|Monthly Tech-Tip |
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, when we speak of deflocculation, we are almost always talking about making a casting slip. Glazes can also be deflocculated (to reduce water content and densify laydown).
Deflocculation is the process of making a clay slurry that would otherwise be very thick and gooey into a thin pourable consistency (it is the opposite of flocculation). The magic of this process can only be appreciated when you see a thick mud transform in watery liquid with the addition of a few drops of dispersant (under mechanical mixing action). Deflocculants are electrolyte-sourcing liquids or powders (like sodium silicate, Darvan) that are added in small amounts. They work their magic by imparting electrical charges to clay particles making them repel each other (more accurately it is said to be a condition where repulsive forces predominate). Slurries are analysed as over or under deflocculated only after it is determined that they are at the correct specific gravity. Over-deflocculated slurries tend to be sticky, livery, produce a powdery cast surface and settle into a hard layer at the bottom (see info on slight over-deflocculation below). Under-deflocculated slurries gel. A casting slip is a delicate balance of water, powder dynamics, deflocculant, temperature and mix energy (as any of these vary so does the properties of the slurry). Once you have used a slip that has been properly formulated, mixed to the right specific gravity and deflocculated for casting you will never go back to using an inadequate one.
Deflocculants change the viscosity of a slurry without changing its specific gravity (adding water to a slurry changes its viscosity by reducing the specific gravity). Thus, to deflocculate a slurry properly it is very important to be able to measure its specific gravity and viscosity accurately. Typical casting slurries have a specific gravity between 1.7 and 1.8 (although some are up to 1.825). A bucket of 1.8 SG slurry is very heavy (1.8 times heavier than water).
Here is an example of a middle temperature casting recipe that delivers about 1.75 specific gravity (in our circumstances):
Dry porcelain powder: 5 Kg
Water: 2.1 kg
Darvan #7: 38 grams
Different types of deflocculants are used in different percentages (for a given specific gravity). This percentage, 0.75% of the dry powder, is large. Some commercial deflocculants (e.g. those used in sanitaryware) are effective in percentages as low as 0.1% (they cost much more but you use much less). The amount of deflocculant required increases with specific gravity and, at some point, further additions become ineffective. Each product has its own characteristics in this regard, so determining the best compromise between the utility of the slurry and its cost requires patience. It is best to focus on slip quality and stability at first and make small changes with time to reduce costs (e.g. raise the specific gravity). One manufacturer finds that a 30% water content delivers 1.72 specific gravity, the low deflocculant requirement and easy adjustability of the slurry make this conservative approach cost effective.
Casting recipes "recommend" water and deflocculant amounts. Quality recipes indicate what specific gravity the recommended water content produces. This is a very important concept since casting recipes do not travel well. Users "adapt" a recipe to their process. This is so, both because of all the variables between different production sites (e.g. water electrolytes, material moisture and solubles, temperature, mixing equipment, etc) but also because priorities differ. Variations in water and materials also occur at the same location (with different shipments and seasons). A factory places a much higher priority on using the highest possible specific gravity (to minimize mold wetting) and is prepared for the extra burden of slurry maintenance (they also have the powerful mixing equipment needed). A potter is likely willing to use a slip of lower specific gravity to benefit from the easier mixing and maintenance. A potter might propeller-mix a slurry for minutes, a factory for days.
