|Monthly Tech-Tip |
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.
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.
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.
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.
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
Glaze Slurry is Difficult to Use or Settling
Understanding glaze slurry rheology is the key to solving problems and creating a suspension that does not settle out, applies well, dries crack free.
Tests conducted on bodies made from materials, as opposed to the materials themselves.