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Melt Fluidity

Ceramic glazes melt and flow according to their chemistry, particle size and mineralogy. Observing and measuring the nature and amount of flow is important in understanding them.

Key phrases linking here: melt fluidity, melt-flow, melt flow, the melt - Learn more


Glazes become fluid when they melt, they are molten. The fluidity (or viscosity) of this melt needs to be considered, especially when troubleshooting problems. While two different fired glazes may appear to have melted a similar amount (even on a vertical surface), one may be radically more melt-fluid than the other (this becomes evident in a fluidity tester or when the glaze is applied thicker). While it might seem logical that a matte glaze has a fairly stiff (viscous) melt, it might actually be highly melt-fluid and runny (because the matteness is usually a product of crystallization on the surface during cooling or phase separation within the glass). How fluid can the melt of a glaze be? Almost like water in some cases (e.g. in highly fluxed glazes with almost no Al2O3 can run off a tile if it is tiled only a few millimetres). Melt flow testers are a good way to get a true picture of how fluid a glaze melt really is. In a well-designed melt flow tester, a glaze with the correct degree of melt flow will travel halfway down the runway. Testing melt fluidity using balls and cone shapes also provides valuable information.

Glazes do not simply melt, they soften. In non-fritted glazes, certain mineral particles in the powder mix melt alone (or multiple types interact to melt) before the others, these are the fluxes and they form what we call "the melt". As temperature increases, they dissolve refractory particles around them (e.g. quartz/silica, clay). The size of the refractory particles and the time available determine the completeness of the melting. Potters normally fire slow enough that it is assumed that all particles have gone into solution if a glaze appears well melted. But in industry, where firings go from cold-to-cold in a very short time, attention must be given to this. For this reason, the use of frits and ball milling to reduce silica particle sizes is standard practice.

Glaze melt fluidity relates closely to a variety of problems like pinholing, crawling, gloss, blistering, crazing, and even leaching. Logically, glazes for vertical surfaces will be more viscous than tile glazes, for example (which are applied to horizontal surfaces). Molten glaze viscosity can be understood in terms of molecular silicate chains (which also link across to other chains). The chemistry of the melt (and the degree to which materials have released their oxides to it) determines the rigidity of the structure and therefore the viscosity of the melt. Glazes high in powerful fluxes (like boron, lithium, sodium) melt and run more. In functional ware, for example, it is desirable to have enough melt to bring into solution all the material particles and produce a fired surface that has good gloss. However if too much flux is present the fired glaze is not as hard, it can have higher thermal expansion (if it contains high KNaO), may be more prone to blistering, and is more likely to leach. Thus it is best to tune the ratio of fluxes to SiO2 and Al2O3 such that the melt has the right degree of movement and no more. Even special-purpose reactive or matte glazes need to be tuned. In the case of the former, a compromise is needed between the high fluidity needed to produce the visual effect and a more stable and harder stiffer melt. For matte glazes, a less fluid type that relies more on high MgO rather than high Al2O3 only will have less cutlery marking of the fired glass.

Blistering often occurs in glazes of high melt fluidity. This might appear illogical since it would seem that such melts would more readily pass gases of decomposition from the body. However, the problem often happens because these glazes begin to melt (and seal the body surface) at much lower temperatures than one might think. Then they just keep percolating the escaping gases as the kiln is soaked and even continue after the kiln is shut off. Fast-dropping temperatures can freeze these blisters into the glass at an even lower temperature than they first melted at. Employing less gassing materials, applying a denser laydown of glaze, a flux system that melts later, or scheduling fire-down of the kiln will help.

The Potter's dictionary has a very good discussion with diagrams of this under the term 'viscosity'.

Related Information

Comparing glaze melt fluidity balls with their chemistries

Three glaze balls melting down into a pool

Ten-gram GBMF test balls of these three glazes were fired to cone 6 on porcelain tiles. Notice the difference in the degree of melt? Why? You could just say glaze 2 has more frit and feldspar. But we can dig deeper. Compare the yellow and blue numbers: Glaze 2 and 3 have much more B2O3 (boron, the key flux for cone 6 glazes) and lower SiO2 (silica, it is refractory). That is a better explanation for the much greater melting. But notice that glaze 2 and 3 have the same chemistry, but 3 is melting more? Why? Because of the mineralogy of Gerstley Borate. It yields its boron earlier in the firing, getting the melting started sooner. Notice it also stains the glaze amber, it is not as clean as the frit. Notice the calculated thermal expansions: The greater melting or #2 and #3 comes at a cost, their thermal expansions are considerably higher, so they will be more likely to craze. Which of these is the best for functional ware? #1, G2926B. Its high SiO2 and enough-but-not-too-much B2O3 make it more durable. And it runs less during firing. And does not craze.

