This is a zircon-based kiln wash. Even though it paints on in a thin layer, there is no problem releasing the very runny glaze from the shelf.
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.
Glaze is excessively runny on firing