|Monthly Tech-Tip |
A ceramic glaze fault that occurs during firing of the ware, the molten glaze pulls itself into islands leaving bare patches of body between.
Key phrases linking here: crawling, crawled, crawl - Learn more
A condition where pottery or ceramic glaze withdraws into islands (during firing) leaving bare body patches between.
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.
The opacifier, Zircopax, when added to whiten glazes, often causes crawling (likely due to a combination of the effects of its fine particle size on drying properties and its tendency to stiffen the melt). Extra measures and constant attention to detail (e.g. glaze thickness, slurry rheology, avoidance of sharp contours on ware) are needed with such glazes.
Common fixes are to use a clay of similar chemistry but less shrinkage (in the recipe), calcining part of the clay, and reformulating the glaze to source more Al2O3 from feldspar (or frit) and less from clay. For multi-layering, the first coat can be bisque fired on, binders can be added, thinner application done or glazes of lower clay content used (or calcined clay).
Crawling problems are not without band-aid solutions, fixes often recommended, but which do not address the key causes. For example, some authors have recommended adding feldspar or silica to glazes to reduce crawling. But such solutions have minimal chance of helping without large additions. These are almost certain to change the fired character of the glaze. For example, most glazes are already close to crazing (or already crazing). Adding significant feldspar will make that worse. It will also affect gloss and melting temperature (feldspar is a melter). Adding some silica is fine if a glaze is really well melted already. But adding enough to dilute too-high clay content is certain to affect melt fluidity and gloss and thermal expansion. Excess raw clay in a glaze is by far best dealt with by reducing the percentage of that raw clay, not diluting it with something else that changes the character. Others blame dusty bisque ware, saying that it should be washed before glazing. Wetting bisque ware is counterproductive, removing traces of dust to accommodate glazes which are 100% dust mixed with water makes little sense.
Sometimes glazes are made to crawl intentionally. One technique to make this happen is to add 15-20% magnesium carbonate (testing required to determine the amount) to a low-fire transparent glaze.
Most often, the problem of glaze crawling is due to poor or a compromised adhesion of the glaze to the body. Because commercial prepared glazes contain significant gum or binder they can be layered and yet still stick and dry well. Usually. But this time the body is a fritted porcelain, its surface is smooth and dense because it was bisque fired too high (1850F instead of 1550). While the dried glaze surface was free of cracks and appeared stuck on well, its adhesion with the body was obviously not good enough to withstand the rigors of being firing on this smooth and vitreous surface.
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).
It was spray applied on the dried bowl (no bisque fire) an was too thick (not to mention under fired). But the main problem was a glaze recipe having too high a clay content. If a glaze has more than about 25% clay, consider a mix of the raw clay and calcined. For example, you can buy calcined kaolin to mix with raw kaolin. Or you can calcine the clay in bowls in your kiln by firing it to about 1200F.
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.
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).
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.
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.
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.
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).
Are you really sure the problem is with the materials? I had been using an 85% Ravenscrag, 15% frit glaze for many years with no crawling problems. But then it started crawling. I tried mixing with new materials and the old ones. Still crawled. The problem? What was I thinking? An 85% clay glaze is going to crawl so the question should have been: How did I get away with it for so long? I actually do not know! But I am now calcining Ravenscrag as appropriate (as documented at ravenscrag.com) and I love the control this gives me in balancing slurry properties with drying hardness.
This high-Alberta-Slip glaze is shrinking too much on drying. Thus it is going to crawl during firing. This common issue happens because there is too much plastic clay in the glaze recipe (common with slip glazes). Clay is needed to suspend the other particles, but too much causes the excessive shrinkage. The easiest way to fix this is to use a mix of raw and calcined Alberta Slip (explained at albertaslip.com). The calcined Alberta Slip has no plasticity and thus much less shrinkage (but it still has the same chemistry). Many matte glazes have high kaolin contents and recipes will often contain both raw and calcined kaolin for the same reason.
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 (one reason to avoid gum if possible). That fixed it! Meaning 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. Replacing another 5% of the water should fix that. If you need to fix the problem with a gloss white it will likely require less gum, start with replacing 10% of the water.
The body: M370. Glaze: G2934Y (with added green stain). Firing: Cone 6 drop-and-hold. Glazing method: dipping (using tongs). Thickness: The same. Surface: Clean on both. The difference: Wall thickness. The one on the right was cast much thinner so the glaze took a lot longer to dry. Common pottery glazes contain clays which need to shrink somewhat during drying. The bond with the bisque, although fragile, is normally enough to prevent cracking during drying. But drying needs to occur quickly. Quick drying is only possible when the body has enough porosity to absorb all the water quickly. Otherwise, cracks appear and these become crawls during firing. A complicating factor is that stain and/or zircon additions make an already-crawl-susceptible glaze even worse.
One or a combination of the following can be done to minimize crawling on even very thin-walled pieces:
-Apply a thinner glaze layer.
-Heat the bisque before dipping.
-Glaze the inside and outside separately (with drying between).
-Deflocculate the glaze to reduce water content.
-Brush or spray it on in multiple coats.
Crawling of a cone 10R Ravenscrag 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. Ravenscrag.com has information on how to do this.
The glaze on the right is crawling at the inside corner. Why? Multiple factors contribute. The angle between the wall and base is sharper. A thicker layer of glaze has collected there (the thicker it is the more power it has to impose a crack as it shrinks during drying). It also shrinks more during drying because it has a higher water content. But the leading cause: Its high raw clay content increases drying shrinkage. Calcining part of the raw clay destroys its affinity for water (which is what makes it plastic), this is an effective way to deal with this. Or doing a little chemistry to source some of the Al2O3 from materials other than clay (e.g. a frit having a higher Al2O3 content).
Fixing a crawling problem with a measured CMC addition
Why does the glaze on the right crawl?
Something is definitely wrong the glaze thickness here!
An additive that that induces severe glaze crawling
Ask yourself the right questions to figure out the real cause of a glaze crawling issue. Deal with the problem, not the symptoms.
Ceramic Glaze Defects
Calcining is simply firing a ceramic material to create a powder of new physical properties. Often it is done to kill the plasticity or burn away the hydrates, carbonates, sulfates of a clay or refractory material.
Traditional Japanese high feldspar glazes having cream to orange color flashing or blushing. Potters today seek to emulate the Shino appearance using a wide range of recipes.
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?
In ceramics, surface tension is discussed in two contexts: The glaze melt and the glaze suspension. In both, the quality of the glaze surface is impacted.
Glaze slurries can gel if they contain soluble materials that flocculate the suspension. Gelling is a real problem since it requires water additions that increase shrinkage.
Light Magnesium Carbonate
A refractory feather-light white powder used as a source of MgO and matting agent in ceramic glazes
|By Tony Hansen|
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