|Monthly Tech-Tip |
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.
A comparison of the weights of equal volumes of a given liquid and water. Water has a specific gravity of 1.0. A ceramic slurry with a specific gravity of 1.8 is thus 1.8 times heavier than water. The best way to measure specific gravity is to weigh a container and record its weight, then weigh the container full of water and full of the liquid of unknown specific gravity. Subtract the weight of the container from each weight and divide the weight of the liquid being measured by the weight of the water.
Specific gravity is very important in the production of casting slips where low-as-possible (or, more accurately, low-as-practical) water content is needed. Hobbyists and potters typically target around 1.75 wheres in industry 1.8 or higher is needed (especially in sanitaryware). Normal clay-based slurries used in traditional ceramics that are outside this range just do not have good working properties. Achieving a slurry having a specific gravity approaching twice that of water is only possible using the deflocculation process. When slurries are not behaving normally (e.g. settling, gelling, casting too slowly or unevenly, not draining from the mold properly, not releasing, producing a powdery surface) the first step in isolating the reason is a specific gravity measurement. Corrections in water content are always made before assessing the whether the amount of deflocculant is correct. A good example of the logic is a slurry having too low a specific gravity: It can exhibit similar casting issues to an over deflocculated one, the best way to know whether a water or deflocculant change is needed is knowing the specific gravity.
Glazes do not have a universally desirable specific gravity range like casting slips. The same glaze can be used effectively by different people and in different processes having quite different specific gravities. We have seen production glazes with a specific gravity approaching 1.75. Some industries even prepare their glazes up to 2.0 (obviously highly deflocculated), something that would be impossible for the average potter. Commercial brushing glazes can have specific gravities ranging from below 1.3 up to 1.55 (some companies emphasize brushability and their products will be on the lower end and require more coats, but remember that you are paying for water with these). Caution is needed here. Some glaze recipes, when mixed to the apparent correct viscosity (having no additives), will have a fairly high specific gravity (e.g. 1.55-1.6). These commonly settle out into a hard layer on the bottom of the container. Raising the water content (thus lowering the specific gravity) and gelling the slurry (using vinegar for example) is a way to deal with this. Notwithstanding this, doing the opposite, deflocculating the slurry, is also a way to prevent sedimentation (provided it is still viscous). Potters who make their own glaze generally do best with them between 1.45 and 1.50.
A glaze slurries that are 'gelled' work best for potters because they apply more evenly, drip less and do not settle. To be gelled the specific gravity has to be lower-than-normal and then a flocculant added to bring it back to a creamy consistency. For many common partially-fritted glaze recipes this occurs around 1.43 to 1.45 specific gravity. The addition of epsom salts or vinegar to the slurry gels it and imparts the property of thixotropy. The important thing is to determine, by experience with a specific glaze, what specific gravity and flocculation procedure work well, then stick to that for future batches.
Dark colored, paint-on glazes used for detail work need to have a high specific gravity so they can cover well with a single brush stroke (e.g. 1.55). Clear glazes than need to be applied (by painting) in a thin layer need a lower specific gravity (e.g. 1.25).
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 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.
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.
The freshly opened transparent low fire glaze on the left has a specific gravity of only 1.34 (that is a high water content). Yet it is viscous because they add alot of gum. It needs three coats to go on thick enough and takes quite a bit of time to dry each one. When dipping, a very thick layer dries very slow and thin. That being said, transparent glazes do need to be applied thinner, AMACO is likely trying to assure that by producing at this low specific gravity. The center Potter's Choice glaze, made by the same company, is 1.52 (that is a much better deal). And it goes on nice and thick. The Celadon glaze on the right is lower, 1.46. Glaze manufacturers can produce at a broad range of specific gravities, they just adapt the percentage of gum to impart the viscosity they want. While it is sensible to use commercial special-effect paint-on glazes, clear cover glazes are best mixed yourself and applied by dipping or pouring.
AMACO and Crysanthos. 1.26 (67.5% water) and 1.22 (68% water)! The former is well below their recommended specific gravity of 1.4 (it still paints well but needs more coats, and more time to dry and apply them). Strangely, the Crysanthos, although having a lower specific gravity is more viscous and goes on thicker (but thinning down as it dries). With underglazes it is important to get adequate thickness with one brush-stroke, so a high specific gravity is important. This may be reason enough to consider making your own (by adding stain powders to a base). Actually, this technique of adding-stains-to-a-base-transparent is even more practical for making your own glazes, it just takes the right amount of gum to make them paint well.
These are Plainsman P300 mugs fired at cone 6. When the glaze, GR6-E, goes on too thick (as on the left) it is dark maroon and has a pebbly surface that does highlight contours. This went on too thick because the specific gravity of the slurry was too high, about 1.53 (even a one-second dip put to thick a layer on the pieces). When I thinned it down to about 1.45 and flocculated it using espom salts, it went on thinner, yet still evenly, and I got the result on the right.
The glaze in this jar was 'goop', impossible to paint on. I did not know whether I needed to add water or try to deflocculate it (although the former is more likely and in keeping with what Laguna says on its website). I measured the specific gravity, it was 1.7, so clearly it needed water. It took 125cc to bring the specific gravity down to 1.5. However, it was still thick and dried immediately after painting on, clearly it does not contain enough gum for brushing. The next time I will add a mix of 50:50 gum solution and water for better paintability. The bright side: I got considerably more than a pint after adding the water, a big difference from some other commercial glazes which are mostly water.
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).
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.
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.
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.
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.
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.
|Tests||Rheology of a Ceramic Slurry|
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?
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, 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).
In ceramic industry glazes are often sprayed, especially in sanitary ware. The technique is important.
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.
In ceramic slurries (especially casting slips, but also glazes) the degree of fluidity of the suspension is important to its performance.
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.
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
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.
Specific Gravity at Wikipedia
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.