< WORKING WITH PATTERNS
CORES
Once the slushed or painted on wax thickness has hardened in the reproduction mould, the founder can prepare the newly formed wax pattern for removal, or STRIPPING, from the rubber skin. At what point this is done depends to some extent upon the investment system used by the founder. Investment systems are discussed in some detail in the following section INVESTMENT, suffice to say here that there are two systems of particular interest here, the PLASTER & GROG system and the CERAMIC SHELL system.
To create a hollow casting of a hollow wax pattern using the plaster and grog system, the founder must first insert a refractory core into the wax pattern’s hollow cavity. This core is usually composed of the same, or a very similar combination of materials used to create the outer investment. The core is mixed in a similar fashion to plaster of Paris mixing, then poured into the wax pattern’s cavity and allowed to set as a solid refractory mass. Later, more investment material is applied to the outer surfaces of the wax, thus creating a sandwich (the wax now layered between the outer refractory and inner core). When the wax is melted out an air gap is created that allows metal to be run into the mould and then solidify as a hollow cast. If the core is omitted, metal will fill the mould and solidify as a heavy mass rather than a thin walled cast.
One of the problems with introducing a fluid plaster and grog core, is the pressure the fluid material excepts on the wax as it fills it’s cavity. In all but the smallest of works, a poured core can easily rupture or distort the relatively fragile wax pattern, ruining the founder’s earlier work. One obvious way of preventing this is to pour the core into the wax whilst it is still contained in and therefore supported by the rubber reproduction mould and case.
When working with smaller sculptures, the founder can simply prepare a core mixture and pour it into the wax via the same mould entrance used for wax slushing. A clay funnel can help direct the flow of core material into the mould cavity, the funnel also allows the core to be extended out of the cavity, this extension can later be used as a stand to support the wax whilst it is being retouched. Tapping the mould lightly helps dislodge any trapped air pockets created by the entering core mixture.
Larger cores often an internal steel armature inserted. If access is easy and the mould is a simple volume, the founder can insert a basic armature structure into the poured core before it sets. If the mould is more complicated the founder might set the armature in place before the core is poured. This can be done by bending steel bars into shape, then securing each in position on a small mound of core mix. Whatever method is used, it is essential that no part of the core armature touches the internal surface of the wax pattern, if it did touch, the armature might misdirect poured metal into the core mass and cause casting faults.
EXTERNAL CORE ARMATURES > |
