Making ceramics is magical, ceramics mystifications certainly are not. Ceramics and glazes are surrounded by a cloud of thick black smoke. And I do not mean the smoke when firing raku.
Theory behind practice is often unnecessarily made very complicated. Because of ignorance, to impress? I have no idea, but it has always frustrated my quest for clear knowledge. Time for unmasking.
In this blog I grab three ceramics mystifications by the clay.. euh horns.
1. glazes are a mystery
Ask on FB or another forum, to solve a glaze problem and you will get dozens of different answers. Often contradictory. Even in books you get many half-truths and whole untruths.
About glazes and in particular the melting temperature are the most mystifying theories . As there are:
- Closest packing of spheres by Bert Cool
- The acid number (Säurezahl) by Lehnhäuser
- Viscosity to Lakatos using Vogel-Fulcher-Tammann comparison
- Melting temperature using melting coefficient “K”, by Lengersdorff (from Stevanof & Batschwarov)
- EuCal Eutectics spreadsheet by Robert Magnuson
Some indicate a certain direction (such as the viscosity of Lakatos or the acid number of Lehnhäuser). But from a few of the other theories, the added value is unclear and in addition, some are not so scientific. With the advent of PC and Excel, the most complex calculations be made. A blessing for humanity, but that does not mean that all calculations make sense.
Theoretical models can be very useful. A model to understand rheology and viscosity of glazes are very valuable for research. But at the moment they do not give practical guidelines in the development of glazes. Complicated calculations (especially if these are totally whiteout any scientific basis) makes it only more unclear.
Predict melting temperature
From 1900 onward, theoretical calculations where made to predict at which temperature a glaze will melt. From the ’70 of the last century this has largely stopped. It became clear that there are simply too many factors to make an accurate prediction of the firing temperature on the basis of (very different) chemical compounds.
Does this mean that glazes are a mystery and it probably will always remain so? Definitely not!
In our 21st century there is so much (public) data available, that it is just a matter of organizing and interpreting. Simple guidelines allow you to use the U.M.F. (Unity Molecular Formula) for simple insights. Using the Stull chart and target formulas, gets you in the right direction.
By looking up certain types of glazes (glazy.org) and studying, you can design test blends. Theory points you in the right direction and with practical tests you get results quickly. This way you develop your own “wish glazes” (or “Wunsch Glasuren” as Gustav Weiss calls them).
2. Stoneware is a clay type, firing temperature does not matter
A few days ago someone asked on a forum how the stoneware clay that she had fired could be made waterproof. There were many reactions; silicone spray, commercial waterproofing mixtures from different suppliers, grease, etc… All solutions undoubtedly work, but there was only one person who gave the simple answer: higher firing!
Stoneware and porcelain clay becomes impenetrable and waterproof…… when fired high enough. Stoneware must be fired to at least 1200 oC (2192 F). But for an water absorption of less than 0.5% (the industrial standard) usually a temperature of more than 1250 oC (2282 F) is usually necessary.
Is waterproofness the only criterion of stoneware ceramics? By no means!
Stoneware is not only waterproof, because of the higher firing temperature it is also much stronger. From approx. 1200 ( 2192 F) degrees, the clay transforms into mullite. From about 1250-1280 oC (2282 – 2336 F) this mullite forms a fibrous crystal structure, which makes as a result the object not only impenetrable, but also much stronger.
Mullite selfies
A study from Maximina Romero (2010) used images made with an electron microscope (SEM, yes very cool), at which temperature this mullite emerges during the firing process. This demonstrated that stoneware ceramics are not only produced by selecting the right clay. If you don’t get it fired above 1250-1280 oC (2282 – 2336 F) it will never become (real) stoneware.
Glazes can be adjusted for a lower temperature, stoneware ceramics do not. The Chinese already knew that more than 2000 years ago. But it is very exciting that in the 21st century “mullite selfies” have been made, to explain.
3. Pyrometer determines the firing temperature
Firing of ceramics seems to be black magic. The most wild rumors about firing accidents are circulating (the quartz jump, the emergence of cristobalite, etc).
Of course also in my kiln there are Gremlins. Just when I have to finish something, the kiln fails or the work cracks. Just when my best work is in the kiln, my spirals burn through or….. this blog is too short to handle all the fire horror stories.
So I confine myself here to the ceramics mystifications about kiln temperature. When everything is done the pots are handed over to “the flames”. The outcome is determined by the kiln gods. Sometime a glaze looks this way, another time completely different….
Nowadays, many ceramists have computer-controlled kilns and at the very least a pyrometer. So they can determine at which temperature there work has been fired. When ceramists start using a glaze recipe from school at home, the result can suddenly be quite different. How is this possible? While “it has been fired at the same temperature”?
Heatwork
The “same temperature” means the temperature that the pyrometer indicated (or was typed into the kiln controller). But you must also take into account the speed of firing (especially the last 100 – 150 degrees) and the speed of cooling. Not only the top temperature affects the glaze (and the clay body) also firing time.
This is called “heatwork” and usually measured with cones. These are numbered and bend at a specific temperature and firing speed (more about it in this blog). So if someone fires at 1250 oC (2282 F), but very slowly, cone 10 can bend. While fired another time very quickly to 1280 oC (2336 F), cone 8 doesn’t even bend. As a result, the glaze of the first firing can be shiny (well-melted), whereas at the “higher temperature” it is still stony mat (under fired).
The same difference also applies in different places in the kiln. Because many kilns have a top to bottom difference in temperature of more than 2 to 3 cones (20 to 40 degrees!). This can make a lot of difference in the appearance of a glaze, even without Gremlins.
More ceramics mystifications
The last word has not been said about ceramics mystifications. Where ignorance prevails, Urban legends are emerging.
The technology of ceramics and glazes is very complicated and complex. This should certainly not be underestimated. But it makes no sense to be taken off track by speculations and wild theories. I try to discover the broad insights, to find general guidelines that are unambiguous and clear.
I have a lot of respect for the researchers and/or the ceramists who develop new theories (with a scientific bases). There is still a lot to learn and discover. So I read everything and listen to everyone, but I only use what proves itself in practice. And the results from my own kiln.
So don’t believe anything in this blog, but find out for yourself…
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