Traditional masks of Nepal can be devided into two sorts: "tribal" masks which belong to the ethnic groups such as Gurung, Magar, Tharu, Rai etc and which are also referred to as "shaman's" masks; and "classical" masks which represent Hindu and Buddhist gods and goddesses.
The classical Nepali masks always represent deities or demons and never the dead or the ancestors. Besides, masks are more often used in processions and in rituals rather than in theatrical performances.
There are three categories of masks: first, masks representing faces of gods which are worn by human beings mostly during ritual dances, a second category, masks which serve as ornamental motifs for decorating a temple, a chariot during a procession, a vase used as a ritual object. A third category includes masks in metal or stone representing also the face of a god and which are worshipped like statues. They are called statue-masks. The metal used for making them are silver, brass or bronze. These masks are often decorated with jewels and when they are not worshipped are kept in a wooden box inside the temple.
Akash Bhairav statue-mask, Indra Chowk, Kathmandu
The masks which are used in ritual dances are made of such materials as papier mache, wood plastered with clay and linen, and are painted in lively colours. So, such masks are never old as they can't survive long in such climate. Besides, according to tradition many masks are supposed to be destoryed after use and be re-made the following year. The oldest known Nepali masks date from the 17the century. The deities represented by masks are numerous: Shiva, Bhairava, Ganesh, Kumari, Varahi, Durga, Laxmi, Sima and Duma, these last two are said to be popular names for lion and tiger. Some people explain that these two masks are a couple, the white-faced Sima being the male; the others think they are two goddesses.
Lakhe dance during Indra Jatra festival
A dancer in a ritual mask
A dance mask, Nepal, 19c, V&A museum
The masks are made of clay mixed with bits of cotton and a gum-like paste made from wheat-flour. The clay mask forms are left to dry on the moulds for about four days. Later the masks are painted. Each colour has a certain significance: red is associated with anger, blue black with energy and power, white with purity and death. Previously only vegetable and mineral colours were used. Now the painters use mainly chemical colours.
A few words about tribal masks.They have been very little studied. Most of the masks are made of wood and are of human dimensions. In contrast with classical masks, the tribal mask maker it seems had more liberty of expression and could use his imagination more freely.
Tribal mask, Nepal, 19c, musée du quai Branly, photo Thierry Ollivier, Michel Urtado
The masks of the Magar and Gurung tribes are of hardwood covered with a glossy, high patina from exposure to smoke and butter fat. Masks of the lowland Tharu people are often of a softer wood, pigmented with polychrome or white kaolin clay. The Rai people are known to make house-protecting masks from tree fungus while some other masks are created from felt and goat skin.
An interesting article about tribal masks of Nepal here