What recipe should I use for making a casting slip from a powdered clay?
Casting slip recipes vary depending on the clay, we recommend the following as a good starting point but be prepared to adjust to ensure the correct weight and fluidity is achieved.
- 25kg powdered clay
- 30-75g 140°tw, or 35-87.5g 120°tw sodium silicate
- 30g soda ash
- 10lt water
The clay should be added to the water and mixed using a blunger or mixer such as our Five Gallon Mixer ref: 7061-01. https://www.potclays.co.uk/machinery-equipment/machinery-equipment-mixers/five-gallon-mixer-(supplied-with-heavy-duty-plastic-tub)
Hold back some of the water to avoid adding too much at first. Also be cautious with the sodium silicate, it must be added slowly and given time to work as it can take 10 minutes for its effect to take effect. Sodium silicate is a deflocculant, it allows the clay particles to be compressed without sticking together forming lumps, it holds the clay in suspension and adjusts the fluidity of the slip. If too much sodium silicate is added the mix will become over-deflocculated becoming very thick and impossible to use, it cannot usually be reclaimed. Once made, the fluidity of the slip may be adjusted and fine tuned with dispex which is a much weaker de-flocculant so will not cause over-deflocculation.
To check the correct mix of your slip, weigh a litre, this will give you the relative density, for example a litre of slip weighing 1.70kgs has a density of 1.7. You can use this to replicate mixtures in the future. Additions of clay will increase the density and additions of water will decrease it. The more clay in the mix the more de-flocculant will be needed to maintain fluidity.
It is worth testing a new batch of slip before going into production
