A. There are a number of different ways to replicate broken or missing ornaments on frames. From a conservational approach, an ornament should be repaired or replaced with the same material if possible; a carved wood section should be replaced with carved wood; compo should be replaced with compo, etc. Many times however, one may run across ornamental decoration that is made from plaster so what I find myself using must of the time is a plaster-like gypsum product called Hydrocal that is somewhat harder than what is sold as Plaster of Paris today.

To create the mold, something as simple as Roma Plastilina Clay, Grey Green #2, may be used. It's oil-based so it stays permanently malleable and so your casting impression only works once and therefore cannot be reused. The material I use most often is a two part silicone putty called Knead-A-Mold (although I believe they've changed the name) much like what dentists sometimes use.  

Casting an ornament can go from very easy to very difficult, depending upon the frame you're working on. Great care must be taken as irreparable damage can be done if not performed properly. The condition of the frame and the finish needs to be determined and what kind of protective precautions need to be taken. One must be very careful, especially if the finish is an antique water gilded surface.

An ornament that is being used to provide a duplicate ornament may first be given a thin coat of shellac to help protect the surface followed by a light dusting of talcum powder to act as a resist. If this is not practical or if it may cause problems by interfering with the existing tone of the frame, use only the talc. The problem one faces with mold casting is the possibility of removing the finish from the ornament being copied. If the frame is valuable it is better to leave it in the hands of someone experienced to avoid future heartache unless one can properly repair aged water gilding in the event of a mishap.

Meanwhile, to create a mold, roll some of the clay or equal parts of the two part modeling putty in your hands and knead it until it becomes workably soft. Press the modeling material onto the ornament which is to be used to create a copy for the broken or missing ornament section. Make sure the image being copied is one that faces in the right direction.

Plastilina Clay may be removed once it has been sufficiently pressed around the ornament; knead-A-Mold takes about 6-15 minutes to set while on the ornament. It moves and settles in to the crevices, which is good and therefore provides more detail than the clay. However, make sure the silicone is supported while in place so it doesn't move away from the ornament being casted. Small sections of putty can be fitted around the mold to provide support. Carefully remove the mold from the frame and allow the silicone version to cure for an hour.

Prepare the hydrocal in a 1:5 hydrocal to distilled water ratio. Be mindful as it sets quickly. Pour the hydrocal into either the clay or silicone mold, occasionally tapping the mold on a tabletop to help disperse air bubbles. After the cast has dried sufficiently, carefully remove the cast from the mold (approximately one hour, depending on the weather) and let it harden.

The cast may then be trimmed with an exacto knife, sanded where necessary, and placed into the area of the missing ornament secured with the use of polyvinyl acetate (PVA glue - also known in the US as Elmer's Glue, good in part due to its reversibility).

There are of course other casting methods but the above method can be used for many day to day applications.