This is an example of serious crazing in a glaze. The lines have gotten darker with use of the bowl! That means the color is organic, from food. This cannot be healthy.

First, the layer is very thick. Second, the body was only bisque fired to cone 06 and it is a raw brown burning stoneware with lots of coarser particles that generate gases as they are heated. Third, the glaze contains zircopax, it stiffens the melt and makes it less able to heal disruptions in the surface. Fourth, the glaze is high in B2O3, so it starts melting early (around 1450F) and seals the surface so the gases must bubble up through. Fifth, the firing was soaked at the end rather than dropping the temperature a little first (e.g. 100F) and soaking there instead.

Bloating in an over fired middle temperature high iron raw clay (Plainsman M2). It is still stable, dense and apparently strong at cone 4 (having 1.1% porosity). But between cone 6 and 7 (top bar) it is already bloating badly. Such clays must be fired at low enough temperatures to avoid this volatility (if accidentally over fired). This clay only reaches a minimum of 1% porosity (between cone 4 and 5), it is not possible to fire it to zero porosity. This is because of the particulate gas-producing particles (it is ground to 42 mesh only).

This cone 6 white glaze is crawling on the inside and outside of a thin-walled cast piece. This happened because the thick glaze application took a long time to dry, this extended period, coupled with the ability of the thicker glaze layer to assert its shrinkage, compromised the fragile bond between dried glaze and fairly smooth body. To solve this problem the ware could be heated before glazing, the glaze applied thinner, or glazing the inside and outside could be done as separate operations with a drying period between.

Example of serious glaze shivering using G1215U low expansion glaze on a high silica body at cone 6. Be careful to do a thermal stress test before using a transparent glaze on functional ware.

This cone 6 white opacified glaze has an addition pigment-bearing granular mineral to create speckle (e.g. illmenite, manganese granular, ironstone concretions). This speckling mechanism can be transplanted into almost any glaze. Unfortunately, the metallic particles that produce the speck are often heavy and settle quickly in the glaze slurry. This can be prevented somewhat by flocculating the slurry.

The glaze is simply insufficiently melted. In cone 10 reduction the iron acts as a flux and it melts very very well, so there are no pinholes. But here it simply does not have enough fluidity to heal the disruptions caused as gases escaping from the body below bubble up through it.

This reduction celadon is crazing. Why? High feldspar. Feldspar supplies the oxides K2O and Na2O, they contribute to brilliant gloss and great color (at all temperatures) but the price is very high thermal expansion. Any glaze having 40% or more feldspar should turn on a red light! Thousands of recipes being traded online are high-feldspar, some more than 50%! There are ways to tolerate the high expansion of KNaO, but the vast majority are crazing on all but high quartz bodies. Crazing is a plague for potters. Ware strength suffers dramatically, pieces leak, the glaze harbours bacteria, crazing invites customers to return pieces. The fix: A transparent base that fits your ware. Add colorants and opacifiers to that. Another fix: substitute some of the KNaO for a lower expansion flux (like MgO, SrO, CaO, Li2O) and add as much SiO2 and Al2O3 as the glaze will take (using glaze chemistry software).

Example of dunting, where a crack has released the stresses produced by uneven thermal contraction during cool-down in the kiln. This usually happens by cooling too quickly through quartz inversion.

This one can take more temperature than most. It looks OK at cone 5 (bottom bar). But at cone 6 bloating (bubbles) begin to occur. This body, while smooth to the touch, contains some iron and sulphate particulates that generate gases during firing, these are the catalyst for the bloating (the clay matrix becomes dense enough that it can no longer vent the gases of decomposition through it).

These are very hard, high in iron and can be as large as volkswagens. Tiny iron concretion particles cause specking in fired ware, especially in reduction.

This is an example of two types of crystals that have formed on the surface of a fritted glaze after a long period of storage (Ferro Frit 3249 in this case). Frits are formulated to give chemistries that natural materials cannot supply. To do that they have to push the boundaries of stability (solubility). Any frit that has an inordinately high amount (compared to natural sources) of a specific oxide (in this case MgO) or lacks Al2O3 (like Frit 3134) are suspect.

