< PLASTER & GROG
PLASTER & GROG – MIXING AND APPLICATION
Prior to application, grog grits are first dry mixed with plaster of Paris. Dry mixing enables the two investment ingredients to be prepared in advance of wetting and to be completely inter-mixed. The mesh size, and proportion of grog added to the plaster binder varies according to the importance of the layer in reproducing detail. As noted in the introductory pages, finer grades of (grog) material are used for the first layer. The first layer also typically contains a higher proportion of plaster binder than found in either secondary back up coats, or cores. First layers might be expected to be proportioned at about 60% plaster of Paris to 40% fine grog refractory. Secondary layers and cores, which are made up of a coarse refractory body, may be bound by as little as 30% plaster of Paris. The first layer typically contains very few if any extra additions, as added material can reduce the sensitivity of the plaster and grog investment in reproducing surface detail. In contrast, secondary layers, and in particular the outer wall thickness of the mould, may contain substantial quantities of added in material.
Before the investment mix is applied, the wax assembly is treated by washing with a DEGREASING AGENT. The agent is normally a solvent such as methylated spirits or a proprietory alcohol, shellac is also sometimes used. The waxes used in art founding are commonly a product of hydrocarbon oils, degreasing the wax this allows water carried investment to be applied to the wax without surface tension repelling the refractory and inhibiting adhesion.
Plaster and grog investments are usually applied to the wax assembly in two distinct stages. First, the dry investment ‘pre-mix’ is added to a quantity of water, employing the same dispersal technique used for fine plaster mixing (see MASTER PATTERNS). The critical first layer should be reasonably fluid and mobile when applied, though it should not have too high a water content, as this would weaken the mould. The fluid investment is carefully applied to the surface of the wax assembly, taking particular care to ensure the material penetrates into any detail and undercuts on the wax pattern. Application is normally by hand, flicking the investment over wax surfaces, sometimes assisted by gently pushing material into pockets of detail with a soft fine brush, this helps to break up any trapped air pockets. As well as brushing, the founder can blow a gentle jet of compressed air over the freshly coated assembly, this helps disturb any underlying air pockets.
When applying any investment, the founder must work quickly and methodically, concentrating on evenly coating the wax pattern itself, rather than achieving complete coverage of the attachments – pouring cup, runners and risers. The casting attachments are less critical than the wax reproduction and these items can be later covered by layers of secondary investment if necessary. After it’s application, the first layer is allowed to harden over the assembly without being distressed or keyed. Leaving this first deposit undisturbed avoids disrupting it’s contact with the wax surface.
Once the first (PRIMARY) layer of investment has set firm, the founder can build up a further wall thickness to withstand kiln firing and metal pouring. The ‘back up’ (SECONDARY) investment contains a coarser refractory than first layers, and may also contain any additives selected by the founder to improve mould strength and aid gas permeability.
The secondary layer can be built up by mixing the plaster and grog with water, then manually applying the investment by hand over the first layer. More commonly though, the founder forms a restraining wall around the wax assembly using a strong, flexible linoleum or similar sheeting – this sheet is easily be rolled and adjusted to to enclose the partially invested assembly. The linoleum is set up in a way that allows a substantial gap between the extremes of the wax assembly and the side walls of the container. The thickness of investment which eventually fills this gap prevents molten metal from running too close to the outer investment wall and rupturing the mould. The allowance between the outer edges of the wax assembly and the container varies according to the scale of the work, however; it is rarely less than a minimum 1-2 inches (25-50 mm).
Once the temporary container has been set up and secured, a quantity of second layer plaster and grog is dry mixed and then added into water. The investment is then carefully decanted into the linoleum container and over the wax assembly. Again, some care is taken to avoid the trapping of air spaces within the mould, especially in areas lying close to the first layer and underlying wax assembly.
After the poured investment has set firm, the temporary linoleum container can be removed from around the completed mould. Any stray refractory fouling the cup, riser tops, core vents or drains is cleared, and sharp edges on the mould are chamfered off, with any loose debris brushed away. The wax assembly is now completely encased within a green plaster and grog BLOCK MOULD – so called because of it’s regular cylindrical or ‘block’ like appearance. The pouring cup and riser tops are exposed on the uppermost face of the block mould. In some cases the founder will further reinforce the investment by binding the block by wrapping around a medium gauge metal wire, though if this is done care is taken not to initiate cracks in the mould. The investment mould can now be set aside to air dry prior to kiln burnout and firing
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