The Chicago Cultural Center ceiling has lost one of it's historic ceiling rosettes and it's my job to reproduce and replace it. You can see the empty panel in the photo, along with the existing rosette I will use to make the replacement. The rosette was carefully removed from the ceiling and cut into it's two principle pieces.
Step 1: Molding the Base
The larger base section of the rosette is cleaned and temporarily affixed to a piece plywood. I then sealed the center hole and area around the base flower with molding plaster. Any damage to the original should be repaired at this step. Next I applied a release agent (I use Mann Ease Release 200 spray) to make the unmolding easier, especially where there is new porous plaster.
The flower is then covered with three coats of silicone rubber. I use Smooth-On Brush On brand. It is a simple one part A to one part B mix (by volume, no scales necessary to measure) with about a twenty minute working "pot life". The first coat of rubber should be brushed on thin enough for you to get in all the little nooks and crannies of the flower, being careful not to miss spots or trap any air bubbles. Any air bubbles now will be bumps in the new reproduced flower later. This layer of rubber must be allowed to set until firm.The subsequent layers may be applied heavier without as much caution. After the third coat of rubber you should have 3/8" to 1/2" of rubber over the entire surface. . All undercuts and crevices on the original should be filled with rubber until the mold is smooth. Smooth-On makes an additive called Thi-Vex which when added to the rubber makes it easier to build up volume and fill in voids.The smoother your last coat of rubber the easier it will be to unmold it from the original.
The rubber should be allowed to set,preferably overnight. The rubber mold is relatively thin still and as such will require a "mother mold" or shell to help it hold it's shape. I created this shell by first coating the rubber mold with a dose of release agent spray. I then laid on some molding plaster suffused with fibers of hemp to a thickness of approximately 1/2" .
Once the plaster shell has hardened it is carefully pried loose and removed. The rubber mold is peeled off of the original flower model and placed inside the shell.
Enough molding plaster to fill the mold is mixed. It should have the consistency of heavy cream. Working quickly, the plaster is then applied to the inside of the mold as a thin layer, thick enough to coat all the rubber without being thick enough to trap air pockets. Any bubbles in this coat of plaster will show up as voids in the finished piece. To the remaining plaster is now added a small bit of hemp fiber for additional strength, this mixture is pored into the mold to cover the previous thin coat. I worked the plaster mix onto the sides of the mold by hand to keep the piece light but it could have been poured in solid.
To replicate the smaller center flower section I took a different approach. I cleaned the original,piece and attached it to the bottom of a bucket. I then sprayed the piece and the inside of the bucket with release agent. Smooth-On Brush On rubber was then poured into the bucket until it covered the original. The bucket was agitated to force any bubbles in the rubber to the surface.The mold was allowed to set overnight pulled out of the bucket and was cut off the original piece. I was then able to use a part of the bucket as a shell mold to hold the two pieces of rubber tightly together while casting. Casting this smaller piece was different from the casting larger part of the flower in as I was able to simply pour in the "heavy cream" plaster mixture and shake the bubbles out by maneuvering the bucket.
The two pieces were attached to the ceiling separately. The large heavy base flower was put up with a toggle bolts, the lighter smaller center rosette was then attached to the base flower using molding plaster as adhesive. After some little filling around the edges and a coat of primer it is impossible to tell the new rosette from the reinstalled original.