|Monthly Tech-Tip |
Thixotropy is a property of ceramic slurries. Thixotropic suspensions flow when you want them to and then gel after sitting for a few moments. This phenomenon is helpful in getting even, drip free coverage.
Knowing about thixotropy enables you to mix non-gummed glazes that stay in suspension much better. But it is not only about staying suspended. While some glazes do not settle out they that have a slurry that behaves like a bucket of motor oil, silky smooth but they just drip and drip and drip. Thixotropic glazes (or engobes) DO NOT do that. They go on evenly, of adequate thickness and do not run and drip (so they dry quickly, even on porous bisque, and are just much nicer to use). The secret to all of this is not intuitive. It involves adding more water and then gelling the slurry using a flocculant (vinegar, epsom salts, calcium chloride) to reach a point at which the slurry is both creamy but also thixotropic. Being able to measure the specific gravity of the glaze slurry accurately is very important in accomplishing this.
Thixotropy refers to a decrease in the viscosity when a slurry is subjected to shearing (movement) followed by a gradual recovery to the viscous state when the movement stops. Or, in simpler terms, it is the tendency of undisturbed slip to thicken with time. A highly thixotropic slurry gels as soon as motion stops (fluid when in motion, gelled when not). Products we use every day are like this (e.g. mayonnaise, toothpaste, paints, ketchup, hair gel, these are thixotropic non-Newtonian fluids). Thixotropy measuring devices record the difference between two readings: In motion and after sitting for a fixed period.
Thixotropy is a luxury we can afford with cover glazes, ones that you want to be able to dip bisque ware into and get even coverage, quick drying with ONE LAYER (no other glaze layers will be dipped or brushed on without bisque firing in between. However we cannot afford the luxury of thixotropy where additional layers must be dipped or brushed on. In these cases the cover dipping glaze needs a binder, typically CMC gum. Gummed dipping glazes have different rheology and priorities and are dealt with on other pages of this site. There is also a kind of thixotropy where slurries of dramatically higher water content than we discuss here (e.g. 1.2 specific gravity) can be gelled by adding a high-surface-area bentonite or smectite (like Veegum or Bentone), this is done with commercial brushing glazes or slips. However, on this page I am talking mostly about creating thixotropy without additives (other than a tiny amount of flocculant) in raw or lightly fritted slurries, these most often require near equal parts of water and powder. That being said, if your recipe contains any bentonite it will be easier to gel.
Thixotropy does come automatically with high-clay-content glazes, these typically produce a creamy slightly-gelling slurry without needing additives. Ravenscrag Slip is an example, at about 1.45 specific gravity (in a 50:50 roast:raw mix) it’s properties are good enough that it can be used as is. While there is some dripping it gels enough to go on evenly. A more extreme example is Alberta Slip, it is very plastic and will certainly cause cracking during drying in high percentages. It is necessary to calcine or roast part it. The proportion of calcine:raw is adjusted to produce a compromise between the creamy slurry with good application properties (that higher raw content imparts) with one that dries quickly without cracking (that higher calcine content produces). The proportions of raw and calcine give fine control over slurry properties simply by visual inspection of slurry behaviour, without the need to know specific gravity or for any additions to gel the slurry to make it thixotropic.
One final class of glazes deserves mention: Gerstley Borate glazes. This material is notorious for making slurries overly thixotropic (because it is somewhat soluble). It is both a clay and a flux. When proportions go beyond about 20% it will gel the slurry, forcing you to add more water. That does not just lower the specific gravity, but increases drying shrinkage, and thus drying cracks. This type of glaze often simply needs to be "endured". That being said, lower percentages of GB can impart nice thixotropic properties, although these can change with aging.
Now, let's get back to typical glaze recipes: Those made from mixtures of ceramic minerals (like kaolin, feldspar, silica, calcium carbonate, dolomite, talc, etc and frits). 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". If I stir the glaze vigorously and pull out the stirring stick it will stop motion in two seconds (in a small 1-2 gallon bucket). And, it bounces 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 behaviour. It is done by incorporating a flocculant.
First, one must discover the "specific gravity starting point": That is the specific gravity the slurry needs to have before you start adding the flocculant (you may have seen many people advise adding the flocculant, but they overlook the detail of having the right specific gravity first). First mix the powder into about 75% its weight in water (which produces a slurry that is too thick). Begin adding water (while propeller mixing) until it reaches the consistency of a thick cream, about five-second-stop viscosity. Check with a bisque tile to see how thick it goes on after a two-second dip. Likely too thick, it needs a little more water. This is the consistency at which I used to use most glazes (as noted above, some glazes, e.g. Ravenscrag recipes, do not need gelling agents).
Now, second, the slurry needs to be gelled (be thixotropic) so that it will stop motion in two seconds and bounce back slightly (after mixing ceases for 1-2 gallons). However at the viscosity of the last paragraph even a small amount of flocculant will turn the whole bucket into useless jelly. So water is needed (remember, I am still describing how to discover the right specific gravity that will tolerate the addition of the flocculant). How much water? It might be more than you think. Enough that about 5 capfuls (caps from my liter-sized vinegar bottle) are required to gel a gallon of the glaze (or about 5 pinches, 2-3 grams, of powderized epsom salts). When this much is required the system is not too sensitive (it is not so easy to over gel the slurry).
