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Viscosity - what is it?
by Teresa Desantis
Before you continue reading this, note the following: Use these guidelines, test in your studio, and set your own standards!
What is a viscosity cup? It is simply a cup with a hole in the bottom which holds a given quantity of slip. There are many different viscosity (also known as "zahn") cups out there, and there are many different ways of using the cup. The various ways of using the cup involve differences in the ways the outflow of slip from the cup is timed. Some people will time the slip from when the cup is full until when the stream "breaks" into a drip pattern. Others will time the slip's outflow over a certain range. (For instance, here at Seeley's, for quality control purposes with our one-of-a-kind zahn cup, we time the outflow of the cup into a milliliter flask, between the 50 and 150 ml. marks.) But the easiest way to use a zahn cup is to simply time the slip leaving the cup from the time the cup is completely full to when it is completely empty. This is the method which I recommend, and which was used in the writing of this article.
What is viscosity? Viscosity is a comparative measure of the fluidity of the slip. Viscosity readings can only be compared when you are using a single type of zahn cup. If you see suggested numbers or ranges for slip viscosity in print, be sure you know which type of viscosity cup was used. For the numbers to be of use to you, you must be using the same model viscosity cup, otherwise the numbers won't be helpful. (The viscosity numbers discussed in this article are geared around the new viscosity cup that we sell; but the general concepts can be applied no matter what style cup you have.) Also, when testing viscosity, it is important to take into account the important variables of slip temperature, agitation, and age, discussed in detail below, so your readings will be of most use to you.
The zahn cup is best used to generate a number for comparative purposes only. Why? Every single batch of slip is affected dramatically by several important factors. The most important of these factors are:
1.) the temperature of the slip,
- 2.) how vigorously you have stirred or agitated the slip before you run the viscosity test, and
- 3.) how much time has elapsed since the manufacture of the slip?
1.) The temperature of the slip is something that you can easily control. For optimum casting and viscosity test results, slip should be tested and poured in "room temperature" conditions. Slip that is cool or cold will always be significantly thicker and more viscous than the same slip at room temperature. Conversely, if you warmed the slip to beyond the point of room temperature, it would be even thinner than it would be at room temperature. So, always let your slip warm up to room temperature before you test it, or make any decisions about adjusting it.
2.) How vigorously you have stirred or agitated the slip before you run your viscosity test, and how long the slip rests before the test starts will also dramatically affect your viscosity readings. Why is this? Casting slips are influenced by their rheology. What, then, is rheology? Rheology is really the "black magic" of slip mechanics. Things are happening in the slip at the molecular level. When the slip is vigorously agitated, it acts more liquid. If you let the same slip stand alone for a time, it will want to thicken up. If you then slowly agitate the slip, it will be less fluid than if you agitate it vigorously. Each slip formula has its own unique "viscosity under agitation" curve (engineers call this "the slip behavior under shear".) This viscosity curve is further modified by any changes in the specific gravity or the deflocculant content of the slip.
What does this mean for you? It means that if you change any one of these factors, the viscosity number that you get will be different. Add water, and the number will be different. Add deflocculant, and the number will be different. Shake the slip up, the number is different. Let it sit for an hour, and the number is again different. Age the slip and the number is different. Even taking the same slip viscosity more than once under the exact same conditions will often yield different numbers. (For this article, I did just that, and got these readings: 27/25/30/30/29/29.) If you can get this much variation under controlled conditions, just imagine what happens when you start testing and adjusting the slip in the studio! That's why it’s best just to treat the viscosity reading which you get as a relative number, to be used for comparison purposes only.
3.) The final important factor to consider in evaluating viscosity is the age of the slip, that is, how much time has elapsed from the date of manufacture. Dramatic changes happen as soon as the batch of slip is made and is pumped into its container. For instance, porcelain that has just been manufactured, and is still under agitation in the mixing tank, has a much lower viscosity than it does after it has been pumped into containers and has begun to age. Actual porcelain in our mixing tanks measures a zahn viscosity of 25-34 seconds; after just 24 hours of ageing in the jug, the viscosity has increased to 1 minute and 25 seconds! After the slip has aged for 3 weeks or more, the viscosity may be as high as 2-3 minutes, or more. This is all in the same batch of slip, with no additions or changes made to it. This ageing isn't simply occurring because the slip is sitting still; there are actual chemical changes that are happening in the slip which cause this irreversible thickening change to occur. With this in mind, if you asked someone at Seeley's what the viscosity should be, and they gave you the figure of "25 seconds", which is for newly manufactured slip, and then you tried to adjust your already aged slip to match this number (probably by adding tons of water and deflocculant), you see that you would be in for a real disaster! Again, a zahn reading and the zahn numbers that you see in print are best used for comparative purposes only.
Use the zahn test in your studio to check the viscosity of the slip as it is received from the manufacturer. (Remember, before beginning any tests, let the slip warm up to room temperature, then stir it thoroughly and for a similar time and manner before each test.) Do a viscosity check right before you start to use the slip. Record what the number is, and then carefully observe how the slip behaves during the casting process. Then, and only then, make any changes to the slip, if necessary.
Let's say that you receive the slip, let it warm to room temperature, stir it until it is well mixed, perform a viscosity test, and the slip tests out at 1 minute. So 1 minute is our starting point. Now cast with the slip. How does it perform? Does it cast perfectly? Does it drain well? If the slip performs well, then leave it alone: your current baseline viscosity for the slip is a 1 minute viscosity.
Maybe instead the slip casts too fast. Maybe it is also difficult to drain because it is too thick. If this is the case, it sounds like the viscosity could use some adjusting. If your slip is casting too fast and draining too slow, your viscosity is high, but your solids content could also be too high. If possible, also run a specific gravity test of the solids content. (This was discussed in the last issue.) If it is on the high side (1.75 and above for porcelain and stoneware, 1.85 and above for earthenware), then you may want to add water first to drop the solids content slightly. Adding water will lower both your viscosity and your specific gravity. If the slip is casting too fast and draining too slow, but the solids content is fine (basically above 1.68, and below the high numbers listed previously), then you can just add deflocculant to lower the viscosity and to thin the slip. Adding deflocculant alone will lower viscosity without significantly changing the specific gravity of the slip.
When in doubt and your slip is thick, what to do? Normally, I add water first, or water with a small amount of deflocculant added. What kind of amounts to add? Normally, for a gallon of slip, I will take a 1/4 -1/2 cup of water with about 1-3 teaspoons of deflocculant in it, and SLOWLY and incrementally add this to the slip while stirring, until the viscosity of the slip drops into the useable range. DON'T GO OVERBOARD, because you don't want to overdo it and have overly thin slip instead. Just add the water/deflocculant mixture by dribs and drabs until the slip is just right. When you feel you are getting close to where you should be, try pouring a typical-sized casting and see how the slip behaves. If you did indeed go too far, the slip will seem watery, and will take much longer to set up, forming only a slight thin shell as you cast and wait and wait. If you've gone too far, the only remedy is to mix in some thicker slip, or to set the slip aside for a few days and see if you can skim some water off the top. (Sometimes possible, but not always.) But if you play it safe and go slow in your additions, everything should be fine.
I hope that this article has helped to "de-mystify" the subject of viscosity and to make it more useful to you. Good luck and happy casting!
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