Fluid melt glazes and over-melting, over fired, to the point that they run down off ware. This feature enables the development of super-floss and cyrstallization.
Fluid melt glazes have a melt that is fluid. When the kiln is at temperature, the glaze, if applied thick enough, melts enough to run down vertical surfaces, even off the ware. In bowl shapes, fluid melt glazes will collect in a lake at the bottom (which, of course, comes with its own issues). The most extreme example of fluid melts are crystalline glazes. They contain almost no Al2O3 and the high levels of ZnO flux enables them to run to the extent that ware needs to be sitting in a bowl (where runoff accumulates in a molten lake). That being said, fluid melt glazes an also be practical on function ware. This is because a glaze can have a melt fluidity that runs when thick, but when applied at normal thickness, it will stay put. By employing a melt flow tester one can tune fluidity quite accurately.
To have a fluid melt, a glaze must either have much more fluxing oxides than normal for the temperature, or have little or no Al2O3. In these two situations it almost always means that the thermal expansion will be high. This is especially the case since most glazes have significant K2O and Na2O, both having super high expansions. That means crazing. Workable exceptions to this include trading KNaO for low expansion fluxes (e.g. MgO, Li2O) coupled with a high boron content. Of course lead or bismuth can also be employed, but each his its issues. The price of lithium cannot be ignored, it is among the most expensive materials used in ceramics now. But using powerful fluxes enables maintaining the SiO2 and Al2O3 at normal levels (thus holding on the durability).
Typically, when we speak of fluid-melt glazes, we are referring to the base transparent, without colorant. That means that most existing bright glazes could be made brighter and more brilliant by transplanting their colorants into a fluid melt base. By separating the base (from its color variations) development or adjustment efforts are simplified and can be targeted at it fitting the clay body, having good application properties, being durable and having a tolerable level of fluidity. As noted, this type of glaze is going to be more expensive. Potentially a lot more expensive! But when the brilliant surfaces and colors it can produce become evident, the
Fluid melt glazes can be glossy or matte. If they are matte it is because crystallization occurs during cool-down. This will almost always happen if the glaze contains significant levels of metal oxide colorants. In extreme examples, a quickly-cooled piece (where insufficient time is available for crystallization) can be a brilliant deep brown or black whereas the same piece, when slow-cooled, might be a light yellow or gold color (where tiny crystals cover the entire surface).
Fluid melt glazes are much easier to create at higher temperatures, the kiln turning ordinary fluxes into super fluxes. Cone 10R copper reds owe their character to the fluid melt. Cone 10R tenmoku glazes, likewise, have a brilliant glassy surface due to the powerful fluxing properties of iron in reduction atmospheres. At cone 10 it is easier to create craze-free surfaces because SiO2 and Al2O3 levels (which both have low thermal expansions) can be much higher. At medium temperatures, a fluid melt can be as easy as simply over firing a low temperature (cone 04-06) glaze to cone 6 (of course it will likely craze).
In 2015 I documented a project comparing common cone 6 fluid-melt base glazes, picked a favourite (Panama Blue) and over-hauled the recipe to fix it's slurry issues and serious crazing. While fluid-melts almost run off ware when applied thick, they host stains & opacifiers to produce brilliant super-gloss surfaces. But the typical chemistry of these is susceptible to crazing, scratching and leaching. In 2019 I reformulated again, moving the thermal expansion down from 7.3 to an incredible 5.8 (the base can now survive a 325F-to-icewater test on our toughest-to-fit porcelain with no crazing). And it is melt-fluidity-controllable, durable (having 30%+ more Al2O3/SiO2) and sources Li2O, MgO and KNaO from frits. Follow the link here to see the entire history of this development effort (beware, there are multiple pages, each with many columns).
This is not just a typical transparent cone 6 glaze with 2% copper carbonate added (and 2.5% tin oxide). Knowing what is different about this clear base, its trade-offs and how it was developed are important. The porcelains are Plainsman P300 and M370. The liner glaze is G2926B, it is a gloss but has a much lower melt fluidity than the outer glaze, G3806C (as a functional transparent its main job is to fit the body and be hard and durable). But in order for that outer glaze to accommodate the copper and still be super glossy it must have a much higher melt fluidity. It was tricky to develop since that fluidity comes with high sodium and lower silica, that raises the thermal expansion and moves it toward crazing.
I am comparing 6 well known cone 6 fluid melt base glazes and have found some surprising things. The top row are 10 gram GBMF test balls of each melted down onto a tile to demonstrate melt fluidity and bubble populations. Second, third, fourth rows show them on porcelain, buff, brown stonewares. The first column is a typical cone 6 boron-fluxed clear. The others add strontium, lithium and zinc or super-size the boron. They have more glassy smooth surfaces, less bubbles and would should give brilliant colors and reactive visual effects. The cost? They settle, crack, dust, gel, run during firing, craze or risk leaching. In the end I will pick one or two, fix the issues and provide instructions.
