•The secret to cool bodies and glazes is a lot of testing.
•The secret to know what to test is material and chemistry knowledge.
•The secret to learning from testing is documentation.
•The place to test, do the chemistry and document is an account at https://insight-live.com
•The place to get the knowledge is https://digitalfire.com
Rheology of a Ceramic Slurry - RHEO
This test procedure was employed in the Foresight Ceramic Database and now is available for those having an account at Insight-Live.com.
Accumulating test data using the variables defined in these procedures enables us to create tools that enable you to compare the physical properties of materials and recipes.
This test collects data about the physical properties related to the rheology of a ceramic slurry. Procedures have not been written yet, this is an outline of the theory and it is generalized to cover as wide a range of slurries and testing budgets as possible. Remember that it is not generally important that the test results you obtain be absolute and compared with industry standards, just that they be compared with a standard within your facility for the specific slurry being tested. Start with simple measuring techniques and then graduate to instrumentation if needed.
Specific gravity, in the traditional ceramic manufacturing context, normally refers to the solids content of a slurry, its density compared to water (e.g. a material with a specific gravity of 2.0 is twice as heavy as water). The term density general refers to the weight of the solid mineral itself. This measurement thus has no units, however since water is 1 gram per cc the units for specific gravity are implied. Ceramic slurries (such as alumina) can be more than twice as heavy as water. Typical casting bodies are around 1.8, glazes are about 1.5. Deflocculation is required to achieve the necessary specific gravity in bodies and the use of deflocculants is a delicate business, it is extremely important to be able to recognize whether the wrong viscosity of a slurry is because of incorrect solids content or incorrect deflocculation.
Viscosity is a measure of the flow of a ceramic slurry. When slurries do not have the correct solids content and viscosity then product quality, reject rates, efficiency of manufacture and materials waste are all at stake. The viscosity instrumentation industry has a confusing array of methods and devices to measure viscosity, but ceramic slurries are unique and it is important to use an appropriate device. Immersion probe viscometers are the best for most situations and manufacturers provide procedures to assure that measurements are reliable and representative. However a timed-drainage-flow tester purchased or made from a hypodermic syringe also works well if the slurry is not too thick (this device is described in the article on understanding the deflocculation process). Values from such a method are, of course, relative only to your own lab (since the size, shape, opening of tubes, timing, procedures will be different).
Thixotropy is a more difficult property to quantify. Instruments are available to measure it in absolute terms. However most facilities see this property in relative terms and thus a simpler measurement technique can be done. Generally most industries maintain slurries in a state of controlled flocculation, that is, in a state that tends slightly toward gelling when left standing (to prevent sedimentation). The target time-to-gel is well known to staff (for example 5 minutes). Thus you can develop a way to judge the amount of time a test slip takes to gel to a specific degree. Or you can agitate the slurry and measure the viscosity using a tube viscometer, then leave another sample in a tube for 5 minutes and flow test that, then record the two drain times.
SG - Specific Gravity (V)
Value in g/cc. Measure this by weighing a specific volume of the slurry and dividing the weight in grams by the number of cc's. Unless slurries are quite runny hydrometers do not give accurate readings.
VISC - Viscosity (V)
Specify the viscometer reading along with the RPM and the barrel yield if using a Brookfield viscometer, otherwise the seconds of drainage time using a flow tester.
THIX - Thixotrophy (V)
Specify the time until gel, values for two or more flow tests at timed intervals, or the value from your instrument.
200R - 200m Residue (V)
Wash 100 cc of the glaze through a 325 screen, dry and weigh the residue.
Out Bound Links
DFDM - Deflocculation Demand
A comparison of the weights of equal volumes of a given liquid and water. 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 t...
Knowing about thixotropy will enable you to mix a glaze that stays in suspension much better. It does not drip alot when a piece is dipped into it. It goes on evenly and does not run. It dries quickly (on porous bisque) and is just much nicer to use. The secret to all of this is not intuitive. It in...
The term viscosity is used in ceramics most often to refer to the degree of fluidity of a slurry or suspension (the term 'shear' is often used when discussing viscosity, theoretically engineers understand viscosity in terms of layers particles or molecules that exhibit a friction that resists latera...
Understanding the Deflocculation Process in Slip Casting
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.
Brookfield Viscometers for Ceramic Slurries
Malvern Instruments Rheology page
5: BDT - Body Tests
In Bound Links
By Tony Hansen