WHAT IS IN THE SLIP THAT MAKES CASTING SUCCESSFUL?

A slip must be in a balanced colloidal state. This colloid suspension, in which clay, organic matters and silica are dispersed evenly within a water medium, is achieved by using chemicals called 'deflocculants'. This allows the slip to flow freely during casting, even when the viscosity is relatively high.

Ball clays offer a balanced colloid suspension due to their content of organic matters and free silica. They also have finer clay particles than kaolins. This is why ball clays are generally preferred in casting slip mixtures.

However, everything comes with its advantages and disadvantages in ceramics. Ball clays fire dark cream in colour. If you aim to prepare a white porcelain body, then you are restricted in how much ball clay you can use.

Choosing a better clay for the purpose of casting is not difficult. As already mentioned, the aim is to create a colloidal suspension in the slip. The clay’s response to the increased amount of deflocculant is crucial and this can be found out by a simple test.

Take a single test clay 50% in the mixture and add 25% felspar or nepheline syenite and 25% silica (under 200 mesh is OK). For the test, the amount of water is not too critical. So you can take 40% water and 60% dry material or prepare the slip as you normally do. I recommend these steps below;
1. Weigh the material (50% clays + 50% non-clays) and make a dry mixture.
2. Weigh the water and put into a bucket (40% water of total slip weight, ie. 600 g material + 400 g water)
3. Dissolve 0.1% Soda Ash (based on the total dry material,ie. 0.6 g for 600 g material) in the water.

Thick Paste


4. Add all the dry material to the water and mix till you get a thick paste
(within 24 hours, this paste becomes a thick slip but you do not have to wait that long).
5. A couple of hours later, start adding increasing amounts of Sodium Silicate (Water Glass). Add 0.1% of Sodium Silicate to the slip at a time, mixing well each time and observe the change in viscosity - the slip should becomes thinner with every addition. Keep adding until you have mixed a total of 0.4% Sodium Silicate (on dry base) into the slip.

Thinner Slip


It may be a problem weighing amounts under 1% when the batch is small (for instance, the Sodium Silicate in the example test above). To weigh the deflocculant accurately, a 10% solution of the deflocculant may be prepared in advance (10 g Soda Ash or Sodium Silicate + 90 g water in solution). If you then take and add 10g of this solution you know you are adding 1g of Sodium silicate.

When you do the trials for different clays and kaolins, you will observe that some clays respond to the chemicals better and their slip gets thinner than the others with a similar amount of Water Glass (Sodium Silicate) addition. So, the clays can be ranked according to their response to deflocculants.

In practice, we prefer not to work with either very thin or very thick slips. If it is thin, casting time will be unneccesarily long. If it is thick, pouring the slip in and out of the mold will be difficult.

With experimentation you may soon find that using kaolin and clay together in a slip mixture can improve control over the slip casting process.

WHAT DOES SUCCESSFUL SLIP CASTING MEAN?

Successful casting means that the slip should
- be cast in a short period of time (so that there is quicker wall formation against the inner surface of the mold)
- ensure the optimum cast quality of the wall (not too hard and not too soft)
- be poured out of the mold by leaving a smooth surface of inner wall (casting surface).



A good slip must
- be stable (should not get too thick after slip preparation and during casting - while the slip is in the plaster mold),
- have the highest slip density possible for a given mixture (the lowest ratio of water to dry material).




The cast pieces should
- be easily released from the mold,
- dry quickly and be crack/distortion (warpage) free,
- have enough dry strength.

In addition, the temperature of the slip and the mold should not be extreme.

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.