All Pictures

Click here for case-studies of Insight-Live fixing problems

You will see examples of replacing unavailable materials (especially frits), fixing various issues (e.g. running, crazing, settling), making them melt more, adjusting matteness, etc. Insight-Live has an extensive help system (the round blue icon on the left) that also deals with fixing real-world problems and understanding glazes and clay bodies.

Related Pictures

Is that recipe you found online really what you think it is?

It contains significant Frit P-25, I googled that to Digitalfire, it contains 1.8% fluorine! When that gases off during firing are you really equipped to deal with fluorine gas in your studio? One answer is to substitute my own frits (which of course do not contain fluorine). I did the calculations in my account at insight-live.com (see the recipes side-by-side). In the formula, I substituted the F (fluorine) for a mix of CaO/MgO. In the recipe, I substituted Ferro Frit 3110 (it is also a high-sodium frit) for the P-25, then I juggled the rest of the recipe to match up the oxides. More Gerstley Borate supplies the lost B₂O₃, more ball clay supplies the lost Al₂O₃ and less silica is needed (because the ball clay brought along more). I ended up with a recipe that is going to suspend much better (more clay) and was able to eliminate the whiting (because the rest of the new recipe is supplying more CaO). This recipe has a high thermal expansion, that means crazing. Would it not be easier to simply add a red stain to a clear glaze you already have that is working well on your clay bodies?

What can you do using glaze chemistry? More than you think!

There is a direct relationship between the way ceramic glazes fire and their chemistry. These green panels in my Insight-live account compare two glaze recipes: A glossy and matte. Grasping their simple chemistry mechanisms is a first step to getting control of your glazes. To fixing problems like crazing, blistering, pinholing, settling, gelling, clouding, leaching, crawling, marking, scratching, powdering. To substituting frits or incorporating available, better or cheaper materials while maintaining the same chemistry. To adjusting melting temperature, gloss, surface character, color. And identifying weaknesses in glazes to avoid problems. And to creating and optimizing base glazes to work with difficult colors or stains and for special effects dependent on opacification, crystallization or variegation. And even to creating glazes from scratch and using your own native materials in the highest possible percentage.

Stuck at home with no ceramic supplies? Time to organize!

Are your records in a messy binder? You could be using an account at insight-live.com! Move your recipes first, assigning each a code number. Then, in your studio/lab, label every fired sample, bucket, jar, glaze test, bag with the corresponding code number. Put in pictures for each recipe. Enter your firing schedules. Research the solutions to issues you are facing with glazes at the Digitalfire Reference Library (ask us questions using the contact form on each of the thousands of pages there). Then start planning improvements and tests. Choose a recipe you need to improve/evolve, duplicate it, increment the code number, make changes, enter explanatory notes. With this preparation you will hit the ground running back at work.

Substitute Ferro Frit 3134, using glaze chemistry, in three glaze types

Can't get frit 3134 for glaze recipes? Can you replace it with frit 3124? No, 3124 has five times the amount of Al₂O₃ (the second most important oxide in glazes) and half the amount of B₂O₃ (the main melter). This ten-minute video presents a glaze chemistry approach that is easier to do than you probably think. On three different recipe types, you will learn to source the needed oxides from two other Ferro frits, 3110 (or Fusion F-75) and 3195 (Fusion F-2) and end up with at least 15% kaolin in each (to suspend the slurry). Each requires a unique approach. Two of the calculations produce improved slurry properties and one yields a recipe of significantly lower cost. If you have a recipe that needs this, get an insight-live.com account, enter it there and I can help you do the calculation.

How I calculated a feldspar-to-frit replacement in a cone 10R clear glaze

A screen shot of side-by-side panels in my account at Insight-live.com. On the left is the original G1947U recipe. On the right I have substituted frit 3110 as a higher-concentration-source of KNaO. This enabled actually increasing KNaO to get a better gloss and melt. I have introduced calcined kaolin (to ratio with the raw kaolin to control slurry and drying properties). I added frit 3249 to introduce low-expansion MgO to counterbalance the higher levels of KNaO. That frit, like the 3110, will not only melt things better simply because it itself was pre-melted, but it also brings along a little boron. That supercharges melting and enables an enhancement: The addition of more SiO₂. The low thermal expansions of MgO, SiO₂ and B₂O₃ counterbalance the increase that will occur as a result of the higher KNaO (0.15-0.26). Sound boring? When you see the unexpected results you might think differently (see the linked post below). I never considered using frits at cone 10R before, this success led to an improvement in my main silky matte glaze also.

