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Understanding the Deflocculation Process in Slip Casting
Section: Clay Bodies, Subsection: Testing
Understanding the magic of deflocculation and how to measure specific gravity and viscosity, and how to interpret the results of these tests to adjust the slip, these are the key to controlling a casting process.
The only exposure many have had to the casting process is the local art ware shops where fragile low-fire objects like ceramic frogs and nativity sets are made. These companies pour liquid clay slurry into plaster molds and let it sit. The plaster absorbs water from the slurry building up a layer against the mold surface. When this layer has reached the desired thickness the slurry is poured back out of the mold. The mold then sits for a few minutes (or hours if it is large) and the clay article slowly stiffens, shrinks and pulls away from the plaster. The mold is then taken apart and the leather hard clay item removed. A simple mix of clay and water will not work well at all for casting. Not only will it quickly waterlog the molds but it will shrink too much (and therefore crack in the molds) and it will take much to long to cast. It will also gel and settle out and generally make the process miserable. Optimal casting requires that the slurry have minimal water content (e.g. 30% instead of 60%), good flow and draining properties and that it not settle out or gel too quickly. When the slurry is right the casting process works amazingly well.
The artware operations buy molds, glazes, and clay slurries from a supply industry that gears itself to providing all the needed support so that almost anyone can manage the process with minimal technical knowledge about deflocculation. A big part of the success of the hobby casting industry is the standardization on a clay slurry recipe (called 'slip') of 50% talc and 50% ball clay (with smaller variations and additions). While this body falls far short of vitrification when fired, its working properties are very good, it dries very well and is about as 'idiot proof' as you can get in forming ceramics (although not idiot-proof enough as we shall see). Many people buy their clay slurry already mixed but most bigger operations get dry clay and use a mixer to blend it with the correct amount of water and deflocculant (sodium silicate, soda ash, Darvan, etc). The existence of this ‘standard’ clay:talc recipe enables manufacturers to provide an incredible array of glaze, stain, slip, and decorative products that everyone can use. This standard body also allows the manufacturers to take primary responsibility for formulations relieving the user to concentrate on the task of learning to use all those products. Thus most hobby ceramists are quite happy and few have the desire to go out ‘on their own’ to develop bodies and matching glazes for higher temperatures. They appear to live in the blissful world of a reliable ceramic process.
Actually, I have probably overstated the level of knowledge that most hobby casters have about slip rheology. In actual fact, most expect to be able to execute the process while being willfully unaware of the basic principles of mixing and maintaining slip clay. They want to rely completely on following printed and word-of-mouth procedures. Amazingly many have no idea what the term 'deflocculation' even means and are unaware of the magic that occurs in the bucket when they mix the slurry. They are unfamiliar with what ball clay is and wonder why on earth a clay body would have so much of a powder they put on baby's bumbs. They are also blissfully unaware that the body fires to much higher than 10% porosity and is thus extremely weak compared to any normal ceramic. Not surprisingly, they are unable to stray from the standard body (e.g. to make a terra cotta, porcelain, stoneware, etc).
Potters, on the other hand, are generally fascinated by the basic processes they use. Many potters have developed their own processes, built their own kilns, and even designed their own clay and glaze recipes. They often regard these developments with the same pride as the creativity they put into their ware. Many potters find the casting process distant and even beneath their dignity both because of the fact that molds and commercial glazes are used, but also because of the general ignorance in the hobby casting industry. By contrast, technicians in the industrial casting industry (e.g. Sanitary ware) are much more keenly aware of not only the things the hobby casters try to get away with ignoring but things potters don't know about materials and process either. If you listen to one of these technicians talk about their work you will hear terms like specific gravity, viscosity, thixotropy and rheology. On an even more technical level the cutting edge of the casting industry pressure-pumps slip into precisely fitting molds made from hi-tech non-plaster materials. There is an obvious need for a slip with optimal flow properties in such a process.
