This photo shows how ordinary white earthenware plastic clay together with other dry ingredients is made fluid by the addition of a small measured proportion of water and two soluble alkalies known as deflocculants.

There is further information on how to make solid clay flow like cream on a related page: clay slip.

In his book, "Mold Making for Ceramics" (ISBN 0-8019-7359-7) Donald Frith describes in detail the chemical and practical processes of making and using clay slip in a manufacturing environment (in America).

The void inside the Shire mould takes approximately six pints of slip. Slip is nearly twice the weight of water. One pint of slip weighs 35 ounces. Therefore the slip in the mould weighs about 13 pounds. This weight of slip needs to be kept securely inside the mould otherwise there would be a mess all over the floor. Instead of strong rubber bands (cut from lorry tyres) that I use on smaller moulds, here I have strapped the mould up tight with adjustable luggage straps.

The mould weighs more than 60 lbs (I am not sure quite how much more as it is more than all the calibrated weights I have). With the slip inside it is too heavy for me to lift to pour out the slip that is still liquid on the inside. I rotate it clockwise so that I can control the angle. The filling hole which is in the top right corner is then over an empty bucket beneath the slats of the casting bench. The slip must pour out at the same rate as air is allowed back inside. If I did not blow air in through a tube, the suction as the slip pours out creates a vacuum that can (and often has done) collapse the soft clay lining thus pulling it away from the plaster mould.