How To: Casting Historic Plaster Ornaments
Yale University, Sprague Hall
Cast plasterwork has been a traditional ornamentation used to enhance interiors for hundreds of years. Originally, plaster was cased using stiff, inflexible molds which required multiple steps to loosen the finished plaster ornament. Unfortunately, this often resulted in damage to the finished plaster piece. New technology has been available that enables flexible molds to be crafted from rubber. These molds are far easier to work with and distinct advantages over carved wood or metal casts, used historically.
Creating a Mold from Original Ornament
If you are lucky enough to have original plaster ornament, then the process can progress faster. If you do not have an original, or an original cannot be freed from the surface without causing potential damage, then additional steps will have to be taken. We will discuss these later.
1) Create a mold shape: identify the size of the ornament you plan to cast. Create the shape of the mold by using wood: make sure the form will hold and interior seams are sealed with either glue, wax, caulk or similar. Ensure that the mold will completely cover the object, allowing “extra space” on each side; one-inch is a good rule. This will provide additional support to the rubber, allowing it to be reused without fear it will crack, or completely split.
2) Secure the mold: make sure that the mold is completely secure on all sides. When rubber is poured in during subsequent steps, it will quickly and evenly spread out, potentially spilling from unsecured cracks in the corner.
3) Secure the ornament to the bottom of the mold: it is extremely important that the original object you are duplicating be secured to the bottom of the mold. When the casting liquid is poured in, it is not uncommon for unsecured objects to rise within the medium, completely encasing them; thus, ruining the mold.
Examples of typical mold making products and release agents, manufactured by Smooth-On.
4) Apply a releasing agent to the object: also extremely important is the application of a releasing agent to the object. Manufactures offer various aerosols and brush applied agents. However, baking shortening oil and petroleum jelly are also excellent substitutes. Without a releasing agent, the rubber mold may not release from the object, thus wasting time, materials and destroying the original object.
5) Mix casting agent: based on the size and composition of the original ornament, some casting rubber molding agents are better than others. Read all instructions prior to mixing and poring. This can be messy business, so use old clothing or a coverall. The rubber is very sticky and is difficult to remove from surfaces. Also, wear durable latex gloves (or similar).
6) Pour mold agent: add the rubber molding agent to the object. Pour slowly and evenly. Fill in circular motions and stop occasionally to shake out and possible air bubbles that might have formed. Ensure that you are pouring enough of the rubber that the original ornament is completely covered and all the nooks and crannies are completely filled. Tap the outside of the box to force any air bubbles to the surface.
7) Removal of original ornament: once the molding agent has dried completely (follow all instructions) remove the outer form material and very carefully free the original ornament from the interior of the mold.
8) Plaster pour: once the mold has been inspected and is ready for use, make sure it is sprayed with the proper release agent prior to adding any plaster. The release agent should be appropriate for both the plaster mix and the mold composition. Pour the plaster into the mold; often shaking to ensure that no air bubbles are present in the plaster. The plaster mix should be loose and pourable having the viscosity of paint, as a reference.
Examples of cast plaster ornament.
9) Plaster removal: carefully remove (demold) the new casting after it has dried (typically 1 hour). If you properly prepped the surface of the mold and are using a flexible rubber mold, the ornament should prop right out.
10) Touch-up mold: while the casting material is excellent for recreating the shape and general features of the original ornament, touch-up work might need to be done to ensure all the features of the original are accurately highlighted. Air bubbles are usually the main cause of imperfections. These touchups can be done by applying wet plaster to the surface and filling in voids.
Missing ornamental plaster. (left) Reinstalled, replicated ornament. (middle) Completed restoration. (right)
Hartford Judicial Courthouse
Re-Creating Ornamental Plaster without Original Ornament:
While the previously listed steps will have to be followed, if no original ornament exists or one cannot be removed from the flat plaster surface, then an exact duplicate can be carved and inserted in its place. This can be done with historic photographs or original architectural drawings.
Understanding Design & Materials
These steps are an abridged version of those taken by professionals but accurately characterize the processes undertaken during ornamentation duplication.
Replicating historic plaster ornamentation is a difficult undertaking, requiring great experience in architectural history and design to achieve professional results. Similarly, a specific understanding of polymer and plastic chemistry is needed to ensure that the mold and its contents are released without a hitch.
This is just one process for replicating and restoring ornamental plaster. Learn more about our process and plaster restoration services here.