Sooner or later people in ceramic production realize that for many ceramic shapes there is no better fabrication method than casting. When people do adopt the process they usually encounter an unexpected situation: The complexity of making molds and tooling up the operation turns out to be easier to master than developing a slurry body and getting it to cast properly. Knowing the 'basic science' is by far the best way to attack this problem. A casting body needs to fire the way you want, fit your glazes and cast properly. Thus there is a real need to understand how these factors intertwine with each other.
First consider how casting body dry ingredient recipes differ from plastic bodies. Casting bodies do not need to be nearly as plastic as jiggering, throwing or pressing bodies since they do not require shaping. Plasticity is only needed to impart shrinkage so the piece pulls away from the mold and to give it adequate dry strength. Too much plasticity is actually bad because plastic materials are less permeable to the passage of water through them, the casting process is slower (casting speed is an important factor in the process) and ware can split in molds with undercuts. Since ball clays are plastic you might then be wondering: “why does the hobby casting standard body have 50% ball clay?”. The answer is two-fold. Large particle low plasticity white burning ball clays are used and they are much more permeable. In addition, the talc also greatly increases the ability of the body to channel water. Obviously it would cast even faster if some kaolin were used instead, but the tough nature of the greenware imparted by the ball clay is seen as more important. In industry, stoneware and porcelain casting bodies employ far less plastic clay because they know how to deal with lower dry strengths and they employ special hardeners in the clay if needed.
No matter what temperature or type of ware you make, the casting process has some compelling advantages over plastic or dust forming methods. Consider:
Understanding Slip Rheology
Most people take the properties of liquids like oil or hand creme, for example, for granted. They do not realize that an entire industry exists to produce instruments people use try to understand and control the way these liquids, solutions and suspensions behave. It is not an accident that each time you buy a specific brand of hand creme that it feels exactly the same. The term 'rheology' encompasses physical presence of things like in ceramic slurries. Describing a clay slip involves talking about the fluidity, the nature of the way it flows and resists changes in movement, its viscosity and its specific gravity. Once you understand the rheology of the slip you use you can take any sample, do some measurements and say exactly what needs to be done to bring its properties back into line. Ceramic slurries for use in casting are fragile entities, they flow and work the way they do as a result of a finely tuned recipe and mixing process and small changes can render them useless or difficult-to-use. There is thus compelling reason to study this more closely.
The Magic of Deflocculation
While a plastic pottery clay might have 21% water, a slurry made from the same material may need 60% water! As you can imagine, such a mixture would be unworkable in a mold. Not only would the mixture settle and the fluid level drop drastically during casting, but molds would become waterlogged quickly and excessive shrinkage would result in splitting ware before release from the mold. Obviously a way is needed to minimize the water content of the slurry. Certain electrolytes like sodium silicate (materials that can supply sodium ions or charged particles) exhibit the remarkable property of turning a thick, viscous clay-water mixture into one so thin that it runs like water. This has to be seen to be believed. Imagine putting a powerful propeller mixer into a bucket of water and adding dry clay until the slurry turns to mud so thick it will no longer agitate. The addition of a few drops of electrolyte will instantly transform it back to a water-like consistency! You can add clay till it is too thick and do it again, and again. The amount of dry material that can be mixed into a small amount of water is truly remarkable to anyone accustomed to using plastic clay. In fact, it is possible to produce a pourable slip with only a little more water than it takes to make a clay suitable for plastic modeling and shaping.
Unfortunately a different amount of deflocculant and water are needed to produce an optimal slurry for different recipes and even different water supplies. The question you are probably wondering is: How does one know how much electrolyte, water, and clay to use? As a general rule, most clay based slips can be deflocculated to have excellent fluidity yet have a specific gravity as high as 1.8. A specific gravity of 1.8 means that the slurry is almost twice as heavy as water. First time slip mixers are often surprised at the weight of the slip. Beginners often find that targeting 1.75 at the start is best. Then, as they become more familiar with the process, they can reduce water to take it higher. Whenever there is any kind of problem with a clay slurry an engineers first question is almost always “What is the specific gravity”? The specific gravity is thus the 'level playing field' of the ceramic slip casting process.
