This time I will write a bit on the thermal expansion of glazes.
The main causes of crazing and shivering are in the thermal expansion of glazes not being in equilibrium with the fired clay used.
In the case of crazing, the glaze contracts more than the clay upon cooling and 'breaks' when the tension becomes too high.
In the case of shivering, the reverse occurs and the glaze becomes 'to big' for the clay and splits off from the fired clay body. This can occur sometimes long after cooling, up to years.
Obviously, the aim is to mix glazes so that their thermal expansion is in equilibrium with the fired clay. Again here, the book of J. Hesselberth and R. Roy ( Mastering Cone 6 glazes) is the reference for further reading. The authors spent quite a long article on this topic with plenty of practical experiments. How they measure thermal expansion is not meant for day by day potters but gives a good scientific proof for how glazes should be made up.
As an important factor for the glaze quality and as an aid to learn more in the future, I use this parameter in the documentation of my ceramic work.
Again, by the use of a glaze calculation software, the values for thermal expansion can be obtained. Also again, this can be obtained by calculating by hand, provided that one has the thermal expansion coefficient for each ingredient.
The graph at the end of the text is a bit hybrid this time. It combine Si/Al ratio and thermal expansion. It also contains SiB/Al ratio. Boron has an exceptional behavior in glazes. In the unity formula it is grouped under Amphoteric but, it act as a glass former too. This makes it more complex. The SIB/Al is less a good indicator for gloss. I should maybe write: 'for matt' . The power for Boron to 'dissolve' Alumina is enormous. So, more glossy glazes are to expect.
Some authors tend to classify Boron as a flux or as a glass former also.
I keep the traditional Seger (unity) formula where Boron is an amphoteric (sometimes called: stabilizer). Although I am using Boron for ages in my glazes, I am still learning how to tackle its mechanism. Therefore, I keep record of the SiB/Al to gather data before drawing conclusions.
Michael Bailey spent a whole chapter on the use of Boron in his book: 'Glazes Cone 6' (Ceramic Handbooks, A&C Black -London)
To construct the bar chart, you obviously need the aforementioned parameters from a glaze calculation software or calculated by hand.
We are coming close to the end of this series. Next time I will discuss how to put everything together in a database for documenting ones work.
Visit my website for more on glaze chemistry:
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Keeping Record of your work Part V Thermal expansion and its role in glaze flaws
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