Unlike crystalline minerals, glazes do not have a specific melting temperature, they soften over a range of temperatures. And after they have been melted they become increasingly fluid and homogeneous. The softening process is not a linear one, this is especially so because raw glaze powders are a mix of many different mineral (and possibly man-made) particles, each of which has its own melting behavior. Flux particles melt first but not just because they are fluxes. The mere proximity of other particles with which they like to interact creates a union that melts readily at temperatures much lower than either individual one would melt on its own. These interactions are also greatly enhanced when the particles are smaller and when groups of different particles react together (the more different types and sizes there are the more complex is their interactions). And then, as viscous glass melts evolve, they develop their own changing dynamics that can accelerate or slow the dissolution of remaining larger and more refractory unmelted ones. When sufficient time is available, the entire mass is melted. However as the last particle dissolves the melt exists as a discontinuity of chemistries and viscosities (phases). Message: OnLine INSIGHTAs yet more time passes the homogeneity of the mix increases. The higher the proportion of fritted materials the more predictable the process is.
How do metal oxides compare in their degrees of melting?
Metallic oxides with 50% Ferro frit 3134 in crucibles at cone 6ox. Chrome and rutile have not melted, copper and cobalt are extremely active melters. Cobalt and copper have crystallized during cooling, manganese has formed an iridescent glass.
A frit softens over a wide temperature range
This is unlike some raw materials which melt suddenly.
Frits melt so much better than raw materials
Feldspar and talc are both flux sources (glaze melters). But the fluxes (Na2O and MgO) within these materials need the right mix of other oxides with which to interact to vitrify or melt a mix. The feldspar does source other oxides for the Na2O to interact with, but lacks other fluxes and the proportions are not right, it is only beginning to soften at cone 6. The soda frit is already very active at cone 06! As high as cone 6, talc (the best source of MgO) shows no signs of melting activity at all. But a high MgO frit is melting beautifully at cone 06. While the frits are melting primarily because of the boron content, the Na2O and MgO have become active participants in the melting of a low temperature glass. In addition, the oxides exist in a glass matrix that is much easier to melt than the crystal matrix of the raw materials.
Melting range is mainly about boron content
Fired at 1850. Notice that Frit 3195 is melting earlier. By 1950F, they appear much more similar. Melting earlier can be a disadvantage, it means that gases still escaping as materials in the body and glaze decompose get trapped in the glass matrix. But if the glaze melts later, these have more time to burn away. Glazes that have a lower B2O3 content will melt later, frit 3195 has 23% while Frit 3124 only has 14%).
Why fast fire glazes employ zinc - a melt fluidity test tells us
We are comparing the degree of melt fluidity (10 gram GBMF test balls melted down onto a tile) between two base clear glazes fired to cone 6 (top) and cone 1 (bottom). Left: G2926B clear boron-fluxed (0.33 molar) clear base glaze sold by Plainsman Clays. Right: G3814 zinc-fluxed (0.19 molar) clear base. Two things are clear: Zinc is a powerful flux (it only takes 5% in the recipe to yield the 0.19 molar). Zinc melts late: Notice that the boron-fluxed glaze is already flowing well at cone 1, whereas the zinc one has not even started. This is very good for fast fire because the unmelted glaze will pass more gases of decomposition from the body before it melts, producing fewer glaze defects.
Glossy blacks are best made adding a black stain to a quality base transparent
The glaze on the left is called Tenmoku Cone 6 (a popular, and old, CM recipe). It is 20% calcium carbonate, 35% Custer feldspar, 15% OM4 Ball Clay and 30% silica, 10% iron oxide. If you have any experience with glaze you will note two things that a fishy here: There is no boron, lithia or zinc sourcing material. How can this melt enough at cone 6? It looks melted, but the ease of scratching it shows it is not. So, it appears that if we saturate an incompletely melted glaze with a lot of refractory brown colorant on a dark body the effect can be black. A better idea is the glaze on the right. We start with a stable, reliable base transparent, G2926B. Then we add 5% Mason 6666 black stain (stains are smelted at high temperatures, quenched and ground, they are inert and relatively safe). A bonus is we end up with a slurry that is not nearly as messy to use and does not turn into a bucket of jelly.
Can a cone 6 functional glaze having only whiting and feldspar melt enough?
This flow test compares the base and base-plus-iron version of a popular CM recipe called "Tenmoku Cone 6" (20% whiting, 35% Custer feldspar, 15% Ball Clay and 30% silica, 10% iron oxide). Although iron is not a flux in oxidation, it appears to be doing exactly that here (that flow is just bubbling its way down the runway, the white one also fires to a glassy surface on ware). It looks melted in the tray on the right but notice how easily it is scratching on the tile (lower left). This demonstrates that looks can be deceiving. Cone 6 functional glazes always have some percentage of a power flux (like boron, lithia, zinc), otherwise they just do not melt into a hard glass. Maybe a glaze looks melted, but it has poor durability.
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