Thus, mixing a new recipe for the first time is a matter of discovering the amount of water and deflocculant you will need to make it work in your process with your priorities. Depending on the power of mixing equipment available a target viscosity (TG) is set. This TG compensates for the mixing equipment (more viscous if mixing equipment is powerful and slip mixing can be done for many hours, since that will thin the slurry). Generally this means starting with about 95% of the recommended amount of water and about 75% of the recommended deflocculant. Material is added quickly at first (while mixing) and more and more slowly as the slurry gets heavier and heavier. As the slurry becomes too viscous in the latter stages of adding the dry powder it must be determined if water or deflocculant is needed. A little of the deflocculant held in reserve is added. If a quick response is noted (a thinning of the slurry) more powder is added, otherwise the water held in reserve is added. This process is continued until all the powder has been added (but hopefully not all the deflocculant). The objective is to mix all the powder into the slurry while assuring its viscousity is a little higher than the TG. Then, something very important is done: The specific gravity (SG) is measured. Since the viscous slurry will not float a hydrometer a weight-volume method of measuring it must be employed. If the SG is above target, water is added and another measurement done. If it is below target a note is made for the next batch and this batch is accepted as off-spec (meaning that less deflocculant will be needed and it will soak molds faster). If the SG is near target, then more deflocculant is added (but only to reach the target viscosity). Finally, the slurry is power mixed for the longest possible time and fine-tuned. Notes are kept regarding changes needed in mixing the next batch.
Potters are mostly unaware of the lengths industrial technicians go to to achieve optimal slurry properties. They must constantly fine-tune the contents of slurry tanks (because the age of the slurry, the slip returns, dissolving sulphates from molds and the recycling of scrap all change the rheology). If sulphates are present they add a little barium carbonate to precipitate them. Then, using a lab viscometer to measure the viscosity and thixotropy, they bring the slurry into a condition of slight under-deflocculation and higher-than-needed specific gravity (this is practical because they have powerful slurry mixing equipment). Deflocculant is added to reduce the thixotropy to 5 degrees over the required value. Then water is added to increase fluidity to the required value (this usually brings thixotropy in line also). Vigorous mixing is then done (especially if barium was added). Aging is also often done (from 1 to 5 days) to get optimal casting performance.
When one has done a few of these discovery methods of determining the best powder, water, deflocculant recipes he/she is empowered to evaluate and adjust existing slurries and add scrap material to slurry mixes.
Unfortunately it is very common for slip casters to know nothing about the above, believing that a recipe can simply be followed and by magic, a delicately balanced slurry will be produced. Many will work for years with substandard slip without knowing it, others will throw away all scrap rather than reprocessing it simply because they do not understand slip rheology. A common mistake is mixing slips using plastic clays intended for modelling or sculpture (these are not permeable and shrink too much, far better casting slkips can be made using mixes of materials that emphasize permeability instead of plasticity). Notwithstanding these things, when people can accept a lower specific gravity (e.g. 1.6 or 1.7) production can actually be done on easily-cast shapes with poor quality slip.
If you are starting out with casting slips, perhaps targeting a lower-than-normal-but-easier-to-mix specific gravity of 1.7 would be a good start. Be careful not to make the slip too thin or it will settle out. Aim for bringing it into a state of controlled flocculation. This is where it will gel after a period of time (this prevents it settling out). Such slip stops short of complete deflocculation in the interests of achieving a slurry with better working and suspension properties. It also avoids slight over-deflocculation, when that happens the slip casts slower and the inside surfaces feel powdery rather than smooth in the leather hard state. Pieces also tear more readily at the rim when trimming them with a knife or when they are pulling themselves away from the mold.
What if you only have a clay body and no amounts for the water or deflocculant. Then use the above recipe as a guide. You risk loosing the first attempt so make a smaller amount. Keep good notes and learn for the next one. An account at insight-live.com is a good place to log this sort of information so it is always there when you need it.
This is the L3798E cone 6 buff-burning stoneware slip (it is 35% KT1-4 ball clay, 25% silica, 12% nepheline, 15% EPK, 13% Redart). This recipe was easy to deflocculate to 1.8 specific gravity and yet it was very thin and runny. It pours nicely and does not gel. The former recipe was using a plastic ball clay and this switches it to a larger particle ball clay intended for casting. Thus the amount of deflocculant needed is less. But we did not reduce it, that's why it is so fluid. While not fluid enough to settle out or have poor draining properties, it is nevertheless, slightly over deflocculated. Although pieces can be extracted from the mold quickly, casting time is longer than it should be (for this one 15 minutes for a 2mm thick wall). Another issue is the inside surface: It should feel soapy and smooth in the leather hard state, but it feels sandy! This underscores the need for controlled flocculation (using less deflocculant) to maintain at a slightly more viscous fluidity so that it has thixotropy.