The manner in which the 1.8 target was determined helps explain the relationship between the viscosity of the slurry and its specific gravity. Imagine starting with a pail of water, inserting your variable speed propeller mixer and adding clay until the slurry becomes too thick. You would then add a few drops of deflocculant to thin it and add more clay. However on the third cycle you would notice that it did not take as much clay before becoming too thick mix again and that as the slip becomes heavier and heavier (I mean heavier by weight) the mixer has to work harder and harder to pull a whirlpool downward. Also bubbles within the mix rising and breaking at the surface do so less and less easily. After each add-clay:add-deflocculant cycle you would increase the mixer speed enough to pull a whirlpool but not so fast as to pull air bubbles into the mix. As it gets heavier you will note that as the entire slip mass is in motion during mixing it looks very smooth and can appear almost motionless. When you reach the stage at which a small addition of deflocculant does not cause the whirlpool to pull deeper the slurry has reached its density limit. Remarkably most ceramic slurries hit this limit right around 1.8 (other slurries like alumina or zircon can be taken to a much higher specific gravity).
An interesting point to remember here is that the amount of deflocculant is key to both the specific gravity and the viscosity. However it is not generally viewed this way. It is the proportions of water and clay that determine the specific gravity, the deflocculant is simply there to make it possible to mix them in the desired proportions. However the final fluidity of the slurry is 'fine tuned' by the deflocculant, thus there is a direct relationship between its amount and the viscosity. In other words, small changes in the amount of deflocculant cause changes in the viscosity. It is important to realize that the same is not true of specific gravity (changing the amount of deflocculant in a slurry has no measurable effect on its specific gravity).
Now I am going to tell you something that it sometimes takes many years to learn: The viscosity:deflocculant relationship comes clearly into focus when one realizes that it is not actually desirable to optimize a slip to the highest possible specific gravity or even to the lowest possible viscosity for a given specific gravity. Professionals learn that it is better to maintain the slurry in a state of 'controlled flocculation'', that is, stop short of full fluidity. Why? Because such a slurry is less likely to settle out. If you stop too far short of full deflocculation the slurry will gel after sitting for a few minutes (it is actually possible to make a slip that appears to pour well and then have it gel so severely in an open mold that you can turn it upside down and it does not run out). The real trick is to deflocculate slip such that it gels after an hour or so. The gelling prevents it from settling out no matter how long it sits.
An important and easy-to-overlook point is power mixing. Obviously, slip should be mixed for an adequate amount of time to stabilize it. If a slip's flow properties change over time, inadequate mixing is a possible reason. How much time should it be mixed? Hours. Hours using a good mixer that can put a lot of energy into the slurry. In industry it is not unusual to mix 12 hours or more, and the big powerful mixers they use heat the slip until it is very warm to the touch! But there is another important factor to consider here: Warm still flows better, alot better, than cold slip. So if you fine tune the viscosity using deflocculants based on the properties of warm slip, then when it cools it is not going to flow enough.
As we can see from this, the specific gravity and fluidity (or viscosity) of the slurry are the two principle properties to understand. Most casting problems (and successes) can be traced to confusion and misunderstanding these two properties. My experience is that even some clay slip suppliers incorrectly document them in their instruction booklets! By careful control of these properties you can maintain slip consistency despite variables like changing water supply, temperature, mixing time and integrity, water content of powders, evaporation from the tank, and additions of dry and leather hard scrap. Yes, you will have a good measure of control over your casting slip if you know how to measure the aforementioned properties. You should be able to say; "Show me the slip and I’ll measure its specific gravity and viscosity and tell you if it is right and if it is not I will tell you how to fix it". Let’s review what these are and how to measure each. Inexpensive devices for measuring specific gravity and viscosity are included in the Lehman Slip Testing Kit, it available from many suppliers (Lehman is a longtime manufacturer of slip mixing machines, tables, and pumps). However you can also make your own testing devices.
