As many of you know, most model horses produced by sculptors for the hobby are cast in resin--that's why we call them "Artist's Resins", after all! Resin is an amazing material and comes in many varieties. The resins used in the hobby vary slightly, but nearly all are a two part, self-hardening thermoset plastic. Thermoset means that once the plastic has set up, you can't just reheat it to melt it down and use it again. Cellulose acetate (what Breyers and Stones are made of) is a thermoplastic plastic, so heating it will make it soften and then melt. Heating a thermoset plastic like resin may soften it up to a point, but it will not melt nicely so you can use it again.
Producing a model in resin is quite expensive. Uncured or unmixed resin is toxic to varying degrees, and getting good castings really requires some expensive equipment. The silicone rubber molds used to make resins are pricey, both in materials and in the very skilled labor needed to make them. The molds don't last forever, either--depending on a variety of factors, perhaps 65 castings can be expected out of each mold.
Resin is not particularly fragile, but nor is it incredibly strong. Heavy bodies on thin legs (like horses!) can cause gradual warping of the legs over time. For this reason, as well as the comfort of painters and economical reasons, hollow-cast resins are popular and common. Wire reinforced legs are also common. Producing a hollow-cast, wire-reinforced resin horse with minimal seams and bubbles requires an immense amount of skill!
When I began creating micro minis, I considered producing them in resin but quickly chose pewter instead. Many people in the hobby are not particularly familiar with pewter, but I think it's a lovely material.
Pewter is an alloy of tin and other metals. It has a relatively low melting point, around 350-450° F. That low melting point means that it can be poured into rubber molds, unlike many other metals which would burn the rubber. Typically, a vulcanized rubber mold is used to produce small pewter pieces like my micro minis. The molds my caster uses can hold 6-7 horses at a time and are vulcanized using heat and pressure, in somewhat the same way that a car tire is produced! This makes for durable molds: instead of 65 horses, a mold can often make hundreds of castings of each horse. I believe that my new horses are going to be cast using different molds, which don't last so long but do allow for complex poses without distortion (due to the high pressures and temperatures involved a certain amount of distortion is inevitable in vulcanized rubber molds).
The more durable molds are far from the major cost savings of producing small horses in pewter, though--after all, I don't really need to be able to produce many hundreds or thousands of each horse. No, the real savings are in the per-casting price, the price I have to pay for each horse on top of the fixed mold cost. For a resin micro mini, the last time I checked that cost per piece was higher than the price at which I sell the pewter micro minis. I would need to significantly increase the price (think doubling or more) to produce them in resin. This is the most important reason I have for using pewter. Even though the price of tin and therefore the cost of pewter (and the cost to me of my little horses) has nearly doubled in the last year I have not yet had to increase my prices on the micro minis.
Pewter can be painted and finished just like resin. In fact, miniature makers have been using pewter for far longer than anyone has used resin! In many cases the pewter horses have fewer and cleaner seams than resins, and the seams they do have can be cleaned the same way as resins--small files, sandpaper, and carbide scrapers.
One of the issues raised about the pewter micro minis is the surface texture or porosity some exhibit. While this can be sanded or polished down, it does add an additional step and could lead to loss of detail. For the newest horses, I am going to be using a different caster who I have been told can do amazing things with pewter. I haven't sent anything to him before, so we will see, but I have very high hopes that these will be cleaner and smoother than ever before.
Another issue comes in repairing broken pewter minis. I honestly can't say I've repaired many myself, as I'd rather simply replace the horse than spend a lot of time fixing it. However, for a painted model, simply replacing it is not so simple! Breaks can be repaired by drilling, pinning, and gluing, but that is very tricky on the tiniest micro mini legs. Depending on the horse, it might be pretty hard to simply glue certain breaks, especially if it is a very delicate leg that needs to support weight. In those cases, I would remove the broken area up to the nearest place thick enough to be drilled and have a pin inserted, then resculpt the broken area over the pin. While the additional weight of the pewter does make this more difficult than in resin, the much greater strength of pewter also means far fewer breaks than would be likely to occur in tiny, delicate resin legs. Some micro minis are more prone to breakage than others, as I have been seeing now that sales of them are handled in-house, and I'm trying to design future releases to be even less fragile while still having refined legs and details.
So, hopefully that helps clarify some of the issues with using resin for such little horses! Please feel free to post questions or comments, I'd love to hear what you think!