These are cone 6 commercial glazes made by a popular US manufacturer. The body is a cone 6 casting porcelain made by another popular manufacturer. Zoom the photo, they are all crazing! Which company is at fault? Neither has the responsibility (or is able, especially with stonewares and porcelains) to match their product to that of every other company. The pattern we see here points-the-finger at the body. Mid-fire porcelains craze glazes much more if they lack sufficient silica (20% is minimum). It is difficult for manufacturers to achieve this since much more feldspar is needed to vitrify the body. And the potter does not know the recipe of the porcelain. What to do then? One option is to get a porcelain from another supplier, with assurances from them about glaze fit. Better yet, mix your own. Casters need a mixer anyway, so why not? We can help you with a recipe if you need it. Actually, mixing your own glaze also would get rid of those micro-bubbles and give a glassier surface.
The glaze on the left (as shown in my account at insight-live.com) is a crystal clear at cone 04. The high frit content minimizes micro-bubbles. The high B2O3 melts it very well (this has 0.66 B2O3, that is three times as high as a typical cone 6 glaze). The recipe on the right is the product of a project to develop a low-thermal-expansion fluid-melt transparent for cone 6 (with added colorants fluid melts produce brilliant and even metallic results and they variegate well). While the balance of fluxes (the red numbers in the formula) is pretty different, look how similar the B2O3, Al2O3 and SiO2 levels are (yellow, red and blue backgrounded numbers in the formula), these mainly determine the melting range. That means that a fluid-melt cone 6 glaze is actually just a low temperature glaze being overfired to cone 6.
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.
This is the same glaze on the outside of these two pieces. It develops the variegated deep blue character only when thick. But if it were applied thick enough on the left piece it would run off onto the kiln shelf. However the recesses in the texture-rolled surface of the one on the right have caught the flow, creating the thicknesses needed to get the color. Another factor is that the piece on the right is buff stoneware. Thus the clay contains some iron and it is bleeding into the glaze to help develop the color.
The outer green glaze on these cone 6 porcelain mugs has a high melt fluidity. The liner glaze on the lower one, G2926B, is high gloss but not highly melt fluid. Notice that it forms a fairly crisp boundary with the outer glaze at the lip of the mug. The upper liner is G3806C, a fluid melt high gloss clear. The outer and inner glazes bleed together completely forming a very fuzzy boundary.
The boron and zinc fluxes make the melt of this glaze highly fluid at cone 7R. That comes with consequences. Notice the Al2O3 and SiO2 in the calculated chemistry. They are at cone 04 levels. The significant ZnO increases surface tension of the melt, this helps bubbles form at the surface (like soap in water). Al2O3 and SiO2 could be added (via more clay), this would stiffen the melt so the large bubbles would be less likely to form (this glaze melts so well that it could accept significantly more clay without loss in gloss). A drop-and-soak firing is another option, in this case a drop of more than 100C might be needed (see the link below to learn more).
This transparent glaze adds a little manganese and iron, just enough to give color, but still maintain transparency to highlight the decorative crack-network in the engobe below. However this glaze is not as brilliant and transparent as it could be. As you can see in the surface reflections, it has an "orange peel" texture on the glass surface. This is due to a combination of factors (e.g. not enough melt fluidity, gassing of the manganese during melting, cooling the kiln too quickly). If the colorants were transplanted into a more fluid-melt transparent, this glaze could be improved. Photo courtesy of J. Decker.
This glaze creates the opaque-with-clear effect shown (at cone 7R) because it has a highly fluid melt that thins it on contours. It is over fired. On purpose. That comes with consequences. Look at the recipe, it has no clay at all! Clay supplies Al2O3 to glaze melts, it stabilizes it against running off the ware (this glaze is sourcing some Al2O3 from the feldspar, but not enough). That is why 99% of studio glazes contain clay (both to suspend the slurry and stabilize the melt). Clay could likely be added to this to increase the Al2O3 enough so the blisters would be less likely (it would be at the cost of some aesthetics, but likely a compromise is possible). There is another solution: A drop-and-soak firing. See the link below to learn more. One more observation: Look how high the LOI is. Couple that with the high boron, which melts it early, and you have a fluid glaze melt resembling an Aero chocolate bar!
In ceramics, glazes melt to produce a liquid glass. That glass exhibits surface tension and it is important to understand the consequences of that.
Ceramic glazes melt and flow according to their chemistry and mineralogy. Observing and measuring the nature and amount of flow is important in understanding them.