Adjusting Glazes for Application
Zinc Silicate Crystalline Glaze Pottery
A chronicle of my recent progress and a way for me to keep it straight in my head!
From the March/April and May/June 1998 issues of Clay Times
One of the more frustrating aspects of ceramics is glazing. Glazing is a pretty non-intuitive act: what you see is definitely not what you get.
Besides that, the very act of glazing is a difficult dance that requires a person to do everything correctly and in the right order to get a perfect glaze application. Having your glazes well prepared won't change any of that, but it will prevent some of the other frustrations that occur, such as having the glaze settle out like a rock or having it drip and "curtain" down the sides of the pot.
In order to understand how to adjust a glaze, you first need to understand a couple of basic concepts about the glaze itself. The first is that all the particles that are in it are not the same size. Yes, they all feel fine to our fingers, but the range of sizes is enormous. If the smallest clay particles are the size of a pea, then by comparison the largest particles (e.g. frits and stains) are the size of a six-story building, with everything in between present as well.
If you looked at the particles under a microscope, you'd see that the largest ones look like rocks and boulders, while the smallest ones (the clays) look like tiny, flat, hexagonal platelets. The rocks and boulders do not float, so by themselves they would settle like, well, the proverbial rock. The small clay particles would also quickly settle except for one thing: they tend to be attracted to one another in such a way that they form soft, loose, open, "flocs". These flocs are what slow down the settling of the coarser particles. Notice that I said "slow down". All glazes settle over time; the question is how quickly they do it, and how hard the layer is afterward. A glaze that is being kept in suspension by these flocs of clay particles is called a flocculated glaze.
A flocculated glaze settles slowly and into a soft, open layer. A deflocculated glaze (at least one that is inadvertently deflocculated) will settle into a thin, rock-hard layer that can only be budged with dynamite (well, that's a bit of an exaggeration, but not much).
If you are going to apply a glaze to soft bisque (i.e. a porous bisque that was fired to earthenware temperatures), and if you intend to apply it by dipping or pouring, then you definitely want the glaze to be in a flocculated state. On the other hand, if you are applying glaze to a vitreous bisque that has been fired to high temperatures (commercial china is made this way), or if you plan on brushing the glaze, then you may want it to be deflocculated.
Besides being very tiny, clay particles in water act like little magnets. I won't bother explaining why (it would take too long for this column), but if you understand how to manipulate this then you can have a lot of control over how the wet glaze acts.
Just as magnets can either attract or repel, so can clay particles in water. How they act depends upon what ions are present. If the glaze contains a soluble alkali (such as sodium, potassium, or lithium), then the clay particles will repel each other and the glaze will be in a deflocculated state. What this means is the tiny particles will not floc together to hold up the large ones. Since none of the tiny particles can individually hold up the large particles, the glaze acts as if there were no tiny particles present and the larger particles quickly settle.
In addition, when clay is in a deflocculated state, it settles into a structure not unlike a deck of cards: parallel to each other and tightly packed together.
In addition to how it affects settling, a deflocculated glaze also acts differently when you apply it. When a pot is dipped into the glaze, the clay particles in the glaze will align into parallel layers on the surface, sealing off the pores of the bisque ware. Ideally, a glaze should seem to dry almost instantly after withdrawing it from the bucket. A deflocculated glaze will stay wet on the surface, allowing the glaze to form a maddening number of drips and forcing the potter to stand forever waiting to put the pot down. At its worst, a deflocculated glaze will only go on in a thin layer and, when dry, will have a crystalline, drippy look. In some cases, entire sections of glaze layer will slip downward on the piece, an effect called curtaining.
So why would anyone deflocculate a glaze? Well, most of the time it's not done on purpose. The alkali can dissolve out into the water from some of our materials. Yes, we try to use only insoluble materials in our glazes (i.e. materials that will not dissolve in water), but in practice some of our materials do exhibit some solubility. The amount of solubility is very small, but since the clay content in most glazes is also small (often less than 10%), even a little solubility is enough. Some of the common materials that can cause this are wood ash, soda feldspar, nepheline syenite, many common frits, and lithium carbonate. I like to think of these as our problem children: they have qualities that make us want to keep them, but they can still make our lives miserable!
The answer to the problem is to add something to the glaze that will counteract the deflocculant and bring the clay back into a flocculated state. That something is called a flocculant. While the soluble alkalis cause a glaze to deflocculate, soluble alkaline earth (such as calcium and magnesium) cause the opposite reaction. [Note to all you chemists: yes, I know that they're in the form of salts, but I'm trying to keep this simple.]
The most common flocculants potters use are epsom salts (magnesium sulfate) and calcium chloride, though acids (such as muriatic acid, available from paint stores and pool supply houses) can also be used. Epsom salts work in most instances and can be found at just about any drug store or grocery store. Calcium chloride is a bit stronger (I prefer it), but a little harder to come by. In cold parts of the country you can get it as a de-icer for sidewalks (it will say "calcium chloride" on the label). Be careful you don't buy sodium or potassium chloride, which are also used as de-icers. If you are in a part of the country with high clay soils, you can get it at gardening stores. In just about any part of the country it is used as an additive to concrete, so you can always get some from a ready-mix concrete plant. (As an aside, the first time I got some from a concrete plant, the old guy behind the desk asked me how I was planning to use it. Being a bit of a wise guy, I simply answered "For flocculating," to which he dryly asked, "Is that legal in Iowa?")