Comparing the melt fluidity of two shipments of Custer Feldspar

A GLFL test for melt-flow to compare Custer Feldspar from Feb/2012 (right) with Mar/2011 (fired at cone 6). Custer Feldspar does not melt like this by itself at cone 10. It was mixed 80:20 Feldspar:Ferro Frit 3134. This test demonstrates that the material has been very consistent between these two shipments.

Melt fluidity tests as done by a frit manufacturer

The height down to which the cone melts is measured and recorded. Courtesy of Ashok Srivastava.

Melt fluidity differences are not obvious by just comparing glazed ware

The glaze on two porcelain mugs looks the same, but the melt flow is very different

These two Plainsman M370 mugs were fired at cone 6, the left one with G2934 matte glaze, the right one with G2934Y4 matte. They look and feel identical in the hand. The two glazes have the same chemistry. But they employ different materials to source that chemistry. The secret of of the matteness is high MgO (magnesia content). In the glaze on the left that MgO is sourced by dolomite, a lot of it. The glaze on the right sources it from a special frit, Ferro 3249. The impact of this difference is visible in the melt flows, the fritted one is obviously melting and flowing better. On other clays, especially stonewares, the G2934 can have a dry surface that cutlery marks. Thicker applications make it worse. But the Y version exhibits no such issues. Its mattness, durability, cleanability and hardness are so good that it is being used in floor tile.

A pottery glaze so melt-fluid it can eat through a firebrick. The fix struck boron-blue gold!

A glaze eats though a firebrick

The melt fluidity tester was fired at cone 6. The glaze on the left is G2826A2, a 50:30:20 Gerstley Borate glaze historically used for reactive glazes. The one on the right is G2926A3, an adjusted version that cuts the B2O3 level and adds lots of SiO2. The result is much more sane, although still very melt-fluid glaze. This is also a lesson in the chemistry that produces boron-blue, the one on the left does not and the one on the right does. This is the most decorative boron-blue we have ever seen, especially on dark bodies. Why? High B2O3 is not the key, it is lower. CaO is lower but it was higher in the original 50:30:20 recipe and that had plenty of boron blue. The SiO2 appears to be the enabler, it is much higher. And we are using 325 mesh silica, so it dissolves in the melt better.

Melt fluidity of Albany Slip vs. Alberta Slip at cone 10R

Albany vs Alberta Slip melt

Albany Slip was a pure mined silty clay that, by itself, melted to a glossy dark brown glaze at cone 10R. By itself it was a tenmoku glaze. Alberta Slip is a recipe of mined clays and refined minerals designed to have the same chemistry, firing behavior and raw physical appearance (but not plasticity). This is a GBMF melt flow test showing them side-by-side.

Copper oxide needs a fluid-melt transparent to produce a glossy glaze

Fired at cone 6. A melt fluidity comparison (behind) shows the G3808A clear base is much more fluid. While G2926B is a very good crystal clear transparent by itself (and with some colorants), with 2% added copper oxide it is unable to heal all the surface defects (caused by the escaping gases as the copper decomposes). The G3808A, by itself, is too fluid (to the point it will run down off the ware onto the shelf during firing). But that fluidity is needed to develop the copper blue effect (actually, this one is a little more fluid that it needs to be). Because copper blue and green glazes need fluid bases, strategies are needed to avoid them running off the ware. That normally involves thinner application, use on more horizontal surfaces or away from the lower parts of verticals.

This flow tester proves the colorant is not fluxing or bubbling the glaze

Copper carbonate fluxing testing

These cone 6 glazes are the same (G3806G), except the one on the right has 3.5% copper carbonate added. Copper is commonly thought to flux glazes, making them melt more. But in this case it percentage is not high enough, the clear base is running just as much as the stained one. Another observation: I was suspicious that the micro-bubbles in the glass matrix were coming from the copper carbonate gassing during firing. But not so, as you can see, the flow on the left has them also, actually, it has even more.

