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Dealing With the Problem of Powdering and Cracking Glazes

Section: Glazes, Subsection: Trouble Shooting

Description

Powdering and dusting glazes are a dust hazard, nuisance and impediment to ware quality. Shrinkage and cracking on glaze drying can shut you down. Both have the same cause.

Article

When glazes 'powder' onto your hands and create dust during handling it can be more than just aggravating. The causes of dusting generally contribute to other problems (slurries settle quickly and lay-down varies in thickness). By contrast, when glazes shrink excessively and crack and fall off during drying it is totally frustrating. These two glaze problems are actually closely related, that is, they have a common cause as we shall see. I've seen normally impatient people demonstrate a remarkable tolerance for these situations. After all that glaze recipe 'came from the Gods and we can't mess with it'. Right? On the contrary, this situation is one that can be dealt with logically.

It might seem the glaze chemistry could not possibly have anything to do with problems like this. But think again. This is exactly the kind of problem where it really shines. Why? Because many of the solutions involve altering the glaze recipe without changing its overall chemistry.

First, what causes dusting? The answer is lack of particle binding (a binder is needed to 'glue' the particles together). What about glaze shrinkage and cracking? Too much particle binding and associated shrinkage. Let us consider a little background.

Glaze slurries are suspensions of mineral powders (a bunch of microscopic rocks floating in water). What makes them float? The same thing that hardens the glaze powder: Clay (e.g. kaolin, ball clay, bentonite). Clay particles are thin and flat and very small. One gram of clay has an unbelievably large total surface area compared to other minerals used in ceramics. Clay particles have a curious surface chemistry that produces opposite electrical charges on the faces and edges. This results an affinity for water on the faces, this is what produces plasticity in clay bodies, the water glues together yet lubricates movement of the particles. In high water systems, like glaze slurries, suspended clay particles hang on to each other directly (edges against faces) and indirectly (faces against faces using water as the glue). This is often referred to as 'a house-of-cards arrangement' and it can accommodate large amounts of other mineral particles within the matrix and still exhibit the same properties (to a lesser degree of course). Conceptually the other mineral powders are just 'dead microscopic rocks' along for the ride!

The mechanism of the 'bonding' that takes place during dewatering (drying) is not commonly understood. As interparticle water is removed during drying clay particles move closer together (and pull others with them). The packing results in shrinkage of the entire matrix. Large particle clays (like kaolins) shrink 5% or less from plastic to dry whereas fine particled clays might shrink 25% or more (shrinkage is more complex than simply particle size but for our purposes we will not get into that). However mere particle proximity does not in itself create a bond. The chemistry on the surface results in the migration of some chemical species across the boundary. While this creates a very weak bond, the fact that there are billions of particles bonded together in such a fashion creates a clay surface that we perceive to be a fairly hard product. The finer the clay particles the harder it will be. However clay bodies and glazes also contain all kinds of other particles in the mix that do not bond, and as noted they reduce the number of clay-to-clay bonds (which is bad) but also reduce the drying shrinkage (which is good). Therefore a dried matrix, whether clay body or glaze layer, is a bunch of rock particles held together by billions of weakly bonded clay particles.

Now, the question is: What bonds a dry glaze layer to a piece of bisque ware? Well there is no obvious dry adhesion mechanism or boundary chemical reaction that glues glaze particles to the bisque wall. The mechanism of the bond relates to the sticky nature of the wet glaze and the microscopically rough surface of the bisque ware. During hardening the glaze layer loses its wet adhesion and simple mechanical contact is the only microscopic bond. The layer stays on because all the minute surface cracks and pores give it places to hang on to. As you can imagine, this bond is weak at best.

Since all glazes shrink during drying, it is not clear how the weak bond with the bisque is able to withstand the pulling forces associated with the shrinkage. Well some glazes hardly shrink at all because they lack clay content and that is of course why they dust off excessively. However glazes that harden properly during drying always crack, you just do not see the cracks. Micro-cracks must develop to relieve the stress. However when there is too much shrinkage they become visible cracks. With even more stress the glaze cracks to form 'islands' with curled up edges (like a dried up lake bottom). You can see this effect clearly if you watch a slurry of pure kaolin or ball clay dry on a bisque surface.