Besides settling, flocculation also affects the application of the glaze. When clay and water are in a flocculated state the clay has a structure like a house of cards: very open and porous. The result is that the glaze can build up quickly on the surface of the pot because there is nothing to impede the flow of water into the pores of the bisque. This becomes obvious when the pot is withdrawn from the bucket of glaze: the glaze coat will seem to dry almost instantly, with little or no dripping.
On a practical level, how (and when) does one go about flocculating a glaze? In practice, I find I need to flocculate almost all of my glazes, if only just a little. If the glaze tends to jell, never settles out, or has a very high clay content (e.g., Albany Slip glazes) then flocculation is not needed. For any glaze that contains one of the problem children and/or just settles out quickly, it may be in order.
As a first step, take a look at the recipe and see how much clay (including kaolin, ball clay, or red clay) it contains. If there is less than 10% clay then you will probably need to add some bentonite so that you will have enough fine particles. If you don't have any clay for it to act upon, all the flocculant in the world won't have any effect. I usually recommend that you only add one, or at the most two, percent bentonite if you are going to flocculate. Too much clay and flocculant together will result in what looks like a bucket of wax.
Gerstley borate acts much like clay (in the bucket!), and can also be flocculated. If there is at least 10% gerstley present, you may not need any bentonite. A good example of this are copper red glazes which often have as much as 15% gerstley borate but have little or no clay in order to keep the alumina content low. I often find no need for bentonite and merely flocculate the gerstley borate.
How do you do it? You can just add the dry powder to the bucket of glaze and mix very well, but that is not the best way because you will tend to use too much. Instead, first make a concentrated solution of the Epsom salts (or calcium chloride) with hot water in a blender reserved for studio use. Add more and more of the salts to the water until no more will dissolve. Pour off the solution, leaving any particles behind.
You can add this solution, a little at a time, while you stir the glaze. As it flocculates you will notice a thickening of the glaze, sometimes enough that you will have to add more water to the mixture. It can take several tablespoons of this solution to flocculate a 5-gallon bucket of glaze (10,000 grams).
If you want to use muriatic acid (which is a dilute solution of hydro-chloric acid) you should exercise caution and first take the bucket outside. When the acid is added to the glaze it will release a lot of fumes that smell like sulfur and aren't particularly good for you (this is a momentary reaction). It usually takes a couple of tablespoons of acid to flocculate a 5-gallon batch of glaze. Please take care in how you use and store the acid. It's not as bad as the stuff you see in the old cartoons where the guy sticks his hand in and comes out with only bones, but it will irritate your skin and could blind you if splashed into your eyes. Once it's in the glaze, it's perfectly safe: it just neutralizes the pH. This is the same reason it's used in swimming pools.
In a pinch you can even use plaster, which is calcium sulfate (another alkaline earth salt). The only reason I don't use it all the time is that it is too coarse to completely dissolve. Otherwise, it works well with no adverse side effects.
You may need to re-flocculate a glaze after it has been mixed up for a long time. This is because more of the alkali can be leached out with time. Simply flocculate as before. It will usually require less solution each time.
In the first column of this series, I discussed what it means when we talk about flocculation/deflocculation and how to flocculate a glaze for dipping or pouring onto porous bisque. In this column I'm going to discuss how you might make and use a deflocculated glaze.
If a flocculated glaze applies so easily, why would anyone want to use a deflocculated one? The answer is that a flocculated glaze applies well, but only in certain circumstances, and only in certain ways. Have you ever tried to brush on a flocculated glaze, either as the main glaze coat or as an overglaze? The moment the brush touches the pot it sticks, snagging on the surface. Any attempt at brushing leaves behind a rough, bumpy, uneven coat of glaze.
Have you ever tried to glaze a load of pots that were inadvertently over-bisqued and had little or no porosity? The glaze refuses to stick to them and simply runs off like water off a sheet of glass. So what qualities would the glaze need in order to work well in these situations?
The obvious answer is that we would like the glaze to act just like paint, flowing easily off the brush and coating even the glassiest surface. Well, when we adjust a glaze for those kinds of situations that's exactly what we are making: paint.
Flocculating a glaze for application is a pretty easy process: just see if you have any clay in it, add flocculant, and "presto," you're ready to dip. It's not quite so easy to prepare a deflocculated glaze: it takes more than just adding a deflocculant. Many of us have experienced using a glaze that became inadvertently deflocculated, usually because it contained a material such as nepheline syenite. After a while the glaze consists of a bucket of clear water with what seems to be a layer of concrete in the bottom.
