Fettle and finish.
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1. Even with a new mould, some seepage may occur between mould parts. It is important to clean the edges of the plaster mould carefully, so that the seepage is not encouraged with the next cast. |  |
2. Fettling is the action of removing the flashing. Scrape rather than cut, use a modelling tool to push the flashing down before sponging. Rinse the sponge often and keep changing the water in the bowl. |
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3. The action of casting slip encourages the platelets of clay to lie flat, parallel to the inside surface of the mould. With a wide gap, the platelets bend themselve around the corner. |
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4. The water has been fired off, and the platelets compressed. This pushes the 'invisible' break line above the surface again.
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5. This illustrates one reason why a mould has to be discarded after a certain number of casts have been taken from it. It gets so worn on the corners that it is impossible to disguise the break-line. When deciding on the break-lines right from the very start of modelling, use where possible natural ridges, such as backbones, veins, and the edges of ears. | 6. The other reason that moulds have to be turned into garden paths is that they become saturated with the deflocculant alkalies from the casting slip. Other impurities in the slip, such as brown 'livering' which is oxidisation, coat the inside surface of the mould and prevent the plaster from absorbing the water from the slip. |
There is more on Shrinkage on the next page. Or back to Step Seven.