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Knowing about thixotropy will enable you to mix a glaze that stays in suspension much better. It does not drip alot when a piece is dipped into it. It goes on evenly and does not run. It dries quickly (on porous bisque) and is just much nicer to use. The secret to all of this is not intuitive. It involves adding more water and then gelling the slurry (by vinegar, epsom salts or calcium chloride additions) to reach a point at which the slurry is both creamy but also thixotropic. It also involves being about to measure the specific gravity of your glaze slurry accurately.
Thixotropy refers to a decrease in the viscosity of a slurry when it is subjected to shearing (movement) following by a gradual recovery to the viscous state when the movement stops. A thixotropic slurry is more fluid when in motion and gelled when not. Products we use every day like mayonnaise, toothpaste, paints, ketchup, hair gel, etc., are thixotropic non-Newtonian fluids.
As noted, each glaze has a rheological "sweet spot", a slurry condition at which it is at its best. What is that condition? When motionless it gels quickly and does not settle. During use it is fluid and goes on evenly and does not drip. It does not dry too quickly on bisque. This sweet spot has a certain specific gravity coupled with a "two-second viscosity". And what is that? It means if I stir the glaze vigorously and pull out the stirring stick it will stop motion in two seconds or sooner. And bounce back slightly. That bounce-back is the thixotropy, the two seconds is the viscosity. You cannot just mix water with a powder and get this behavior. How do we get the slurry to gel? By adding vinegar or epsom salts. How?
First, you need to discover the "specific gravity starting point": That is the specific gravity the slurry needs to have before you start adding vinegar. You may have seen many people advise adding vinegar or epsom salts, but they overlook this detail (having the glaze at the right specific gravity). You may not be able to discover the ideal value right away, it might take time. I will tell you how I do it. First I mix the powder into about 75% its weight in water (which produces a slurry that is too thick). I begin adding water (while propeller mixing) until it reaches the consistency of a thick cream, about five-second viscosity. I check with a bisque tile to see how thick it goes on after a 2-3 second dip. Often too thick, so it needs a little more water. OK, NOW I HAVE THE CONSISTENCY AT WHICH I USED TO USE GLAZES. Some work OK like this (those with about 15-20% clay), but many do not. Some settle, some do not.
Now we need to gel this slurry. Again, what do I mean by "gel"? I mean that it will stop in two seconds and bounce back slightly after I stop stirring vigorously. However at the viscosity of the last paragraph even a small amount of vinegar will turn the whole bucket into useless jelly. So I need to add water. How much? Enough that about 6 capfuls (from the vinegar bottle) are required to gel a couple gallons of the glaze. When this much vinegar is required I am not dealing with too much sensitivity (it is not so easy to over gel the slurry). What if I add two capfuls and it reaches the two-second gel? Then I need to add more water. You might be surprised at now much extra water it will need.
Now, I have my two-second slurry. What next? Using dipping tongs I glaze a mug, holding it under for 2-3 seconds, pulling it out and turning it over at 45 degrees to drain. If it does not go on evenly or dries too fast in some areas I need more water. Then more vinegar. Repeat. OK, now I have it going on perfectly, it applies in an even layer of gelled glaze cover that dries in a minute or two. This next thing I say is key to this whole page: I MEASURE THE SPECIFIC GRAVITY. And record it. That is what I mix it to next time before I start adding the vinegar.
The glaze is not working right, what do I do?
Is the specific gravity correct? I cannot do anything else until that is right!
What if I have too much water in the glaze, will the vinegar still gel it?
No. But epsom salts might. Or calcium chloride.
My glaze contains carbonates and they are reacting with the vinegar. What can I do?
Use epsom salts or calcium chloride.
Can you give me a ball-park specific gravity?
For many raw or partially fritted glazes I target 1.43-1.45 specific gravity (these take about two or three cap fills of vinegar per gallon).
What about bisque temperature?
It does not matter! With my glazes dipping time is always the same (assuming ware is dry); immerse it and remove it right away. On any clay, porous or dense, bisqued or green the thickness will be right! Getting control of this concept after struggling for years has revolutionized my ability to apply glazes.
Do I still need to add bentonite to my glazes?
Any glaze that has at 15% kaolin or ball clay does not need bentonite when gelled like this (provided the clay is a good glaze suspender). Actually that is wrong, even non-clay materials will gel and suspend (e.g. feldspar).
I mixed the glaze again, got it to the right specific gravity and it is thin as water and settling in seconds after I stop stirring. Why?
You have not added the vinegar. When you do it will turn back into the beautiful gel you had last time.