5 different brand names of iron oxide at 4% in G1214W cone 5 transparent glaze. The specks are not due to particle size, but differences in agglomeration of particles. Glazes employing these iron oxides obviously need to be sieved to break down the clumps.

Five different brand names of iron oxide at 4% in G1214W cone 5 transparent glaze. The glazes have been sieved to 100 mesh but remaining specks are still due to agglomeration of particles, not particle size differences.

Testing for pinholes and dimples is often best done using a transparent glaze over a large surface and looking at the surface in the light. In this case, an open bowl is used. Heavy pieces like this are difficult to fire evenly and encourage under fired areas where pinholes are more likely to appear.

Cone 6 iron bodies that fire non-vitreous and burn tan or brown in oxidation can easily go dark or vitreous chocolate brown (or even melting and bloated in reduction). On the right is Plainsman M350, a body that fires light tan in oxidation, notice how it burns deep brown in reduction at the same temperature. This occurs because the iron converts to a flux and the glass development that occurs brings out the dark color. On the left is Plainsman M2, a raw high iron clay that is quite vitreous in oxidation, but in reduction it is bloating badly. When reduction bodies are this vitreous there is a much great danger of black coring.

Cracking during heatup. They start inward on the concave angles. It is important to create shapes not prone to cracking and smooth, compress and round abrupt contours and areas prone to cracks (to deny them a place to start).

An example of how a glaze that contains too much plastic has been applied too thick. It shrinks and cracks during drying and is guaranteed to crawl. This is raw Alberta Slip. To solve this problem you need to tune a mix of raw and calcine material. Enough raw is needed to suspend the slurry and dry it to a hard surface, but enough calcine is needed to keep the shrinkage low enough that this cracking does not happen. The Alberta Slip website has a page about how to do the calcining.

These are two strong bar magnets that are suspended below the chamber of a hammermilll that grinds stoneware clays. This iron they hold is both natural in the clays and from wearing of the hammers during grinding.

These mugs have experienced very serious shivering. This is an Albany Slip glaze with 10% lithium carbonate, it is known to have a very low thermal expansion. This problem can be solved by reducing the amount of lithium or adding high-expansion sodium or potassium. However these fixes will likely affect the appearance.

This liner glaze is 10% calcium carbonate added to Ravenscrag slip. Ravenscrag Slip does not craze when used by itself as a glaze at cone 10R on this body, so why would adding a relatively low expansion flux like CaO make it craze? It does not craze when adding 10% talc. This is an excellent example of the value to looking at the chemistry (the three are shown side-by-side in my account at The added CaO pushes the very-low-expansion Al2O3 and SiO2 down by 30% (in the unity formula), so the much higher expansion of all the others drives the expansion of the whole way up. And talc? It contains SiO2, so the SiO2 is not driven down nearly as much. In addition, MgO has a much lower expansion than CaO does.

A closeup of a cone 10R rutile blue (it is highlighted in the video: A Broken Glaze Meets Insight-Live and a Magic Material). Beautiful glazes like this, especially rutile blues, often have serious issues (like blistering, crazing), but they can be fixed.

This crack likely starting during bisque. It started at a sharp angled indent on the outside (that coincided with a thin wall section) and grew around the perimeter (not visible). From there it branched to the base.

This crack began as stresses created during uneven drying (the rim was allowed to get ahead of the base). A thinner section (that happened during throwing) was exploited by the stresses and a crack appeared during heatup, likely during the bisque.

An example of how calcium carbonate can cause blistering as it decomposes during firing. This is a cone 6 Ferro Frit 3249 based transparent (G2867) with 15% CaO added (there is no blistering without the CaO). Calcium carbonate has a very high loss on ignition (LOI) and for this glaze, the gases of its decomposition are coming out at the wrong time. While there likely exists a firing schedule that takes this into account and could mature it to a perfect surface, the glaze is high in MgO, it has a high surface tension. That is likely enabling bubbles to form and hold better.

An example of how a carbonate can cause blistering. Carbonates produce gases during decomposition. This glaze (G2415B) contains 10% lithium carbonate, which likely pushes the initial melting temperature down toward the most active decomposition temperatures.

