Search Results - Glossary

• Glaze Crawling

  A ceramic glaze fault that occurs during firing of the ware, the molten glaze pulls itself into islands leaving bare patches of body between.

• Glaze Layering

  In hobby ceramics and pottery it is common to layer glazes for visual effects. Using brush-on glazes it is easy. But how to do it with dipping glazes? Or apply brush-ons on to dipped base coats?

• Glaze Shrinkage

  Raw ceramic glazes contain clay to harden them on drying and to suspend the slurry. The more clay there is the more the glaze shrinks as it dries on the piece.

• Glaze thickness

  Many ceramic glaze benefits and issues are closely related to the thickness with which the glaze is applied. Many glazes are very sensitive to thickness, so control is needed.

Search Results - Articles

• A Low Cost Tester of Glaze Melt Fluidity

  Use this novel device to compare the melt fluidity of glazes and materials. Simple physical observations of the results provide a better understanding of the fired properties of your glaze (and problems you did not see before).

Low Budget Testing of Ceramic Glazes

There is more to glazes than their visual character, they have other physical properties like hardness, thermal expansion, leachability, chemistry and they exhibit many defects. Here are some simple tests.

Search Results - Troubles

• Crawling

Search Results - Pictures

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  A fluid reduction rutile glaze is crawling

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  Alberta Slip crawling because it shrinks too much on drying

  Example of Alberta Slip which has been sprayed on dry ware and single fired. This happened because the slip shrunk during drying creating a network of cracks. These cracks become the crawl-points during firing. …

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  ChatGPT is surprisingly wrong about the causes of glaze crawling.

  ChatGPT trained on the entire internet and yet gave 100% wrong answers and neglected the key thing that causes 90% of crawling! How can the internet be so wrong? Consider the suggestions it gave: -Dust or oil on the bisque: This almost never happens. Besides, glaze is a mix of dust and water! -Too much feldspar in a glaze can cause it to shrink excessively during firing: No, high feldspar causes thermal expansion/contraction of the fired glass, not physical shrinkage of the melt. -If the clay body surface is not roughened, the glaze may not adhere properly: No, glazes don’t crawl any more on porcelains than other bodies. -If the glaze is too thick in some areas and too thin in others, it can crawl in the thin areas: No, it crawls where thick because that’s where it cracks during drying. -Over-firing or under-firing: No. Glazes fired to the ideal temperature crawl just as much. -If the pottery is dried evenly or the drying process is too rapid: No, rapid drying of glaze on bisque is important to prevent cracking. This crawling happened because the glaze cracked along the inside of that corner during drying. Such cracking is by far the number one cause of crawling, the melt pulls back from either side of the crack. The specific gravity of the slurry was too high, the resulting greater thickness right at the corner gave the shrinking glaze power to pull a crack. Adding water to bring the SG back down to 1.4 and then Epsom salts to gel it to thixotropic gave the slurry much better dipping and drying properties and totally solved this issue. …

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  Commercial brushing glaze on a non-gummed dipping glaze: Crawling

  Non-gummed dipping glazes go on evenly and dry quickly on bisque ware (if properly gelled). But they only work well as a single layer. If you try to paint commercial gummed brushing glazes over them the latter will compromise their bond with the body, cracks will develop during drying and bare patches like this will result during firing. For multi-layering the base dipping glaze must be gummed (e.g. 1% CMC gum). It will go on thinner, drip longer and dry much slower, but that is the price to pay if you want to layer over it. …

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  Crawling can happen when paint-on glazes are layered over dipping glazes

  This bowl was dipped in a non-gummed clear dipping glaze. Such glazes are optimized for fast drying and even coverage. However their bond with the bisque is fragile. The blue over-glaze was applied thickly on the rim (so it would run downward during firing). But during drying, it shrunk and pulled the base coat away at the rim (likely forming many tiny cracks at the interface between the clear and the bisque. That initiated the cascade of crawling. When gummed dipping glazes are going to be painted over, a base-coat dipping glaze should be used. What is that? It is simply a regular fast-dry dipping glaze with some CMC gum added (perhaps half the amount as what would be used for painting). There is a cost to this: Longer drying times after dipping and less even coverage. And gum destroys the ability to gel the glaze and make the slurry thixotropic. …