Specific gravity is defined as the comparison of a liquid’s weight with the weight of an equal volume of water. In other words it is the weight per unit of volume of the slip. In metric it is simple: water weighs one gram per cc (ccs and milliliters are the same). If a slurry weighs 1.8 grams per cc, then it has a specific gravity of 1.8; it is 1.8 times heavier than water. A slip with a specific gravity that is too high is said to be "heavy" (the more water in a slip, the lower its specific gravity will be, the more solids, the higher it will be). As mentioned, slip with too much water will soak the molds more quickly, give slow casts, and result in excessive shrinkage that contributes to splits in the ware.
Measuring Specific Gravity:
You must have a reliable way to measure specific gravity. In fact, it is good to have two ways to measure it so you can confirm measurements that appear unusual. Here are two ways to do it:
Some slip suppliers quote specific gravity in ounces per pint (i.e. 29 oz/pt), however, this measure is not intuitive in my opinion. The British and US pints are not the same size and this method does not relate well to the weight of water. Avoid it, do it right and use grams per cc. Also, if you are mixing a large tank of slip, use the most accurate method possible. If someone tells you that a problem slip has a specific gravity of x, ask them how they measured that.
Viscosity refers to the mobility of the slip; its "thickness" or "runniness". A slip that has high viscosity is thick like syrup and one that has low viscosity is fluid. A deflocculant is used to lower the viscosity of a slip (provided of course that the slip does not have so high a solids content that a deflocculant simply cannot thin it any more). A slip with a viscosity that is too high is said to be "thick".
The 60 cc veterinarian’s syringe is also an effective tool for measuring this property. To work properly it needs to have a large diameter needle insert (e.g. 3 mm). Pull out the needle and time how long it takes the slip to run out from the 60 cc mark to the 10 cc mark. Calibrate the test on a sample of slip that performs well. A viscometer also comes with the Lehman Slip Test Kit. http://lagunaclay.com sells a combination viscometer/specific gravity checker with a page of instructions.
The following two statements are important and if you understand them you are ready for action:
The most common method of preparation is to achieve the specific gravity first using the minimum needed deflocculant, then fine-tune the viscosity using a little more of the same deflocculant. Normally, beginners should try to achieve a specific gravity of 1.75, while experts will be able to work comfortably at 1.8 for many body types.
Various typical recipes for standard low-temperature whiteware and porcelain slips call for differing amounts of water. This underscores the importance of being able to measure specific gravity and viscosity. Below, I have provided some guidelines on mixing a slip. The first few times you mix a new recipe concentrate on creating a workable process and figuring out optimal amounts of water and deflocculant. After this, a period of fine tuning will perfect a process that can be committed to paper for repeating on a routine basis. However be ready to adjust things for different times of the year, different materials batches, for reprocessing scrap, etc. The notes to follow thus refer to the early stages of learning to mix a particular slip recipe.
Thixotropy refers to changes in a slips runniness or viscosity over moments, minutes or hours of time. Or, it refers to differences in the way the slip responds to stimuli to make it move once it has become motionless for varying amounts of time. Thixotropy thus involves time and interparticle reactions that happen in steady state slurries. This is a simplistic and incomplete description of the phenomena but for our purposes I use the term to embody the idea that slip should gel after sitting for a half hour or so. As discussed above, you want to achieve a state of controlled flocculation, a compromise between a fluid slip and one that will gel and therefore no settle out. This involves first thorough mixing (putting energy into it) to get a slurry to a state of equilibrium where its viscosity and thixotropic behavior are stable. Then it involves carefully noting the specific gravity and viscosity and flow behavior of the slurry and determining if slightly more or less deflocculant would be better. Obviously you cannot add less deflocculant, thus you would have to add water and powder body mix if deflocculant levels are too high.
Although the chemistry and physics of how they work is quite complex, for now just think of a deflocculant as a source of ions that charge clay particles to repel each other electrostatically and thus produce a slurry that is thinner than it would otherwise be. Manufacturers and suppliers generally compare their products to the standard solium silicate:soda ash mix and many products are quite well documented. The products mentioned below (obviously an incomplete list) are simply a summary of my own personal experience.