Bloating. These teapots have been refired to cone 6.

These are thick pieces, they need time for heat to penetrate. Both were soaked 15 minutes at cone 6 (2195F in our test kiln). But the one on the left was control-cooled to 2095F degrees and soaked 45 more minutes. Pinholes and dimples are gone, the clay is more mature and the glaze is glossier and melted better. Why is this better than just soaking longer at cone 6? As the temperature rises the mineral particles decompose and generate gases (e.g. CO2, SO4). These need to bubble through the glaze. But on the way down this activity is ceasing. Whatever is gassing and creating the pinholes will has stopped by 2095F. Also, these are boron-fluxed glazes, they stay fluid all the way down to 1900F (so you could drop even further before soaking).

A cone 6 stoneware with 0.3% 60/80 mesh manganese granular (Plainsman M340). Fired from cone 4 (bottom) to cone 8 (top). It is normally stable to cone 8, with the manganese it begins to bloat at cone 7. The particles of manganese generate gases as they decompose and melt, these produce volumes and pressures sufficiently suddenly that closing channels within the maturing body are unable to vent them out.

This is G2415J Alberta Slip glaze on porcelain at cone 6. Why did the one on the right crawl? Left: thinnest application. Middle: thicker. Right thicker yet and crawling. All of these use a 50:50 calcine:raw mix of Alberta Slip in the recipe. While that appears fine for the two on the left, more calcine is needed to reduce shrinkage for the glaze on the right (perhaps 60:40 calcine:raw). This is a good demonstration of the need to adjust raw clay content for any glaze that tends to crack on drying. and both have pages about how to calcine and calculate how much to use to tune the recipe to be perfect.

This mug is pinging loudly and literally self-destructing in front of my eyes! Why? The glaze is under so much compression (the inside is pushing outward, the outside inward). Spiral cracks are developing all the way up the side. Small razor-sharp flakes are shivering off convex contours. Why? I accidentally fired a low-temperate talc body at cone 6 (the glaze is the Alberta Slip base cone 6 glossy). The clay body is not overly mature, but it just has an extremely high thermal expansion (talc is added to increase the expansion to fit low fire commercial glazes, they would craze without it). Shivering is serious, it is a mismatch of thermal expansion between body and glaze. It can happen at any temperature.

An excellent example that demonstrates the brittleness typical of vitrified terra cotta bodies. This bowl was fired to cone 02 and rung like a tuning fork when struck with a spoon. The body is dense like a porcelain and at appeared to be incredibly strong (this body is much more vitreous than an average terra cotta would be). However after a few more taps with the spoon it broke in two! It is brittle! Very hard, but brittle. At first I thought it might be that the glaze is under compression, but when I dropped the halves they did not shatter in the manner characteristic of compressed glaze, and they broke with razor sharp edges (like a vitrified porcelain does). So firing for this body must stop short of the most dense matrix possible to avoid this brittleness.

Both of these glazes were made as 1000 gram batches and then mixed with the necessary amount of water to produce a slurry of the correct consistency. The one on the left is a fritted glaze with 20% kaolin, the one on the right is a Gerstley Borate based raw glaze (30% GB + feldspar, silica, ball clay). The GB glaze required much more water and gelled shortly after (it also tends to dry slowly and crack during drying on the ware). The fritted glaze has very good slurry and application properties.

I poured 4 teaspoons of two glazes onto a board and let them sit for a minute, then inclined the board. The one with Gleason Ball clay (right, much higher in coal and finer particle size) has settled and the water on the top of running off. The one with Old Hickory #5 ball clay has not settled at all and the whole thing is running downward. Below I have begun to sponge them off. Old Hickory No. 1 Glaze Clay is even better than #5 for suspension. The most amazing thing about this: There is only 7% ball clay in the recipe.

This cone 6 brown functional stoneware has been fired across a range of temperatures. Cone 4 is too porous. From cone 7 it is expanding and density is not improving, it will likely warp or bloat. Cone 7 is losing the red color, there is no room for over-firing (by accident). The porosity at cone 6 is so much better than cone 5 and color is still stable. Therefore, cone 6 is the one we want.

