[ Home ] [ Up ] [ auscraft ] [ no.10 ] [ cb1 ] [ limoges ]

BASIC PREPARATION OF CASTING SLIP
It is most important to have a good quality casting slip for economic production. To consistently achieve this quality, accurate measurements of deflocculents, materials, water and specific gravity (litre weight) of slip are necessary. Slip making is effected by changes in slip and air temperature, humidity, soluble salts in the water supply and/or raw materials. In order to produce casting slip easily, these variations need to be understood.
In our recipes, a range of quantities is given. This is to allow for differences in both water supplies and soluble salts. We recommend that you start with the minimum quantities, and if adjustments are required, slowly and carefully add the balance until the desired slip consistency is attained. On aging, if the slip thickens, even after having being agitated,
1. Check and if necessary adjust the litre weight.
2. Add small amounts of Dispex - NOT sodium silicate - to restore fluidity.
Stir well after each addition.
MIXING METHODS
Weigh or measure out the required amount of clay and deflocculents. Blend deflocculents into equivalent  amount of hot water, then mix into the measured amount of water for total batch. Then add the clay slowly and blend to a smooth mix, adding deflocculents as required and as explained above. Then slip should then be sieved through at least an 80 mesh sieve before use.
Deflocculants
The amounts of Sodium Silicate or Dispex added to a casting slip are very critical and too much or too little may each result in the slip being too thick. If this happens, take a 1 litre sample of the slip and add one or two drops of Dispex. If the slip becomes thinner after stirring then this shows that more is required to achieve good fluidity. If the slip becomes thicker then this shows that too much has been added already. Therefore more clay and water needs to be mixed into the slip in the same proportion as given in the casting recipe – no more Sodium Silicate or Dispex. The recipe can depend on the quality of the water supply, which may vary from one location to another and also may be affected by minor variations in the body’s raw materials.
Maturing the slip
On standing overnight the slip may thicken up slightly. This will easily reblend when mixing is resumed, even by hand. We recommend that at the end of the day's production a thin film of water, approximately 2mm thick, is placed on top of the slip. This will reduce water evaporating from the mix, especially in warmer weather. Simply mix again the following day.
Litre Weight Checks
The most important part of successfully mixing casting slips is getting the correct litre weight. For accurate and easy litre weight measurement contact Walker Ceramics for a Litre Weight Bottle and chart.
Recycling Scrap
Great care is needed to avoid contamination, particularly from plaster moulds. Scrap should be kept in sealed containers in a plastic condition. Dry scrap will promote air inclusion in the slip. Recycle by adding a maximum of 20% of scrap. More than 20% can effect the quality of the slip. We recommend adding Barium Carbonate (BaCO₃) to the slip to remove sulphates, which may cause peeling faults. The amount of Barium Carbonate to add should be 0.1% (25gm per 25 kg bag) of the total dry weight of clay and scrap.
Faults and Remedies
Everybody who casts suffers from time to time from some form of casting trouble. Experience throughout the industry, combined with the extensive work done on casting slips in our laboratory has enabled us to connect the various faults with measurable properties of the slip.
Of course, other factors besides slip properties can cause casting faults, but usually they can be easily noticed; for example, wet moulds causing slow casting, careless filling causing 'pinholes' or 'casting-spot', etc.,
The table shown gives a brief description of each of the common troubles and the suggested remedies. This only gives the direction in which to move and it is up to the individual to determine how far they need to go.
Say you were suffering from flabbiness and you increased the dispersal addition to correct this then found that you began to get brittle ware with casting spot, then you would know you had gone too far. The following table will help you in recognizing and fixing common problems. Thixotropy is the property of slips becoming thicker when they are at rest i.e. "thixotropy too high" means that the slip thickens up very quickly!