Now, we have a two-second slurry. What next? Using dipping tongs glaze a piece, holding it under for 2 seconds, pulling it out and turning it over at 45 degrees to drain. If it does not go on thick enough, or drips too much, more flocculant is needed. If it gels too fast (not going on evenly) it needs more water. Then more flocculant. Repeat. OK, now we have it going on perfectly, it applies in an even, thick enough layer of gelled glaze cover that dries quickly.
This next thing I say is key to this whole page: MEASURE THE SPECIFIC GRAVITY. And record it. That is what to mix it to next time before adding the flocculant.
Questions about making a 1.45SG slurry:
The glaze is not working right, what do I do?
Is the specific gravity correct? You 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?
Vinegar does not seem to work well for me, what can I do?
Use epsom salts or calcium chloride. See link below.
Can you give me a ball-park specific gravity?
For many raw or partially fritted glazes I target 1.43-1.45 specific gravity. However, some glazes want to be fluid at higher specific gravities, so this is not a hard-fast rule (see above on how to discover the right specific gravity).
Does the temperature of my slurry affect is thixotropy?
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 10% 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 by flocculating (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 flocculant. 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 based on my water (with its electrolytes), my materials, my studio, my recipes. Yours could well be different.
It is weeks later and my glaze has gone thin. What can I do?
Put in more flocculant. It will stabilize more each you do this.
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 (to densify laydown), the opposite of what I am doing here (well, not quite, as you will see in a moment). With 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.
We slip cast ware and vessel walls are thin. At 1.43 specific gravity the glaze waterlogs the ware, it dries slow, cracks and then crawls in the firing. What can we do?
Find a way to enable quicker drying. Heat ware before immersing, bisque lower or glaze the insides first and let them dry and then the outsides. As noted in the previous point you may need to deflocculate the glaze to lower water content (perhaps to 1.5 specific gravity). Using Darvan it is still easy to make it quite watery, then you can gel it.
Couldn’t I deflocculate the glaze to enable lower water content and yet still have it runny and then add the flocculant to gel it?
Yes. Once you get better at this. But it produces a more fragile slurry theology, you will be adjusting it more often. And it is heavier so will form downward runs more easily on verticals.
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. Engobes must be gelled more than glazes. When the consistency is right you will be amazed at how even and smooth 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.
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.
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 (powdered), 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.
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.
This is a white engobe (L3954B) drying on two dark burning cone 6 stoneware leather-hard mugs (Plainsman M390). Those lumps are on the left cannot be screened out, they are agglomerates. That slip has excessive flocculant (powdered Epsom salts are added to gel it so that it stays put on the piece after dipping). About 4 drops of Darvan were added to one gallon of the slurry, this immediately made it smooth and a perfect consistency for application. It remains stable on ware (without runs). Engobes require tight control to have the right viscosity and thixotropy (which can be achieved over a range of specific gravities (about 1.45-1.6). When they are right they are a joy to use, when they are not ware is ruined.
First, the hydrometer is long, the only container I have is this graduated cylinder. I had to fill it just the right level so it reads near the top. OK, fine. But the hydrometer needs to bob up and down to find home. However this glaze has a creamy consistency, that prevents free movement. OK, I will carefully help it find home by pushing it down a little. But then it doesn’t want to bob back up! Ok, I’ll pull it up and push it down and put it where I think it should float. Not great. Next problem: The glaze is opaque, I can’t see the reading. Yikes! A better way would be to throw out the hydrometer and just tare the empty cylinder on a scale, fill it to 100 and read the SG as the weight/100. If this glaze was free-flowing and watery it would be a different story, the hydrometer would be useable.
This is the L3954B engobe. But it has 15% Mason 6600 black body stain (instead of the normal 10% Zircopax for white). There is no cover glaze, yet it is durable and absolutely coal black (so a lesser stain % is possible). Lots of information is available for L3954B (including mixing instructions showing exact amounts for water, powder, Darvan). Engobes are tricky to use, follow the link above or below to learn more. L3954B is designed to work on regular Plainsman M340 (this piece), M390 and Coffee Clay, these bodies dry better than porcelains and are much less expensive, coating them with an engobe to get a surface like this makes a lot of sense. This engobe is actually a highly plastic body, but it does not contain enough feldspar to be a porcelain (this is on purpose to match the firing shrinkage of the stonewares).
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.
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 viscous to enable free movement). Just counterbalance the empty graduated cylinder to zero (you can buy these at amazon.com), fill it to the 100cc mark and the scale reading divided by 100 is the specific gravity. Be careful on cheap plastic graduated cylinders like this, check them with water and mark the true 100cc mark if needed. You could actually use any container, just fill it with water and mark the level, then fill to the same level with slurry and divide the slurry weight by water weight.