Obviously, we need to add more than just a deflocculant. In order to get a clue as to how to treat the glaze, we need only look at a recipe for paint. A recipe for acrylic-latex paint looks very much like a recipe for glaze. In fact, the average university glaze room would have almost everything necessary to make paint. The recipe would start with "acrylic latex polymer" (which you probably don't have, but could get at an art store-it holds the paint together after it dries) but after that things are more familiar: "fillers and opacifiers ," (talc, wollastonite, quartz, kaolin, titanium dioxide), "thixotropes" (fine particle clays like veegum and bentonite), "dispersing agents" (which we would refer to as deflocculants), "gums and other thickeners" (materials such as CMC and Glycerin), preservatives (chemicals that prevent the gums from spoiling), and conditioners (things that prevent foaming, help the paint lay down flat, etc.).
Looking at a typical glaze recipe you'd see that we already have the "fillers," that are the basic recipe of ground minerals and clays that make up glazes. It's the other things, the "thixotropes, dispersing agents, and thickeners" that we need to add.
It might be in order to review what some of these materials are in order to help us choose which ones we may want to use.
Thixotropes are very fine particle volcanic clays such as Veegum, Macaloid, and bentonite. Bentonite is the least expensive one of these materials, but it sometimes is difficult to deflocculate. It is available in several different grades, with white bentonite being the best. Veegum is a mineral based material, not an organic gum. It comes in several different variations: Veegum T, which is for all purposes, whether it is plasticising a clay body or suspending a glaze; Veegum Pro, which is Veegum that has been treated to make it disperse more readily, a quality that is not needed in clay bodies, but might be helpful in a glaze; and Veegum Cer, which is a mixture of Veegum and an organic gum, CMC. I'll talk more about CMC later on. Macaloid is a different brand of what is essentially the same thing as Veegum.
Thickeners are soluble materials, usually organic, that make the water (our vehicle) thicker and slicker. Most of them also have a slight tendency to deflocculate. These include gums like CMC (sodium carboxymythelcellulose), gum arabic, and gum tragacanth. Sometimes people use other liquids that can be mixed with the water and have the same effect as the gums. These include glycerin (which you can buy at any drug store) and propylene glycol. You can sometimes find propylene glycol at drug stores (it's what the druggist mixes with a child's medicine to make it drinkable), or it can be found in an impure form as automobile antifreeze. Antifreeze is made of either ethylene glycol or propylene glycol. Either works as a glaze additive, but ethylene glycol antifreeze shouldn't be used if either children or pets have access to your studio: it is sweet tasting and poisonous, a bad combination. Propylene glycol antifreeze is considered safer. Read the label before you buy it for this purpose.
The third thing that can be added is the deflocculant. Sometimes this isn't even needed, especially if you are using enough gum or glycerin. Good deflocculants for this purpose include Darvan 7, sodium silicate, and soda ash.
I know this sounds very complicated, but it needn't be. One of my students was recently painting some majolica and complaining about the brushing quality (or lack thereof) of the typical gerstley borate and stain overglaze. At my suggestion she added a bit of glycerin that she bought at the drug store and raved about the improvement.
It's difficult to give an exact recipe for what to add: it would vary according to what was in the glaze. If it has a lot of clay in it already, you may not need to add any Veegum. If it has a minimal amount, such as 10% kaolin, then you would probably want to add 1 or 2% Veegum.
Next you will need to add some gum or glycerin. If you're going to use gum, then first you will have to mix it into a syrup. Add 50 grams of the dry gum to one liter (a bit less than a quart) of very hot water in a blender reserved for studio use. Blend this mixture very well, allow it to stand for a day, then blend again. Each gum gives a different feel to a glaze. Some potters prefer to use CMC, while glass painters seem to prefer gum arabic. It seems to be a matter of taste.
Weigh out 500 grams of the dry glaze and add it to water in the blender. It is difficult to tell you how much water as it varies according to the glaze. It is usually in the range of 350 to 500 ml (a bit less than 2 cups). Use just enough water to make a thick mixture. Try adding the gum or glycerin a bit at a time, checking occasionally to see how the glaze brushes on a piece of bisque ware. The glaze should thin down as you add the syrup. One-half to 1% gum (figured as dry gum as a percentage of the dry weight of the glaze) is usually considered the upper limits for gums. Too much gum can cause a glaze to crawl. Glycerin doesn't seem to have this problem, and you could (in theory) use just glycerin as the vehicle.
In addition to the things I have mentioned, there are other additives that are added to paint to lower the surface tension and improve brushing qualities. One that is commonly found at paint stores is Floetrol, made by the Flood Company. Some of my students have added this to commercial glazes to improve brushing.
I can think of few situations in which brushing is the preferable way to apply a glaze to an entire piece. Non-lead glazes almost always exhibit a high surface tension when melted, and any variation in application is magnified by the firing. There are many situations, such as overglaze decoration, in which improving a glaze's application qualities can really help an artist realize his/her vision.
The best way to approach this is to gather all the ingredients, and, in the words of my 5-year-old, "play chemistry set." Just make sure you keep good records so you can repeat any positive results.
Pete Pinnell may be reached via e-mail at: email@example.com
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