The slip on the right has way too much Darvan deflocculant. Because the new recipe substitutes a large-particle kaolin for the original fine-particled material, it only requires about half the amount of Darvan. Underestimating that fact, I put in three-quarters of the amount. The over-deflocculated slurry cast too thin, is not releasing from the mold (therefore cracking) and the surface is dusty and grainy even though the clay is still very damp. On my second attempt I under-supplied the Darvan. That slurry gelled, did not drain well at all and it cast too thick. On the third attempt I hit the jackpot! Not only does it have 1.8 specific gravity (SG), but the slurry flowed really well, cast quickly, drained perfectly and the piece released from the mold in five minutes. Interestingly, on a fourth mix I made an error, putting in too much water, getting 1.6SG. The casting behavior was similar to the over-deflocculated slip (even though the Darvan content was much lower). A good casting slip is a combination of a good recipe, the right SG and the correct level of deflocculation.
A hydrometer is being used to check the specific gravity of a ceramic casting slip in a graduated cylinder. Common traditional clay-containing ceramic slips are usually maintained around 1.75-1.8. In this case the slurry was too heavy, almost 1.9. Yet it is very fluid, why is this? It has both too much clay and too much deflocculant. While it is possible to use such a slip, it will not drain as well and it will gel too quickly as it stands. It is better to settle for a lower specific gravity (where you can control the thixotropy and it is easier to use). It might have been better to simply fill a 100cc cylinder and weigh it to get the specific gravity (slurries that are very viscous do not permit hydrometers to float freely).
In this instance, the slurry forms a skin a few minutes after the mixer has stopped. Casting recipes do not travel well. Over-deflocculation is a danger when simply using the percentage of water and deflocculant shown. Variables in water electrolytes, solubles in materials, mixing equipment and procedures, temperature and production requirements (and other factors) necessitate adapting recipes of others to your circumstances. Add less than the recommended deflocculant to try and reach the specific gravity you want. If the slurry is too viscous (after vigorous mixing), then add more deflocculant. At times, more than what is recommended in your recipe will be needed. After all of this you will be in a position to lock-down a recipe for your production. However flexibility is still needed (for changing materials, water, seasons, etc).
This deflocculated slurry of 1.79 specific gravity (only 28% water) has just been poured into a mold. The mold is dry, the wall thickness of the bowl will build quickly and the liquid level will sink only slightly. The mold can be drained in minutes (for a wall thickness of 3-4 mm). The clay is not too plastic (too fine particle sized) so it is permeable enough to enable efficient water migration to the plastic face. If the specific gravity of this slip was too low (too high a percentage of water) the liquid level would sink drastically during the time in the mold, take longer to build up a wall thickness and water-log the mold quickly. If the slip contained too much deflocculant it would cast slower, settle out, form a skiln and drain poorly. If it had too little deflocculant it would gel in the mold and be difficult to pour out.
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.
If a glaze has already been mixed and gelled to give it thixotropy these things won't bob up and down to home in on the right level. If the glaze is watery enough there are other issues. The one on the right has a 1.0-1.7 scale. Since most pottery glazes need to be 1.4-1.5 specific gravity (40-50 on this scale) it is difficult to get a very accurate reading. And it is long, you will need a container tall enough to float it and enough glaze to fill it. The small hydrometer appears better, it has a scale of 1.2 to 1.45. But it really bobs up and down (so it is even more important that the slurry be runny and thin to give it the freedom to do so). It is better to weigh a measured volume of glaze slurry and calculate the SG instead. The easiest is a 100cc graduated cylinder (from Amazon.com), if 100 ccs weighs 140 grams, that is 1.4 specific gravity.
