Glazes melt. If they do not melt enough then the surface is not glassy and smooth and easy-to-clean. It stains, cutlery marks or leaches metals in to food and drink. On the other hand, if a glaze melts too much it runs down off the ware during firing. But there are other consequences. Glazes that run likely do so because they have excessively high levels of fluxing oxides. Or inadequate Al2O3. Both of these suggest the presence of chemical imbalances that contribute to leaching and lack of durability (just like under-firing). Running glazes often also crystallize on cooling, this may be a sought-after visual (difficult to keep consistent). Or it may be a defect.
Many so called "reactive glazes" are in fact over-fired glazes. It is common, for example, to see glaze recipes labelled as cone 6 whereas they are in fact low fire glazes being over-melted to get variegated surfaces. People often learn how to use them (toleration might be a better term), applying them thick enough to get the effect but thin enough that they do not run too much. It is a balancing act. But as already noted, these are best employed on non-functional surfaces).
Runny glazes almost always craze. This is because of two things: More fluxes are needed to make them melt (and fluxes have high thermal expansions). Less Al2O3 and SiO2 are desirable (these are low expansion).
Glazes do not need to be runny to be glossy. Good gloss glazes have high SiO2 and lower Al2O3, but they are stable on ware and do not run.
Iron oxide is unique in this respect. In oxidation, it is a refractory. But in reduction firing, its fluxing power is geometrically related to percentage, often running much more than would be expected.
This recipe melts to such a fluid glass because of its high sodium and lithium content coupled with low silica levels. Reactive glazes like this produce interesting visuals but these come at a cost that is more than just the difficulty in firing. Recipes like this often calculate to an extremely high thermal expansion. That means that not only will this form a lake in the bottom of ware when used on the inside, but the food surfaces will craze badly. The low silica will also contribute to leaching of the lithium and any colorants present.
The glaze is running down on the inside, so it has a high melt fluidity. "High melt fluidity" is another way of saying that it is being over fired to get the visual effect. It is percolating at top temperature (during the temperature-hold period), forming bubbles. There is enough surface tension to maintain them all the way down to the body, and for as long as the temperature is held. To break the bubbles and heal up after them the kiln needs to be cooled to a point where decreasing melt fluidity can overcome the surface tension. The hold temperature needs to be high enough that the glaze is still fluid enough to run in and and heal the residual craters. A typical drop temperature is 100F.
Crystallization (also called devritrification). You can see the tiny crystals on the surface of this copper stained cone 6 glaze (G3806C). The preferred orientation of metallic oxides is crystalline. When kilns cool quickly there is simply not enough time for oxides in an average glaze to organize themselves in the preferred way and therefore crystals do not grow. But if the glaze has a fluid melt and it cools slowly through the temperature at which the crystals like to form, they will. There is another issue here also: There are tiny dimples in the surface. This is because copper carbonate was used here instead of copper oxide. During firing, it generates carbon dioxide (because it is a carbonate) that bubbles out of the melt, leaving behind dimples that may or may not heal during cooling.
The original cone 6 recipe, WCB, fires to a beautiful brilliant deep blue green (shown in column 2 of this Insight-live screen-shot). But it is crazing and settling badly in the bucket. The crazing is because of high KNaO (potassium and sodium from the high feldspar). The settling is because there is almost no clay. Adjustment 1 (column 3) eliminates the feldspar and sources Al2O3 from kaolin and KNaO from Frit 3110. The chemistry of the new chemistry is very close. To make that happen the amounts of other materials had to be juggled (you can click on any material to see what oxides it contributes). But the fired test reveals that this one, although very similar, is melting more (because the frit releases its oxide more readily than feldspar). Adjustment 2 (column 4) proposes a 10-part silica addition (to supply more SiO2). SiO2 is the glass former, the more a glaze will accept, the better. Silica is refractory so the glaze will run less. It will also fire more durable and be more resistant to leaching.
This is a zircon-based kiln wash. Even though it paints on in a thin layer, there is no problem releasing the very runny glaze from the shelf.
Iron is among the most powerful of fluxes in reduction firing. This is normally a glossy glaze, but the kiln was slow-cooled, resulting in total crystallization of the surface. The crystals are larger and layered at the neck. Their presence, as a thin layer on top, has completely matted the rest of the surface. Enough glaze ran downward off the piece that the vase was left sitting in a pool of molten glass.
|Oxides||Al2O3 - Aluminum Oxide, Alumina|
|Oxides||SiO2 - Silicon Dioxide, Silica|
Fluxes are the reason we can fire clay bodies and glazes in common kilns, they make glazes melt and bodies vitrify at lower temperatures.