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.