This is G2926B clear cone 6 glaze deflocculated with Darvan. Because the Darvan is thinning it, 2.5kg of glaze powder is suspended in only 1100g (1100ml) of water (half the normal amount). While the slurry in the bucket flows well and appears like it should work, a one-second dip produces twice the desired thickness. It dries slowly and it is very difficult to prevent runs. The lesson: Make sure the specific gravity (SG) of your glazes is right. What should the SG be? Measure it when your glaze is working well. Or take note of it in instructions that come with the recipes you use. For bisque ware: 1.43-1.45 with a flocculant (like Vinegar or powdered Epsom Salts) added to gel the slurry slightly.
This is slurry is made of 80% Redart and 20% KT1-4 ball clay. It has very good casting properties IF it is deflocculated well (smaller pieces can be cast in 10 minutes and extracted from the mold in another 10). However high-iron slurries are notorious for gelling, either right away during mixing or later. Sometimes a slurry seems to pour well into a mold but 10 minutes later it has gelled so much that vigorous shaking is need to loosen it enough to pour it back out (even then much may hang on in corners). A batch of slurry may need extra deflocculant several times over the first few days of being used. Davan 811 is often used for these. More deflocculant is needed, in this case about 1 gram for each 100 grams of powdered clay.
Casting slips require a minimum of water. Amazingly, it is possible to get 3000g of M370 powder into 1100g of water! And the fluid slurry produced, 2250cc, still fits in the container. How is this possible? That water has 11 grams of Darvan 7 deflocculant in it, it causes the clay particles to electrolytically repel each other! An awareness of “the magic” can help give you the determination to master deflocculation, the key enabler of the slip casting process. Determination? Yes, the process is fragile, must must develop the ability to “discover” the right amount of Darvan for your clay mix and water supply. And the ability to recognize what is wrong with a slurry that is not working (too much or little water, too much or little deflocculant).
To-the-brim the bucket holds 8.8 liters (2.43 Canadian gal, 1.9 US gal). The slip itself weighs 14 kg (30 lb). It has a specific gravity of between 1.75 and 1.8. The slurry was power-mixed in a larger bucket.
A Ford Cup being using to measure the viscosity of a casting clip. These are available at paint supply stores. This is a #4, it holds 100ml and drains water in 10 seconds (it has a 4.25mm opening). This casting slip has a specific gravity of 1.79 and we target a 40-second drain. That being said, if you are not working in a factory, if will be sufficiently to eye-ball the viscosity as you gain experience. If you are in a high-production situation, the seconds-value that this test produces gives you something to write down in testing records, producing an audit-trail for quality control and problem solving later. One thing to note: A slurry can gel while it is draining, if this happens the value produced is not valid. First adjust the rheology so it maintains viscosity throughout the drain time.
These are cone 6 commercial glazes made by a popular US manufacturer. The body is a cone 6 casting porcelain made by another popular manufacturer. Zoom the photo, they are all crazing! Which company is at fault? Neither has the responsibility (or is able, especially with stonewares and porcelains) to match their product to that of every other company. The pattern we see here points-the-finger at the body. Mid-fire porcelains craze glazes much more if they lack sufficient silica (20% is minimum). It is difficult for manufacturers to achieve this since much more feldspar is needed to vitrify the body. And the potter does not know the recipe of the porcelain. What to do then? One option is to get a porcelain from another supplier, with assurances from them about glaze fit. Better yet, mix your own. Casters need a mixer anyway, so why not? We can help you with a recipe if you need it. Actually, mixing your own glaze also would get rid of those micro-bubbles and give a glassier surface.
