As I stated earlier, I don’t recommend re-fitting ornate Plaster Of Paris frames. However, if it means the difference between repairing an existing frame you already own and purchasing a new one there is a way to fix chipped or broken edges. First, I want you to notice that on most plaster frames such as the one shown in the photo, there is a repeating pattern on the corners and sides. To do this repair, you will need to make a clay mold of the unbroken matching side of the frame, fill it with plaster and use the new piece to repair the broken side.
What kind of clay should you use to make your mold?
The easiest clay to work with is made by Crayola and it air dries. Soft and pliable, Crayola Air-Dry (brand name) modeling clay allows the formation stable arts and crafts without the need for an oven or kiln. Smoother, finer, and less sticky than traditional clay, Air-Dry Clay softens easily with water and is a quick clean up. It’s ideal for traditional methods. It works just fine for this type of quick project and doesn’t require much more than your kitchen table and sink as a workspace. Most art supply houses carry it, or you can order it from http://www.dickblick.com/products/crayola-air-dry-clay/.
Working with Plaster of Paris
Plaster of Paris is a great material to use for basic sculptures and craft projects because it is easy to prepare and sets in a few minutes. Mixing Plaster of Paris is easy. The powder is very light and fine. to avoid getting the powder to the eyes and nose, wear a dust mask. Never mix Plaster of Paris with your bare hands.
Cover your work area with a plastic mat or with newspapers. Find a mixing container (preferably a disposable one) that will hold the size of the concoction you intend to make up.
The ideal ratio for the mixture is 2 parts Plaster of Paris to 1 part water.
Start with the water. Measure out the Plaster of Paris in another container. Break up any lumps of powder with a spoon.
Start adding the Plaster of Paris powder to the water in your mixing container by sprinkling or sifting the powder over the water. Do not add the powder in one clump; instead try to sprinkle the powder over as much area as you can.
Do not mix yet. Instead, tap the side of your mixing container with a spoon to disperse the powder into the water and remove any air bubbles.
Continue adding the Plaster of Paris, patting the sides of the container as you add the powder. Your cue to stop is when you notice that the powder has almost covering the surface and is no longer being as easily absorbed. Gently blend the Plaster of Paris mixture until it reaches a smooth consistency. Do not stir strongly or you may create air bubbles.
*If colored Plaster of Paris mixture is desired, add some poster paint once the mixture is free of lumps and has a smooth consistency. Continue mixing from side to side until the color is uniformly dispersed.
Allow the mixture to stand for a minute before pouring it into your mold. Don’t attempt to wash your left over residue down the drain! It will clog your pipes! Left over mixture should be allowed to harden and then thrown into the garbage can.
Using the Mold
Step 1: find the broken pattern on the side of the frame or corner. Using plaster carving tools try to make the broken edge as flat as possible.
Step 2: make a clay mold of the opposite unbroken side. You can find the instructions for making a simple clay mold on this site: http://www.instructables.com/id/Simple-Clay-Mold/
Soften the clay and then push it onto the unbroken side of the frame. You should use a large enough piece of clay so that when you turn the mold over, it will fit squarely on a flat surface. If the clay has dried before you pour the plaster, it might be wise to take the precaution of spraying the inside of the mold with cooking spray. If you do this, take a small brush or Q-tip and smooth out any bubbles in the oil before pouring the plaster. Allow the plaster to set.
Step 3: Fill it with Plaster of Paris and let it set hard, peel off the mold and glue the replacement piece to the broken place. Since it is on the opposite side, you will probably need to reverse it in order for it to fit properly.
It will also be necessary to fit the new piece to the broken space by doing some scraping with plaster knives and probably some sanding once it is glued down to make the new piece fit smoothly with the frame. I then recommend re-painting or if you are using gold leaf covering the entire frame for better color matching.