All Glossary

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

Details

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 different fired glazes may appear to have melted a similar amount (even on a vertical surface), one may be radically more melt-fluid than another (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 functional glaze will travel about halfway down the runway (while reactive glazes will run off the end). 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

Melt fluidity differences are not obvious by just comparing glazed ware

This picture has its own page with more detail, click here to see it.

These two Plainsman M370 test 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 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 fluidity tester, the fritted one is melting and flowing much 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.

Melt fluidity tests as done by a frit manufacturer

This picture has its own page with more detail, click here to see it.

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

Frits vs. raw materials in glazes: It is not just about the chemistry

This picture has its own page with more detail, click here to see it.

The difference between sourcing fluxing oxides from frits vs. raw materials is significant to say the least. The oxides MgO and CaO normally come from natural mineral that melt high. But common frits that source them soften low. The chemistry in the two cone 6 glazes (compared on this melt flow tester) are the same. But G2934Y4 sources the KNaO from Ferro Frit 3110 instead of feldspar and alot of the MgO from Ferro Frit 3249 instead of talc. Even though Y4 has 10% calcined alumina it still flows much better! Alumina, as a source of Al₂O₃, is a super refractory material (compared to kaolin, the normal source), the glaze should flow less - but the frits overcome even that to create this amazing fluidity in a matte glaze. The lesson: All glazes have a chemistry, but that cannot be taken in isolation from the materials that source it. Glaze chemistry is a relative, not absolute science.

Comparing the melt fluidity of two shipments of Custer Feldspar

This picture has its own page with more detail, click here to see it.

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.

Comparing glaze melt fluidity balls with their chemistries

This picture has its own page with more detail, click here to see it.

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. There is a better explanation, compare these 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). 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 release 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 of #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 (left). Its high SiO₂ and enough-but-not-too-much B₂O₃ make it more durable. And it runs less during firing. And does not craze.

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

This picture has its own page with more detail, click here to see it.

The melt fluidity tester was fired at cone 6. The glaze on the left is G2826A2, a 50:30:20 Gerstley Borate base glaze recipe (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, producing a highly decorative boron-blue, 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.

Testing two brands of tin oxide in a melt flow tester

This picture has its own page with more detail, click here to see it.

This melt fluidity tester compares two different tin oxides in a cone 6 transparent glaze (G2926B). Opacifiers affect not just opacity in glazes, but also liquid properties of the melt. The length, surface character, opacity and color of these flows provide an excellent indication of how similar the two materials are. This is the GLFL test.

Testing a new brand of dolomite

This picture has its own page with more detail, click here to see it.

Dolomite is a key material for glazes, especially mattes. We were forced to adopt a new brand and needed confidence it was equivalent. Three tests were done to compare the old long-time-use material (IMASCO Sirdar) with a new one (LHoist Dolowhite). The first melt flow tester compares them in a very high dolomite cone 6 recipe formulated for this purpose; the new material runs just slightly more. The second tester is uses the G2934 cone 6 MgO matte recipe with 5% black stain; the new material runs a little less here. The third test is the high dolomite glaze on a dark burning clay to see the translucency and compare the surface character. They are very close. These three gave us the confidence to proceed.

This flow tester indicates copper is not fluxing or bubbling this glaze

This picture has its own page with more detail, click here to see it.

These cone 6 glazes are the same (G3806G), except the one on the right has 3.5% copper carbonate added. Copper is commonly fluxes glazes, making them melt more. But in this case it is not, the clear base is running just as much as the stained one. Either the percentage is not high enough or the host transparent glaze is resistant. 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 on this melt fluidity tester, the flow on the left has many more (it appears less melted because of this). In this specific glaze it seems probable that the copper bubbles (generated as it decomposes) act as a fining agent to coagulate and help clear the others.

Melt fluidity and coverage: RedArt Slip vs. Albany Slip vs. Alberta Slip

This picture has its own page with more detail, click here to see it.

