A company using a rubbery casting slip and depending on outside technical support discovers how simple it is to fix a slip that is drastically wrong using simple testing.
I joined a company that manufactured sanitaryware. It had been built and run by an overseas company who then sold it to a South African holding company. The plant was a conventional one, the slip being prepared in a sliphouse, the clays turned into slops using blungers, and a body slip made up and then filter pressed. The filtercakes were then reconstituted into a casting slip. This was the best way, rather than direct slip making, as the clays were crude and needed to be cleaned up first.
White glaze was produced at the plant, coloured glazes were bought in. Casting was by hand using casting benches and firing was carried out in electric tophat kilns.
When I arrived the slip was like porridge and casting times were long. It was something to watch casters handling separate cast rims, they flexed like rubber in their hands! I immediately had an idea what was wrong but needed to prove it. The original owners had used their overseas technical facilities and so the plant did not have a Lab to speak of. One test was being done - particle size by hydrometer, and that was wrong. So to investigate the slip problem I bought a torsion viscometer, a burette and stand, a small lab stirrer and a stopclock. Then I proceeded to do deflocculation curves on each clay. Of the four clays it became obvious that there were problems with two of them. One of them was a china clay obtained from a small company in the area. While it should have deflocculated easily, instead it gelled to the extent that I could invert the container of clay slip (with added deflocculant) and it stayed in place, not even shaking could remove it! The other clay was supplied by a brickworks and it also thickened rather than thinned as deflocculant was added, but not to the extent of the China clay. By now I definitely knew the problem was that of Montmorillonite in the clays, which causes gelling. I sent the clays for XRD analysis, reminding the laboratory to add glycerol to the them to cause the Monmorillonite to swell on its C-axis (otherwise it might not be detected). It was detected in both of the 2 clays.
The offending clays were replaced and the slip became fluid and the casting rate was much faster and the cast items were hard. A shorter body resulted but the casters soon adapted and the formulation continued to be used for years.
Monmorillonite contains Magnesium in the Brucite layer, I wondered how both of the errant clays, which were essentially kaolinitic, came to have Montmorillonite in them. I am not a Geologist but I reckoned that perhaps there was Magnesium containing rocks near to these clays. This turned out to be true, in the Cape there were such rocks exposed above the China clay deposit and only the clay near to the surface contained Montmorillonite. In addition, the brick clay was in a Dolomitic area.
Out Bound Links
By Peter Clowes