Cone 6 transparent glaze testing to fit Plainsman M370: Left and right: Perkins Studio Clear. The far left one is a very thick application. Center: Kittens Clear. The porcelain for all is Plainsman P300. Why? Because P300 is much more likely to craze the glaze because it has a lower silica content (about 17% and only kaolin whereas M370 has 24% silica plus the free quartz that comes with the 20% ball clay it also contains). If a thick layer works on P300 it is a shoe-in to fit M370. If it also passes the oven:icewater test.

A cone 10R grey stoneware mug that has begun to craze on the inside. The greyer coloration around the craze lines indicates that water is soaking into the slightly porous body. This mug has lost the ring it had when it was new (it is only about a year old). It could be refired to be as good as new but would soon return to this condition. The only real solution is to reformulate this glaze to reduce its thermal expansion.

Worthington Clear is a popular low fire transparent glaze recipe. It has 55% Gerstley Borate plus 30% kaolin (Gerstley Borate melts at a very low temperature because it sources lots of boron). GB is also very plastic, like a clay. I have thrown a pot from this recipe! This explains why high Gerstley Borate glazes often dry so slowly and shrink and crack during drying. When recipes also contain a plastic clay the shirinkage is even worse. GB is also slightly soluble, over time it gels glaze slurries. Countless potters struggle with Gerstley Borate recipes. How could we fix this one? First, substitute all or part of the raw kaolin for calcined kaolin (using 10% less because it has zero LOI). Second: It is possible to calculate a recipe having the same chemistry but sourcing the magic melting oxide, boron, from a frit instead.

The engobe on the left, even though it has a fairly low water content, is running off the leather hard clay, dripping and drying slowly. The one on the right has been flocculated with epsom salts, giving it thixotropy (ability to gel when not in motion but flow when in motion). Now there are no drips, there are no thin or thick sections. It gels after a few seconds and can be uprighted and set on the shelf for drying.

This demonstrates the difficulty you can encounter when trying to get an engobe working with a clay body. Here the slip/glaze is flaking off the rim of pieces at cone 04 (does not happen at 06). The front bi-clay bar demonstrates the white and red clays dry well together (the slight curve happened on the drying). They also fire well together (the curvature did not change on firing). The back two thin bars seem to demonstrate thermal expansion compatibility: a thick layer of glaze is not under enough compression to curve either bar during firing. While the white clay contains 15% frit and forms a good bond with the red body, that bond is not nearly as good as the one between the glaze and the white slip. Yet it is still flaking off the rim at the slip/body interface. Why? At first it seemed that failure was happening at quartz inversion (because the body had less quartz than the white slip). However now it appears that the combination of compressions of the slip and glaze are sufficient to break the slip-body bond on convex contours. The compression of the slip and glaze likely did not demonstrate well on the bars because at this low a temperature they are not vitreous enough to be easily curled.

I used a binder to form 10 gram balls and fired them at cone 08 (1700F). Frits melt really well, they do not gas and they have chemistries we cannot get from raw materials (similar ones to these are sold by other manufacturers). These contain boron (B2O3), it is magic, a low expansion super-melter. Frit 3124 (glossy) and 3195 (silky matte) are balanced-chemistry bases (just add 10-15% kaolin for a cone 04 glaze, or more silica+kaolin to go higher). Consider Frit 3110 a man-made low-Al2O3 super feldspar. Its high-sodium makes it high thermal expansion. It works in bodies and is great to incorporate into glazes that shiver. The high-MgO Frit 3249 has a very-low expansion, it is great for crazing glazes. Frit 3134 is similar to 3124 but without Al2O3. Use it where the glaze does not need more Al2O3 (e.g. it already has enough clay). It is no accident that these are used by potters in North America, they complement each other well. The Gerstley Borate is a natural source of boron (with issues frits do not have).

Example of a glaze (G1916J) shivering on the rim of a mug. But the situation is not as it might appear. This is a low quartz cone 03 vitreous red body having a lower-than-typical thermal expansion. The white slip (or engobe) has a moderate amount of quartz and is thus put under some compression by the body. But the compression is not enough to shiver off (e.g. at the rim) when by itself. However the covering glaze has an even lower expansion exerting added compression on the slip. This causes a failure at the slip-body interface.

