The craft requires artists to use a special bamboo pen with a hook-tipped brush inside a glass container that usually features a very small opening.

"The inner wall of those containers is often frosted, providing a rough surface for coloring, as a smooth surface cannot hold the pigments effectively," Li explains.

Traditional snuff bottles were most popular for the art, but any glass items of the same texture can be used.

"The old-fashioned snuff bottles with interior paintings are now sought after as collectibles and gifts, while colored glazed beads featuring the art can be accessories," Li says.

Compared with working on the paper, interior painting is much more complex.

"One needs to plan and conceptualize in advance, taking into account the curvature of the inner wall of the bottle. The process is intricate and requires dexterity and skill," she adds.

All the lines and outlines in the art form require absolute precision.

"When drawing the lines, you need to go slowly without bending them. It requires a delicate touch and extensive practice," Li says.

The mouth of a snuff bottle is usually not big enough for a soybean to pass through. Consequently, the complex patterns drawn inside represent an extraordinary feat.

An artist has to apply sketching, stippling, cross-hatching (an artistic technique used to add shadow and dimension to drawn objects) and coloring to present the art.