Soda Ash &Sodium Silicate: Sodium silicate is a liquid deflocculant that has been the standard for many years. Powdered soda ash is a ‘slip softener’ and produces protective colloids to deal with the anti-deflocculant effects of organic materials in the clays. These two materials work well together. Many technicians still refuse to use anything else and dispute claims that sodium silicate is responsible for clogging up mold pours and degrading performance, they say that if you can minimize the amount of sodium silicate to as close to 0.2% as possible using continuous and long mixing times that mold degradation in minimual.
Darvan No. 7, Allied Colloids #311: These are examples of more modern sodium polyacrylate dispersants, and manufacturers compare them with sodium silicate making the following claims:
A Sample Slip Mixing Procedure
Casting speed and ease of mold release are important factors in a casting operation. If you have a mold drier and make small items it should be possible to fill the molds many times each day. It is a very good idea to have a rating system so that you can take a sample of slip and compare its performance to the past. Make a small mold of a cone shape and keep it dry and replace often. To test leave the slurry in it for a set amount of time (e.g. 2 minutes). As soon as the liquidous state of the slip layer has disappeared, use a sharp knife to trim a clean line around the lip down to the plaster. Put the piece upside down on blocks and measure how long it takes until the piece falls out. When dry, measure the average thickness of the walls. This will give you a good measure of casting time and thickness buildup.
a clay slurry
A Good Mixer is Very Important
You need a good propeller mixer to mix slip properly. Slips should be mixed for hours so you need a mount and a mixer that is not going to over heat (an electric drill cannot possibly do this, they are designed to be run for a few seconds at a time). Good mixers are expensive, a small commercial one starts at about $500-$750! Thus you might consider watching for a bargain at an auction sale or on the internet. At Ebay.com, for example, searching for 'lab mixer' or 'duty mixer' should turn up something. Heavy duty mixers are used in so many industries and are thus a common item. It is important to have a unit that can run for many hours, has variable speeds (so as to avoid mixing air bubbles into the slip by pulling a deep whirlpool(, and is powerful enough to keep the whole mass in constant motion without overheating. A lot of science goes into selecting a propeller adapted each type of liquid. It have found that the standard three or four narrow blade type works well.
Mix the slip for several hours for best results. It is only by putting energy into the slurry that you can thoroughly wet every particle and extract the best performance. Less deflocculant may be necessary if the slip is mixed for a longer period. This is an advantage because some deflocculants, like sodium silicate, attack your molds and the less that you use, the better. After the slip stands overnight and is mixed for a few minutes the next day, it will usually cast better.
More About Controlled Flocculation
Ideally, a slip should be adjusted to a state of "controlled flocculation" where it is fluid, yet the finer particles remain agglomerated somewhat. Quality slips are intentionally less fluid than they could be. In a totally deflocculated slip the particles are so free to move that they can settle out in a very hard layer. During a cast, the smaller ones will be drawn toward the mold surface causing differences in particle size distribution and drying shrinkage across the thickness of the clay wall, this can result in drying cracks. In thin slips poor mold release can also occur because the fine clay is able to penetrate very small recesses in the mold surface and "hold on", resisting release. If you have these problems, vinegar can be added to a slip to flocculate it a little (it is quite remarkable to see what a cap full of vinegar will do for a bucket of settling slip). If a slurry is in just the right state, it should gel slightly while standing for an hour or two, this holds all particles in suspension nicely. Re-mixing will loosen it and the slip should flow freely again.
Clays That Do Not Deflocculate
Do not assume that any clay recipe will respond to deflocculants. There are many clays that contain soluble salts that impede or totally block their electrolytic action. Slurries of these materials simply gel immediately or over time. This problem is most likely with fire clay, stoneware or earthenware sedimentary type clays, the vast majority of kaolins and ball clays will deflocculate normally.
No matter what the problem with the slip is, your first question should always be: "What is the specific gravity"? Until you know this and know it reliably, you cannot fix the problem. For difficult slips be content with a specific gravity of 1.75 to solve problems in the short term. Target 1.78 when you are confident and have a body of experience.
