Some older houses have plaster moldings and ornaments (also called enrichments) on the walls and ceilings. Homeowners who live in these homes are sometimes faced with the problem of repairing or restoring these beautiful decorations when they crack or break.
Restoring adornments, like long runs, surround moldings, cornices or large medallions, is generally a job for a professional with extensive restoration experience. It is possible for the average do-it-yourselfer to shape some pieces (about 3 feet long or less) in the workshop and glue them in place on the wall. The technique is not especially difficult, but it does require patience, attention to detail and a certain amount of trial and error. The process consists of four steps: preparing the model, making the mold, pouring the plaster and mounting the enrichment in place. The model is the piece to be copied. It may be either an existing plaster ornament or a new piece modeled in clay or carved in wood. If the model is loose, that is, not attached to a wall, then you can do the entire casting process on a workbench. The first step is to mount the model securely on a flat surface, called a bearing. For a bearing, you can use a piece of scrap plywood that extends at least 6 inches beyond the perimeter of the model. Glue the model to the bearing with carpenter's wood glue.Next, coat the model and the surrounding bearing surface with a parting agent. The parting agent is a liquid coating that keeps the mold from adhering to the surface of the model. You can use liquid hand soap or dish detergent. Both of these are effective parting agents and are easy to obtain. Brush the agent on with a soft-bristle brush so that it coats the entire surface of the piece, particularly the recesses and cavities. Now build a temporary fence around the model with four pieces of 1-by-2 wood. (Note: the fence should be higher than the model. For a model in high relief, it may be necessary to use 1-by-3 or 1-by-4 wood.) The fence is basically a rough frame that will hold the molding liquid until it sets. Seal the bottom edge of the fence with modeling clay; this will keep the molding liquid from seeping out.
You're ready to pour the mold. Traditional artisans made molds of gelatin, but this has largely been replaced by modern mold-making rubber materials. These include silicon, polysulfide, latex rubber and polyurethane. These materials are usually not available at local home centers. Sometimes you can buy them at large art supply stores or sculpture suppliers. You can also order them by mail from Smooth-On Inc., 1000 Valley Road, Dept. OHJ, Gillette, N.J. 07933.
Silicon is very expensive. Polysulfide is useful because it can be brushed on. This is important if you're trying to duplicate an enrichment that is part of a wall decoration. In this case, it's not necessary to mount the model on a bearing or build a fence around it. Simply brush on suc-ces-sive layers (sometimes as many as 20 layers are required) until the mold is at least one-eighth-inch thick. Polysulfide molds do not keep their shape for very long, however, so it's necessary to keep a cast in the mold at all times.
Latex rubber can be brushed on or poured, but the mold tends to shrink when it dries, so the finished cast will be slightly smaller than the original.
Polyurethane is probably the best choice for mold material. It comes in a two-part formulation. Mix the two parts together (follow the manufacturers' directions as to proportions) and pour it in a smooth steady stream over the model. The mold should cover the entire model by about one-eighth to one-quarter inch. Allow the mold to dry; this takes about 16 hours, then peel it away from the model. Now you can pour liquid plaster into the mold to make the cast. It's best to use casting plaster for this. Casting plaster can reproduce intricate details because it's made up of fine gypsum particles. It also contains starch that helps create a hard, durable surface.
Mix the plaster according to the package directions. But before pouring it, use a plant mister to moisten the mold with water. It is not necessary to use a parting agent. Now pour the plaster into the mold. Use a small brush or a narrow stick to work the plaster into all the nooks and crannies. Reinforce a large cast by cutting a piece of crinoline (available at fabric stores) and embedding it in the wet plaster. Before the plaster hardens, draw a straight edge across the surface to level it.
Allow the cast sufficient time to harden - this will vary with the thickness of the piece - then peel away the mold.
Before mounting the cast on a plaster wall, score the back by scraping it with a nail or an awl. Do the same to the wall surface. This roughened texture will give the surfaces a "tooth" that the adhesive will be able to grip. You can use ordinary gypsum plaster (plaster of Paris) as an adhesive. Mix the plaster to the consistency of thick cream and spread it on the back of the cast. Press the cast onto the wall and hold it for a minute or two until the plaster-glue sets.