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  Crawling glaze on slip cast ware is common

  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. There are several measures that can be taken 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). …

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  Crawling glaze on the convex edges of sanitaryware

  Sanitaryware glazes are high in zircon, thus a stiff glaze melt is a part of their very nature. That means glaze crawling is a part of their nature! And, preventing it is a major effort by producers. Crawling most commonly happens inside acute contour changes (that thicken the layer), but crawl points can even occur on relatively flat surfaces. However, this time it appears on the outside of an abrupt curve. Factors that can cause crawling can compound on corners. During drying, soluble salts and binders in the clay tend to concentrate at edges and corners - these can affect glaze laydown (especially its thickness and adhesion). The slip casting process favours the concentration of the finest clay particles at the mold face, but corners see the most surface disruption during cleaning and tooling at the leather hard stage (which can expose coarser particles below the surface). Glazes containing clay must shrink somewhat during drying, a corner like this will be the first place a crack appears (and thus a crawl), especially if adhesion is not as good. …

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  Crawling glazes withdraw into islands during melting leaving bare body patches

  This problem is almost always caused by glazes shrinking too much during drying, and then cracking. Those cracks become the crawl points during firing. Excessive shrinkage is normally a product of too much raw clay in a glaze. Even glazes having marginally high clay can crack if applied too thick. It is likewise with multi-layer application without consideration for the specific needs of that process (e.g. failure to use a base coat glaze for the first layer). Multi-layering of glazes rewets the first layer, stressing its bond with the body and pulling it away from the body as it shrinks, base coat glazes have better adherence. Crawling is quite prevalent in once-fired ware since glaze bonding is more tenuous. To state again: Glazes contain clay to suspend their slurries. Clay shrinks when it dries. Some shrinkage can be tolerated but when it is excessive something has to give. As glaze layer thickness increases it is afforded more and more power to impose its shrinkage on the bond with the body. At some point, that bond will be compromised in places where cracks occur to release the tension. Even if these do not appear on dry ware, bond compromise can still exist. …

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  Crawling in G2934Y Zircopax white glaze: Here are some fixes.

  G2934Y, a variation of the G2934 base, is a good stain matte base glaze but it is not without issues. It has significant clay content in the recipe and high levels of Al2O3 in the chemistry, these make it susceptible to crawling. This base is normally fine as is but when opacified or certain stains are added (especially at significant percentages) it can crawl. This has 10% Zircopax. Even though the glaze layer thickens at the recess of the handle join it is still crawling. We also get this on the insides of mugs where wall and foot meet at a sharp angle. This was initiated because the glaze cracked here during drying. Normally it would heal but the zircon stiffens the melt, making it less mobile. The easiest solution is to adjust the specific gravity of the glaze to 1.44 and flocculate it to thixotropic, this assures that the application is not too thick. Another measure is to add a little CMC gum (by replacing some of the water with gum solution). Lastly, use a blend of tin oxide and Zircopax, as in the G3926C version of the recipe, to opacify it. …

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  Crawling on a thickly applied glaze (cone 6)

  Example of glaze crawling on the inside of a stoneware mug. Notice how thick it is. Thickly applied glazes have more ability to assert their shrinkage during drying and thus compromise their bond with the body below. The cracks that appear become bare patches after firing. …

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  Crawling on sanitary ware. Laydown is the first suspect.

