NNP

Release Agent.  A coating in a mold to ease the removal of a casting; a mold release. Release agents are also called parting agents and are used in most all casting applications irrespective of the type of mold or the composition of the casting. (Green sand castings are the only molds which do not need a mold release, because of the water in the sand.)

In electrogalvanic casting the release agent of bronze powders metallizes the surface, not only does the bronze powder coat the surface of the object to be cast but also makes the surface electrically conductive for the deposition of the metal. Clay models are usually coated with talc but graphite can also be used. Plaster models are coated with green soap when pouring other plasters in the casting, but light oil, silicon spray can also be used as release agents. See plaster casting.

Casting mold preparation.  The ideal surface to be cast is one that has no pores, is very hard, has no undercuts and all relief and lettering has a bevel of 5° or more. Plaques to be cast in flexible molds do not require the latter two. The hardest, finest plaster is used for coin and medal models (as Densite). Any organic material (as wood or ivory) has pores; these need to be closed off. The face of the mold is coated with shellac.

After the pattern is as hard as it can be and all pores are covered over, then the mold release is applied. Messy powders, as graphite and bronze powders, are applied with a brush. A stipple brush is used for green soap on plaster, light oil is applied with a cloth. Liquids that can be sprayed on are the easiest, but are susceptible to applying too heavy a coating. Theoretically only two microns of coating are needed for separating the mold and its casting. Release coatings need only a very light coating but it must cover the entire surface.

Popular Release Agents

Pattern or Mold                                                   Casting

Composition               Release Agent           Composition

Clay or                        Graphite (plumbago)  Plaster

Plasticine                  or talc

Epoxy                          Silicon spray               Plaster, metal

Galvano Patterns         Bronze powders          Electrodeposition:

metal, plaster,                                               Copper, silver,

epoxy                                                            gold

Gelatin                        Alum (for hardness)    Plaster

Green sand                  None required             Usually metal

Plaster                        Green soap, light         Plaster, metal

oil, commercial

mold release

Rubber, Latex,            Silicon spray               Plaster, metal

PVC

Wax                                                                Plaster, metal

Wood, Ivory                Shellac (for                 Plaster, metal

sealing pores)

excerpted with permission from

An Encyclopedia of Coin and Medal Technology

For Artists, Makers, Collectors and Curators

COMPILED AND WRITTEN BY D. WAYNE JOHNSON

Roger W. Burdette, Editor