Why do these cone 04 and 6 clear glazes have so similar a chemistry?

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 may actually be just a low temperature glaze being overfired to cone 6.

Why would a low fire transparent require four frits?

To get the needed chemistry to avoid boron blue clouding (calcium borate crystals). The one on the right clouds, the other does not. Why? Differences in the chemistry (as seen in my account at insight-live.com). G2931K, on the left, has greater Al₂O₃ (which impedes the growth of crystals), lower CaO (starves their growth) and more boron (for better melting). There is actually no practical way to adjust the recipe on the right (by supplying MgO with talc and fiddling with frit percentages) to achieve this. Frit 3124 lacks Na₂O and B₂O₃. 3134 has excessive CaO and almost zero Al₂O₃. Talc does not melt well enough. But Frit 3249 supplies the needed MgO and has lots of B₂O₃ and low CaO. And Frit 3110 has low CaO and supplies the needed Na₂O.

A large glaze batch mixing error rescued using glaze chemistry

The person used Frit 3134 instead of 3124, it makes up 70% of the recipe. The glaze melted much more and ran off the ware. That sounds like an impossible-to-fix problem. It just so happens that these frits have very similar chemistry except for one thing: 3134 has almost no Al₂O₃. That means that kaolin can be added to the bad batch to source the missing Al₂O₃ (and replenish the shortage of SiO₂ at the same time). Extra silica is also needed to restore the full SiO₂. The new chemistry is not an exact match, the B₂O₃ is a little higher, but this should not be an issue. Of course a raw:calcine mix of kaolin is needed (to prevent the glaze from shrinking too much on drying and therefore cracking). From this calculation we can see that for every 100 grams of the original powder we need to add 10 EPK, 15 calcined kaolin and 17 silica. Of course, one would need to know the water content of the slurry, that is calculated as (weight wet - dry weight)/wet weight * 100. If the slurry was 50% water, for example, then every 200 grams of slurry would contain 100 grams of powder.

Cannot get Nepheline Syenite? Here is what to do.

Nepheline Syenite is similar to a feldspar. I have them open side-by-side here in my Insight-live.com account (the blue panels). In the "Analysis" column notice that Minspar has 9% more SiO₂ and 5% less Al₂O₃. Minspar has 12% fluxes and Nepheline has more than 15%. If a recipe does not contain a significant percentage of Nepheline these differences might be tolerated but what if there is 30% or more? There is no combination of materials that has the chemistry of Nepheline (there is no way to take 10% of the SiO₂ out of this feldspar, for example). But it is possible to take SiO₂ out of a glaze containing Nepheline Syenite. Notice the two green recipe panels below: The changes made to the one on the right harmonize the oxide chemistry with the original on the left. Those changes were significant: 15% more feldspar, 12% less silica and 2% less kaolin. Notice how easy this was using a little chemistry!

Calculating a substitute for Minspar

Why do this? We did not have it in stock and customers needed to mix recipes. When the chemistries of the two feldspars are very similar substitution is often not a problem, especially when a recipe only calls for 5 or 10%. However, when a recipe calls for a significant percentage the situation becomes much trickier (in our cone 6 test recipe, "Perfect Clear", 40% Minspar is needed). Feldspars are almost a glaze in themselves, just needing silica and alumina to shift their chemistry toward 'glazedom'. In this project I calculated a mix of materials, in my Insight-live.com account, that sources the same chemistry as Minspar. I made a cone 6 GLFL test comparing the Minspar and Minspar substitute (left) and comparing the Perfect Clear glaze with each feldspar (right). As you can see, the similarity in melt flow is stunning! This is a real demonstration of just how practical and valuable glaze chemistry calculation can be.

