What was the substance used to stuff in the mummy's nostrils?
Over many years, different substances were used. This site states that beeswax was often used to plug the mummy's nostrils.
http://www.archaeologyexpert.co.uk/EgyptianMummies.html
To find out the substance used to stuff in the mummy's nostrils, you can refer to a reliable source such as books, scholarly articles, or museum collections that specialize in ancient Egyptian mummification. The substance commonly used in Egyptian mummification to stuff the nostrils was a mixture called natron.
Natron is a naturally occurring mineral made up of hydrated sodium carbonate, sodium bicarbonate, and small amounts of sodium chloride and sodium sulfate. It was available in large quantities in ancient Egypt and had various applications, one of which was preserving bodies during the mummification process.
During the embalming process, after the brain was removed through the nostrils using special hooks, the mummification practitioners would pack natron into the nostrils. This would help absorb moisture and prevent the decomposition of the body. Natron would act as a drying agent, effectively desiccating the tissues and preventing the growth of bacteria and other microorganisms.
By studying historical sources or consulting experts in mummification, you can gain further detailed information about the use of natron and the mummification process in ancient Egypt.