This chart compares the gassing behavior of 6 materials (5 of which are very common in ceramic glazes) as they are fired from 500-1700F. It is a reminder that some late gassers overlap early melters. The LOI (loss on ignition) of these materials can affect your glazes (e.g. bubbles, blisters, pinholes, crawling). Notice that talc is not finished until after 1650F (many glazes have already begin melting by then).

The white slip (applied to a leather hard cup) on the left is dripping downward from the rim (even though it was held upside down for a couple of minutes!). Yet that slurry was viscous with a 1.48 specific gravity, on mixer-off the motion stopped immediately. Why? Because it was not thixotropic (it did not gel). The fix? I watered it down to 1.46 (making it very thin and runny) and did a cycle of adding a pinch of epsom salts (about 0.5 gm) and mixing vigorously watching for it to thicken enough to stop motion in about 1 second on mixer shut-off (bounce backward!). It is extremely difficult not to overdo the epsom salts (gelling it too much) so I keep ungelled slurry aside and pour some back in to dilute to overgelled batch. That works perfect to fine-tune the degree of thixotropy so it gels after about 10-15 seconds of sitting. So to apply it I stir it, wait a couple of seconds and dip the mug. By the time I pull it out it is ready to gel and hold in place.

There is a direct relationship between the way ceramic glazes fire and their chemistry. Wrapping your mind around that and overcome your aversion to chemistry is a key to getting control of your glazes. You can fix problems like crazing, blistering, pinholing, settling, gelling, clouding, leaching, crawling, marking, scratching, powdering. Substitute frits or incorporate better, cheaper materials, replace no-longer-available ones (all while maintaining the same chemistry). Adjust melting temperature, gloss, surface character, color. Identify weaknesses in glazes to avoid problems. Create and optimize base glazes to work with difficult colors or stains and for special effects dependent on opacification, crystallization or variegation. Create glazes from scratch and use your own native materials in the highest possible percentage.

On the inside you can see the razor sharp flakes of glaze that have already fallen off the outside of this test piece. They reveal that the clear glaze, which looks fine on the inside, is under too much compression. The weaker interface between the slip under layer and the body provides a point of failure. This reminds us of the morality of making pottery, we have a responsibility to make safe ware.

The top glaze is VC71, a popular matte cone 6 glaze used by potters. Bottom is G2934 matte, a public domain recipe produced by Plainsman Clays. The latter is a high-MgO matte, it melts well and does not cutlery mark or stain easily. As evidence that it is a true matte, notice that it is still matte when fired to cone 7 or 8. VC71, while having a similar pleasant silky matte surface at cone 6, converts to a glossy if fired higher. This suggests that the cone 6 matteness is due to incomplete melting. For the same reason, it is whiter in color (as soon as it begins to melt and have depth the color darkens).

These mugs are quite thin walled. A glaze has just been applied to the inside. Notice how it has water logged the bisque (you can see the contrast at the base, where the clay is a little thicker and has not changed color yet). Although there may be enough absorbency that a glaze could be applied now, it would still not be a good idea because it would completely waterlog the piece and result in a very long drying time. This is bad, not only because of process logistics, but also because slow drying glazes almost always crack and lift from the bisque (causing crawling).

Pinholing is often related to the smoothness of the underlying body surface. The lower half of this vase was tooled during the leather hard stage, all the pinholes occur there. Even though this glaze contains 10% grog, the pinholes are not appearing on the upper half because the slip generated during throwing has left a smooth surface.

This is Polar Ice cone 6 porcelain that has been over fired. The electric kiln was set to do its standard cone 6 fast fire schedule, but a cone in the kiln demonstrates that it fired much higher (perhaps to cone 7 judging by the bend on the cone). This is a translucent frit-fluxed porcelain that demands accurate firing, the over fire has produced tiny bubbles and surface dimples in the glaze. The mug rim has also warped to oval shape. The lesson: If you are firing ware that is sensitive to schedule or temperature, use large cones and adjust if needed. If it fires too hot like this, then program to fire to cone 5 with a longer soak, or cone 5.5 (if possible). Or, program all the steps yourself; that is definitely our preference.

