|Monthly Tech-Tip |
We use EPK in production clay bodies and sell it to customers for their use in glazes and bodies. In the past we just accepted materials as they arrived, assuming quality and equality with previous shipments. But times have changed, large companies have gobbled up the smaller ones and suitability and consistency for ceramics are no longer a focus for them. Even where materials are meant for ceramics we are not convinced that suppliers even measure important properties like plasticity, drying shrinkage, fired maturity, fired color, etc. Sometimes we measure these things just to be able to judge a material's suitability for us, other times we have to react and adjust recipes in which it is used (or warn customers of changes).
A shipment EP Kaolin has arrived for use in some of our production porcelain and stoneware bodies. Of course, this needs to be tested before being put into product. But how? The first step is to create a new recipe record in my Insight-Live account, and find their production date code stamp on the bag. Hmmm. It does not have one! OK, then I need to record the date on which we received it. We need to save a bag on every pallet and sieve 50 grams through 100 mesh (to spot contamination). Then we'll make test bars (of all the samples mixed) to fire across a range of temperatures (to compare fired maturity with past shipments). We do a drying performance disk also to assess soluble salts.
We are using the 80, 100, 150 and 200 mesh screens in our version of the SIEV test. In this wet screen particle size distribution test we will wash the material through them (while fine powders like this do not pass sieves that have microscopic openings water will do it no problem).
Letting the powder slake in a generous amount of water first creates a thin slurry that will pour through the sieves instantly. Since the material has a very fine particle size only a few will be trapped by the sieves.
When doing a seive analysis we first wash it all through the finest sieve to remove all minus 200 mesh material. If this is not done the bottom screens can blind (and the whole stack fills up with water).
To recover the material on the screen we wash it backwards into a container (turning the sieve to catch it all).
The recovered oversize from the 200 mesh screen is being washed through the whole stack (from coarsest to finest testing sieves). For each, we wash to material together for easier removal and weighting when dry.
Set them on edge like this (on a plaster table in this case) speeds drying.
EPK slurry is sticky and dewaters slowly. Very slowly. This is actually a signature aspect of its utility and something we take note of for different batches. After a couple of hours water will be low enough to be able to knead the material.
I have rolled it to 3/8 in thickness and placed it on a 11.5cm wide board (as specified by the SHAB test procedure). EPK is plastic enough that the slab can be handled without danger of it falling apart. And it can be cut without issues of tearing of the edges.
I have cut bars for an SHAB test, they are 1 inch wide (for firing to cone 7, 8, 9, 10 and 10R). A smaller half-length bar for the LDW test has been made and weighed. The disk is 12cm in diameter and 3/16in thick and used for the DFAC test.
The 5 bars have a 10cm length marker than can now be measured and numbers entered into Insight-live (for the SHAB test);. The smaller bar for the LDW test can now be weighed (and that entered also). The disk has curled (because EPK has a high drying shrinkage), the nature of the crack can be assessed and its value entered for the DFAC test.
A 10cm mark was made in the plastic bar during preparation (we dip the ends of the marker-tool into talc powder, as a parting agent, to get a crisp mark). These bars are made from the same clay, a shipment of EPK kaolin, each bar is measured dry, fired at its assigned temperature and measured again (to supply data for the SHAB test). As fired length numbers become available and are entered into the recipe record, Insight-live is able to calculate and display more of the drying and fired shrinkages. Values for the DFAC test disk and the LDW test bar can also be assessed an entered at this time.
This shipment of light EPK (there are two versions, light and dark) is just barely plastic enough to make this cylinder with a slight belly. Notice the splits on the rim, these are indicative of the shortness (low plasticity). The cohesiveness of the material, as much as plasticity, gives it a toughness to survive the stresses of throwing (an unique aspect of EPK).
All of the test bars have been fired in this project to characterize a shipment of EPK (from cone 7-10 and 10R bottom to top). The dry and fired lengths and the dry and fired weights testdata was measured and entered into our Insight-live account. The results are shown in the red-titled columns (the drying shrinkage, firing shrinkage, absorption). The weights from the sieve analysis are also shown. And the calculation of water content of the plastic material (a very high 33.4%) and LOI (14.5%). We also like to take and upload a photo of the oversize material from the sieve analyses (to compare with past shipments) and of their manufacturers certificate of analysis.
Notice the data that the manufacturer considers important to include on the certificate that they send with the shipment: Particle size, particle surface area and moisture content. Of course, they would not be able to do all the tests we do. But our product is affected by the properties of the material, so we have to test.
A kaolin that gels slurries (thus handy to suspend ceramic glazes). It is plastic and fires white enough that it is also valuable in porcelain bodies.
|By Tony Hansen|
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