This is a 50:50 kaolin:silica wash. While economical, this is not an ideal recipe. It gets too hard and shrinks too much during drying and firing. It is better to use a recipe that remains somewhat powdery to the touch. Pure calcined alumina with a little calcined kaolin (about 10%) gives almost zero shrinkage. Add more kaolin if it fires too powdery or less if fires too hard. Add 1% CMC gum to produce a very paintable material (it will take some time to dry).
A high-melting powder applied to kiln shelves to prevent ware from sticking to the shelf during firing. It is also used as a parting agent for to keep elements of fired ware from sticking to each other.