The specific gravity I have determined is different than yours. Why?
The information above is my water (with its electrolytes), my materials, my studio, my recipes. Yours will be different.
It is weeks later and my glaze has gone thin. What can I do?
Put in more vinegar. It will stabilize more each you do this.
Vinegar does not seem to work well for me, what can I do?
Use epsom salts instead. Or, better yet, calcium chloride. See link below.
I put too much flocculant in, now my glaze is jelly. What can I do?
Add a little Darvan to thin it. If the specific gravity is a little high, surprisingly small amounts of water can rethin it. Or, you may need to add more powdered glaze mix (and water).
I used to have a lot higher specific gravity in my glazes than this, are you sure this is right?
I am not making a rule for all types of ceramics. For sanitary ware, for example, it is common to deflocculate glazes, the opposite of what I am doing here (well, not quite, as you will see in a moment). Via tight controls on production parameters they are able to maintain the rheology of the slurry at a state of controlled flocculation at 1.75 specific gravity (basically there is not quite enough deflocculant in the mix). In this state the slurry also gels when not in motion. With the right equipment they can spray heavy green ware achieving a dense thick lay down that dries hard and even. You would not be able to use that glaze with any of the application techniques common in a studio environment.
Thixotropy is more of an anti-settling strategy, ideally the slip remains fluid when in use but if allowed to stand it turns to a gel in half an hour, for example. And with casting slip we obtain that thixotropy, not by adding an acid, but by deliberately adding a smaller amount of deflocculant than what would take it to the minimum viscosity possible (again, this state is called controlled flocculation).
Thixotropy is vitally important when using slips (engobes), especially when applied to vertical surfaces or leather hard clay (which is normal). Being able to gel an engobe is the foundation of being able to effectively apply it. Flocculated slips stay put. But slips must be gelled more than glazes. When you get the consistency right you will be amazed at how even and silky the application will be. These more gelled slips (and glazes) also work better for spraying because you can apply a thicker wetter layer. Be aware also that a fresh application of slip re-wets the ware and will take hours to be able to handle (it ads a day to the drying cycle).
These are sometimes called thixotropic. This usually refers to material that is very elastic, can be pulled and twisted like taffy, and does not set until left still for a time.
Adding water actually made this white engobe run less? How?
The white slip (applied to a leather hard cup) on the left is dripping downward from the rim (even though it was held upside down for a couple of minutes!). Yet that slurry was viscous with a 1.48 specific gravity, on mixer-off the motion stopped immediately. Why? Because it was not thixotropic (it did not gel). The fix? I watered it down to 1.46 (making it very thin and runny) and did a cycle of adding a pinch of epsom salts (about 0.5 gm) and mixing vigorously watching for it to thicken enough to stop motion in about 1 second on mixer shut-off (bounce backward!). It is extremely difficult not to overdo the epsom salts (gelling it too much) so I keep ungelled slurry aside and pour some back in to dilute to overgelled batch. That works perfect to fine-tune the degree of thixotropy so it gels after about 10-15 seconds of sitting. So to apply it I stir it, wait a couple of seconds and dip the mug. By the time I pull it out it is ready to gel and hold in place.
Can you bisque fire at cone 02? Yes. But why? How?
The buff stoneware mug on the right was bisque fired at cone 02, the one on the left at cone 06. The cone 02 mug was immersed in the clear glaze for 1 second and allowed to dry. The other was glazed on the inside first, allowed to dry, then glazed on the outside with a 1 second dip. Of course, the cone 02 one took longer to dry. In spite of this, the glaze is thicker and more even on the one bisque fired to cone 02. How is the possible? The secret is the thixotropy of the glaze. When that is right, a one second dip will give the same thickness and evenness whether dry or bisque, 06 or 02. Why bisque fire to cone 02? To get a glazed surface free of pinholes on some stoneware clays.
When to use vinegar and when to use epsom salts to flocculate a slurry
Slurries with more clay (like engobes, slips) generally respond better to epsom salts. However the extra clay also makes them more likely to go moldy, so you may need to add a few drops of Dettol to kill the bacteria (if they are stored for any length of time). Vinegar works better for glaze surries, but only if they have sufficient specific gravity. Many people like to make an epsom salts solution and add that, but if you have a good mixer you may find it more intuitive to add the crystals and wait 30 seconds for the viscosity to respond.
I want this engobe to gel in ten seconds. Why?