As you can see, we want a glaze to have enough clay so that it forms a hard dry layer but not so clay that it shrinks excessively and cracks off the bisque. There are a number of strategies you can employ if your glaze is powdering on one extreme or shrinking and cracking off on the other. It follows that a powdering glaze needs either more clay or a finer more plastic clay whereas a glaze that is shrinking and cracking needs less clay or less plastic clay. Typically pottery glazes need a minimum of 20% kaolin (equivalent to 10-15 ball clay or 5% bentonite) to harden adequately.

-If your cracking and shrinking glaze employs a relatively plastic kaolin (like #6 Tile or Sapphire), try switching to a less plastic one like EPK or Pioneer. This will not affect glaze chemistry much. A similar switch of one ball clay for another is not as likely to work since pretty well all common ball clays are very plastic. If your glaze is powdering then switch from the less plastic material to a more plastic one. However I must say that if your glaze has a problem, this one change is not going to solve it, more will be needed.

-Add 3-5% bentonite for powdering glazes, remove it from cracking glazes. Bentonite is super fine and super plastic and therefore dries very hard and shrinks alot. The small amount of bentonite does not affect the glaze chemistry too much. Remember you can't add bentonite to an existing slurry, it agglomerates into balls that even a propeller mixer won't break up; you need to shake it up with the powder in a new batch to separate the particles).

-Add CMC gum to powdering glazes. Like bentonite, it needs to be added during dry mixing. Gum is very sticky and it hardens, using it is a way of 'gluing' a glaze on the ware. Strangely gum also helps suspend, but I have no idea why. Gum burns away so it has no effect on glaze chemistry. One problem: gummed glazes dry slower and drip-drip-drip after glaze dipping pull-out. Experiment with the amount, try 0.5% to start. Do not use gum unless you need it.

-Use kaolin instead of ball clay for cracking glazes (and vice versa for powdering ones). Since kaolin has less silica you will need to use ceramic chemistry to figure out how to compensate for the change in alumina and silica. No big deal? Think again, the amount of SiO2 and Al2O3 is the primary determining factor to many fired glaze properties and kaolin is the number one source of Al2O3.

-Check the specific gravity of your glaze (its weight per cc). If it is too low (below 1.4) then there is too much water in the slurry. Perhaps your water supply contains electrolytes that are flocculating the mix, this is, thickening it. Try using distilled water. Also, look out for slightly soluble materials in your glaze, they might be the source of electrolytes. High nepheline syenite glazes can do this. If you have a big container of flocculated glaze there is not much you can do with it except throw it out. You might try adding a small amount of deflocculant like Darvan or Sodium Silicate (e.g. 0.1%) to thin it but then you still have to figure out how to get all the water out and it might be thick again next week!

One last note about powdering glazes: Because they generally lack clay they settle out also. Often a layer of water forms at the surface only a minute or two after stirring (generally not easily seen). Although an adequately thick layer may still build up on the piece during dip, on pull out the water layer may wash glaze off on the last-to-leave sections (usually the rim). The principles mentioned above apply, if you don't want to stir it every minute then the glaze needs reformulation so the chemistry stays the same but more plastic materials are used to source alumina.

Another note about glaze bonding: If you fire your bisque too high it might not be absorbent enough to build up a good layer of glaze on dipping and still dry out quickly. If a glaze needs to dry over a long period on water logged ware, then it will usually crack. Likewise, if your ware has very thin walls then there simply will not be enough porosity in the matrix to pull the water out of the glaze quickly enough (normally a glaze should lose its wet sheen within 30 seconds, many do in less than 10 seconds). One solution is to heat the bisque and dip it hot into the glaze using dipping tongs (of course that is not an option if ware is delicate). Alternatively, you could heat and then spray the glaze onto it. Also, remember that smooth porcelain surfaces do not provide as many imperfections for the glaze to hang on to.

There you have it. The moral of the story is that you need to understand the purpose of each material in the glaze (and even the water) to fix problems. And glaze chemistry is pretty hard to avoid, here again it is one of the most valuable tools to solve the problem.

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Authors

Tony Hansen (Owner)

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