High B2O3 imparts better melt fluidity, but also fewer micro-bubbles

Glazed tiles and melt fluidity comparisons between the two clear glazes

A cone 6 firing. The glaze on the left has a B2O3 molar content of 0.54 whereas the one on the right has 0.64 (other oxide levels are the same). This is triple the typical amount of boron in a cone 6 glaze, the result is obvious: High melt fluidity for both. But G3904A has a significant characteristic that is different: The flow is more transparent because of the lower micro-bubble population. It's melt is less viscous, that enables the bubbles to pass, exit and the surface to heal. Why don't all glazes use more boron? Cost. Frits are expensive and they are the best source of boron. There is also a cost to durability (although mitigated when there is plenty of Al2O3 and SiO2 present, as is the case here). These recipes were part of an interesting project to fix a recipe where the potter mistakenly used Frit 3134 instead of 3124 when mixing a large batch of glaze. I calculated how much kaolin and silica to add to bring the chemistry back into line with the original. This was possible because frit 3134 chemistry is an approximate oxide-subset of 3124. The resultant glaze is potentially better than the original.

Melt flow test demonstrates glaze:clay interaction

Melt flow testers made from different clays

The melts being compared here are our code number 6998, a production run of Alberta Slip. The same sample batch and ball weight is being compared in these two flow testers fired side by side in a cone 10R kiln. Why are the flows behaving so differently? It is the clay from which the flow testers were cast. The one on the left is made from L4404A, a highly refractory casting slip. The one on the right is M370, a medium-temperature porcelain (it survives pretty well to cone 10 but is obviously very vitreous). The difference in the flows (the width and length) is a product of the interaction with the material being tested and the tester itself. On the M370 tester the flow is adhering to the clay surface so well that it has spread and thinned enough so that few bubble-breaks are visible. This interaction has even slowed the flow. But the L4404A flow tester is clearly better, minimizing interaction and better revealing the fluidity of the melt.

FeO (iron oxide) is a very powerful flux in reduction

This cone 10R glaze, a tenmoku with about 12% iron oxide, demonstrates how iron turns to a flux in reduction firing and produces a glaze melt that is much more fluid. In oxidation, iron is refractory and does not melt well (this glaze would be completely stable on the ware in an oxidation firing at the same temperature, and much lighter in color).

Highly melt fluid glazes that run are in style but do they produce quality ware?

A melt fluid pottery glaze

This is an example of a highly fluid glaze melt that has pooled in the bottom of a bowl. While it may be decorative, this effect comes at a cost. The melt fluidity is often a product of high KNaO - so not surprisingly it is crazing like mad! This crazing weakens the piece, much, much more than you might think. when glaze are thick like this the crazing becomes cracks, they go right down through that thick layer at the bottom of this bowl. And they want to continue right down into the body - and will do so at the first opportunity (e.g. sudden temperature change, a bump). Also, fluid glazes like this are much more likely to leach (because they lack SiO2 and Al2O3). That is crystallization in the middle, who knows how chemically stable that is? Yes, these are commercial glazes having a seal that suggests they are somehow immune to leaching, but multiple different layers are interacting here, there is no possible way to predict the resistance to leaching.

The glaze broke the bottom off the pot!

An example of a highly fluid cone 6 glaze that has pooled in the bottom of a mug (and crystallized). Glazes normally need to be under some compression to avoid crazing (by having a lower-than-the-body thermal expansion), but if they are thick like this the body does not have the strength to resist the extra outward pressure the glaze can be exerting at the base from the inside. The result here is a separated base. Conversely, if the glaze is under tension (having too high an expansion), the cracks that develop within it to relieve the tension are deep and wider and thus more likely to propagate into the body. The ultimate result: Poor ware strength and durability. A better solution is to use a liner glaze.

Glaze bubbles behaving badly! We see it in a melt fluidity test.

These melted-down-ten-gram GBMF test balls of glaze demonstrate the different ways in which tiny bubbles disrupt transparent glazes. These bubbles are generated during firing as particles in the body and glaze decompose. This test is a good way to compare bubble sizes and populations, they are a product of melt viscosity and surface tension. The glaze on the top left is the clearest but has the largest bubbles, these are the type that are most likely to leave surface defects (you can see dimples). At the same time its lack of micro-bubbles will make it the most transparent in thinner layers. The one on the bottom right has so many tiny bubbles that it has turned white. Even though it is not flowing as much it will have less surface defects. The one on the top right has both large bubbles and tinier ones but no clouds of micro-bubbles.

An extremely runny glaze at cone 6. The runniness is manageable, it has other issues!

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 the obvious cost of being runny like this. But this problem can be managed with glaze technique, a catch glaze on the outside and a liner glaze (to prevent the formation of a lake on the inside bottom (which leads to glaze compression problems). A bigger problem is that recipes like this often calculate to an extremely high thermal expansion. That means food surfaces will craze badly. Another issue that underscores the value of using a liner glaze: Low silica often contributes to leaching of the lithium and any heavy metals present in colorants.