  This is glaze crawling and it underscores the need for attention to the details of all production parameters. This one small glaze defect makes this pedestal sink either a refire, a second or unsaleable. This is most common on abrupt surface contours but that is not the case here. The cause of this is likely several factors combining. The glaze is opaque white because it contains a high percentage of zircon opacifier. Zircon glazes tend to do exactly this so their successful use is doubly dependent on minimizing the percentage added and on attention to other details to compensate. This glaze has been applied thickly to ensure good coverage (thicker laydowns bring more crawling problems). The glaze is likely low in clay and thus the physical bond of the dried glaze layer depends on the binders being used, their percentages, the integrity of the way they were mixed in, and their shelf life. The ability of the glaze laydown to dry-bond with the body depends on the condition of the surface (e.g. water content, dry or bisque fired, smoothness, dustfreeness, quality of materials used in the body and integrity of body preparation, etc), the presence of surface contaminants (e.g. soluble salts) and the way in which it was applied and its thickness. The glaze melt's ability fire-bond and form an interface with the body that produces a smooth surface is dependent on its melt fluidity and ability to form an interface with the body. There is another way to look at this problem: The process runs along crawling multiple tipping points: A viscous glaze melt, glaze application to dry rather than bisque ware, a thick glaze application, a large surface area intolerant of any defects and a glaze application technique (spraying) prone to irregularities of thickness. Rather than trying to identify the specific problem it might be better to simply make changes to move the process back from the tipping points. …

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  Crawling sanitary ware glaze sourcing Al2O3 from only feldspar

  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 Insight-live.com). The change also provided opportunity to substitute some of the KNaO with lower expansion CaO. This reduced the thermal expansion and reduced crazing issues. …

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  Fixing a crawling problem with a measured CMC addition

  The problem: This dipping glaze is crawling (as shown on the glazed tile). Let's assume I have already checked to make sure the specific gravity is right and the slurry is thixotropic. Because it has settled a little there is an opportunity for plan B: Remove some of the water and replace it with gum solution. Shown here is a precise calculation of the exact water content of the slurry to replace 10% of the water with gum solution. But it is better to take a more conservative and easier approach: Replace one-twentieth of the water with gum solution (too much gum and the glaze will drip excessively and dry too slowly). Rather than be overly precise let's just guess: I have 5000g of slurry that is about 50% water, so that is 2500g of powder, so I need to remove 125g of water and replace it with 125g of Laguna gum solution. A good way is to use a sponge: Wet and wring it out first and then repeat touching it to the water surface and wringing it out into a container to get 125g. A propeller mixer is needed to mix in the added gum solution (it won't just stir in). …

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  Fixing a crawling problem with Ravenscrag Tenmoku

  Crawling of a cone 10R Ravenscrag Slip iron crystal glaze. The added iron oxide flocculates the slurry raising the water content, increasing the drying shrinkage. To solve this problem you can calcine part of the Ravenscrag Slip, that reduces the shrinkage. …

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  G2934 cone 6 matte glaze + 5% Zircon is crawling

  The crawling is happening at sharp concave surface contours (e.g. inside bottom corners on mugs) and on pieces like this. G2934 has good melt fluidity so that is not the issue. Here are some questions to consider: -Did the glaze crack on drying? That is a sure sign it will crawl. Did you use calcined kaolin as specified in the recipe? That helps reduce the shrinkage as it dries (to eliminate the cracking). -This looks like it is going on pretty thick - was it correctly mixed as a thixotropic slurry (high water content, 1.44 specific gravity, and gelled)? Or was it mixed with low water content (e.g. 1.55 specific gravity). -These crawled areas were disturbed during spout forming, what can be done to repair the surface then? -This is firing pretty glossy so kiln cooling is likely pretty fast. A drop and hold firing can help (as long as the glaze is not too thickly applied). -Slower cooling means it will likely fire too matte, that is why we make an 80:20 blend of G2934 and G2926B glossy - to enable tuning matteness to any firing schedule. -Mix up some of it as a brushing glaze. Apply a thin layer onto the bisque in the areas likely to crawl. Then dip the whole piece in the dipping glaze version. -Mix the batch as a base coat dipping glaze, then it will adhere better to the bisque. -Reduce the amount of calcined kaolin somewhat in favor of raw kaolin (perhaps 5%) - that may produce a slurry with better coverage and adherence. …

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  Glaze crawling on single-fire - solved by bisque firing

  This problem only happened once in a while at a production pottery. On the rims of thick or thin pieces. The ware is glazed inside and out, the rim sponge-cleaned after each. Finally, the rim was dipped (after a period of drying). Glaze thickness and layering did not affect the appearance of the issue. They control glaze rheology carefully (according to specs). It appeared the problem had been solved when it was discovered that a worker was using hand cream and getting it on the rim while handling the ware (the touch points being the crawl points). Further, ware was not being kept clean and dust-free. But these were not the issue. In the end this was solved by bisque firing. Yes, they were glazing ware in the dry, or green state. The fact that they could accomplish this on thin-walled ware is amazing. Their experience is a testament to the value of bisque firing for smaller production facilities having limited equipment and resources. …

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  Glaze is crawling on overlap when it did not do so before. Why?

