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' comes from 'safeguard'. The use of electric kilns made saggars redundant.
I use the saggar in the opposite way. The wood and other materials are put in the saggar together with the work. I fire the saggar in an electric kiln. This means you cannot use salts, chlorides and sulfates s colorants because they are harmful for your elements.
As combustibles I use natural materials like: