A 20th Century Shop Tour--Personnel, Materials, and Processes

Before discussing how decorative forms such as cornices, medallions, and ceiling coffers are repaired onsite and in the shop by ornamental plasterers, the "shop tour" explains traditional casting processes used in conjunction with updated materials. A shop tour can be exciting, but confusing to the layman without some explanation of modeling, molding, and casting activities. For a prospective client, a visit to the plaster studio or site can be of value in choosing a qualified plastering contractor.

Shop and Personnel. Generally, a highly functional shop should look well organized--that is, not in disarray with remnants of past projects lying about to impede current production. Old molds may be in abundance, but hanging from the wall or otherwise "on file." Machinery (saws and drill presses) and hand tools should appear well maintained. In short, one might evaluate such a studio as one does an auto mechanic's shop: does it inspire confidence? This is the time to look around and ask questions. What is the shop's past project work experience? Is the firm mostly involved in new construction work or total reconstruction? More important than the way the shop looks, is the personnel sufficiently experienced in making repairs to historic decorative plaster? What about training and apprenticeships? How did the staff learn the trade? The more that is known about the total operation the better.

This plaster studio is well organized, with ample work space. Note the plaster casts hanging neatly on the wall. Photo: Berry and Homer, Philadelphia.

Molding Rubber. Familiarity with contemporary molding rubbers is desirable. There are several formulations currently on the market. In the past, flexible molds were made with hide glue melted in a double boiler and poured over plaster originals which had been prepared with an appropriate parting agent. Of the newer rubbers, latex (painted on the model coat by coat) is time consuming and has little dimensional accuracy; polysulfide distorts under pressure; and silicone is needlessly expensive. Urethane rubber, with a 30-durometer hardness, is the current choice. Urethanes are manufactured as pourable liquids and as thixotropic pastes so that they can be used on vertical or overhead surfaces. The paste is especially useful for onsite impressions of existing ornament; the liquid is best used in the shop much as hide glue or gelatin was historically. Urethane rubber has the ability to reproduce detail as fine as a fingerprint and does not degrade during most ornamental plaster projects. No flexible molding material lasts forever, so spare casts should be maintained for future remolding.

Molding Plaster. Molding plaster will also be in evidence; it is the product most similar to that used historically. This plaster is finely ground to accept the detail of the rubber molds, not so hard as to prohibit tooling, and combines readily with finish lime. High-strength plaster is available in varying densities, some with added components for specific purposes. Most shops maintain these varieties, but use molding plaster for typical work.