  By far the number one cause of crawling is cracking of the glaze layer during drying. Of course, glazes having too high a percentage of plastic ball clay or bentonite will shrink and therefore crack more. But in this case, the G2934 matte recipe contains less than 20% kaolin and it worked well in the past (left plate). But this time the crawling happened on the overlap (upper right). The problem is with the specific gravity, it is too high (resulting in faster thickening for the same dip duration). This dipping glaze works best with a specific gravity of 1.44. If the slurry is too runny Epsom salts (or other flocculant) can be added to make it thixotropic. When the rheology is right it works extremely well. …

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  Here is why Gillespie Borate is crawling some glazes

  This is a variation on the 50:30:20 cone 6 pottery glaze recipe, it contains 22% Gillespie Borate (GB) and 12% calcined kaolin. Our objective was to reduce its melt fluidity. But the crawling is so bad that it is almost unusable. The reason was not obvious until we fired a sample to 1550F and 1650F. At the former the integrity of the glaze layer is great, but by 1650F it does this (many of the edges of these are curling upward). Ulexite, which GB contains, is known for the behaviour of suddenly shrinking and then suddenly melting over a narrow range of temperatures. Since GB is plastic and suspends slurries well we thought calcined kaolin would be better than raw kaolin (to minimize drying shrinkage). However, the improvement is minimal. …

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  Mag carb causes crawling in higher amounts

  Example of two crawling glazes. Both have magnesium carbonate added to make this happen (around 10%). On the left at cone 04 on a terra cotta body, on the right at cone 6 on a porcelain. Magnesium carbonate also mattes glazes. …

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  Majolica glaze crawling on this slip-cast terra cotta

  Mitigating factors are poor adherence of the glaze to the smooth-surfaced L4170B terra cotta bisque, high zircon content in the glaze (it reduces melt fluidity) and the very thick application (this is a variation on G1916Q as a majolica glaze). Which is the most important of those factors? The adherence to the bisque. An addition of Veegum CER solved the problem (50g of the solution to 0.75 litres of glaze slurry, with added water to thin it out). Unlike with CMC gum, this slurry still goes on thick and evenly. Although the piece on the left is darker red, that is a photo issue, these were both the same body and fired at cone 04. …

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  Multi-Layering on a large bowl has produced crawling

  Glaze layering is tricky. We recommend a gummed base coat dipping glaze where layering will be done over it. This alone could have solved this issue. …

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  This serious glaze crawling problem was solved with a simple addition

  This is G2934Y white (with 10% Zircopax). I initially blamed the zircon for the crawling. But, since the slurry had settled somewhat I was able to remove about 15% of the water and replace it with CMC gum solution. The gum addition was not enough to slow down the drying much but it really improved reduced the problem! This likely means that adherence of the dried layer to the smooth bisque was the issue. This being said, there were still a couple of small spots where it crawled. The ultimate solution thus appears to be discovering the percentage of CMC gum and water needed to get the least loss of drying speed and while achieving integrity of glaze coverage. It is best to add the CMC gum as a powder at mixing time and blender mix the slurry thoroughly to be sure that it fully dissolves (watch for a rheology change on aging, that will demonstrate if mixing was thorough enough). …

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  Why is the glaze crawling on these tiles?

  An example of crawling in a zircon opacified glaze on a tile. The immediate source of the problem is likely at the decoration stage. The water from the blue overglaze is rewetting the white under glaze, expanding and reshrinking it. This compromises the white glaze's bond with the body, resulting in cracking and lifting of the edges of the cracks. A number of things can be done to improve the situation: Adding a binder to the white glaze, reducing the clay content or using less plastic clays in its recipe, reducing the water content of the overglaze, heating the tiles before glazing and/or decorating so they dry faster and reducing the surface tension of the glaze melt. …

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