|Monthly Tech-Tip |
Ask yourself the right questions to figure out the real cause of a glaze crawling issue. Deal with the problem, not the symptoms.
Crawling is where the molten glaze withdraws into 'islands' leaving bare clay patches. The edges of the islands are thickened and smoothly rounded. In moderate cases there are only a few bare patches of clay, in severe cases the glaze forms beads on the clay surface and drips off onto the shelf. The problem is by far most prevalent where bisque-applied glazes contain excessive plastic clay content or are applied thickly or in multiple layers. It is also common in once-fire ware where it is more difficult to achieve a good bond with the body surface.
If the dried glaze forms a network of cracks it is a sign that the glaze is shrinking too much. The fault lines provide places for the crawling to start (especially where the islands the cracks delineate raised edges that are no longer in contact with the body). There are a number of possible contributors:
- If very fine-particled materials are present (i.e. zinc, bone ash, light magnesium carbonate) these will contribute to higher shrinkage during drying. Try using calcined zinc, synthetic bone ash or another source of calcia, talc or dolomite to source magnesia instead of magnesium carbonate.
- It is normal to see 20% clays (ball clay, kaolin). If significantly more is present try using a less plastic clay (i.e. kaolin instead of ball clay, low plasticity kaolin instead of high plasticity kaolin, or a mix of calcined and raw kaolin). Ultimately you must tune the glaze recipe's clay content to achieve a compromise of good hardness and minimal shrinkage while maintaining the chemistry (Want to learn how? Please email).
- If a glaze has been ball milled for too long it may shrink excessively (for example, zircon opacified glazes can be ground more finely than tin ones). Highly ground glazes may produce a fluffy lay down.
- If a slurry has flocculated (due to changes in water, dry material additions like iron oxide, or addition of an acid, epsom salts, calcium chloride, etc) it will require more water to achieve the same flow and will therefore shrink more during drying and require a longer period to dry. Try using distilled water. Always measure the specific gravity to maintain solids content and use deflocculants/flocculants if necessary to thin/thicken the slurry (remove water from an existing glaze slurry by pouring some on a plaster batt, then mixing the water-reduced mass back in).
- Gerstley Borate is plastic and therefore contributes to glaze shrinkage, especially if the recipe also contains kaolin or ball clay. It also tends to gel glazes so they need excessive water. Use boron frits instead. Try a boron frit (do calculations to make these adjustments, there is no frit that has the same chemistry).
- Many potters add bentonite to every glaze they make, thinking that it will help suspend. But when glazes already have sufficient, or even excessive clay content, the extra bentonite may increase the shrinkage enough to cause drying cracks.
- Again, be "recipe flexible", tune the raw clay content. Use enough clay in the glaze mix to both suspend the slurry and toughen the dried layer, more than that risks excessive shrinkage. Less and the glaze does not harden and forms a powdery surface. A fool-proof way to reduce shrinkage is to calcine (see the link below) part of the clay. If there is 35% kaolin in the mix, then try using 15% calcined kaolin and 20% raw kaolin (calcine the clay yourself if needed, it is easy). If there is 70% Alberta Slip, try using 35% raw and 35% calcined. Adjust the proportion as you get experience in working with the glaze. One detail: If there is significant clay content, adjust for the LOI (weight lost during firing) when calcining. Kaolin, for example, loses 12% weight on firing, so use 12% less calcined). Alberta Slip and Ravenscrag Slip have 9% LOI.
It is possible to create glaze slurries that gel and flow extremely well, dry hard and do not crack by using the right kaolin (i.e. EPK) or ball clay (i.e. Old Hickory #5, No. 5 Glaze) in adequate amounts. It is very important to realize that many ball clays and kaolins to not produce glazes of good slurry properties (without additives), a simple substitution can make a remarkable difference. It may not seem that glaze chemistry is related to this subject of getting the right type and right amount of clay in a recipe. But it very much is. Why? Because the ability to juggle a recipe to source Al2O3 from a clay or a feldspar/frit, as needed, enables controlling the amount of clay in the recipe while maintaining its fired character. Chemistry also gives you the ability to switch between different types of clay (that have different oxide makeups).
The mechanism of the bond between dry glaze and body is simply one of physical contact, the roughness of the ware surface copmbined with the ability of the liquid glaze to flow into all the tiny surface pores and irregularities and the degree to which it is able to dry hard without shrinking too much, these determine its ability to 'hang on'.
Matte glazes are more prone to crawling. Why? Because they usually have high Al2O3. The major contributor of that oxide is clay, especially kaolin. Matte glazes commonly have 35% kaolin in the recipe. Use part calcined material, part raw kaolin to deal with this problem.
Many pottery glazes have high feldspar and low clay contents, simply because they were improperly formulated. Using chemistry, you can shift the recipe to supply part of the Al2O3 from kaolin instead of the feldspar, reducing the feldspar percentage (this involves corrections in the amount of silica and other materials also).
Slip glazes can have 70, 80 or even 90% of a slip clay in them. Alberta Slip and Ravenscrag slip are examples. These materials melt by themselves to make glazes. But they are clays, they shrink. Follow the instructions from the manufacturer on how to use them properly.
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.
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.
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.
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).
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. Albertaslip.com and Ravenscrag.com both have pages about how to calcine and calculate how much to use to tune the recipe to be perfect.
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).
The glaze on the left is 85% of a calcine:raw Alberta Slip mix (40:60). It was on too thick so it cracked on drying (even if not too thick, if others are layered over it everything will flake off). The center piece has the same recipe but uses 85% pure raw Alberta Slip, yet it sports no cracks. It should be cracked much worse than #1. How is this possible? 1% added CMC Gum (via a gum solution) was added! This is magic, but there is more. It is double-layered! Plus very thick strokes of a commercial brushing glaze have been applied over that. Yet no cracks. CMC is the secret of dipping-glazes for multi-layering. The downside: More patience during dipping, they drip a lot and take much longer to dry.
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).
Understanding Glaze Slurry Properties
It is possible to have a glaze slurry that is a joy to use, but only if you understand the physics of the materials in the glaze recipe.
Creating a Non-Glaze Ceramic Slip or Engobe
It can be difficult to find an engobe that is drying and firing compatible with your body. It is better to understand, formulate and tune your own slip to your own body, glaze and process.
Alberta Slip 1900F Calcined
A pure source of BaO for ceramic glazes. This is 77% BaO and has an LOI of 23% (lost at CO2 on firing).
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.
In ceramics, glazes are slurries. They consist of water and undissolved powders kept in suspension by clay particles. You have much more control over the properties than you might think.
Ceramic Glaze Defects
A ceramic glaze fault that occurs during firing of the ware, the molten glaze pulls itself into islands leaving bare patches of body between.
In ceramics, glazes melt to produce a liquid glass. That glass exhibits surface tension and it is important to understand the consequences of that.
Glaze Pinholes, Pitting
Analyze the causes of ceramic glaze pinholing and pitting so your fix is dealing with the real issues, not a symptom.
|By Tony Hansen|
Follow me on