An extreme example of blistering in a piece fired at cone 03. The glaze is Ferro Frits 3195 and 3110 with 15% ball clay applied to a bisque piece. Is LOI the issue? No, this glaze has a low LOI. Low bisque? No, it was bisqued at cone 04. Thick glaze layer? Yes, partly. Holding the firing longer at temperature? No, I could hold this all night and the glaze would just percolate the whole time. Slow cooling? Close, but not quite. The secret I found to fix this was to apply the glaze in a thinner layer and drop-and-hold the temperature for 30 minutes at 100F below cone 03. Doing that increased the viscosity of the glaze melt to the point that it could break the blisters (held by surface tension) while still being fluid enough to smooth out the surface.

This cone 04 mug has survived a 300F to Ice-water thermal shock test, one which a similar recipe failed badly. This is G1916T glaze on Plainsman Buffstone fired to cone 04 (the failing recipe was G1916Q). The difference? This one switches the Frit 3110 for Frit 3249.

This Cone 10 matte mug has been refired to attach decals. During the refire the Quartz-containing body passed up through quartz and cristobalite inversions while the glaze did not (all of its quartz was converted to silicates during the previous glaze firing). The sudden expansion in these two zones stretched the glaze and cracked it. Had that glaze been better fitted (under some compression) it would have been able to survive.

The mug on the left was bisque fired and then glazed, the one on the right was glazed in the green (dry) state. The glazes are the same inside and out but the porcelain one the right is based on New Zealand kaolin (vs. American kaolin on the left). Three secrets for success for the one on the right were: It was glazed inside and out in two operations with a drying phase between, it was heated to about 150F before each application and it was fired with a soaking period (at about 1900F) on the way up to top temperature (cone 6).

The mug on the right was bisque fired and then glazed, the one on the left was glazed in the green (dry) state using our standard meet-two-colors-at-the-rim glazing method. This method lends itself well to single fire glazing. Notice the glaze did not go on as thick on the once-fired piece (extra attention is needed to make sure it gets on thick enough without cracking the piece). In addition, there are a few pinholes whereas the bisqued piece is flawless. Single firing ware requires extra attention to firing, climbing to a point just before the glaze begins to melt and soaking there to enable hydrates and carbon to escape.

An example of the importance of allowing a bisque piece to dry before glazing the outside face. This hand-built caserole lid is thin and was glazed on the inside first. That wetted the bisque enough that when the outside was poured there was not enough absorbency remaining to build a sufficient thickness on some of the areas of thinner cross section. The problem is exacerbated by the fact that the underlying red body is darkening the color of the thinner glazed sections.

An example of splitting, where a thrown clay develops a crack during, or minutes after, throwing. This often happens with clay bodies that have a wide range of particle sizes, for example those made from native clays not ground to 200 mesh. The larger particles create networks into which water can penetrate and begin and propagate a split. It is wise not to leave water or high water content slip on any surfaces experiencing tensile stress during forming.

This is an admirable first effort by a budding artist. They used a built-in cone 6 program on an electronic controller equipped electric kiln. But it is over fired. How do we know that? To the right are fired test bars of this clay, they go from cone 4 (top) to cone 8 (bottom). The data sheet of this clay says do not fire over cone 6. Why? Notice the cone 7 bar has turned to a solid grey and started blistering and the cone 8 one is blistering much more. That cone 8 bar is the same color as the figurine (although the colors do not match on the photo). The solution: Put a large cone 6 in the kiln and program the schedule manually so you can compensate the top temperature with what the cone tells you.

When electric kilns, especially large ones are tightly packed with heavy ware, the shady or undersides of the pots simply will never reach the temperature of the element side, no matter how long you soak. In this example, the inside of this clear glazed cone 6 bowl has a flawless surface. The base is pinholed and crawling a little and the surface of one side (the shady side), the remnants of healing disruptions in the melt (from escaping gases) have not smoothed over. The element side is largely flawless like the inside, however it is not as smooth on the area immediately outside the foot (because this is less element-facing). Industrial gas kilns have draft and subject ware to heat-work by convection, so all sides are much more evenly matured.