Recognizing Under Deflocculation
If the slip is gelling after a few minutes or hanging motionless near the sides of the container while mixing, more deflocculant is probably needed. Be careful not to add too much; this is a common mistake (you will have to make more of the powder mix and add it, and more water, to counterbalance the oversupply of deflocculant). It is thus best to err on the side of under rather than over deflocculation when mixing a new batch.
If the slip has not thinned after an addition of deflocculant, then there is already enough present. An easy way to tell if a slip thins is to use a good mixer and watch the depth of the whirlpool. Set the speed so that there is a moderately deep vortex but (not quite deep enough to suck air into the mix). Add a little deflocculant and watch if the vortex deepens. Sometimes a very small addition of water will thin a troublesome slip dramatically, take advantage of this and don't struggle to work with a slip of high specific gravity if a slightly lower value is much better.
Recognizing Over Deflocculation
If the slip does not gel at all or settles out in a layer on the bottom of the container then there is too much deflocculant. Too much deflocculant is also indicated by a thin slow cast, a wavy and gritty looking inner surface after draining, rapid formation of a skin on the slip after the mixer is stopped, poor mold release, splitting and cracking of the ware during drying and brittle ware. It is quite amazing how poorly over-deflocculated slip can perform and it is not unusual to see people falsely blame the problem on changes in body ingredients or on the body recipe. Over-deflocculation is a phenomenon independent of specific gravity, a slip of 1.7 or even 1.85 or higher can be over-deflocculated. Also, you might feel that because your slurry does not have some or all of the problems mentioned above it is OK, but this is not true. The solids in an over-deflocculated slurry of 1.7 may settle out as it stands whereas this may not happen if the slurry has been taken over 1.8. I have seen a very fluid slurry of 1.87 that did not settle, yet exhibited serious skin formation and very poor casting. Because the slurry was very fluid, it seemed counter intuitive that it needed alot of water and less deflocculant, but when the specific gravity was correctly measured this became evident. This again underscores the key point: make sure your slurry has your target specific gravity (1.78 for example) and then judge what to do with it after that is achieved.
Incredible as it may sound there are actually people and companies who do not mix scrap clay back into new batches. This is invariably because of bad experiences resulting from not understanding how to maintain slip rheology. Scrap clay already has deflocculant in it, thus it seems logical that it should be possible to adjust only the water content to accomodate it. Actually this is not true. For whatever reason you invariably have to make adjustments in deflocculant amount also. The bottom line is that you need to understand the things explained on this page so that you can test any slip and determine what needs to be done to adjust it. With this ability you can add any amount of scrap and still have an optimal casting slip.
Recognizing Clay Recipe Problems
Even if you achieve an optimum slurry, it will not necessarily cast well if the clay recipe itself is not right. Recipes that contain a lot of fine clay minerals (i.e. ball clay, bentonite) will cast slowly because the clay is excessively impermeable to the passage of water and ware will resist release from the mold (although ceramic slip with its 50% typical ball clay is an exception because the high talc in the recipe helps vent the water). High clay formulations will produce ware that will shrink more and crack more. Recipes that have inadequate clay or clays of very low plasticity will shrink too little and will not release from the mold very well. They will produce fragile ware that fractures when being removed from the mold or during handling.
If you would like to formulate your own casting body (e.g. A terra cotta, a stoneware, a porcelain) it is obvious that decisions you make about its makeup will affect the way in which it performs during the process. As already noted, casting body recipes look very different than plastic bodies. This process enables you to make use of cleaner materials not suited to plastic bodies and you can discontinue the use of problematic materials. I have written an article specifically about this called “Understanding Slip Casting Body Recipes”.
Out Bound Links
In Bound Links
A veterinarian hypodermic syringe, notice the large diameter end that enables slip to freely flow
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A hydrometer being used to check the specific gravity of a ceramic slurry. In this case the slurry was too heavy, almost 1.9, yet very fluid. Thus it has both too much clay and too much deflocculant.
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An example of how an over deflocculated ceramic slurry forms a skin a few minutes after the mixer is stopped.
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