These three melt flows and mugs were fired at cone 6 (using the C6DHSC firing schedule). The benchmark recipe is 80% clay and 20% Ferro Frit 3195 (our standard GA6-B recipe).
-The center melt flow (and matching buff stoneware mug below) employ the original Albany Slip.
-The one on the right employs Alberta Slip. Notice that, although having a very similar melt flow, it needs an iron oxide addition to darken the color (e.g. 2%).
-The one on the far left uses an Albany Slip substitute made from 80% Redart, 6.5% calcium carbonate, 6.5% dolomite and 6.5% nepheline syenite (our code L3613D). The chemistry of RedArt is different enough from Albany that some compromises were needed to avoid over-supplying the iron even more (and firing darker yet). Although this Redart version runs in a very similar pattern on the melt flow, the character of the glaze on the mug reveals it needs a little more melting (increasing the frit percentage would take care of that).

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

This picture has its own page with more detail, click here to see it.

This is a GLFL test, it employs a slipcast melt flow tester to show the flow patterns of two glazes (or materials), side-by-side. 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 at high temperatures. Alberta Slip is a recipe of mined clays with added refined minerals that give it a similar chemistry, firing behavior and raw physical appearance. As you can see, the melt fluidity is very similar.

How does Gillespie Borate compare with the original Floating Blue recipe?

This picture has its own page with more detail, click here to see it.

The original Floating Blue recipe, our code number G2826R, has been popular for 50 years. But also troublesome (because of a fragile mechanism, poor slurry properties and inconsistencies in Gerstley Borate and rutile). Gillespie Borate, it's 2023 apparent successor, appears to solve most of its issues. These specimens of the recipe were fired using the cone 6 C6DHSC schedule. We have "vintage" Gerstley Borate from the 1990s, that is what was used here.
Top left: Floating Blue using Gerstley Borate (GB) (top) and Gillespie Borate bottom on a buff burning body.
Top right: Same but on a red burning body.
Centre: Melt fluidity GLFL test of the two glazes (GB) on the left.
Bottom: The two recipes and their calculated chemistries.
Clearly, the Floating Blue itself is firing greener than usual. And the Gillespie Borate version is much bluer. You may be used to something in between these two. The green tones could likely be restored by a reduction in the cobalt and increase in the iron oxide. The best news is that at 1.47 specific gravity, Gillespie Borate produces a far better slurry, there is no gelling. And no sign of settling into a hard layer.
The chemistry comparison at the bottom highlights some concerns, the difference is not insignificant. B₂O₃, Al₂O₃ and SiO₂ are all lower (this could be part of the reason for the differences in color also). For better or worse, the melt fluidity is the same: Very high. This is likely because the percentage of Ulexite is higher (that melts better than Colemanite).

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

This picture has its own page with more detail, click here to see it.

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 difference: The flow is more transparent because of the lower micro-bubble population. It's melt better 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 a 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.

What Bismuth Subnitrate does between 1100-1250F

This picture has its own page with more detail, click here to see it.

I did this using our standard melt flow tester. I prepared 10-gram balls by mixing the bismuth subnitrate powder with 1% CMC gum (like what we do for the GBMF test). These balls were fired in an electric oxidation kiln to 1100F, 1200F, 1250F and 1300F (593-704C). It is not difficult to see why this is a potential ingredient in low-temperature frits. Does anyone know why it burns yellow? Does this turn transparent at higher temperatures?

Melt flow test demonstrates glaze:clay interaction

This picture has its own page with more detail, click here to see it.

The melts being compared here are our code number 6998, a production run of Alberta Slip. Both are the same sample batch and ball weight. These two flow testers were 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.

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

This picture has its own page with more detail, click here to see it.

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.

Two transparents having opposite melt fluidity/surface tension balances

This picture has its own page with more detail, click here to see it.

