Efficaincy of glaze. Perfect example.
I've recently been doing a large amount of tests with the glazes recently. playing with silica, alumina, boron, just to see the limits of where they change based on molar amounts. Because we know that alumina and silica do not melt anywhere near cone 10 or 6 we use fluxes such as alkaline earth and alkaline metal (fluxes) to lower the temperature so the glaze can “melt / mature” at lower temperatures like cone 6 and 10. Simple. What many people do not take into account is that while flux is a main melter in glazes the amount of alumina and silica are major factors in melting temperature as well. Yes you need glass formers like silica (usually) and alumina (clay or Kaolin) to make a glaze in general but too much of them will make a glaze work harder regardless of the amount of flux. TLDR instead of adding more flux you can reformulate it to have less silica and/or alumina. Those things (outside matt and gloss) don’t matter much to the color. Many artists break their back to figure out how much flux and in what form they need to keep color when they might just need a lighter load for the glaze to be more efficient with it’s fluxes. Recently I've been playing with the amount of lithium as a flux in a new glaze line I'm developing. The hypothesis was that I could decrease the amount of lithium needed by increasing the amount of boron or decreasing the silica and alumina, and can I create general parameters of silica and alumina with regards to how efficient I can make a glaze? The answer is yes.
Here is what usually happens. If you work around a glaze you will inevitably end up working around the silica and alumina amounts. If not on purpose than on accident. This will, to some extent, change the amount of energy needed for the desired reaction. Here is a good example.
This glaze was the result of playing with the amount of silica, alumina, and flux to find the correct ratio that won’t burn out the glaze.
Here is a close up. These iron crystals need enough flux to be developed but not so much that the glaze burns out. But it’s not as simple as adding lithium carb while reworking the glaze recipe.
Let’s take a glaze that has both too much silica and not enough flux at the same time for the effect I'd like. Van Gilders tenmoku glaze made for cone 6 ox for example.
This is a good result of Van gilders tenmoku (from the John Britt glaze book I have) The recipe itself has a little too much silica and alumina. This is effecting the amount of work the Flux has to work to develop the iron in the glaze. You see lots of brown here. The glaze is trying to become red but the work load is too much.
This is my glaze. It’s in the same family as the above glaze but has less silica and alumina. Do you see how much more red I'm getting? This glaze is very sensitive. My glaze and the glaze from Van gilders could be considered brothers. One just with the fat trimmed.
Of course if we double down on lowering the amount of alumina and silica to decrease the milky temperature without adding any flux we have to do the reverse as well. We have to “confirm the kill” so to speak. This next glaze is around (if not the same) chemical composition but has much more silica in it, increasing the amount of work the flux has to do to get the glaze to melt. The last test in this line has relatively the same amount of flux, boron, iron, all that. Because the silica and alumina levels do not effect the color too much (outside of how efficient the flux / melt it) I could reformulate the glaze to have much more silica and alumina. This increases the amount of work the flux has to do to melt everything.
I will admit there probably is a limit to how much you can decrease the silica and alumina amounts before the point where it might run off the pot or become unusable. The glaze wouldn't be hard enough to use functionallly. But the point stands. As a final look lets look at two pictures side by side with roughly the same amount of flux and lithium carb with different Al2O3 (alumina) And SiO2 (silica) amounts.
I kept everything else the same. The Iron, phosphorus, calcium, Mg0 levels, everything is the same if not very close to it. Look at the comparison above. Even with 1% more lithium the bottom glaze is struggling. Same water amount, Same frost porcelain clay to boot. To end off this blog I will say whenever I help people to develop a glaze I almost always look at how much silica and alumina they have in the glaze ( given it’s not trying to make a Matt glaze) and reduce them. Usually it’s silica. So many minerals have extra silica in them, there is no shortage of silica in the glaze recipe. Wanna know the real kicker? All of these tests were done in my tester kiln with faulty heat work. None of these have a hold or slow cool cycle and to top that of the kiln cooled down so fast the glaze didn't get a chance to “develop” with extra heat coming from the pieces around them or any heat being held in the kiln to let them “cure”. My tester kiln is very small so 1 piece in, 1 piece out. None of these tests were done under preferable conditions.
WHen I took my glaze chemistry classes they told me this would happen, but of course I had to test it myself. Here you see the proof. The amount of silica and alumina are increasing the work load for the rest of the glaze to an extent. Keep in mind the Cone system is developed off of the amounts of silica and alumina in a glaze. The chart is pretty easy to remember. For example.
Note: This picture rom my ceramic materials workshop classes I took a while back. Credit to Matt and Rose Katz.
What people don’t tell you ( or at least I believe more emphasis should be put on it) is glaze STARS melting/ developing at these ratios. If you have 0.5 alumina and 5.0 silica and glaze will START to melt/ develop at cone 5. The glaze is often fully developed or matured about 4 cones latter. This is why many recipes, regardless of boron, will often have ratios like this pictures in the first part of this blog. They have a ratio of almina to silica of 02 : 2.0 or so. I was always taught that in order for a glaze to develop at cone 6 you needed a specific amount of boron in it. While that is mostly true, that is not what the original cone system is based on. So when I develop a glaze I take the amount of Alumina, Silica, Boron, and temperature into account. It’s not as simple as “Just dump boron in a cone 10 glaze” How early the glaze develops and the efficiency of the glaze are also effected by the amount of (mostly) Alumina and silica present.
I know this was a long blog so lets make a quick review.
The mount of alumina : silica in a glaze is mostly was the cone system is set (see chart above) example 0.5 alumina to 5.0 silica would start to melt/ mature at cone 5. Keep in mind a glaze STARTS to melt / develop at this range. This means you often want a glaze that is developed for cone 5 to really have the alumina and silica ratios of a cone 2 or 3 glaze. 0.2 : 2.0
Because the heat required to melt a glaze, and alumina takes more heat to melt than silica. Here will almost always be more silica than alumina in a glaze. Notice the ratios for the cone chart above are 0.5 alumina and 5.0 silica. About 10X more silica. Unless it’s a specialty glaze this is how most base glazes work.
A good amount of Boron still needs to be present for a good melt. Usually when I present this information people say “ than why do we need boron”? We kind of do. Perhaps not a huge amount but Boron is not only a good flux / glass former but it also does a good job at melting a glaze without the use of such extreme fluxes and work around s that would often come with going over certain limits. So when I make a glaze I put in an amount of boron while also looking at the Alumina to silica ratio. Boron or not.
