Page 110 - QRedition5.0 062021

Quality Requirements Edition 5.0 European Enamel Association

8.5.3.2 Examination of test sieve residue 8.5.4 The flow characteristics of enamel slip (slurry)

Purpose and scope Elucidation
In order to reduce the share of defects in enamelling, it is recommendable to investigate Enamel slips do belong to the muds and are in the widest meaning not a Newtonian liquid.
the collected sieve residue, that is left over from the test of fineness of grinding, thoroughly. For the most part, muds are colloidal systems i.e. one phase is very fine distributed but
Such residue delivers references to mill damage and other dragged-in contamination. embedded in another phase. The media can be in a solid, liquid or gaseous state. Colloids
are not in a thermodynamic equilibrium. One of their most important properties is, that their

After the test of fineness of grinding, the sieve residue is in the measuring tube. Pour a part internal surface is much larger than their external one. Systems with solid state particles
of the supernatant water, shake and mix the sieve residue with the rest of the water and embedded in liquids are called suspensions. Independence of the size of the solid state
pour it onto a filter paper or white cloth. Should the sieve residue consist not only of enamel particles, one has to differentiate between coarse dispersive, colloid dispersive and
particles but also of other materials, then further measures are recommended to find out molecular dispersive systems. The transitions are floating and are numerically not defined.
which foreign matter it is and from where it originates. White, very hard particles are mostly If in a colloidal system, the gaps between the solid state particles are repleted with liquid
porcelain or steatite caused by broken milling balls or mill lining. Smudgy or sticky, greyish and are then getting smaller than their average diameter, the system solidifies; the liquid
looking pellets which can be crushed, are clay minerals. In this case, after the mill was suspension becomes a gel. The viscosity of muds mainly depends on the velocity gradient,
loaded, clay formed a big clot in the mill, not being wetted by the mill water. Also particles the flow duration and pressure. In aqueous solutions temperature only plays a
of rust are no rarity. Their origin can be of various kinds. In any case, one should search subordinated role and is sufficiently described as room temperature, quite different to oil
for the sources of contamination, because black, grey or white specks in the enamelling emulsions. In this case, the functional relationship of the oil temperature strongly influences
often arise from contaminated slips. In most of these cases these contamination can be the rheological properties.
recognized in the sieve residue.
Enamel slips are intrinsic viscous liquids with a flow limit and superimposed thixotropy.

To make sure, that the sieve residue is not causing defects in the enamelling, take a bigger They consist of a mixture of ground frits, water and additions like clay, quartz, colouring
quantity of slip, screen it and stir about 5 - 10 g of this residue into 100 ml of slip in use. oxides, setting agents and substances which subsequently influence the dry enamel layer.
Test plates enamelled with this mixture will indicate the defects which can derive from the

sieve residue. Various demands are imposed upon the flow behaviour of the enamel slip. It is supposed to
be relative viscous, it is not allowed to settle and must have an easy workability for
application. It must be uniformly distributed on the article. The applied wet layer should not
move when transported. The dried biscuit must be free of cracks and has to have enough
biscuit strength to withstand the transfer onto the firing chain without any damage.

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