In the old days kilns were fired with wood. This resulted in flying ashes that fell on glazed pieces. To protect the glazed pieces they were put in a closed ceramic pot (saggar) during firing. This method was used in Asia (Japan, China, Korea) but also in Great Britain. The word 'saggar' is probably derived from the word 'safeguard'. The use of electric kilns made saggars redundant.
I use the saggar in the opposite way. Bisque fired pieces are put in the saggar together with colorants and combustibles. During the firing (in an electric kiln) the combustible burn and the colorants bring color to your work.
As combustibles I use natural materials like:
You can read everything in my book: "Saggar firing in an electric kiln"