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 very viscous with a 1.48 specific gravity. Why? Because it was not thixotropic. The fix? I watered it down to 1.46 (making it runny) and added pinches of powdered epsom salts (while mixing vigorously) until it thickened enough to stop motion in about 1-2 seconds on mixer shut-off. But that stop-motion is followed by a bounce-back. That is the thixotropy. It is easy overdo the epsom salts (gelling it too much), I add a drop or two of Darvan to rethin it if needed. When the engobe is right it gels after about 10 seconds of sitting, so I can stir it, dip and extract the mug, shake to drain it and then it gels and holds in place. Keep in mind, this is a pottery project. In industry they deflocculate engobes to reduce water content. But a deflocculated slurry can still be gelled (if it is runny).
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.
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.
It is going to be applied to leather hard earthenware and it needs to be thixotropic (gelled when not in motion, liquid when in motion). 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 second gel-time is up I get a perfectly even 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. Achieving this behaviour requires very careful additions of powdered 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 (if that happens I add a drop or two of Darvan). This process works across a range of specific gravities (about 1.45-1.6), the higher the SG the trickier it is (but the faster it dries).
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 (which you should crush to a powder) and wait 30 seconds for the viscosity to respond.
Base-Coat Dipping Glaze
These are ceramic glazes intended for dipping but which contain a gum to enable them to adhere to the body better and tolerate over-layers without danger of flaking or cracking.
In ceramics, glazes are suspensions. 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.
Engobes are high-clay slurries that are applied to leather hard or dry ceramics and fire opaque. They are used for functional or decorative purposes.
In traditional ceramics and pottery dipping glazes can be of two main types: For single layer and for application of other layers overtop. Understanding the difference is important.
Gas fired rustic ceramic ware is cooled from red-hot in a closed container with organic material. The zero-oxygen atmosphere produced reduces carbonate metal decoration to its metallic form.
In ceramic slurries (especially casting slips, but also glazes) the degree of fluidity of the suspension is important to its performance.
The flocculation process enables technicians in ceramics to create an engobe or glaze slurry that gels and goes on to ware in a thick yet even, non-dripping layer.
The deflocculation process is the magic behind the ceramic casting process. It enables you to make a slurry of far lower water content and thus lower shrinkage.
In ceramics, the specific gravity of casting slurries and glazes tells us their water-to-solids. Body slurries especially require tight control of this property for performance reasons.
In ceramics some clays of are of such exceedingly small particle sizes that they can stay in suspension in water indefinitely. But unlike common colloids, clays have a secret weapon.
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, this term refers to the flow and gel properties of a glaze or body suspension (made from water and mineral powders, with possible additives, deflocculants, modifiers).
Powdering, Cracking and Settling Glazes
Powdering and dusting glazes are difficult and a dust hazard. Shrinking and cracking glazes fall off and crawl. The cause is the wrong amount or type of clay.
Uneven Glaze Coverage
The secret to getting event glaze coverage lies in understanding how to make thixotropy, specific gravity and viscosity work for you
G1916Q - Low Fire Highly-Expansion-Adjustable Transparent
An expansion-adjustable cone 04-02 transparent glaze made using three common Ferro frits (low and high expansion), it produces an easy-to-use slurry.
G2926B - Cone 6 Whiteware/Porcelain Transparent Base Glaze
A base transparent glaze recipe created by Tony Hansen for Plainsman Clays, it fires high gloss and ultra clear with low melt mobility.
GR6-M - Ravenscrag Cone 6 Floating Blue
Plainsman Cone 6 Ravenscrag Slip based version of the popular floating blue recipe. It can be found among others at http://ravenscrag.com.
G2934 - Matte Glaze Base for Cone 6
A base MgO matte glaze recipe fires to a hard utilitarian surface and has very good working properties. Blend in the glossy if it is too matte.
G3806C - Cone 6 Clear Fluid-Melt Clear Base Glaze
A base fluid-melt glaze recipe developed by Tony Hansen. With colorant additions it forms reactive melts that variegate and run. It is more resistant to crazing than others.
How to Liner-Glaze a Mug
A step-by-step process to put a liner glaze in a mug that meets in a perfect line with the outside glaze at the rim.
Where do I start in understanding glazes?
Break your addiction to online recipes that don't work. Get control. Learn why glazes fire as they do. Why each material is used. Some chemistry. How to create perfect dipping and drying properties. Be empowered. Adjust recipes with issues rather than sta
Adjusting the Thixotropy of an Engobe for Pottery
How to fine-tune the thixotropy of a ceramic engobe for pottery
How to Apply a White Slip to Terra Cotta Ware
I will show you some secrets of making a base engobe (or slip) apply to leather hard terracotta ware in a thick, perfectly even layer.
Thixotropy and How to Gel a Ceramic Glaze
I will show you why thixotropy is so important. Glazes that you have never been able to suspend or apply evenly will work beautifully.
|Tests||Rheology of a Ceramic Slurry|