It is going to be applied to leather hard earthenware and it needs to be thixotropic. Why? I do not want it to run down from the rims of the mugs after dipping. The process: Stir the engobe, pour-fill the mug, pour it out and push it upside down into the engobe. If I can pull it back out before the 10 seconds is up I get a perfectly even gelled layer that does not move. A good test is to stir it then pull out the spatula slowly. If it hangs on in a even layer with only a few drips it is perfect. I also I tip the measuring cup to check it is gelling. Achieving this behaviour requires very careful additions of epsom salts (and thorough mixing between). As the slip approaches this 10 second threshold even a slight bit more salts will turn it into a bucket of jelly! I almost always overdo it! So I keep some thinner slip aside to dilute it down to perfect, not much is usually needed.
White spots and blisters in a high zircon glaze at cone 6
This is also a common problem at low fire on earthenware clay (but can also appear on a buff stonewares). Those white spots you see on the beetle also cover the entire glaze surface (although not visible). They are sites of gas escaping (from particles decomposing in the body). The spots likely percolate during soaking at top temperate. Some of them, notably on the almost vertical inner walls of this bowl, having not smoothed over during cool down.
What can you do? Use the highest possible bisque temperature, even cone 02 (make the glaze thixotropic so it will hang on to the denser body, see the link below about this). Adjust the glaze chemistry to melt later after gassing has finished (more zinc, less boron). Apply a thinner glaze layer (more thixotropy and lower specific gravity will enable a more even coverage with less thickness). Instead of soaking at temperature, drop 100 degrees and soak there instead (gassing is much less and the increasing viscosity of the melt overcomes the surface tension). Use a body not having any large particles that decompose (and gas) on firing. Use cones to verify the temperature your electronic controller reports.
The same engobe. Same water content. What is the difference?
The engobe on the left, even though it has a fairly low water content, is running off the leather hard clay, dripping and drying slowly. The one on the right has been flocculated with epsom salts, giving it thixotropy (ability to gel when not in motion but flow when in motion). Now there are no drips, there are no thin or thick sections. It gels after a few seconds and can be uprighted and set on the shelf for drying.
Measuring glaze slurry specific gravity
This is the easiest way to measure the specific gravity of a glaze if it is not in a container deep enough to float a hydrometer (or if it is too thick to float it properly). Just fill to the 100cc mark and the scale reads the specific gravity. Be careful on cheap plastic graduated cylinders like this, check them with water and correct the true 100cc mark if needed (using a felt pen). You could actually use any tall narrow container you have (if you mark the 100cc level).
Specific gravity of a glaze using a scale and measuring cup
The specific gravity of a glaze slurry is simply its weight compared to water. Different glazes optimize to different specific gravities, but 1.4 to 1.5 is typical (highly fritted glaze are higher). To measure, counter-weigh a plastic measuring cup on your scale and fill it with 500 grams of water and note how high the water fills it (hopefully to the 500cc mark!). Fill the container with your glaze to the same place. Divide its weight by the number of ccs (in this case, 500) and you have the specific gravity. The more you weigh the more accurate is the test.
Fundamentals of Fluid Mechanics - book
Many aspects of ceramic production relate to the control of fluids (mostly suspensions). This is also true of material production. If you want to solve problems and optimize your process this is invaluable knowledge. This book is available at amazon.com.
Feldspar applied as a glaze? Yes! The way I did it will change how you glaze.
Custer feldspar and Nepheline Syenite. The coverage is perfectly even on both. No drips. Yet no clay is present. The secret? Epsom salts. I slurried the two powders in water until the flow was like heavy cream. I added more water to thin and started adding the epsom salts. After only a pinch or two they both gelled. Then I added more water and more epsom salts until they thickened again and gelled even better. They both applied beautifully to these porcelains. The gelled consistency prevented them settling in seconds to a hard layer on the bucket bottom. Could you do this with pure silica? Yes! The lesson: If these will suspend by gelling with epsom salts then any glaze will. You never need to tolerate settling or uneven coverage again! Read the page "Thixotropy", it will change your life as a potter.
What to do when glazes dry-drip like this on the rims of ware
These are the same glazes. The one on the left had a specific gravity of 1.45 and the slurry was creamy and appeared to be good. However when this bisque porcelain mug was pulled out of the slurry (after the dip) the glaze dried so fast that it would not even out around the lip (even though I rolled it). To fix this I added water to take it to 1.43 specific gravity, they I added epsom salts to gel it back to the same creamy consistency it was. This time it went on evenly, dried more slowly and stayed even. Notice the darker color, is it still damp. Although the piece dries enough to handle in less than 30 seconds, it does take longer to dry completely.
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