The buff stoneware mug on the right was bisque fired at cone 02, the one on the left at cone 06. The cone 02 mug was immersed in the clear glaze for 1 second and allowed to dry. The other was glazed on the inside first, allowed to dry, then glazed on the outside with a 1 second dip. Of course, the cone 02 one took longer to dry. In spite of this, the glaze is thicker and more even on the one bisque fired to cone 02. How is the possible? The secret is the thixotropy of the glaze. When that is right, a one second dip will give the same thickness and evenness whether dry or bisque, 06 or 02. Why bisque fire to cone 02? To get a glazed surface free of pinholes on some stoneware clays.

This is a cone 10 glossy glaze. It should be crystal clear and smooth. But it contains strontium carbonate, talc and calcium carbonate. They produce gases as they decompose, if that gas needs to come out at the wrong time it turns the glaze into a Swiss cheeze of micro bubbles. One solution is to use non-gassing sources of MgO, SrO and CaO. Or, better, do a study to isolate which of these three materials is the problem and it might be possible to adjust the firing to accommodate it. Or, an adjustment could be make to the chemistry of the glaze such that the melting happened later and more vigorously (rather than earlier and more slowly). The latter is actually the likely cause, this glaze contains a small amount of boron frit. Boron melts very early so the glaze is likely already fluid while gases that normally escape before other cone 10 glazes even get started melting are being trapped by this one.

The original recipe had a very low clay content, sourcing almost all of its Al2O3 from feldspar instead. Although the glaze slurry was maintained at 1.78 specific gravity (an incredibly high value) and thus would have had very low shrinkage, it did not stick and harden well enough to the ware. Why? Lack of clay content in the glaze. The fix was to source much more of the Al2O3 from kaolin instead of feldspar. The reduction in feldspar shorted the glaze on KNaO and SiO2 so these were sourced from a frit and pure silica instead (the calculations to do this were done in The change also provided opportunity to substitute some of the KNaO with lower expansion CaO. This reduced the thermal expansion and reduced crazing issues.

The left half of this cone 6 buff burning native-clay stoneware (Plainsman M340) was sponged at the dry stage. That exposed iron-bearing particles that are normally pushed under the surface. The result is a denser population of fired specks. While not usually a problem on flat surfaces, this can be an issue when rims of functional pieces are sponged and glazes stretch thin there during firing.

This is also a common problem at low fire on earthenware clay (but can also appear on a buff stonewares). Those white spots you see on the beetle also cover the entire glaze surface (although not visible). They are sites of gas escaping (from particles decomposing in the body). The spots likely percolate during soaking at top temperate. Some of them, notably on the almost vertical inner walls of this bowl, having not smoothed over during cool down. What can you do? Use the highest possible bisque temperature, even cone 02 (make the glaze thixotropic so it will hang on to the denser body, see the link below about this). Adjust the glaze chemistry to melt later after gassing has finished (more zinc, less boron). Apply a thinner glaze layer (more thixotropy and lower specific gravity will enable a more even coverage with less thickness). Instead of soaking at temperature, drop 100 degrees and soak there instead (gassing is much less and the increasing viscosity of the melt overcomes the surface tension). Use a body not having any large particles that decompose (and gas) on firing. Use cones to verify the temperature your electronic controller reports.

Using stonewares it is easy to get pretty sloppy in the studio because a particle of iron or cobalt in a glaze or body is no big deal. But on a ice white, translucent, transparent-glazed piece it is a really big deal. These specks are particles of cobalt that were trapped in my 80 mesh glaze screen from previous use. I use a soft brush to coax the glaze through the screen faster, but even that was enough to dislodge some of the cobalt particles. The lesson: I need a dedicated glaze screen for use with this transparent glaze, it gets used for nothing else.

Typical zero-boron high temperature glazes will not soften in a 1500F decal firing. But low temperature glazes will (especially those high in boron). Even middle temperature ones can soften. G3806C, for example, is reactive and fluid, it certainly will. Even G2926B, which has high Al2O3 and SiO2, has enough boron to soften and sometimes create tiny pits. In serious cases they can bubble like the mug on the right. Why? Steam. It was in use and had been absorbing water in the months since it was first glaze fired at cone 03. The one on the left was not used, but it did have some time to absorb water from the air, it is showing tiny pits in the surface. Even if moisture is not present, low fire bodies especially may still have some gases of decomposition to affect the glaze. One more thing: Fire the decals at the recommended temperature, often cone 022.

