WHY DOES THE SLIP BECOME THICKER?

1. WHERE THERE IS NO WATER LOSS: Even if unused slip is stored in a sealed bucket, thixotropy (increase in viscosity in time without loss of water) takes place and as a result, the slip becomes thicker and pouring can be a problem. You may observe that some parts of the slip start shifting over other parts rather than flowing evenly.

Thixotropy is, in fact, desired in the slip casting process because it speeds up the formation of a wall against the inner surface of the plaster mold. The level of thixotropy is an important factor in casting quality, which can be adjusted by the amount of deflocculants (mainly sodium bearing chemicals) in the slip.

As soon as a slip is prepared, thixotropy starts building up and the slip becomes thicker and thicker over time depending on the amount of deflocculants in the slip. Therefore, measuring thixotropy after 5,10 and15 minutes is generally the standard test in the industry.

Remedy:
When thixotropy takes place, the slip can be turned back to its original condition by shaking or stirring it vigorously. Reaction is immediate].

2. WHERE THERE IS WATER LOSS: Various factors such as casting on a hot day or in a warm environment, or not covering the slip properly for a period of time, may lead to some water loss. This will mean the viscosity (resistance of a liquid against flow) of the slip will increase (become thick).

Remedy:
In the case of water loss, slip viscosity can be reduced by adding a small amount of water and mixing well.

3. WHERE THERE IS CONTAMINATION FROM THE MOLD: If you use a batch of slip over and over again or you go to use some left over slip that has been stored for a while, the flow properties of the slip may change dramatically. The level of thixotropy will be even higher due to contamination and slip viscosity can not be re-adjusted by simply stirring.



The slip casting molds are made from plaster of Paris, which is Calcium Sulfate + Water in formula. While pouring the slip back, the slip can easily come into contact with plaster (see photo above) and some plaster dust or small particles may drop into the slip bucket and contaminate the slip. When this occurs, calcium and sulfate ions act as strong flocculants (forcing solid particles to flock together rather than flow individually) and increase viscosity as well as thixotropy.

In addition to that, water absorbed by plaster during the casting and drying of a mold may carry some sulfate salts (white scum) to the surface of the mold (see photo below). If not removed effectively, these salts may eventually mix with the slip. Therefore, it is always recommended to clean the mold surfaces with a damp sponge before casting to avoid contamination.



Remedy:
In order to reverse the effect of the calcium and sulfate ions and to make the slip thinner again, a small amount of diluted deflocculant (sodium silicate or other deflocculants, but not soda ash) must be added to the casting slip. If this does not work very well, then some water may be added, too.
If you can, try to collect the used slip (poured out of molds) in another bucket. Do not mix newly prepared/unused slip with the already used one. Then add some diluted sodium silicate and a small amount of water (if necessary) to re-adjust the viscosity of old/used slip.