A runny glaze is blistering on the inside of a large bowl

This rutile 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 overfired 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 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. Where is that? Only experimentation will demonstrate, try dropping a little more (e.g. 25 degrees) over a series of firings to find a sweet spot. The hold temperature needs to be high enough that the glaze is still fluid enough to run in and heal the residual craters. A typical drop temperature is 100F.

Our G2926B glaze may not work on dark burning clays

These two glazes, applied to the outsides of these mugs, both fire as brilliant glass-like super-transparents. But on this high-iron stoneware, from which both pieces are made, only one is working well. G3806C (on the outside of the piece on the left) melts more, it is fluid and much more runny. This melt fluidity gives it the capacity to pass the micro-bubbles generated as the body gases during firing. G2926B (right) works great on porcelain and buff stoneware but it cannot clear the clouds of bubbles coming out of this body (the bubbles are actually partially opacifying it). Even the normal glassy smooth surface has been affected. The moral: Potters need more than one base transparent recipe. Being able to host colors, opacifiers and variegators is nice, but sometimes just a transparent that works well is needed. An interesting trade-off of reactive melt-fluid glazes is that, while they develop more interesting surfaces, their lower SiO2 and Al2O3 contents make them susceptible to crazing, settling of the slurry and cutlery marking.

The same glaze fired at 1850F and 2200F

This is G3806F copper green glaze. It is highly melt fluid at cone 6. As the glaze begins to melt notice that it splits on the sharp concave sections on the handle join. The underscore the needs for the melt to be sufficiently fluid to flow back in and heal these areas.

White majolica bases have very, very low melt fluidity

This is Linda Arbuckle's base recipe (66% frit 3124, 23% feldspar, 13 kaolin/bentonite, 9 zircon, 4 tin oxide mixed to 1.62 specific gravity). It is fired at cone 05 creating a super gloss. This is applied very, very thickly (double the thickness of what a stoneware glaze would be). Yet notice how the air-vent holes did not heal during firing, these would have filled in easily had this been on stoneware or porcelain. For this reason, majolica glazes must be applied to ware not having any abrupt concave contours (this also happened on the handle-joins and where foot-meets-wall inside). Bisque must be clean to assure good adherence. Any glaze coverage issues must be repaired carefully before firing. This piece was bisque fired at cone 06, that is the reason for the air vent holes. Had it been bisque fired at cone 04 (higher than the glaze firing) these would not have occurred. However that would have extended the dip-time, this one was held under 10 seconds whereas a cone 04 bisque would have required 20 seconds.

How can you make Ravenscrag Floating Blue dance more?

Here it is fired to cone 8 where the melt obviously has much more fluidity! The photo does not do justice to the variegation and crystallization happening on this surface. Of course it is running alot more, so caution will be needed.

The same glaze fires very differently depending on kiln cooling rate

Both mugs use the same cone 6 oxidation high-iron (9%), high-boron, fluid melt glaze. Iron silicate crystals have completely invaded the surface of the one on the left, turning the gloss surface into a yellowy matte. Why? Multiple factors. This glaze does not contain enough iron to guarantee crystallization on cooling. When cooled quickly it fires the ultragloss near-black on the right. As cooling is slowed at some point the iron will begin to precipitate as small scattered golden crystals (sometimes called Teadust or Sparkles). As cooling slows further the number and size of these increases. Their maximum saturation is achieved on the discovery, usually by accident, of the likely narrow temperature range they form at (normally hundreds of degrees below the firing cone). Potters seek this type of glaze but industry avoids it because of difficulties with consistency.

How to make this glass pooling effect in pottery bowls

A small pottery bowl with melted glass centre

These are called "Little Dishes" and are sold in tourist areas. They are made in the USA. They appear to be fired at cone 10R (because of the appearance of the glazes and bare clay on the foot). After glaze application and pigment banding, a thick layer of glass powder (glass cullet) is poured in. Since this type of glass has a high CTE it crazes thoroughly during cooling. Of course, that weakens the piece, but since these are decorative the aesthetics are considered more important. This effect can be achieved at any temperature by just using frit powder, Ferro frit 3110 will melt and craze like this at cone 04 or even lower. Suspended bubbles will be a problem, use a firing schedule with a hold to give them time to surface. Also, use a body having low LOI so that it is not generating gas bubbles. Bisque firing pieces at a higher temperature will accomplish the same (for example, if you are making these at cone 04 then bisque fire at cone 03).

