Characteristics of Casting Slips
By Vikki Liro
Are you interested in creating your own ceramic pieces from scratch? Then it’s time to learn about casting slip, the liquid clay mix used for producing molded ceramic pieces.
In slip casting, the mold is made from a porous material like plaster. Casting slip is typically one half clay and one half fillers but the characteristics of casting slip and the resulting ceramic piece can vary widely depending upon on the clay used in the mix.
In slip casting, the mold is made from a porous material like plaster. Casting slip is typically one half clay and one half fillers but the characteristics of casting slip and the resulting ceramic piece can vary widely depending upon on the clay used in the mix.
Critical to the success of slip casting is the concept of deflocculation, which is the chemical process that allows the clay to remain suspended in the slip mixture. Typical deflocculants are soda ash or sodium silicate. Without the proper amount of deflocculants for the type of clay being used, the resulting piece may not release from the mold or at the other extreme may shrink excessively and become brittle. Another problem with an incorrect mix of clay and deflocculant is that the casting slip will gel too fast.
The route to success in slip casting is to marry creativity in design and texture with a practical knowledge of the chemistry of various clays. Quite simply, over the centuries various regions of the world have become famous for pottery and ceramic wares due to their proximity to clay sources with different chemical compositions. So, for example, Polish pottery from the Boleslawiec region that is regaining world renown today relies on local sources of white stoneware clay high in molochite content and fine white sand.
In general, porcelain white clays are preferred for use in casting slip because they combine well with deflocculant compounds. Ceramic engineers have the complex task of chemical analysis of clays and determining the chemical properties of the packaged clay products that are available on the market. Then, the ceramic artist must be able to understand the chemical composition of these clays in order to produce the creative ceramic artwork.
The technical data sheet that accompanies a clay product provides essential information about how well the clay will deflocculate. Generally, one of the goals in producing slip cast greenware is to create a strong piece that can withstand handling during the finishing process. The size of the individual clay particles within the casting slip influences the strength of the resulting greenware and a variety of particle sizes that bond together is required for a strong product.
The technical data sheet for clay should contain particle size information typically expressed in microns. The data sheet indicates a percentage of particles below 5 microns and the percentage below 0.5 microns. A low percentage of fine particles translates to a coarser clay and a coarse clay in turn is known to release water easily. Thus, coarse clay is preferred for use in casting slip.
Another important piece of information on the technical data sheet is the exchange rate. Without a review of high school chemistry and positive and negative ions, suffice it to say that the exchange rate is an indicator of the chemical reactivity between the clay and deflocculating materials. Contrary to intuition, clays with a lower exchange rate are preferred for casting slip. To help solidify this information, red earthenware clay which is good for pottery but unsuitable for casting slip may have an exchange rate of 22.0 while a porcelain clay like kaolin (China clay) has an exchange rate of just 4.5.
While you may concentrate on getting the right mixture of clay and deflocculant for your prize artistic project, remember that water is also a key ingredient in casting slip. Water from different sources also possesses different chemical compositions. Much of the country deals with hard water with a high mineral content. The mineral content of the water can influence how well your casting slip deflocculates. Deionized water is preferred for use in casting slip. Deionized water is devoid of harsh chemicals but still retains organic matter.
