Repairing Historic Ornamental Plaster

Cornice. A plain run or ornamented plaster cornice which has undergone damage or severe deterioration can often be repaired. Footage which is beyond repair should be identified and be carefully demolished to expose the underlying structure beneath to which the molding was secured. To replace the missing lengths, the first step is to obtain a cross-section, or profile, through the cornice from finish ceiling to finish wall lines. This is best accomplished using one of these methods:

1. A section through the cornice may be determined by sawing through the molding, inserting a sheet metal blank in the slot and tracing the profile directly on the template. This is considerably more accurate than the profile gauge, but will require repointing the saw kerf; alternatively, the cut may be made on one of the deteriorated pieces, provided it was removed as an intact unit.

2. The section may be obtained by making a thixotropic rubber impression of the molding, casting the result in fresh plaster and sawing through the cast to transfer the cross-section to a sheet metal template.

With the section determined, it is drawn onto 22-gauge galvanized sheet metal, cut with tin snips and carefully filed to the line. The template is checked periodically against the original profile to assure a perfect match. With the template blade finally complete, it is nailed to stock and slipper, ready for running the replacement footage.

Short lengths of new cornice are best run on a bench using gypsum and lime; the reproduction molding should be somewhat longer than the required length. The new footage is cut and fit in place to match the existing cornice, then securely countersunk-screwed to studs, joists and/or blocking. The resulting joints are pointed with flat mitering rods, flush with adjacent members.

Longer lengths of cornice may be run in place, much as they were historically. Care should be taken that the position of the running mold engages with the existing work at either end of the run. Yet another method is to bench run the cornice to five or six feet, make a rubber mold of the model, and precast the replacement parts either at the site or in the shop.

If the damaged cornice is ornamented, samples of the enrichment should be removed, making sure that whole original units are obtained. This is a difficult process, since these units were stuck into plain-run recesses called "sinkages" using plaster as an adhesive. In order to insert a flat chisel behind the ornament to break the bond, some units may have to be sacrificed. Sacrifice should be minimal. The excised enrichment should then be removed to the shop for rubber molding and casting either with or without the paint buildup, depending on the demands of the project. Whereas molding with several layers of paint make it hard to discern new casts from originals, paint-stripped molding reveals the remarkable talents of the period model-makers. As noted, contemporary rubber materials have "fingerprint detail" capability. Modern casts are then applied to the new or original runs, again using plaster as an adhesive.