Ravenscrag Plum Red recalculated to use frit instead of ulexite

The original recipe, from which Ravenscrag Plum Red was derived, employed Gerstley Borate to source the boron (the melter). That was a problem because it gels glaze slurries (there is already 15% iron oxide present and that gels also). Thus ulexite was instead to source the boron. That was many year ago but now, ulexite is almost impossible for potters to get. We reformulated again, this time sourcing the boron from a frit (actually two frits). Frit 3249 was used to source some of the MgO needed (this glaze has high MgO levels) since it melts so well and also sources lots of boron. There is now a little less Ravenscrag Slip to suspend the slurry, but that is not a problem, the iron will gel it.

Does adding boron alone always increase glaze melt?

Boron (B₂O₃) is like silica, but it is also a flux. Frits and Gerstley Borate supply it to glazes. In this test, I increased the amount of boron from 0.33 to 0.40 (using the chemistry tools in my insight-live.com account). I was sure that this would make the glaze melt more and have less of a tendency to craze. But as these GBMF tests for melt flow (10 gram GBMF test balls melted on porcelain tiles) show, that did not happen. Why? I am guessing that to get the effect B₂O₃ has to be substituted, molecule for molecule for SiO₂ (not just added to the glaze).

Comparing glaze melt fluidity balls with their chemistries

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.

This cone 6 transparent looked good, but I still improved it alot

The green boxes show cone 6 Perkins Studio Clear (left) beside an adjustment to it that I am working on (right). I am logged in to my account at insight-live.com. In the recipe on the right, code-numbered G2926A, I am using the calculation tools it provides to substitute Frit 3134 for Gerstley Borate (while maintaining the oxide chemistry). A melt-flow GLFL test comparison of the two (bottom left) shows that the GB version has an amber coloration (from its iron) and that it flows a little more (it has already dripped off). The flow test on the upper left shows G2926A flowing beside PGF1 transparent (a tableware glaze used in industry). Its extra flow indicates that it is too fluid, it can accept some silica. This is very good news because the more silica any glaze can accept the harder, more stable and lower expansion it will be. You might be surprised how much it took, yet still melts to a crystal clear. See the article to find out.

A pottery glaze is settled, running and crazing. What to do?

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 in the picture) eliminates the feldspar and sources Al₂O₃ from kaolin and KNaO from Frit 3110 (preserving the glaze's chemistry). To make that happen the amounts of other materials had to be juggled. But the fired test revealed that this one, although very similar, is melting more (because the frit releases its oxides more readily than feldspar). Adjustment 2 (column 4) proposes a 10-part silica addition. SiO₂ is the glass former, the more a glaze will accept without losing the intended visual character, the better. The result is less running and more durability and resistance to leaching.

Substituting a frit to source B2O3 leads to a dead end

In the new recipe I am trying to match the oxides (the white, yellow, red, blue boxes). B₂O₃ is priority, I have supplied enough of the new frit to almost match it, but that far overshoots the Na₂O (even if all of the feldspar is removed). While differences in the Al₂O₃ and SiO₂ can be compensated by adjustments in the feldspar, clay and silica, the new frit has far more Na₂O and only half the B₂O₃. The match shown here is the best I am able to achieve, its biggest problem is the significant rise in the calculated thermal expansion, that is going to mean issues with crazing. The only way to use this frit to better match the chemistry of the original glaze would be in combination with another one that has much higher B₂O₃ and has as little Na₂O as possible.

Fix obvious issues in Glazy recipes before even trying them

Many come to Insight-live after "Glazy recipe fails". A better way is to recognize the potential in a recipe, fix it by material logic and calculation (e.g. limit recipes), then and try it. Glazy "Red Orange #111576" is a good example. It has two things I avoid: Lithium (expense) and a high percentage of red iron (slurry gelling). It is easy to fix both. Spodumene is a better source of Li₂O but it contributes lots of Al₂O₃ and SiO₂. We can "make room" for it by replacing the feldspar with frit 3110 (the latter contributes much more sodium and much less Al₂O₃/SiO₂ than the feldspar). Second, use black iron instead of red. The results using the C6IRED schedule are fabulous. And the cost is way down. Amazingly black iron does not gel the slurry at all! And it is not nearly as messy as the red. Like any iron red, this has a fluid melt so is running (although it is applied thickly). The thermal expansion is still quite low so it should not craze. And the LOI is much lower, that should minimize bubbling.