The very whitest porcelains are made from New Zealand kaolin. However, while Grolleg kaolin does not fire quite as white, it requires up to 10% less feldspar to produce a vitreous porcelain (it contains natural feldspar). That 10% less spar can be made up in kaolin, imparting better workability and dry strength to the body (and Grolleg is known for its dry strength). Assuming that 25% silica is needed for glaze fit, one only needs to discover what blend of feldspar and kaolin in the remaining 75% achieves the desired degree of vitrification (e.g. we like zero porosity just-reached at cone 6). We found 25% nepheline was too vitreous (pieces warped) and at 20% porosity was not yet zero. While the Grolleg version fires a little darker, the better workability imparted by the extra kaolin makes up for that. The plasticity needed for good throwing requires the addition of bentonite (4% for NZK and 3% for Grolleg). Both of these can be made into casting bodies by reducing the amount of bentonite (~ 1% for NZK, 0.5% for Grolleg). Do your testing to discover the % of bentonite needed for the leather hard to pull away from a mold without cracking but not take too long to cast.
Why? Glaze fit. These are available on Aliexpress (as Drip Pottery) and they are made by a manufacturer that has close control of body maturity (and thus strength) and a dilatometer to precisely match the thermal expansion of the glaze. The glaze has to fit better than normal because of the absence of an outside glaze. Too low an expansion and it's compression (outward pressure) will fracture body (because these are thin-walled pieces). Too high and it will craze. And that thick glaze? It will shiver or craze with far less forgiveness than a thin layer. And how did they get the glaze on this thick? They deflocculated it, up to 1.7 or more, glazed the inside, let it dry, then glazed the outside. These pieces are a visual and technical achievement. If you are a potter you had best think twice before attempting the same.
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.
Sanitary ware factories optimize their slips to have the lowest possible specific gravity for production volume reasons. Potters would be happy with 1.7 SG whereas numbers approaching 1.9 SG are common in factories. They often teeter on the edge of issues like this (sections softening causing localize warping) and inexperienced technicians can be unaware of the critical balances needed to prevent loss in production.
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.
This is 8.4L of water (in the bottom of that pail) and a 20kg bag of Polar Ice porcelain casting clay. Amazingly enough it is possible to get all that powder into that little bit of water and still have a very fluid slurry for casting. The volume will increase to only 2/3 of this 5 gallon pail. How is this possible? That water has 100 grams of Darvan 7 deflocculant in it, it causes the clay particles to repel each other such that you can make a liquid with only little more water than is in a throwing clay! All it takes is 15 minutes under a good power propeller mixer.
A video of the kind of agitation you need from a power mixer to get the best deflocculated slurry properties. This is Plainsman Polar Ice mixing in a 5 gallon pail using my mixer. Although it has a specific gravity of 1.76, it is very fluid and yet casts really well. These properties are a product of, not just the recipe, but the mixer and its ability to put energy into the slurry.
This cast bowl (just out of the mold and dried) is 130mm in diameter and 85mm deep and yet the walls are only 1mm thick and it only weighs 89 gm! The slip was in the mold for only 1 minute. What slip? A New Zealand Halloysite based cone 6 translucent porcelain. This NZ material is fabulous for casting slips (it needs a little extra plasticizer also to give the body the strength to pull away from the mold surface as it shrinks).
As the amount of defloccuant is increased the viscosity drops and the slurry becomes more and more fluid. However, at some point, the slurry will begin to become more viscous with increasing deflocculant percentages. This underscores the importance and tuning your casting slip recipes to avoid this problem. It is actually better to deflocculate to a point before the curve reaches its minimum (where the slop is still downward). This "controlled state of flocculation" enables the slip to gel after a period of time (to prevent sedimentation) and avoids the issues that come with over-deflocculation.
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 ceramic industry glazes are often sprayed, especially in sanitary ware. The technique is important.
Water in Ceramics
Water is the most important ceramic material, it is present every body, glaze or engobe and either the enabler or a participant in almost every ceramic process and phenomena.
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).
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.
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.
A method of forming ceramics where a deflocculated (low water content) slurry is poured into absorbent plaster molds, forming a layer against mold walls, then poured out.
Soluble sulfates in clay produce efflorescence, an unsightly scum that mars the fired surface of structural and functional ceramic products.
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