glaze, and that is the test in the upper left corner.
In this test we took this glaze and simply fired it at lower and lower temperatures, Cone, 8, 7, and 6. As you can see, the glaze has some of the characteristics of the original glaze. They are all red, which is good. But if you look closely you will see, that as the temperature goes down, the glaze goes from highly glossy to dull.
This is a great representation of the melting process because you are seeing the glaze melt. At Cone 6, the glaze is simply underfired, full stop. Some might want to call this glaze "Matte" but is absolutely not, it is underfired.
The tile on the lower right, represents a hue problem with a lot of "Matte" glazes. Quite often we hear the phrase "My matte glaze gets glossier, if I fire it a little hotter" What that means is that you don't have a "Matte" glaze, you have an underfired glaze. If you didn't know what you are looking at, you could very easily call the lower right a matte glaze, but in reality it is just an underfired glaze.
So how do we get temperature down??? How do we know if out glazes are Matte or underfired??? Stick with us, we'll show you everything.
More from our upcoming online workshop, The Middle Glazes: The how, what and why of mid temperature glazes.
Delayed crazing-it’s a thing!
Our student Lois Aranow asked us if she was seeing things, because work she had that was originally not crazed, we’re now crazed, even though the firing was a long time ago.
To learn more about how crazing works, check out our youtube channel, https://youtu.be/NkNda1R-xrofor a video describing exactly what crazing is and why it happens. What you can and can’t do about it, and what are total myths.
A lot of you commented that the glazes in our last post were crazed. The main thing to note is that A) Crazing is based on the chemistry B) Crazing is always dependent on the exact clay body you are using. This is why some glazes craze on one body and not another.
These two images are exactly the same tile. The left tile was photographed after the glaze has been cool for about six hours. The right image is the exact same tile four days later.
As you can see, the glaze continued to craze, even though it was completely cool when we took the first picture. This is because crazing is caused by the stress of Thermal Expansion.
Every clay and glaze has its own thermal expansion. As they cool, they settle into their expansion and if the two are radically different, the glaze will crack to relative the stress.
In some cases it can take days, weeks months or even years for the stress to set abs and for the work craze.
There are a hundred myths out there, as how to fix crazing. 99% of those answers are myths and wrong. There is only one answer.
This is a chemistry problem and that is the solution.
Join us in our online classes and workshops to learn more.
We’re going to be launching our next online workshop VERY SOON!!! It’s called The Middle Glazes: The how, what and why of mid temperature glazes. It’s going to blow your mind.
Here we have a cool representation of glaze chemistry at work. These are all variations on 20x5, but in each variation we change the fluxes.
20x5 is the 4321 of Cone 6. A glaze with a formula that is easy to remember. 5 ingredients, each at 20%. But the question is often, “Which Ingredients?” This year shows you that the answer is pretty much “Any”. The original formula by Tony Hansen, was composed of Custer Feldspar, Wollastonite, Frit 3134, EPK, and Silica.
In these tests we keep the Frit 3134, EPK, and Silica. What we changed is the Alkali Metal (Custer Feldspar) and Alkaline Earth (Wollastonite). We substituted other “Feldspars” Neph Sy and Spodumene and other Alkaline Earth sources, Whiting, Dolomite, Strontium Carbonate.
Lots more to dive into here. And we will in future posts.
We just got back from NCECA, and we have something to share with everyone. For the last few years, Rose and I have been working on a project, trying to redefine glaze chemistry as we know it.
We have been attempting to define the exact chemical role of the colorants in the U.M.F. Our NCECA paper will be published later, but for the moment We are publishing our the experimental glaze calculator. This calculator integrated the colorants into their prescribed roles as we've been able to define through our research.
Give it a spin and let us know!