Low-fire glazes must be able to pass the bubbles they and the underlying bodies generate (or clouds of micro-bubbles will turn them white). This cone 04 flow tester makes it evident that 3825B has a higher melt fluidity (A has not even dripped onto the tile). And its higher surface tension is demonstrated by how the flow meets the runway at a perpendicular angle (it is also full of entrained micro-bubbles). Notice that A, by contrast, meanders down the runway, a broad, flat and relatively clear river. Low-fire glazes, for example, must pass many more bubbles than their high-temperature counterparts, the low surface tension of A aids that. A is Amaco LG-10. B is Crysanthos SG213 (Spectrum 700 behaves similarly, although flowing less). These two represent very different chemistry approaches to making a clear glaze. Which is better? Both have advantages and disadvantages - this property has implications, just not for bubble clouding, but on other issues involving glaze performance and even defects.

An example where adding silica really helps a glaze

This picture has its own page with more detail, click here to see it.

The flow on the left is an adjusted Perkins Frit Clear (we substituted frit for Gerstley Borate). It is a cone 6 transparent that appeared to work well. However it did not survive a 300F oven-to-icewater IWCT test without crazing on Plainsman M370. The amount of flow (which increases a little in the frit version) indicates that it is plenty fluid enough to accept some silica. So we added 10% (that is the flow on the right). Now it survives the thermal shock test and still fires absolutely crystal clear.

Frit Melt Fluidity Comparison - 1800F

This picture has its own page with more detail, click here to see it.

Fired at 350F/hr to 1800F and held for 15 minutes (I already did firings from 1300F-1750F in 50 degree increments, all of them are visible in the parent project). Frit 3110, 3134, 3195, F75 have run all the way down. All of the frits have softened and melted slowly over a range of temperatures (hundreds of degrees). By contrast, Gerstley Borate, the only raw material here, suddenly melted and flowed right over the cliff (between 1600 and1650)! But not before Frit 3602 and FZ16 had done so earlier. Frit 3249 is just starting to soften but F69 (the Fusion Frits equivalent) is a little ahead of it. LA300 and Frit 3124 are starting also. F524, F38, F15 will all be over the end by the next firing. The melt surface tension is evident by the way in which the melts spread out or hold together.

Ferro Frit 3249 vs Fusion F-69 at cone 04

This picture has its own page with more detail, click here to see it.

The chemistry of these two supposedly interchangeable frits is very similar (the difference being that 3249 has 3% CaO that is missing in F-69). But that does not appear to account for this difference in melt fluidity at cone 04! However, as temperatures increase 3249 rapidly becomes more active. Inspite of the difference here we have found the two work interchangeablely in our G2934Y recipe. Obviously, the F-69 is going to make glazes melt better, at least at low fire.

Ferro Frit 3124 vs Fusion F621/19 at cone 04

This picture has its own page with more detail, click here to see it.

Fusion Frit F621/19 is recommended as a substitute candidate for Ferro Frit 3124. However, as shown on this melt fluidity test of the two pure frit powders, it appears to have a lower surface tension and flow better. However the character of the flow is opposite to what is expected of a lower surface tension in the melt. There should either be a lower population of micro-bubbles or they should be smaller - but the opposite is the case. There should be fewer breaking at the surface and they should be healing better - but that is not the case. Frits are supposed to be free of carbon or hydrates - or are they? If the frit is sourcing gases because of raw materials not fully decomposing during smelting - how is that even possible? Do you know why this is happening, can you tell me please? Fusion Ceramics does not supply the chemistry of Frit F621/19 and it is not shown on their website in 2021, but I have no reason to believe it contains fluorine.

Inbound Photo Links

Stains having varying fluxing effects on a host glaze

Melting range is mainly about boron content

Melt fluidity: Cornwall Stone vs. Nepheline Syenite

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

The first of 15 "Fool-Proof Recipes" wrecked my kiln shelf!

Melt fluidity is not evident on typical glaze tests

Links

By Tony Hansen Follow me on

Got a Question?

Buy me a coffee and we can talk