Sơn mài is a costly form of lacquer painting in Vietnam. Here's what makes these paintings so expensive

Sơn mài is a traditional Vietnamese form of lacquer painting. Created using a toxic lacquer harvested from one region of the country, it requires months of application and sanding back layers of paint to build up the image. Last year, a sơn mài painting sold at auction for $972,000. So what makes these paintings so special? And why are they so expensive?