If your drying glaze is doing what you see on the left, do not smooth it with your finger and hope for the best. It is going to crawl during firing. Wash it off, dry the ware and change your glaze or process. This is Ravenscrag Slip being used pure as a glaze, it is shrinking too much so I simply add some calcined material to the bucket. That reduces the shrinkage and therefore the cracking (trade some of the kaolin in your glaze for calcined kaolin to do the same thing). Glazes need clay to suspend and harden them, but if your glaze has 20%+ kaolin and also bentonite, drop the bentonite (not needed). Other causes: Double-layering. Putting it on too thick. May be flocculating (high water content). Slow drying (try bisquing lower, heating before dipping; or glaze inside, dry it, then glaze outside).

This was made by a potter who carefully designed his own body and glaze recipe and obtained a high quality kiln in which to fire a line this line of ware. These pieces are being used in a restaurant and this one has failed. Why? It could be pushing up against the limits of this type of clay body (even at its best). Compared to what typical restaurant ware is made from it has more porosity (demonstrated by the color change around the crack). It likely has entrapped carbon inside. It has lower strength (a big issue restaurant ware which is handled so much). Larger particles of high expansion minerals create vulnerability to sudden temperature changes. If this is all true then all of the bowls will eventually crack. That being said, what if they do not all crack? Maybe this crack appeared to relieve stresses within the ware from uneven drying or uneven firing (due to technique or uneven thickness). Some bodies expand upon absorbing water, if that is the case with this then the bare clay surfaces (which permit water entry) could be an issue also.

This is the plus 100 mesh particulate from 50 grams of two different ball clays. Most of the particles are carbon, they will burn out and possibly cause glaze defects. If any of them are metallic, they will produce fired specks.

These are jiggered lids made from Plainsman M340 middle temperature stoneware. The one on the right was sponged in the dry stage to smooth issues that occurred during jiggering. That has exposed speck producing particles that were under the surface. This body is made from quarried materials that are ground to 42 mesh.

Notice the inside of this large transparent glazed cone 6 stoneware bowl. There is a concentration of specks on one part because that area was sponged at the leather hard and dry stages to smooth surface problems that happened during the jiggering process. These specks are normally driven below the surface during forming.

Low fire glazes must be able to pass the bubbles their bodies generate (or clouds of micro-bubbles will turn them white). This cone 04 flow tester makes it clear that although 3825B has a higher melt fluidity (it has flowed off onto the tile, A has not). And it has a much higher surface tension. How do I know that? The flow meets the runway at a perpendicular angle (even less), it is long and narrow and it is white (full of entrained micro-bubbles). Notice that A meanders down the runway, a broad, flat and relatively clear river. Low fire glazes 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 similar to SG13, although flowing less). However they all dry very slowly. Watch for a post on G2931J, a Ulexite/Frit-based recipe that works like A but dries on dipped ware in seconds (rather than minutes).

Custer feldspar and Nepheline Syenite. The coverage is perfectly even on both. No drips. Yet no clay is present. The secret? Epsom salts. I slurried the two powders in water until the flow was like heavy cream. I added more water to thin and started adding the epsom salts. After only a pinch or two they both gelled. Then I added more water and more epsom salts until they thickened again and gelled even better. They both applied beautifully to these porcelains. The gelled consistency prevented them settling in seconds to a hard layer on the bucket bottom. Could you do this with pure silica? Yes! The lesson: If these will suspend by gelling with epsom salts then any glaze will. You never need to tolerate settling or uneven coverage again! Read the page "Thixotropy", it will change your life as a potter.

The materials and body were clean. The problem was very strange because the specks only appeared on the insides of the ware. The problem turned out to be iron powder in the burners (shown in the overlay in this photo). Disassembling and cleaning them solved most of the issue. The rest? Disassembling and cleaning them better!

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