A separating agent for a bivalve cast is all except:

A separating agent is used to prevent materials from bonding together when creating a bivalve cast, which is commonly used in foot orthotic design. The purpose of a separating agent is to ensure that the cast can be removed without damaging it or the underlying structure.

Soap, powder, and petroleum jelly are all substances that can create a barrier between the casting material and the surface it adheres to. Soap can provide a slick surface, making it easier to detach the cast once it has set. Powder, such as talcum or cornstarch, helps to absorb moisture and provide a non-stick surface that aids in the removal process. Petroleum jelly serves a similar function by offering lubrication that prevents adhesion.

On the other hand, water does not act as a separating agent. In fact, water can sometimes promote adhesion between materials due to its ability to enhance the bonding process. Using water as a separating agent could lead to issues with the cast adhering to the model or form being used, thus compromising the integrity of the bivalve cast.

Therefore, identifying water as not being a suitable separating agent underscores its function in maintaining the quality and usability of the cast during the orthotic fabrication process.