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You now have half the mold completed and must wait until the plaster sets before you pour the other half. When it hardens (usually in about an hour), coat the entire surface and inside of the box with petroleum jelly or a heavy oil. After this, mix more plaster of Paris or whatever substance you are using to make the mold. Then fill the box to the top with this mixture. After this second pouring of plaster sets, in an hour or so, you break apart the box holding the cast. Then separate the two halves of the mold with a knife blade, as shown in Fig. 89. This must be done with care in order not to break off any section of the plaster mold. You'll notice a line indicating where the two halves meet and by slowly working a knife blade between them at several points you can usually separate them with no trouble. The original pattern will be found adhering to one of the halves, and this must be worked loose very carefully. You will then have two damp plaster halves as shown in Fig. 90. Set them aside to dry and season for about two weeks. This is important because if you try to pour hot metal into a damp mold it will spatter all over and will also crack the mold. Figure 89. Using knife to separate halves of mold.
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