Every Breyer model starts with an artist’s rendering. Various artists, either staff or commissioned, sculpt the first rendering of each model. Then, Breyer casts a mold out of a mix of copper and steel. That mold is then injected and dried according to the material used. Breyer uses ceramic, cellulose acetate, or other resins to create each model. There’s no mass production, each model is created one at a time, using the same copper/steel mold.
Once the mold is set and ready for painting, everything else is done by hand. First, the model is airbrushed for the larger details. Finer details, like shadows, eyes, or fine markings, are painted by hand. According to Breyer, as many as 20 artists might work on a single model. That explains why not every single model from a particular mold looks exactly the same. It also explains how they get those fine details that look so life-like.
More than one model series might be produced from a single mold. For example, Breyer has one mold for Stormchaser (#700301), Apropos (#189) and Big Ben (#483). If you look at the silhouette of each of these models, it’s the same. That’s because Breyer has only one “Big Ben” mold. They simply paint it differently and add different accessories to make Stormchaser and Apropos.