This is the final version of the earlier sculpture of the American bison. But because bright white sculpture tends to wash out the details and be a bit glaring on the eye, it's common to add various surface treatments. This can be a glaze - of which you can learn more about if you click here. But glazes can a bit complex, bewildering, and not always suitable for outdoor sculptures. On the other hand, simpler effects are possible by adding a "patina".
Now the word itself, patina, is defined as "a surface appearance of something grown beautiful especially with age or use". Well, it is true that if a surface has grown beautiful through age or use, it has acquired a patina. But patinas you see on many sculptures are in fact put on deliberately by - ah - "artificial means". In fact, making patinas themselves can be so complex that there are specialists called patineurs who do nothing but put the patina on the statue. You may also be shocked! shocked! to learn that when you see a particularly beautiful patina it is almost a sure bet that the artist hired a patineur to do the job.
Putting patinas on metal is a complex process which can involve paying particular attention to the concentrations and amounts of the solutions used and the amount of heat applied to the statue before and after applying the chemicals. Putting a patina on clay sculptures can be simpler. You can paint the surface which was a common enough procedure for the Greek and Romans who would have been surprised at the aesthetic praise given today to white marble. Or you can add a thin suspension of clay - "a slip" - to the surface, or as here, using a mineral of the desired color. Either way whether slip or mineral patinas, you almost always have a subsequent firing which can change the final appearance.
The patina here is called "oxide". Of course, chemists will go into spittle flinging diatribes reminiscent of Donald Duck in one of his most uncontrollable temper tantrums since "oxide" can be almost any combination of elements and oxygen. But "oxide" for fired clay sculptures tends to mean iron oxide - a pardonable simplification. An advantage of patinas from minerals or slips (compared to glazes) is they are generally more predictable. Glazes are formulated as complex water suspensions of minerals and can sometimes contain as many as twenty individual components - not counting the water!. They are prepared with some difficulty and only an expert can apply them to a statue in a way where he or she can tell how everything is going to look after the final firing. Sometimes even the experts are surprised.
But if you use something like iron oxide - chemically the same as rust - you just slurry the powder in water and brushed it on. Then you can rub the surface to achieve various effects. Then a final firing and - hey, presto! - you've got your sculpture which if you're lucky - as this case was - deemed worthy bu someone to purchase.
You can see an animated rotating view of this sculpture if you click here.