Modeling and Moldmaking
Artists should use sulfur-free plasteline when modeling a piece requiring a rubber mold.
Plasteline containing sulfur is still used by many artists who model pieces meant to be cast in metal, resin or other castable material. Before the advent of RTV (room temperature vulcanizing) rubber some three decades ago, the common practice was to make a waste mold on the artist's plasteline model so that a plaster positive could be taken from the waste mold. A gelatine mold could then be made from the plaster cast so that a wax replica could be cast from the gelatine mold for casting in bronze by the lost-wax method.
Today the waste mold/plaster cast procedure can be skipped entirely,and a rubber mold can be made directly from the artist's plasteline model. However, polyurethane and silicone base rubbers are deathly incompatible and will not cure if they come in contact with the sulfur in the plasteline. The moldmaker must shellac or lacquer the model to place a block between the plasteline and the liquid rubber,a process which does not always work perfectly. Sometimes the protective coating is loosened or made sticky by the water clay blanket the moldmaker applies to the model in making the plaster mother mold, or other times the release agent applied to the model before the moldmaker pours the rubber reacts with the protective coating and lifts it or turns it gummy, producing a tacky or thin uncured surface on the rubber mold. The end result will be a less than perfect surface on the wax cast made from the rubber mold, requiring considerable extra touching up of the wax replica before it can be used for a lost-wax metal casting.
Using sulfur-free plasteline eliminates the need for a protective coating on the plasteline model and avoids the potential problems associated therewith. Sulfur-free plastelines are now widely available and competitively priced with the older formulas of sulfur containing plasteline.
Armatures for models.
The worst nightmare a moldmaker can experience is having the artist/client's model move or break apart before the moldmaker can pour the rubber. An inadequate armature in the model is almost always the reason. If sculptors knew all the stresses and strains their models would be subject to in the moldmaking process, they would pay more attention to constructing a strong armature before starting to build up their model in clay, plasteline, wax or other modeling media. Generally speaking, the molds that insure the most faithful casting of the artist's work in wax, resin, plaster etc., are those in which the moldmaker covers the entire piece with a 1/4 inch blanket of water clay and makes a reinforced plaster mother mold on top of the clay blanket. In opening the mold the water clay blanket sticks to the plaster mother mold and has to be pulled away from the artist's model. It is at this critical stage that ears, fingers, arms or other extended parts may be ripped away from the model if not properly armatured.
Here are some tips to preparing a model for moldmaking.
Mount the main armature solidly on a 3/4 inch thick piece of plywood using standard plumbing fittings that can be found at any hardware store, e.g. screw-down threaded pipe flanges, threaded 3/8 inch or 1/2 inch galvanized pipe, 90 degree els, tees, etc. Use aluminum armature wire for the extensions from the main armature. The wires can be twisted to make them thicker and stronger or twisted together to go from thicker wire to thinner wire, e.g. going from arm armature to hand and fingers armatures. The armature should be continuous, e.g. main armature to back to shoulder to arm to fingers or any other extreme appendage such as ears or tails. Any armature extension , which goes back to the base, such as legs of animals or humans should be stapled to the base so it cannot move. Do not use steel wire for armatures if there is any chance that the moldmaker will have to sever the section in the moldmaking process, i.e., make a separate mold for the part(s). It is difficult to cut an embedded steel wire with a jeweler's saw.
Please make sure that the modeling is tight so that no rubber can get into the interior of the model.
Here are some tips that will save the moldmaker time and material thus reducing the cost to the artist of having a mold made while insuring the best results.
Mold Board
The starting point of modeling is to select a board to build the sculpture on. Many artists will grab any piece of plywood or planking and screw a pipe flange to it for the main armature. The artist should be aware that the moldmaker needs at least 3" of mold board extending beyond the sculpture on all sides for building the mold. If a piece is brought to the moldmaker on a board that does not satisfy this requirement, the moldmaker has to factor into the price the time it will take to correct the deficiency including but not limited to,
a) making a large enough mold board,
b) securing the artist's board to a new mold board.,
c) cutting and fitting strips of wood the same thickness as the artist's board to extend the sides of the artist's board to the 3" clearance requirement,
d) screwing those strips in place,
e) sealing the cracks between the original board and the extension strips, so the liquid rubber will not leak into that space and ultimately,
f) taking the whole assembly apart after the mold is made in order to give the model back to the artist in the same form in which it arrived.
Tip: always use a flat (not warped) base board of 3/4" plywood large enough to leave at least 3" clearance on all sides around the model.
Preventing rubber leaks into the model.
Another moldmaker's nightmare is to mix sufficient rubber to fill the mold and after pouring the rubber and topping up the pouring cup to see the rubber level go down and down, while air bubbles are coming up in the pouring cup. The moldmaker then refills the cup and the level goes down again and air bubbles keep coming. Pretty soon the extra rubber in the mixing bucket is all used up and the pouring cup is still not staying filled. Quickly the mold maker mixes more rubber and keeps pouring rubber into the mold until finally the pouring cup stays filled.
This problem occurs when the rubber leaks into the model, and the bubbles are the result of the air space within the model being displaced with liquid rubber. Sometimes the bubbles will stay on the surface of the mold or in vented air traps in the mold that had already been plugged by the moldmaker when the rubber was poured. These voids will result in imperfections in the rubber mold so that later when a cast is made in the mold, the wax, plaster or resin cast will have "pearls" (little round balls) in various parts of the surface, adding to the work of touching up the cast after demolding.
Worst case scenario - one of the bubbles will lodge in the nose or eyes of the sculpture thus obliterating the most delicate of the artist's modeling work.
Tip: When modeling in clay, plasteline or wax, after building the necessary volume around the armature, make sure the surface is sealed tightly with the modeling material before addressing the final surface texture and details.
Coryat Casting Co. Inc., 3346 Rt. 9G, Rhinebeck, N.Y. 12572
Tel: (845) 876-2553 Fax: (845)876-0783