High melt fluidity is required to achieve the visual effect of this glaze

A runny pottery glaze

This is G3948A, an iron red cone 6 reactive glaze (similar to several commercially available products). The reason for the variegated surface is the high fluidity of the melt. Adequate thickness is also important, enabling it to run downward to some extent. That means this is not actually over-fired. Using it thus requires consideration of the running behavior, accommodating it in the shape of the ware on which it is used. Obviously, using this on the insides of pieces would result in pooling at the base, which would likely produce glaze compression, cracking the piece during cooling. Use on the outsides may require a catch glaze.

Switching copper carbonate for copper oxide in a fluid glaze

The top samples are 10 gram GBMF test balls melted down onto porcelain tiles at cone 6 (this is a high melt fluidity glaze). These balls demonstrate melt mobility and susceptibility to bubbling but also color (notice how washed out the color is for thin layers on the bottom two tiles). Both have the same chemistry but recipe 2 has been altered to improve slurry properties.

Left: Original recipe with high feldspar, low clay (poor suspending) using 1.75% copper carbonate.
Right: New recipe with low feldspar, higher clay (good suspending) using 1% copper oxide.

The copper oxide recipe is not bubbling any less even though copper oxide does not gas. The bubbles must be coming from the kaolin.

An ultra-clear brilliantly-glossy cone 6 clear base glaze? Yes!

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. Out of this work came the G3806E and G3806F.

Two bases, 2% copper additions. Which is the better transparent?

Wrong. It is the one on the right. While the copper looks so much better in that fluid one on the left, that melt mobility comes at a cost: blisters. As a clear glaze it is no glossier than the other one, but it runs into thicker zones at the bottom and they blister. This is because the high mobility coupled with the surface tension blows bubbles as gases of decomposition travel through (in a normal cooling kiln they break low enough that mobility is insufficient to heal them). The fired glass in the one on the left is also not as hard, it will be more leachable, it will also craze more easily and be more susceptible to boron-blue devritrification. But as a green? Yes it is better.

Add 5% calcium carbonate to a tenmoku. What happens?

In the glaze on the left (90% Ravenscrag Slip and 10% iron oxide) the iron is saturating the melt crystallizing out during cooling. GR10-K1, on the right, is the same glaze but with 5% added calcium carbonate. This addition is enough to keep most of the iron in solution through cooling, so it contributes to the super-gloss deep tenmoku effect instead of precipitating out.

Inbound Photo Links

Melt fluidity: Cornwall Stone vs. Nepheline Syenite

Stains having varying fluxing effects on a host glaze

Melting range is mainly about boron content

Will a cone or ball flow out better in a melt flow test?

A melt flow tester showing how a normal glazes runs when melting compared to this one
The first of 15 "Fool-Proof Recipes" wrecked my kiln shelf!


Glossary Viscosity
In ceramic slurries (especially casting slips, but also glazes) the degree of fluidity of the suspension is important to its performance.
Glossary Physical Testing
In ceramics, glazes, engobes and bodies have chemistries and physics. To fix, formulate and adjust their relative importances in each situation need to be understood.
Glossary Surface Tension
In ceramics, surface tension is discussed in two contexts: The glaze melt and the glaze suspension. In both, the quality of the glaze surface is impacted.
Glossary Melting Temperature
The melting temperature of ceramic glazes is a product of many complex factors. The manner of melting can be a slow softening or a sudden liquifying.
Glossary Fluid Melt Glazes
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.
Glossary Reactive Glazes
In ceramics, reactive glazes have variegated surfaces that are a product of more melt fluidity and the presence of opacifiers, crystallizers and phase changers.
Projects Temperatures
Tests Frit Softening Point
In ceramics, this is the temperature at which a glaze or glass begins to flow, ceasing to exhibit the properties of a solid.
Tests Glaze Melt Fluidity - Ball Test
A test where a 10-gram ball of dried glaze is fired on a porcelain tile to study its melt flow, surface character, bubble retention and surface tension.
Tests Glaze Melt Flow - Runway Test
A method of comparing the melt fluidity of two ceramic materials or glazes by racing them down an inclined runway.
Articles A Low Cost Tester of Glaze Melt Fluidity
This device to measure glaze melt fluidity helps you better understand your glazes and materials and solve all sorts of problems.
Troubles Glaze Blisters
Questions and suggestions to help you reason out the real cause of ceramic glaze blistering and bubbling problems and work out a solution
By Tony Hansen
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