Yes, fiberglass is a resin—a rather toxic resin in its uncured, liquid state, as is, unfortunately, the nature of most resins. Some are more toxic than others. But you’ve read the safety precautions discussed in Chapter 1, and you’ve been paying attention to them in this chapter as well, so you are well prepared for using these materials.
Forton MG (FMG), though technically a gypsum product (Hydrocal is the main ingredient), is considered a resin casting material because it incorporates plastics and fiberglass. FMG consists of a modified gypsum (hence the MG—Modified Gypsum), FGR-95, dry melamine resin powder, a hardener, a liquid polymer called Forton VF-812, and chopped fiberglass. The advantages of FMG over fiberglass are that it is odorless and nontoxic and it cures much faster. Another advantage is that it can be cast directly into an alginate mold, unlike polyester and epoxy resins. The disadvantages are that there are more components to measure and measurement must be precise, so an accurate gram scale is essential; in addition, it is heavier than polyester or epoxy resin molds. By my math, advantages outweigh disadvantages for Forton MG. (Please excuse the pun.)
You can also substitute Densite HS for the FGR-95. FGR-95 is Alpha gypsum from U. S. Gypsum, whereas Densite HS is from Georgia Pacific but is also Alpha gypsum. What is Alpha gypsum? Alpha gypsum is made by processing batches of gypsum under high pressure; it has a lower water-carrying capability and is used where strength is required, as in mold making. Ahhhh… So what’s Beta gypsum? When the hemi-hydrate of gypsum (two gypsum molecules for every one molecule of water) is formed by calcining (heating to the point of burning to ash) in kettles at atmospheric pressure, it’s called Beta gypsum. Ohhhhhh… now don’t you wish you’d paid attention in chemistry class? Sources where you can get Forton MG are listed in the appendix. A similar gypsum-based resin product is called duoMatrix, from Smooth-On.
Forton MG really doesn’t need fillers for strength because it already has them as part of the system. However, you can add metallic powders or marble powders to it to simulate the look and feel of foundry castings or natural stone sculpture. The surfaces can then be patinaed using foundry etching acids or be polished to a high shine.