Use a frit blend ratio to control the amount of kaolin in a glaze recipe

These are the recipes and calculated oxide chemistries of two pottery glazes (as shown in my account at insight-live.com): The original problem recipe and an adjustment to fix it. Recipe #1 sources boron from a soluble material and three plastic materials are combined to increase drying shrinkage enough to cause cracking when drying (and thus crawling). Recipe #2 solves these problems while producing the same chemistry. It sources boron from two frits (one having almost no Al₂O₃) whose ratio to each other can be altered to supply more or less Al₂O₃ to the melt. That enables removing two of the plastics: Ball clay and Gerstley Borate. The remaining 20% EPK is perfect to create a creamy slurry that suspends, applies and dries well.

Why does the inside glaze crystallize with one frit and not the other?

It is about the oxide chemistry, as shown calculated below the recipes in my account at insight-live.com. These glazes are fired at cone 6 using the C6DHSC schedule (we are focussing on the amber glossy glaze on the insides of the mugs). Most oxides want to form silicate crystals (combine with SiO₂) as the glaze cools (if the cooling ramp is slow enough), iron oxide is not the least of these. Alumina (Al₂O₃) stabilizes the melt, that means it helps the melt to solidify as a glassy solid, not a crystalline one (thus, it does not devitrify). Notice the two Al₂O₃ values (black-on-red numbers): The glaze on the right has much less. That is because Ferro Frit 3134 contains almost no Al₂O₃ (notice in the blue panel, only 2%). The alumina in the glaze on the left, sourced more abundantly by Frit 3195, readily releases in the melt, ready to take on its job: Stiffen it and impede the formation of iron silicate crystals during cooling to create a better glass.

Magnesium carbonate vs. oxide: One big difference

Here is a screenshot of side-by-side recipes in my account at insight-live.com. It takes 120 mag carb to source the same amount of MgO as 50 mag ox. I just made the two recipes, went into calculation mode and kept bumping up the magcarb by 5 until the chemistry was the same. Note the LOI of the magcarb version is 40. This one would certainly crawl very badly.

Insight-Live comparing a glossy and matte cone 6 base glaze recipe

Insight-live is calculating the unity formula and mole% formula for the two glazes. Notice how different the formula and mole% are for each (the former compares relative numbers of molecules, the latter their weights). The predominant oxides are very different. The calculation is accurate because all materials in the recipe are linked (clickable to view to the right). Notice the Si:Al Ratio: The matte is much lower. Notice the calculated thermal expansion: The matte is much lower because of its high levels of MgO (low expansion) and low levels of KNaO (high expansion). Notice the LOI: The matte is much higher because it contains significant dolomite.

Comparing two glazes having different mechanisms for their matteness

These are two cone 6 matte glazes (shown side by side in an account at Insight-live). G1214Z is high calcium and a high silica:alumina ratio. It crystallizes during cooling to make the matte effect and the degree of matteness is adjustable by trimming the silica content (but notice how much it runs). The G2928C has high MgO and it produces the classic silky matte by micro-wrinkling the surface, its matteness is adjustable by trimming the calcined kaolin. CaO is a standard oxide that is in almost all glazes, 0.4 is not high for it. But you would never normally see more than 0.3 of MgO in a cone 6 glaze (if you do it will likely be unstable). The G2928C also has 5% tin, if that was not there it would be darker than the other one because Ravenscrag Slip has a little iron. This was made by recalculating the Moore's Matte recipe to use as much Ravenscrag Slip as possible yet keep the overall chemistry the same. This glaze actually has texture like a dolomite matte at cone 10R, it is great. And it has wonderful application properties. And it does not craze, on Plainsman M370 (it even survived a 300F-to-ice water IWCT test). This looks like it could be a great liner glaze.

Links

Got a Question?

Buy me a coffee and we can talk